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Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined ** Domain of definition of a partial function ** Natural domain of a partial function **Domain of holomorphy of a function * ...
of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
, and are present in most of its
habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
s. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria are vital in many stages of the nutrient cycle by recycling nutrients such as the fixation of nitrogen from the atmosphere. The nutrient cycle includes the
decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and ...
of
dead bodies ''Dead Bodies'' is a 2003 Irish drama film by Robert Quinn starring Andrew Scott, Katy Davis, Eamonn Owens, Darren Healy and Kelly Reilly. The screenplay was written by Derek Landy. Plot Tommy McGann (Scott) gets back together with his ex-girl ...
; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and
cold seep A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs, often in the form of a brine pool. ''Cold'' does not mean that the temperature of the see ...
s,
extremophile An extremophile (from Latin ' meaning "extreme" and Greek ' () meaning "love") is an organism that is able to live (or in some cases thrive) in extreme environments, i.e. environments that make survival challenging such as due to extreme tem ...
bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
, to energy. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology. Humans and most other animals carry millions of bacteria. Most are in the gut, and there are many on the skin. Most of the bacteria in and on the body are harmless or rendered so by the protective effects of the immune system, and many are beneficial, particularly the ones in the gut. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. The most common fatal bacterial diseases are respiratory infections. Antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. Bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium, copper and other metals in the mining sector, as well as in biotechnology, and the manufacture of antibiotics and other chemicals. Once regarded as plants constituting the class ''Schizomycetes'' ("fission fungi"), bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
and rarely harbour membrane-bound organelles. Although the term ''bacteria'' traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that
evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...
from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.


Etymology

The word ''bacteria'' is the plural of the New Latin ', which is the latinisation of the Ancient Greek ('), the diminutive of ('), meaning "staff, cane", because the first ones to be discovered were rod-shaped.


Origin and early evolution

The ancestors of bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life. Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage. The most recent common ancestor of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billion–3.2 billion years ago. The earliest life on land may have been bacteria some 3.22 billion years ago. Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea. This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial
symbionts Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya (sometimes in highly reduced form, e.g. in ancient "amitochondrial" protozoa). Later, some eukaryotes that already contained mitochondria also engulfed cyanobacteria-like organisms, leading to the formation of chloroplasts in algae and plants. This is known as
primary endosymbiosis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and pos ...
.


Habitat

Bacteria are ubiquitous, living in every possible habitat on the planet including soil, underwater, deep in Earth's crust and even such extreme environments as acidic hot springs and radioactive waste. There are approximately 2×1030 bacteria on Earth, forming a biomass that is only exceeded by plants. They are abundant in lakes and oceans, in arctic ice, and
geothermal springs A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by cir ...
where they provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. They live on and in plants and animals. Most do not cause diseases, are beneficial to their environments, and are essential for life. The soil is a rich source of bacteria and a few grams contain around a thousand million of them. They are all essential to soil ecology, breaking down toxic waste and recycling nutrients. They are even found in the atmosphere and one cubic metre of air holds around one hundred million bacterial cells. The oceans and seas harbour around 3 x 1026 bacteria which provide up to 50% of the oxygen humans breathe. Only around 2% of bacterial species have been fully studied.


Morphology

Size. Bacteria display a wide diversity of shapes and sizes. Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0  micrometres in length. However, a few species are visible to the unaided eye—for example, '' Thiomargarita namibiensis'' is up to half a millimetre long, '' Epulopiscium fishelsoni'' reaches 0.7 mm, and '' Thiomargarita magnifica'' can reach even 2 cm in length, which is 50 times larger than other known bacteria. Among the smallest bacteria are members of the genus '' Mycoplasma'', which measure only 0.3 micrometres, as small as the largest viruses. Some bacteria may be even smaller, but these ultramicrobacteria are not well-studied. Shape. Most bacterial species are either spherical, called '' cocci'' (''singular coccus'', from Greek ''kókkos'', grain, seed), or rod shaped, called '' bacilli'' (''sing''. bacillus, from
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''baculus'', stick). Some bacteria, called '' vibrio'', are shaped like slightly curved rods or comma shaped; others can be spiral shaped, called '' spirilla'', or tightly coiled, called '' spirochaetes''. A small number of other unusual shapes have been described, such as star-shaped bacteria. This wide variety of shapes is determined by the bacterial cell wall and cytoskeleton, and is important because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, swim through liquids and escape predators. Multicellularity. Most bacterial species exist as single cells; others associate in characteristic patterns: '' Neisseria'' forms diploids (pairs),
streptococci ''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occu ...
form chains, and staphylococci group together in "bunch of grapes" clusters. Bacteria can also group to form larger multicellular structures, such as the elongated filaments of '' Actinomycetota'' species, the aggregates of '' Myxobacteria'' species, and the complex hyphae of ''
Streptomyces ''Streptomyces'' is the largest genus of Actinomycetota and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinomycetota, streptomycetes are gram-positiv ...
'' species. These multicellular structures are often only seen in certain conditions. For example, when starved of amino acids, myxobacteria detect surrounding cells in a process known as quorum sensing, migrate towards each other, and aggregate to form fruiting bodies up to 500 micrometres long and containing approximately 100,000 bacterial cells. In these fruiting bodies, the bacteria perform separate tasks; for example, about one in ten cells migrate to the top of a fruiting body and differentiate into a specialised dormant state called a myxospore, which is more resistant to drying and other adverse environmental conditions. Biofilms. Bacteria often attach to surfaces and form dense aggregations called biofilms, and larger formations known as microbial mats. These biofilms and mats can range from a few micrometres in thickness to up to half a metre in depth, and may contain multiple species of bacteria, protists and archaea. Bacteria living in biofilms display a complex arrangement of cells and extracellular components, forming secondary structures, such as microcolonies, through which there are networks of channels to enable better diffusion of nutrients. In natural environments, such as soil or the surfaces of plants, the majority of bacteria are bound to surfaces in biofilms. Biofilms are also important in medicine, as these structures are often present during chronic bacterial infections or in infections of implanted medical devices, and bacteria protected within biofilms are much harder to kill than individual isolated bacteria.


Cellular structure


Intracellular structures

The bacterial cell is surrounded by a cell membrane, which is made primarily of phospholipids. This membrane encloses the contents of the cell and acts as a barrier to hold nutrients,
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s and other essential components of the cytoplasm within the cell. Unlike
eukaryotic cells Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...
, bacteria usually lack large membrane-bound structures in their cytoplasm such as a
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: * Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
, mitochondria, chloroplasts and the other organelles present in eukaryotic cells. However, some bacteria have protein-bound organelles in the cytoplasm which compartmentalize aspects of bacterial metabolism, such as the carboxysome. Additionally, bacteria have a multi-component cytoskeleton to control the localisation of proteins and nucleic acids within the cell, and to manage the process of cell division. Many important biochemical reactions, such as energy generation, occur due to concentration gradients across membranes, creating a potential difference analogous to a battery. The general lack of internal membranes in bacteria means these reactions, such as electron transport, occur across the cell membrane between the cytoplasm and the outside of the cell or periplasm. However, in many photosynthetic bacteria the plasma membrane is highly folded and fills most of the cell with layers of light-gathering membrane. These light-gathering complexes may even form lipid-enclosed structures called chlorosomes in green sulfur bacteria. Bacteria do not have a membrane-bound nucleus, and their
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
tic material is typically a single circular bacterial chromosome of DNA located in the cytoplasm in an irregularly shaped body called the nucleoid. The nucleoid contains the chromosome with its associated proteins and RNA. Like all other organisms, bacteria contain ribosomes for the production of proteins, but the structure of the bacterial ribosome is different from that of eukaryotes and archaea. Some bacteria produce intracellular nutrient storage granules, such as glycogen, polyphosphate, sulfur or
polyhydroxyalkanoates Polyhydroxyalkanoates or PHAs are polyesters produced in nature by numerous microorganisms, including through bacterial fermentation of sugars or lipids. When produced by bacteria they serve as both a source of energy and as a carbon store. ...
. Bacteria such as the photosynthetic cyanobacteria, produce internal gas vacuoles, which they use to regulate their buoyancy, allowing them to move up or down into water layers with different light intensities and nutrient levels.


Extracellular structures

Around the outside of the cell membrane is the cell wall. Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by peptides containing D- amino acids. Bacterial cell walls are different from the cell walls of plants and fungi, which are made of cellulose and chitin, respectively. The cell wall of bacteria is also distinct from that of achaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, and the antibiotic penicillin (produced by a fungus called '' Penicillium'') is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan. There are broadly speaking two different types of cell wall in bacteria, that classify bacteria into Gram-positive bacteria and Gram-negative bacteria. The names originate from the reaction of cells to the Gram stain, a long-standing test for the classification of bacterial species. Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins. Most bacteria have the Gram-negative cell wall, and only members of the '' Bacillota'' group and actinomycetota (previously known as the low G+C and high G+C Gram-positive bacteria, respectively) have the alternative Gram-positive arrangement. These differences in structure can produce differences in antibiotic susceptibility; for instance,
vancomycin Vancomycin is a glycopeptide antibiotic medication used to treat a number of bacterial infections. It is recommended intravenously as a treatment for complicated skin infections, bloodstream infections, endocarditis, bone and joint infection ...
can kill only Gram-positive bacteria and is ineffective against Gram-negative pathogens, such as '' Haemophilus influenzae'' or '' Pseudomonas aeruginosa''. Some bacteria have cell wall structures that are neither classically Gram-positive or Gram-negative. This includes clinically important bacteria such as mycobacteria which have a thick peptidoglycan cell wall like a Gram-positive bacterium, but also a second outer layer of lipids. In many bacteria, an S-layer of rigidly arrayed protein molecules covers the outside of the cell. This layer provides chemical and physical protection for the cell surface and can act as a macromolecular diffusion barrier. S-layers have diverse functions and are known to act as virulence factors in '' Campylobacter'' species and contain surface
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s in ''
Bacillus stearothermophilus ''Geobacillus stearothermophilus'' (previously ''Bacillus stearothermophilus'') is a rod-shaped, Gram-positive bacterium and a member of the phylum Bacillota. The bacterium is a thermophile and is widely distributed in soil, hot springs, ocean s ...
''. Flagella are rigid protein structures, about 20 nanometres in diameter and up to 20 micrometres in length, that are used for motility. Flagella are driven by the energy released by the transfer of ions down an electrochemical gradient across the cell membrane. Fimbriae (sometimes called " attachment pili") are fine filaments of protein, usually 2–10 nanometres in diameter and up to several micrometres in length. They are distributed over the surface of the cell, and resemble fine hairs when seen under the electron microscope. Fimbriae are believed to be involved in attachment to solid surfaces or to other cells, and are essential for the virulence of some bacterial pathogens.
Pili Pili may refer to: Common names of plants * '' Canarium ovatum'', a Philippine tree that is a source of the pili nut * ''Heteropogon contortus'', a Hawaiian grass used to thatch structures Places * Pili, Camarines Sur, is a municipality in the ...
(''sing''. pilus) are cellular appendages, slightly larger than fimbriae, that can transfer genetic material between bacterial cells in a process called
conjugation Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics *Complex conjugation, the change ...
where they are called conjugation pili or sex pili (see bacterial genetics, below). They can also generate movement where they are called type IV pili.
Glycocalyx The glycocalyx, also known as the pericellular matrix, is a glycoprotein and glycolipid covering that surrounds the cell membranes of bacteria, epithelial cells, and other cells. In 1970, Martinez-Palomo discovered the cell coating in animal c ...
is produced by many bacteria to surround their cells, and varies in structural complexity: ranging from a disorganised
slime layer A slime layer in bacteria is an easily removable (e.g. by centrifugation), unorganized layer of extracellular material that surrounds bacteria cells. Specifically, this consists mostly of exopolysaccharides, glycoproteins, and glycolipids. Therefo ...
of extracellular polymeric substances to a highly structured capsule. These structures can protect cells from engulfment by eukaryotic cells such as macrophages (part of the human immune system). They can also act as antigens and be involved in cell recognition, as well as aiding attachment to surfaces and the formation of biofilms. The assembly of these extracellular structures is dependent on bacterial secretion systems. These transfer proteins from the cytoplasm into the periplasm or into the environment around the cell. Many types of secretion systems are known and these structures are often essential for the virulence of pathogens, so are intensively studied.


Endospores

Some genera of Gram-positive bacteria, such as '' Bacillus'', '' Clostridium'', ''
Sporohalobacter ''Sporohalobacter'' are a genus of anaerobic bacteria belonging to the family Haloanaerobiaceae. The organisms are spore-forming bacteria that grow in hypersaline environments. See also * List of bacterial orders * List of bacteria genera This ...
'', ''
Anaerobacter ''Anaerobacter'' is a genus of Gram-positive bacteria related to ''Clostridium''. They are anaerobic chemotrophs and are unusual spore-formers as they produce more than one spore per bacterial cell (up to five). They fix nitrogen Nitrogen ...
'', and '' Heliobacterium'', can form highly resistant, dormant structures called '' endospores''. Endospores develop within the cytoplasm of the cell; generally a single endospore develops in each cell. Each endospore contains a core of DNA and ribosomes surrounded by a cortex layer and protected by a multilayer rigid coat composed of peptidoglycan and a variety of proteins. Endospores show no detectable metabolism and can survive extreme physical and chemical stresses, such as high levels of UV light, gamma radiation, detergents, disinfectants, heat, freezing, pressure, and desiccation. In this dormant state, these organisms may remain viable for millions of years, and endospores even allow bacteria to survive exposure to the vacuum and radiation in space, possibly bacteria could be distributed throughout the Universe by space dust, meteoroids, asteroids, comets, planetoids or via directed panspermia. Endospore-forming bacteria can also cause disease: for example, anthrax can be contracted by the inhalation of '' Bacillus anthracis'' endospores, and contamination of deep puncture wounds with ''
Clostridium tetani ''Clostridium tetani'' is a common soil bacterium and the causative agent of tetanus. Vegetative cells of ''Clostridium tetani'' are usually rod-shaped and up to 2.5 μm long, but they become enlarged and tennis racket- or drumstick-shaped when ...
'' endospores causes tetanus, which like botulism is caused by a toxin released by the bacteria that grow from the spores. Clostridioides difficile infection, which is a problem in healthcare settings is also caused by spore-forming bacteria.


Metabolism

Bacteria exhibit an extremely wide variety of metabolic types. The distribution of metabolic traits within a group of bacteria has traditionally been used to define their taxonomy, but these traits often do not correspond with modern genetic classifications. Bacterial metabolism is classified into nutritional groups on the basis of three major criteria: the source of energy, the electron donors used, and the source of carbon used for growth. Bacteria either derive energy from light using photosynthesis (called phototrophy), or by breaking down chemical compounds using oxidation (called chemotrophy). Chemotrophs use chemical compounds as a source of energy by transferring electrons from a given electron donor to a
terminal electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Electron acceptors are sometimes mista ...
in a redox reaction. This reaction releases energy that can be used to drive metabolism. Chemotrophs are further divided by the types of compounds they use to transfer electrons. Bacteria that use inorganic compounds such as hydrogen, carbon monoxide, or ammonia as sources of electrons are called lithotrophs, while those that use organic compounds are called organotrophs. The compounds used to receive electrons are also used to classify bacteria: aerobic organisms use oxygen as the terminal electron acceptor, while anaerobic organisms use other compounds such as nitrate, sulfate, or carbon dioxide. Many bacteria get their carbon from other organic carbon, called heterotrophy. Others such as cyanobacteria and some
purple bacteria Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with variou ...
are autotrophic, meaning that they obtain cellular carbon by
fixing Fixing may refer to: * The present participle of the verb "to fix", an action meaning maintenance, repair, and operations * "fixing someone up" in the context of arranging or finding a social date for someone * "Fixing", craving an addictive drug, ...
carbon dioxide. In unusual circumstances, the gas
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane ...
can be used by methanotrophic bacteria as both a source of electrons and a substrate for carbon anabolism. In many ways, bacterial metabolism provides traits that are useful for ecological stability and for human society. One example is that some bacteria called
diazotroph Diazotrophs are bacteria and archaea that fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia. A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples of organisms tha ...
s have the ability to fix nitrogen gas using the enzyme nitrogenase. This environmentally important trait can be found in bacteria of most metabolic types listed above. This leads to the ecologically important processes of
denitrification Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and ultimately produces molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. Facultative anaerobic bacteria perform denit ...
, sulfate reduction, and
acetogenesis Acetogenesis is a process through which acetate is produced either by the reduction of CO2 or by the reduction of organic acids, rather than by the oxidative breakdown of carbohydrates or ethanol, as with acetic acid bacteria. The different bact ...
, respectively. Bacterial metabolic processes are also important in biological responses to
pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, th ...
; for example, sulfate-reducing bacteria are largely responsible for the production of the highly toxic forms of mercury ( methyl- and
dimethylmercury Dimethylmercury (( C H3)2 Hg) is an extremely toxic organomercury compound. A highly volatile, reactive, flammable, and colorless liquid, dimethylmercury is one of the strongest known neurotoxins, with a quantity of less than 0.1 mL capable of in ...
) in the environment. Non-respiratory anaerobes use fermentation to generate energy and reducing power, secreting metabolic by-products (such as ethanol in brewing) as waste.
Facultative anaerobe A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are '' Staphylococ ...
s can switch between fermentation and different
terminal electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Electron acceptors are sometimes mista ...
s depending on the environmental conditions in which they find themselves.


Growth and reproduction

Unlike in multicellular organisms, increases in cell size ( cell growth) and reproduction by cell division are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through binary fission, a form of asexual reproduction. Under optimal conditions, bacteria can grow and divide extremely rapidly, and some bacterial populations can double as quickly as every 17 minutes. In cell division, two identical clone daughter cells are produced. Some bacteria, while still reproducing asexually, form more complex reproductive structures that help disperse the newly formed daughter cells. Examples include fruiting body formation by myxobacteria and aerial hyphae formation by ''
Streptomyces ''Streptomyces'' is the largest genus of Actinomycetota and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinomycetota, streptomycetes are gram-positiv ...
'' species, or budding. Budding involves a cell forming a protrusion that breaks away and produces a daughter cell. In the laboratory, bacteria are usually grown using solid or liquid media. Solid growth media, such as agar plates, are used to isolate pure cultures of a bacterial strain. However, liquid growth media are used when the measurement of growth or large volumes of cells are required. Growth in stirred liquid media occurs as an even cell suspension, making the cultures easy to divide and transfer, although isolating single bacteria from liquid media is difficult. The use of selective media (media with specific nutrients added or deficient, or with antibiotics added) can help identify specific organisms. Most laboratory techniques for growing bacteria use high levels of nutrients to produce large amounts of cells cheaply and quickly. However, in natural environments, nutrients are limited, meaning that bacteria cannot continue to reproduce indefinitely. This nutrient limitation has led the evolution of different growth strategies (see r/K selection theory). Some organisms can grow extremely rapidly when nutrients become available, such as the formation of
algal Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
and cyanobacterial blooms that often occur in lakes during the summer. Other organisms have adaptations to harsh environments, such as the production of multiple antibiotics by streptomyces that inhibit the growth of competing microorganisms. In nature, many organisms live in communities (e.g., biofilms) that may allow for increased supply of nutrients and protection from environmental stresses. These relationships can be essential for growth of a particular organism or group of organisms ( syntrophy).
Bacterial growth 250px, Growth is shown as ''L'' = log(numbers) where numbers is the number of colony forming units per ml, versus ''T'' (time.) Bacterial growth is proliferation of bacterium into two daughter cells, in a process called binary fission. Providing ...
follows four phases. When a population of bacteria first enter a high-nutrient environment that allows growth, the cells need to adapt to their new environment. The first phase of growth is the lag phase, a period of slow growth when the cells are adapting to the high-nutrient environment and preparing for fast growth. The lag phase has high biosynthesis rates, as proteins necessary for rapid growth are produced. The second phase of growth is the logarithmic phase, also known as the exponential phase. The log phase is marked by rapid exponential growth. The rate at which cells grow during this phase is known as the ''growth rate'' (''k''), and the time it takes the cells to double is known as the ''generation time'' (''g''). During log phase, nutrients are metabolised at maximum speed until one of the nutrients is depleted and starts limiting growth. The third phase of growth is the '' stationary phase'' and is caused by depleted nutrients. The cells reduce their metabolic activity and consume non-essential cellular proteins. The stationary phase is a transition from rapid growth to a stress response state and there is increased expression of genes involved in DNA repair, antioxidant metabolism and nutrient transport. The final phase is the death phase where the bacteria run out of nutrients and die.


Genetics

Most bacteria have a single circular chromosome that can range in size from only 160,000 base pairs in the endosymbiotic bacteria '' Carsonella ruddii'', to 12,200,000 base pairs (12.2 Mbp) in the soil-dwelling bacteria '' Sorangium cellulosum''. There are many exceptions to this, for example some ''
Streptomyces ''Streptomyces'' is the largest genus of Actinomycetota and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinomycetota, streptomycetes are gram-positiv ...
'' and '' Borrelia'' species contain a single linear chromosome, while some '' Vibrio'' species contain more than one chromosome. Bacteria can also contain plasmids, small extra-chromosomal molecules of DNA that may contain genes for various useful functions such as antibiotic resistance, metabolic capabilities, or various virulence factors. Bacteria genomes usually encode a few hundred to a few thousand genes. The genes in bacterial genomes are usually a single continuous stretch of DNA and although several different types of introns do exist in bacteria, these are much rarer than in eukaryotes. Bacteria, as asexual organisms, inherit an identical copy of the parent's genomes and are clonal. However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations. Mutations come from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria. Genetic changes in bacterial genomes come from either random mutation during replication or "stress-directed mutation", where genes involved in a particular growth-limiting process have an increased mutation rate. Some bacteria also transfer genetic material between cells. This can occur in three main ways. First, bacteria can take up exogenous DNA from their environment, in a process called transformation. Many bacteria can naturally take up DNA from the environment, while others must be chemically altered in order to induce them to take up DNA. The development of competence in nature is usually associated with stressful environmental conditions, and seems to be an adaptation for facilitating repair of DNA damage in recipient cells. The second way bacteria transfer genetic material is by transduction, when the integration of a bacteriophage introduces foreign DNA into the chromosome. Many types of bacteriophage exist, some infect and lyse their host bacteria, while others insert into the bacterial chromosome. Bacteria resist phage infection through
restriction modification system The restriction modification system (RM system) is found in bacteria and other prokaryotic organisms, and provides a defense against foreign DNA, such as that borne by bacteriophages. Bacteria have restriction enzymes, also called restriction end ...
s that degrade foreign DNA, and a system that uses CRISPR sequences to retain fragments of the genomes of phage that the bacteria have come into contact with in the past, which allows them to block virus replication through a form of RNA interference. The third method of gene transfer is
conjugation Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics *Complex conjugation, the change ...
, whereby DNA is transferred through direct cell contact. In ordinary circumstances, transduction, conjugation, and transformation involve transfer of DNA between individual bacteria of the same species, but occasionally transfer may occur between individuals of different bacterial species and this may have significant consequences, such as the transfer of antibiotic resistance. In such cases, gene acquisition from other bacteria or the environment is called horizontal gene transfer and may be common under natural conditions.


Behaviour


Movement

Many bacteria are motile (able to move themselves) and do so using a variety of mechanisms. The best studied of these are flagella, long filaments that are turned by a motor at the base to generate propeller-like movement. The bacterial flagellum is made of about 20 proteins, with approximately another 30 proteins required for its regulation and assembly. The flagellum is a rotating structure driven by a reversible motor at the base that uses the electrochemical gradient across the membrane for power. Bacteria can use flagella in different ways to generate different kinds of movement. Many bacteria (such as '' E. coli'') have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and makes their movement a three-dimensional random walk. Bacterial species differ in the number and arrangement of flagella on their surface; some have a single flagellum ('' monotrichous''), a flagellum at each end (''
amphitrichous A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have fro ...
''), clusters of flagella at the poles of the cell ('' lophotrichous''), while others have flagella distributed over the entire surface of the cell ('' peritrichous''). The flagella of a unique group of bacteria, the spirochaetes, are found between two membranes in the periplasmic space. They have a distinctive helical body that twists about as it moves. Two other types of bacterial motion are called twitching motility that relies on a structure called the
type IV pilus A pilus (Latin for 'hair'; plural: ''pili'') is a hair-like appendage found on the surface of many bacteria and archaea. The terms ''pilus'' and '' fimbria'' (Latin for 'fringe'; plural: ''fimbriae'') can be used interchangeably, although some ...
, and gliding motility, that uses other mechanisms. In twitching motility, the rod-like pilus extends out from the cell, binds some substrate, and then retracts, pulling the cell forward. Motile bacteria are attracted or repelled by certain stimuli in behaviours called '' taxes'': these include chemotaxis, phototaxis, energy taxis, and magnetotaxis. In one peculiar group, the myxobacteria, individual bacteria move together to form waves of cells that then differentiate to form fruiting bodies containing spores. The myxobacteria move only when on solid surfaces, unlike ''E. coli'', which is motile in liquid or solid media. Several '' Listeria'' and ''
Shigella ''Shigella'' is a genus of bacteria that is Gram-negative, facultative anaerobic, non-spore-forming, nonmotile, rod-shaped, and genetically closely related to '' E. coli''. The genus is named after Kiyoshi Shiga, who first discovered it in ...
'' species move inside host cells by usurping the cytoskeleton, which is normally used to move organelles inside the cell. By promoting actin polymerisation at one pole of their cells, they can form a kind of tail that pushes them through the host cell's cytoplasm.


Communication

A few bacteria have chemical systems that generate light. This bioluminescence often occurs in bacteria that live in association with fish, and the light probably serves to attract fish or other large animals. Bacteria often function as multicellular aggregates known as biofilms, exchanging a variety of molecular signals for inter-cell communication, and engaging in coordinated multicellular behaviour. The communal benefits of multicellular cooperation include a cellular division of labour, accessing resources that cannot effectively be used by single cells, collectively defending against antagonists, and optimising population survival by differentiating into distinct cell types. For example, bacteria in biofilms can have more than 500 times increased resistance to antibacterial agents than individual "planktonic" bacteria of the same species. One type of inter-cellular communication by a molecular signal is called quorum sensing, which serves the purpose of determining whether there is a local population density that is sufficiently high that it is productive to invest in processes that are only successful if large numbers of similar organisms behave similarly, as in excreting digestive enzymes or emitting light. Quorum sensing allows bacteria to coordinate gene expression, and enables them to produce, release and detect
autoinducer Autoinducers are signaling molecules that are produced in response to changes in cell-population density. As the density of quorum sensing bacterial cells increases so does the concentration of the autoinducer. Detection of signal molecules by ba ...
s or pheromones which accumulate with the growth in cell population.


Classification and identification

Classification seeks to describe the diversity of bacterial species by naming and grouping organisms based on similarities. Bacteria can be classified on the basis of cell structure, cellular metabolism or on differences in cell components, such as DNA, fatty acids, pigments, antigens and quinones. While these schemes allowed the identification and classification of bacterial strains, it was unclear whether these differences represented variation between distinct species or between strains of the same species. This uncertainty was due to the lack of distinctive structures in most bacteria, as well as lateral gene transfer between unrelated species. Due to lateral gene transfer, some closely related bacteria can have very different morphologies and metabolisms. To overcome this uncertainty, modern bacterial classification emphasises molecular systematics, using genetic techniques such as guanine cytosine ratio determination, genome-genome hybridisation, as well as sequencing genes that have not undergone extensive lateral gene transfer, such as the rRNA gene. Classification of bacteria is determined by publication in the International Journal of Systematic Bacteriology, and Bergey's Manual of Systematic Bacteriology. The International Committee on Systematic Bacteriology (ICSB) maintains international rules for the naming of bacteria and taxonomic categories and for the ranking of them in the International Code of Nomenclature of Bacteria. Historically, bacteria were considered a part of the Plantae, the Plant kingdom, and were called "Schizomycetes" (fission-fungi). For this reason, collective bacteria and other microorganisms in a host are often called "flora". The term "bacteria" was traditionally applied to all microscopic, single-cell prokaryotes. However, molecular systematics showed prokaryotic life to consist of two separate domains, originally called Eubacteria and Archaebacteria, but now called Bacteria and Archaea that evolved independently from an ancient common ancestor. The archaea and eukaryotes are more closely related to each other than either is to the bacteria. These two domains, along with Eukarya, are the basis of the three-domain system, which is currently the most widely used classification system in microbiology. However, due to the relatively recent introduction of molecular systematics and a rapid increase in the number of genome sequences that are available, bacterial classification remains a changing and expanding field. For example, Cavalier-Smith argued that the Archaea and Eukaryotes evolved from Gram-positive bacteria. The identification of bacteria in the laboratory is particularly relevant in medicine, where the correct treatment is determined by the bacterial species causing an infection. Consequently, the need to identify human pathogens was a major impetus for the development of techniques to identify bacteria. The '' Gram stain'', developed in 1884 by Hans Christian Gram, characterises bacteria based on the structural characteristics of their cell walls. The thick layers of peptidoglycan in the "Gram-positive" cell wall stain purple, while the thin "Gram-negative" cell wall appears pink. By combining morphology and Gram-staining, most bacteria can be classified as belonging to one of four groups (Gram-positive cocci, Gram-positive bacilli, Gram-negative cocci and Gram-negative bacilli). Some organisms are best identified by stains other than the Gram stain, particularly mycobacteria or ''Nocardia'', which show acid fastness on Ziehl–Neelsen or similar stains. Other organisms may need to be identified by their growth in special media, or by other techniques, such as serology. Culture techniques are designed to promote the growth and identify particular bacteria, while restricting the growth of the other bacteria in the sample. Often these techniques are designed for specific specimens; for example, a sputum sample will be treated to identify organisms that cause
pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severit ...
, while stool specimens are cultured on selective media to identify organisms that cause diarrhea, while preventing growth of non-pathogenic bacteria. Specimens that are normally sterile, such as
blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
, urine or spinal fluid, are cultured under conditions designed to grow all possible organisms. Once a pathogenic organism has been isolated, it can be further characterised by its morphology, growth patterns (such as aerobic or anaerobic growth), patterns of hemolysis, and staining. As with bacterial classification, identification of bacteria is increasingly using molecular methods, and mass spectroscopy. Most bacteria have not been characterised and there are may species that cannot be grown in the laboratory. Diagnostics using DNA-based tools, such as polymerase chain reaction, are increasingly popular due to their specificity and speed, compared to culture-based methods. These methods also allow the detection and identification of " viable but nonculturable" cells that are metabolically active but non-dividing. However, even using these improved methods, the total number of bacterial species is not known and cannot even be estimated with any certainty. Following present classification, there are a little less than 9,300 known species of prokaryotes, which includes bacteria and archaea; but attempts to estimate the true number of bacterial diversity have ranged from 107 to 109 total species—and even these diverse estimates may be off by many orders of magnitude.


Phyla


Valid phyla

The following phyla have been validly published according to the Bacteriological Code: *
Acidobacteriota Acidobacteriota is a phylum of Gram-negative bacteria. Its members are physiologically diverse and ubiquitous, especially in soils, but are under-represented in culture. Description Members of this phylum are physiologically diverse, and can be ...
* Actinomycetota * Aquificota * Armatimonadota *
Atribacterota ''Atribacterota'' is a phylum of bacteria, which are common in anoxic sediments rich in methane. They are distributed worldwide and in some cases abundant in anaerobic marine sediments, geothermal springs, and oil deposits. Genetic analyzes sug ...
* Bacillota *
Bacteroidota The phylum Bacteroidota (synonym Bacteroidetes) is composed of three large classes of Gram-negative, nonsporeforming, anaerobic or aerobic, and rod-shaped bacteria that are widely distributed in the environment, including in soil, sediments, and ...
* Balneolota * Bdellovibrionota *
Caldisericota ''Caldisericum exile'' is a species of bacteria sufficiently distinct from other bacteria to be placed in its own family, order, class and phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below ...
*
Calditrichota ''Caldithrix'' is a genus of thermophilic and anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anae ...
* Campylobacterota *
Chlamydiota The Chlamydiota (synonym Chlamydiae) are a bacterial phylum and class whose members are remarkably diverse, including pathogens of humans and animals, symbionts of ubiquitous protozoa, and marine sediment forms not yet well understood. All of ...
*
Chlorobiota The green sulfur bacteria are a phylum of obligately anaerobic photoautotrophic bacteria that metabolize sulfur. Green sulfur bacteria are nonmotile (except ''Chloroherpeton thalassium'', which may glide) and capable of anoxygenic photosynthes ...
*
Chloroflexota The Chloroflexota are a phylum of bacteria containing isolates with a diversity of phenotypes, including members that are aerobic thermophiles, which use oxygen and grow well in high temperatures; anoxygenic phototrophs, which use light for p ...
*
Chrysiogenota Chrysiogenaceae is a family of bacteria. Phylogeny The phylogeny is based on 16S rRNA based LTP_12_2021 See also * List of bacterial orders * List of bacteria genera This article lists the genera of the bacteria. The currently accepted taxo ...
* Coprothermobacterota *
Deferribacterota The Deferribacteraceae are a family of gram-negative bacteria which make energy by anaerobic respiration.Huber, H., and Stetter, K.O. "Family I. ''Deferribacteraceae'' fam. nov." In: Bergey's Manual of Systematic Bacteriology, 2nd ed., vol. 1 (T ...
* Deinococcota * Dictyoglomota * Elusimicrobiota * Fibrobacterota * Fusobacteriota * Gemmatimonadota * Ignavibacteriota *
Lentisphaerota Lentisphaerota is a phylum of bacteria closely related to Chlamydiota and Verrucomicrobiota. It includes two monotypic orders Lentisphaerales and Victivallales. Phylum members can be aerobic or anaerobic and fall under two distinct phenotyp ...
* Mycoplasmatota * Myxococcota * Nitrospinota * Nitrospirota * Planctomycetota * Pseudomonadota *
Rhodothermota The Rhodothermales are an order of bacteria. Phylogeny The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI). See also * Lis ...
* Spirochaetota * Synergistota *
Thermodesulfobacteriota The Thermodesulfobacteriota are a phylum of thermophilic sulfate-reducing bacteria. A pathogenic intracellular thermodesulfobacteriote has recently been identified. Phylogeny The phylogeny is based on phylogenomic analysis: See also * Lis ...
* Thermomicrobiota * Thermotogota * Verrucomicrobiota


Provisional phyla

The following phyla have been proposed, but have not been validly published according to the Bacteriological Code (including those that have '' candidatus'' status): * "''Candidatus'' Abawacabacteria" * "
Abditibacteriota Abditibacteriota is a bacterial phylum previously known as FBP candidatus, which is widespread in extreme environments on Earth, from polar and desert ecosystems to wastewater and contaminated mining sites. The first cultured representative cam ...
" * "''Candidatus'' Absconditabacteria" * "''Candidatus'' Acetothermia" * "''Candidatus'' Adlerbacteria" * "''Candidatus'' Aerophobetes" * "''Candidatus'' Amesbacteria" * "''Candidatus'' Aminicenantes" * "''Candidatus'' Andersenbacteria" * "''Candidatus'' Azambacteria" * "''Candidatus'' Beckwithbacteria" * "''Candidatus'' Berkelbacteria" * "''Candidatus'' Binatota" * "''Candidatus'' Bipolaricaulota" * "''Candidatus'' Blackallbacteria" * "''Candidatus'' Blackburnbacteria" * "''Candidatus'' Brennerbacteria" * "''Candidatus'' Brownbacteria" * "''Candidatus'' Buchananbacteria" * "''Candidatus'' Caldatribacteriota" * "''Candidatus'' Calescamantes" * "''Candidatus'' Campbellbacteria" * "''Candidatus'' Chisholmbacteria" * "''Candidatus'' Cloacimonetes" * "''Candidatus'' Coatesbacteria" * "''Candidatus'' Collierbacteria" * "''Candidatus'' Colwellbacteria" * "''Candidatus'' Cryosericota" * "''Candidatus'' Curtissbacteria" * " Cyanobacteria" * "''Candidatus'' Dadabacteria" * "''Candidatus'' Daviesbacteria" * "''Candidatus'' Delongbacteria" * "''Candidatus''
Delphibacteria Delphibacteria is a candidate bacterial phylum in the FCB group. The phylum was first proposed after analysis of two genomes from the mouths of two bottlenose dolphins. "Dephibacteria" was proposed in recognition of the first genomic representati ...
" * "''Candidatus'' Dependentiae" * "''Candidatus'' Desantisbacteria" * "''Candidatus'' Dojkabacteria" * "''Candidatus'' Dormibacteraeota" * "''Candidatus'' Doudnabacteria" * "''Candidatus'' Edwardsbacteria" * "''Candidatus'' Eisenbacteria" * "''Candidatus'' Elulimicrobiota" * "''Candidatus'' Eremiobacterota" * "''Candidatus'' Falkowbacteria" * "''Candidatus''
Fermentibacteria Fermentibacteria (formerly Hyd24-12) is a bacterial phylum with candidate status. It is part of the FCB group FCB may refer to: Banks * Fairfield County Bank, in the United States * Farm Credit Bank, part of the Farm Credit System in the Un ...
" * "''Candidatus''
Fertabacteria Fertabacteria is a candidate bacterial phylum of the Candidate Phyla Radiation, first proposed in 2017 after analysis of a genome from the mouth of a bottlenose dolphin. Members of this phylum are predicted to have been widely under-detected in 16 ...
" * "''Candidatus'' Fervidibacteria" * "''Candidatus'' Firestonebacteria" * "''Candidatus'' Fischerbacteria" * "''Candidatus'' Fraserbacteria" * "''Candidatus'' Genascibacteria" * "''Candidatus'' Giovannonibacteria" * "''Candidatus'' Glassbacteria" * "''Candidatus'' Goldbacteria" * "''Candidatus'' Gottesmanbacteria" * "''Candidatus'' Gracilibacteria" * "''Candidatus'' Gribaldobacteria" * "''Candidatus'' Handelsmanbacteria" * "''Candidatus'' Harrisonbacteria" * "''Candidatus'' Howlettbacteria" * "''Candidatus'' Hugbacteria" * "''Candidatus'' Hydrogenedentes" * "''Candidatus'' Hydrothermae" * "''Candidatus'' Hydrothermota" * "''Candidatus'' Jacksonbacteria" * "''Candidatus'' Jorgensenbacteria" * "''Candidatus'' Kaiserbacteria" * "''Candidatus'' Kapabacteria" * "''Candidatus'' Katanobacteria" * "''Candidatus'' Kerfeldbacteria" * "''Candidatus'' Komeilibacteria" * "''Candidatus'' Krumholzibacteriota" * "''Candidatus''
Kryptonia Kryptonia is a bacterial phylum with candidate status. It is a member of the FCB group. The phylum was first proposed in 2016 following the recovery of genomes from a large-scale effort to mine metagenomic and single-cell genomic datasets for ...
" * "''Candidatus'' Kuenenbacteria" * "''Candidatus'' Lambdaproteobacteria" * "''Candidatus'' Latescibacteria" * "''Candidatus'' Levybacteria" * "''Candidatus'' Lindowbacteria" * "''Candidatus'' Liptonbacteria" * "''Candidatus'' Lloydbacteria" * "''Candidatus'' Magasanikbacteria" * "''Candidatus'' Margulisbacteria" * "''Candidatus''
Marinimicrobia Marinimicrobia is a candidate phylum of bacteria, previously known as SAR406, MGA or Marine Group A. They have been found mainly at great depths such as the Challenger Deep, the Mariana Trench, and the Puerto Rico Trench. This phylum has a low r ...
" * "''Candidatus'' Mcinerneyibacteriota" * "''Candidatus'' Melainabacteria" * "''Candidatus'' Microgenomates" * "''Candidatus''
Modulibacteria Modulibacteria is a bacterial phylum formerly known as KS3B3 or GN06. It is a candidate phylum, meaning there are no cultured representatives of this group. Members of the Modulibacteria phylum are known to cause fatal filament overgrowth (bulki ...
" * "''Candidatus'' Moisslbacteria" * "''Candidatus'' Montesolbacteria" * "''Candidatus'' Moranbacteria" * "''Candidatus'' Muirbacteria" * "''Candidatus'' Muproteobacteria" * "''Candidatus'' Nealsonbacteria" * "''Candidatus'' Niyogibacteria" * "''Candidatus''
Nomurabacteria Nomurabacteria is a candidate phylum of bacteria belonging to the CPR group so they are ultra-small bacteria. They have been found in a wide variety of environments, mainly in sediments under anaerobic conditions.Castelle, C., Brown, C., Thomas ...
" * "''Candidatus'' Omnitrophica" * "''Candidatus'' Pacebacteria" * "''Candidatus'' Parcubacteria" * "''Candidatus'' Parcunitrobacteria" * "''Candidatus'' Peregrinibacteria" * "''Candidatus''
Poribacteria Poribacteria are a candidate phylum of bacteria originally discovered in the microbiome of marine sponges (''Porifera''). Poribacteria are Gram-negative primarily aerobic mixotrophs with the ability for oxidative phosphorylation, glycolysis, ...
" * "''Candidatus'' Portnoybacteria" * "''Candidatus'' Pyropristinus" * "''Candidatus'' Ratteibacteria" * "''Candidatus'' Raymondbacteria" * "''Candidatus'' Riflebacteria" * "''Candidatus'' Roizmanbacteria" * "''Candidatus'' Rokubacteria" * "''Candidatus'' Ryanbacteria" * "''Candidatus'' Saccharibacteria" * "''Candidatus'' Saganbacteria" * "''Candidatus'' Schekmanbacteria" * "''Candidatus'' Shapirobacteria" * "''Candidatus'' Spechtbacteria" * "''Candidatus'' Stahlbacteria" * "''Candidatus'' Staskawiczbacteria" * "''Candidatus'' Sumerlaeota" * "''Candidatus'' Sungbacteria" * "''Candidatus'' Tagabacteria" * "''Candidatus'' Taylorbacteria" * "''Candidatus'' Tectomicrobia" * "''Candidatus'' Terrybacteria" * "''Candidatus'' Teskebacteria" * "''Candidatus'' Tianyabacteria" * "''Candidatus'' Torokbacteria" * "''Candidatus'' Uhrbacteria" * "''Candidatus'' Veblenbacteria" * "''Candidatus'' Vogelbacteria" * "''Candidatus'' Wallbacteria" * "''Candidatus'' Wildermuthbacteria" * "''Candidatus'' Wirthbacteria" * "''Candidatus'' Woesebacteria" * "''Candidatus'' Wolfebacteria" * "''Candidatus'' Woykebacteria" * "''Candidatus'' Yanofskybacteria" * "''Candidatus'' Yonathbacteria" * "''Candidatus'' Zambryskibacteria" * "''Candidatus''
Zixibacteria Zixibacteria is a bacterial phylum with candidate status, meaning it had no cultured representatives. It is a member of the FCB group Zixibacteria was proposed as a bacterial phylum following the recovery of a genome from representative RBG-1. ...
"


Genera ''incertae sedis''

The following bacteria genera have not been assigned to a phylum, class, or order: * " Fermentobadaceae" Haiying 1995 ** " Guhaiyingella" Haiying 1995 * Not assigned to a family: ** "''Candidatus'' Aegiribacteria" Hamilton et al. 2016 ** Archaeoscillatoriopsis Schopf 1993 ** " Eoleptonema" Awramik et al. 1983 ** "''Candidatus''
Epulonipiscium "''Candidatus'' Epulonipiscium" is a genus of Gram-positive bacteria that have a symbiotic relationship with surgeonfish. These bacteria are known for their unusually large size, many ranging from 200–700 μm in length. Until the discovery of ...
" corrig. Montgomery and Pollak 1988 ** "''Candidatus'' Ovibacter" corrig. Fenchel and Thar 2004 ** "'' Primaevifilum''" Schopf 1983 ** "''
Rappaport The Rappaport family (Hebrew: רפפורט; Yiddish: ראפאפארט) is a prominent Kohanic rabbinic family, who are generally considered to possess the oldest and best recorded Kohanic pedigree. The modern origins of the family can be traced ...
''" Waldman Ben-Asher et al. 2017


Interactions with other organisms

Despite their apparent simplicity, bacteria can form complex associations with other organisms. These symbiotic associations can be divided into parasitism, mutualism and
commensalism Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...
.


Commensals

The word "
commensalism Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fr ...
" is derived from the word "commensal", meaning "eating at the same table" and all plants and animals are colonised by commensal bacteria. In humans and other animals millions of them live on the skin, the airways, the gut and other orifices. Referred to as "normal flora", or "commensals", these bacteria usually cause no harm but may occasionally invade other sites of the body and cause infection. '' Escherichia coli'' is a commensal in the human gut but can cause urinary tract infections. Similarly, streptoccoci, which are part of the normal flora of the human mouth, can cause heart disease.


Predators

Some species of bacteria kill and then consume other microorganisms, these species are called ''predatory bacteria''. These include organisms such as '' Myxococcus xanthus'', which forms swarms of cells that kill and digest any bacteria they encounter. Other bacterial predators either attach to their prey in order to digest them and absorb nutrients or invade another cell and multiply inside the cytosol. These predatory bacteria are thought to have evolved from saprophages that consumed dead microorganisms, through adaptations that allowed them to entrap and kill other organisms.


Mutualists

Certain bacteria form close spatial associations that are essential for their survival. One such mutualistic association, called interspecies hydrogen transfer, occurs between clusters of
anaerobic bacteria An anaerobic organism or anaerobe is any organism that does not require molecular oxygen for growth. It may react negatively or even die if free oxygen is present. In contrast, an aerobic organism (aerobe) is an organism that requires an oxygenat ...
that consume organic acids, such as butyric acid or propionic acid, and produce
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
, and methanogenic archaea that consume hydrogen. The bacteria in this association are unable to consume the organic acids as this reaction produces hydrogen that accumulates in their surroundings. Only the intimate association with the hydrogen-consuming archaea keeps the hydrogen concentration low enough to allow the bacteria to grow. In soil, microorganisms that reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) carry out nitrogen fixation, converting nitrogen gas to nitrogenous compounds. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of over 1,000 bacterial species in the normal human gut flora of the intestines can contribute to gut immunity, synthesise vitamins, such as folic acid, vitamin K and biotin, convert sugars to lactic acid (see '' Lactobacillus''), as well as fermenting complex undigestible carbohydrates. The presence of this gut flora also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion) and these beneficial bacteria are consequently sold as
probiotic Probiotics are live microorganisms promoted with claims that they provide health benefits when consumed, generally by improving or restoring the gut microbiota. Probiotics are considered generally safe to consume, but may cause bacteria-host i ...
dietary supplements. Nearly all animal life is dependent on bacteria for survival as only bacteria and some archaea possess the genes and enzymes necessary to synthesize vitamin B12, also known as cobalamin, and provide it through the food chain. Vitamin B12 is a water-soluble vitamin that is involved in the metabolism of every cell of the human body. It is a cofactor in DNA synthesis, and in both fatty acid and amino acid metabolism. It is particularly important in the normal functioning of the nervous system via its role in the synthesis of myelin.


Pathogens

The body is continually exposed to many species of bacteria, including beneficial commensals, which grow on the skin and mucous membranes, and saprophytes, which grow mainly in the soil and in decaying matter. The blood and tissue fluids contain nutrients sufficient to sustain the growth of many bacteria. The body has defence mechanisms that enable it to resist microbial invasion of its tissues and give it a natural
immunity Immunity may refer to: Medicine * Immunity (medical), resistance of an organism to infection or disease * ''Immunity'' (journal), a scientific journal published by Cell Press Biology * Immune system Engineering * Radiofrequence immunity de ...
or innate resistance against many microorganisms. Unlike some viruses, bacteria evolve relatively slowly so many bacterial diseases also occur in other animals. If bacteria form a parasitic association with other organisms, they are classed as pathogens. Pathogenic bacteria are a major cause of human death and disease and cause infections such as tetanus (caused by ''
Clostridium tetani ''Clostridium tetani'' is a common soil bacterium and the causative agent of tetanus. Vegetative cells of ''Clostridium tetani'' are usually rod-shaped and up to 2.5 μm long, but they become enlarged and tennis racket- or drumstick-shaped when ...
''), typhoid fever, diphtheria, syphilis, cholera, foodborne illness, leprosy (caused by '' Mycobacterium leprae'') and tuberculosis (caused by '' Mycobacterium tuberculosis''). A pathogenic cause for a known medical disease may only be discovered many years later, as was the case with '' Helicobacter pylori'' and peptic ulcer disease. Bacterial diseases are also important in
agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...
, with bacteria causing
leaf spot A leaf spot is a limited, discoloured, diseased area of a leaf that is caused by fungal, bacterial or viral plant diseases, or by injuries from nematodes, insects, environmental factors, toxicity or herbicides. These discoloured spots or lesions ...
, fire blight and wilts in plants, as well as Johne's disease, mastitis, salmonella and anthrax in farm animals. Each species of pathogen has a characteristic spectrum of interactions with its human
hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host ...
. Some organisms, such as '' Staphylococcus'' or '' Streptococcus'', can cause skin infections,
pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severit ...
, meningitis and sepsis, a systemic inflammatory response producing
shock Shock may refer to: Common uses Collective noun *Shock, a historic commercial term for a group of 60, see English numerals#Special names * Stook, or shock of grain, stacked sheaves Healthcare * Shock (circulatory), circulatory medical emerge ...
, massive vasodilation and death. Yet these organisms are also part of the normal human flora and usually exist on the skin or in the nose without causing any disease at all. Other organisms invariably cause disease in humans, such as '' Rickettsia'', which are obligate intracellular parasites able to grow and reproduce only within the cells of other organisms. One species of ''Rickettsia'' causes typhus, while another causes Rocky Mountain spotted fever. '' Chlamydia'', another phylum of obligate intracellular parasites, contains species that can cause pneumonia or urinary tract infection and may be involved in coronary heart disease. Some species, such as '' Pseudomonas aeruginosa'', ''
Burkholderia cenocepacia ''Burkholderia cenocepacia'' is a Gram-negative, rod-shaped bacterium that is commonly found in soil and water environments and may also be associated with plants and animals, particularly as a human pathogen. It is one of over 20 species in the ...
'', and ''
Mycobacterium avium ''Mycobacterium avium ''complex is a group of mycobacteria comprising ''Mycobacterium intracellulare'' and ''Mycobacterium avium'' that are commonly grouped because they infect humans together; this group, in turn, is part of the group of nontu ...
'', are
opportunistic pathogens An opportunistic infection is an infection caused by pathogens ( bacteria, fungi, parasites or viruses) that take advantage of an opportunity not normally available. These opportunities can stem from a variety of sources, such as a weakened immun ...
and cause disease mainly in people who are immunosuppressed or have cystic fibrosis. Some bacteria produce toxins, which cause diseases. These are endotoxins, which come from broken bacterial cells, and exotoxins, which are produced by bacteria and released into the environment. The bacterium '' Clostridium botulinum'' for example, produces a powerful exotoxin that cause respiratory paralysis, and '' Salmonellae'' produce an endotoxin that causes gastroenteritis. Some exotoxins can be converted to toxoids, which are used as vaccines to prevent the disease. Bacterial infections may be treated with antibiotics, which are classified as
bacteriocidal A bactericide or bacteriocide, sometimes abbreviated Bcidal, is a substance which kills bacteria. Bactericides are disinfectants, antiseptics, or antibiotics. However, material surfaces can also have bactericidal properties based solely on their ...
if they kill bacteria or bacteriostatic if they just prevent bacterial growth. There are many types of antibiotics, and each class inhibits a process that is different in the pathogen from that found in the host. An example of how antibiotics produce selective toxicity are chloramphenicol and puromycin, which inhibit the bacterial ribosome, but not the structurally different eukaryotic ribosome. Antibiotics are used both in treating human disease and in intensive farming to promote animal growth, where they may be contributing to the rapid development of antibiotic resistance in bacterial populations. Infections can be prevented by antiseptic measures such as sterilising the skin prior to piercing it with the needle of a syringe, and by proper care of indwelling catheters. Surgical and dental instruments are also sterilised to prevent contamination by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection.


Significance in technology and industry

Bacteria, often lactic acid bacteria, such as '' Lactobacillus'' species and '' Lactococcus'' species, in combination with yeasts and moulds, have been used for thousands of years in the preparation of fermented foods, such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine and yogurt. The ability of bacteria to degrade a variety of organic compounds is remarkable and has been used in waste processing and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills. Fertiliser was added to some of the beaches in Prince William Sound in an attempt to promote the growth of these naturally occurring bacteria after the 1989 ''Exxon Valdez'' oil spill. These efforts were effective on beaches that were not too thickly covered in oil. Bacteria are also used for the bioremediation of industrial toxic wastes. In the chemical industry, bacteria are most important in the production of enantiomerically pure chemicals for use as pharmaceuticals or
agrichemical An agrochemical or agrichemical, a contraction of ''agricultural chemical'', is a chemical product used in industrial agriculture. Agrichemical refers to biocides (pesticides including insecticides, herbicides, fungicides and nematicides) and sy ...
s. Bacteria can also be used in the place of pesticides in the biological pest control. This commonly involves '' Bacillus thuringiensis'' (also called BT), a Gram-positive, soil dwelling bacterium. Subspecies of this bacteria are used as a Lepidopteran-specific insecticides under trade names such as Dipel and Thuricide. Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, pollinators and most other beneficial insects. Because of their ability to quickly grow and the relative ease with which they can be manipulated, bacteria are the workhorses for the fields of molecular biology, genetics and
biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists can determine the function of genes,
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s and metabolic pathways in bacteria, then apply this knowledge to more complex organisms. This aim of understanding the biochemistry of a cell reaches its most complex expression in the synthesis of huge amounts of enzyme kinetic and gene expression data into mathematical models of entire organisms. This is achievable in some well-studied bacteria, with models of ''Escherichia coli'' metabolism now being produced and tested. This understanding of bacterial metabolism and genetics allows the use of biotechnology to bioengineer bacteria for the production of therapeutic proteins, such as insulin, growth factors, or antibodies. Because of their importance for research in general, samples of bacterial strains are isolated and preserved in
Biological Resource Centers A biorepository is a facility that collects, catalogs, and stores samples of biological material for laboratory research. Biorepositories collect and manage specimens from animals, plants, and other living organisms. Biorepositories store many di ...
. This ensures the availability of the strain to scientists worldwide.


History of bacteriology

Bacteria were first observed by the Dutch microscopist Antonie van Leeuwenhoek in 1676, using a single-lens microscope of his own design. He then published his observations in a series of letters to the Royal Society of London. Bacteria were Leeuwenhoek's most remarkable microscopic discovery. They were just at the limit of what his simple lenses could make out and, in one of the most striking hiatuses in the history of science, no one else would see them again for over a century. His observations had also included protozoans which he called
animalcule Animalcule ('little animal', from Latin ''animal'' + the diminutive suffix ''-culum'') is an old term for microscopic organisms that included bacteria, protozoans, and very small animals. The word was invented by 17th-century Dutch scientist ...
s, and his findings were looked at again in the light of the more recent findings of cell theory.
Christian Gottfried Ehrenberg Christian Gottfried Ehrenberg (19 April 1795 – 27 June 1876) was a German naturalist, zoologist, comparative anatomist, geologist, and microscopist. Ehrenberg was an evangelist and was considered to be of the most famous and productive scient ...
introduced the word "bacterium" in 1828. In fact, his '' Bacterium'' was a genus that contained non-spore-forming rod-shaped bacteria, as opposed to ''Bacillus'', a genus of spore-forming rod-shaped bacteria defined by Ehrenberg in 1835. Louis Pasteur demonstrated in 1859 that the growth of microorganisms causes the fermentation process, and that this growth is not due to spontaneous generation ( yeasts and molds, commonly associated with fermentation, are not bacteria, but rather fungi). Along with his contemporary Robert Koch, Pasteur was an early advocate of the germ theory of disease. Before them, Ignaz Semmelweis and
Joseph Lister Joseph Lister, 1st Baron Lister, (5 April 182710 February 1912) was a British surgeon, medical scientist, experimental pathologist and a pioneer of antiseptic surgery and preventative medicine. Joseph Lister revolutionised the craft of ...
had realised the importance of sanitized hands in medical work. Semmelweis ideas was rejected and his book on the topic condemned by the medical community, but after Lister doctors started sanitizing their hands in the 1870s. While Semmelweis who started with rules about handwashing in his hospital in the 1840s predated the spread of the ideas about germs themselves and attributed diseases to "decomposing animal organic matter", Lister was active later. Robert Koch, a pioneer in medical microbiology, worked on cholera, anthrax and tuberculosis. In his research into tuberculosis Koch finally proved the germ theory, for which he received a
Nobel Prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...
in 1905. In Koch's postulates, he set out criteria to test if an organism is the cause of a disease, and these postulates are still used today. Ferdinand Cohn is said to be a founder of bacteriology, studying bacteria from 1870. Cohn was the first to classify bacteria based on their morphology. Though it was known in the nineteenth century that bacteria are the cause of many diseases, no effective antibacterial treatments were available. In 1910, Paul Ehrlich developed the first antibiotic, by changing dyes that selectively stained '' Treponema pallidum''—the spirochaete that causes syphilis—into compounds that selectively killed the pathogen. Ehrlich had been awarded a 1908 Nobel Prize for his work on immunology, and pioneered the use of stains to detect and identify bacteria, with his work being the basis of the Gram stain and the Ziehl–Neelsen stain. A major step forward in the study of bacteria came in 1977 when Carl Woese recognised that archaea have a separate line of evolutionary descent from bacteria. This new phylogenetic taxonomy depended on the sequencing of 16S ribosomal RNA, and divided prokaryotes into two evolutionary domains, as part of the three-domain system.


See also

* Genetically modified bacteria *
Marine prokaryotes Marine prokaryotes are marine bacteria and marine archaea. They are defined by their habitat as prokaryotes that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. All cellula ...


References


Bibliography

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External links


On-line text book on bacteriology (2015)
{{Authority control Bacteriology Domains (biology)