Pathogenic bacteria are
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
that can cause
disease
A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...
.
This article focuses on the bacteria that are
pathogenic
In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ.
The term ...
to humans. Most
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
of bacteria are harmless and many are
beneficial but others can cause
infectious diseases
infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...
. The number of these pathogenic species in humans is estimated to be fewer than a hundred. By contrast, several thousand species are considered part of the
gut flora
Gut microbiota, gut microbiome, or gut flora are the microorganisms, including bacteria, archaea, fungi, and viruses, that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the g ...
, with a few hundred species present in each individual human's
digestive tract
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the Digestion, digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascula ...
.
The body is continually exposed to many species of bacteria, including beneficial
commensal
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 f ...
s, which grow on the skin and
mucous membrane
A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It ...
s, 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 ...
or
innate resistance against many
microorganisms
A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
.
Pathogenic bacteria are specially adapted and endowed with mechanisms for overcoming the normal body defences, and can invade parts of the body, such as the blood, where bacteria are not normally found. Some pathogens invade only the surface
epithelium
Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
, skin or mucous membrane, but many travel more deeply, spreading through the tissues and disseminating by the
lymphatic
Lymph () is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated. At the origi ...
and
blood streams. In some rare cases a pathogenic microbe can infect an entirely healthy person, but infection usually occurs only if the body's defence mechanisms are damaged by some local trauma or an underlying debilitating disease, such as wounding,
intoxication
Intoxication — or poisoning, especially by an alcoholic or narcotic substance — may refer to:
* Substance intoxication:
** Alcohol intoxication
** LSD intoxication
** Toxidrome
** Tobacco intoxication
** Cannabis intoxication
** Cocaine i ...
,
chilling, fatigue, and
malnutrition
Malnutrition occurs when an organism gets too few or too many nutrients, resulting in health problems. Specifically, it is a deficiency, excess, or imbalance of energy, protein and other nutrients which adversely affects the body's tissues a ...
. In many cases, it is important to differentiate
infection
An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...
and
colonization
475px, Map of the year each country achieved List of sovereign states by date of formation, independence.
Colonization (British English: colonisation) is a process of establishing occupation of or control over foreign territories or peoples f ...
, which is when the bacteria are causing little or no harm.
Caused by ''
Mycobacterium tuberculosis
''Mycobacterium tuberculosis'' (M. tb), also known as Koch's bacillus, is a species of pathogenic bacteria in the family Mycobacteriaceae and the causative agent of tuberculosis.
First discovered in 1882 by Robert Koch, ''M. tuberculosis'' ha ...
'' bacteria, one of the
disease
A disease is a particular abnormal condition that adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...
s with the highest
disease burden
Disease burden is the impact of a health problem as measured by financial cost, mortality, morbidity, or other indicators. It is often quantified in terms of quality-adjusted life years (QALYs) or disability-adjusted life years (DALYs). Both ...
is
tuberculosis
Tuberculosis (TB), also known colloquially as the "white death", or historically as consumption, is a contagious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can al ...
, which killed 1.4 million people in 2019, mostly in
sub-Saharan Africa
Sub-Saharan Africa is the area and regions of the continent of Africa that lie south of the Sahara. These include Central Africa, East Africa, Southern Africa, and West Africa. Geopolitically, in addition to the list of sovereign states and ...
.
Pathogenic bacteria contribute to other globally important diseases, such as
pneumonia
Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...
, which can be caused by bacteria such as ''
Staphylococcus
''Staphylococcus'', from Ancient Greek σταφυλή (''staphulḗ''), meaning "bunch of grapes", and (''kókkos''), meaning "kernel" or " Kermes", is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillale ...
'', ''
Streptococcus
''Streptococcus'' is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a sing ...
'' and ''
Pseudomonas
''Pseudomonas'' is a genus of Gram-negative bacteria belonging to the family Pseudomonadaceae in the class Gammaproteobacteria. The 348 members of the genus demonstrate a great deal of metabolic diversity and consequently are able to colonize a ...
'', and
foodborne illness
Foodborne illness (also known as foodborne disease and food poisoning) is any illness resulting from the contamination of food by pathogenic bacteria, viruses, or parasites,
as well as prions (the agents of mad cow disease), and toxins such ...
es, which can be caused by bacteria such as ''
Shigella
''Shigella'' is a genus of bacteria that is Gram negative, facultatively anaerobic, non–spore-forming, nonmotile, rod shaped, and is genetically nested within ''Escherichia''. The genus is named after Kiyoshi Shiga, who discovered it in 1 ...
'', ''
Campylobacter
''Campylobacter'' is a type of bacteria that can cause a diarrheal disease in people. Its name means "curved bacteria", as the germ typically appears in a comma or "s" shape. According to its scientific classification, it is a genus of gram-negat ...
'', and ''
Salmonella
''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...
''. Pathogenic bacteria also cause infections such as
tetanus
Tetanus (), also known as lockjaw, is a bacterial infection caused by ''Clostridium tetani'' and characterized by muscle spasms. In the most common type, the spasms begin in the jaw and then progress to the rest of the body. Each spasm usually l ...
,
typhoid fever
Typhoid fever, also known simply as typhoid, is a disease caused by '' Salmonella enterica'' serotype Typhi bacteria, also called ''Salmonella'' Typhi. Symptoms vary from mild to severe, and usually begin six to 30 days after exposure. Often th ...
,
diphtheria
Diphtheria is an infection caused by the bacteria, bacterium ''Corynebacterium diphtheriae''. Most infections are asymptomatic or have a mild Course (medicine), clinical course, but in some outbreaks, the mortality rate approaches 10%. Signs a ...
,
syphilis
Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms depend on the stage it presents: primary, secondary, latent syphilis, latent or tertiary. The prim ...
, and
leprosy
Leprosy, also known as Hansen's disease (HD), is a Chronic condition, long-term infection by the bacteria ''Mycobacterium leprae'' or ''Mycobacterium lepromatosis''. Infection can lead to damage of the Peripheral nervous system, nerves, respir ...
.
Pathogenic bacteria are also the cause of high
infant mortality
Infant mortality is the death of an infant before the infant's first birthday. The occurrence of infant mortality in a population can be described by the infant mortality rate (IMR), which is the number of deaths of infants under one year of age ...
rates in
developing countries
A developing country is a sovereign state with a less-developed Secondary sector of the economy, industrial base and a lower Human Development Index (HDI) relative to developed countries. However, this definition is not universally agreed upon. ...
.
A
GBD study estimated the
global
Global may refer to:
General
*Globe, a spherical model of celestial bodies
*Earth, the third planet from the Sun
Entertainment
* ''Global'' (Paul van Dyk album), 2003
* ''Global'' (Bunji Garlin album), 2007
* ''Global'' (Humanoid album), 198 ...
death rates from (33) bacterial pathogens, finding such infections contributed to one in 8 deaths (or ~7.7 million deaths), which the
second largest cause of death globally in 2019.
Most pathogenic bacteria can be grown in
cultures
Culture ( ) is a concept that encompasses the social behavior, institutions, and Social norm, norms found in human societies, as well as the knowledge, beliefs, arts, laws, Social norm, customs, capabilities, Attitude (psychology), attitudes ...
and identified by
Gram stain
Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. It may also be used to diagnose a fungal infection. The name comes ...
and other methods. Bacteria grown in this way are often
tested
''Tested'' is the first official live album by punk rock band Bad Religion. It was recorded in the USA, Canada, Germany, Estonia, Denmark, Italy and Austria, in 1996, and released in 1997. It is Bad Religion's second live album. Instead of usi ...
to find which
antibiotic
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...
s will be an effective treatment for the infection. For hitherto unknown pathogens,
Koch's postulates
Koch's postulates ( ) are four criteria designed to establish a causality, causal relationship between a microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884, based on earlier concepts described by ...
are the standard to establish a
causative relationship between a microbe and a disease.
Diseases

Each species has specific effect and causes symptoms in people who are infected. Some people who are infected with a pathogenic bacteria do not have symptoms. Immunocompromised individuals are more susceptible to pathogenic bacteria.
Pathogenic susceptibility
Some pathogenic bacteria cause disease under certain conditions, such as entry through the skin via a cut, through sexual activity or through compromised immune function.

Some species of ''
Streptococcus
''Streptococcus'' is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a sing ...
'' and ''
Staphylococcus
''Staphylococcus'', from Ancient Greek σταφυλή (''staphulḗ''), meaning "bunch of grapes", and (''kókkos''), meaning "kernel" or " Kermes", is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillale ...
'' are part of the normal
skin microbiota and typically reside on healthy skin or in the nasopharyngeal region. Yet these species can potentially initiate skin infections.
Streptococcal infections
''Streptococcus'' is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a single ...
include
sepsis
Sepsis is a potentially life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs.
This initial stage of sepsis is followed by suppression of the immune system. Common signs and s ...
,
pneumonia
Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...
, and
meningitis
Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, intense headache, vomiting and neck stiffness and occasion ...
.
These infections can become serious creating a systemic inflammatory response resulting in massive vasodilation, shock, and death.
Other bacteria are
opportunistic pathogens and cause disease mainly in people with
immunosuppression
Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse react ...
or
cystic fibrosis
Cystic fibrosis (CF) is a genetic disorder inherited in an autosomal recessive manner that impairs the normal clearance of Sputum, mucus from the lungs, which facilitates the colonization and infection of the lungs by bacteria, notably ''Staphy ...
. Examples of these opportunistic pathogens include ''
Pseudomonas aeruginosa
''Pseudomonas aeruginosa'' is a common Bacterial capsule, encapsulated, Gram-negative bacteria, Gram-negative, Aerobic organism, aerobic–facultative anaerobe, facultatively anaerobic, Bacillus (shape), rod-shaped bacteria, bacterium that can c ...
'', ''
Burkholderia cenocepacia'', 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 ...
''.
Intracellular
Obligate intracellular parasite
Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a host. They are also called intracellular pathogens.
Types
There are two main types of intracellular parasites: Facultative and Obligate ...
s (e.g.
Chlamydophila
''Chlamydia'' is a genus of pathogenic Gram-negative bacteria that are obligate intracellular parasites. ''Chlamydia'' infections are the most common bacterial sexually transmitted diseases in humans and are the leading cause of infectious bli ...
,
Ehrlichia
''Ehrlichia'' is a genus of Rickettsiales bacteria that are transmitted to vertebrates by ticks. These bacteria cause the disease ehrlichiosis, which is considered zoonotic, because the main reservoirs for the disease are animals.
''Ehrlichia' ...
,
Rickettsia
''Rickettsia'' is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The genus was n ...
) are only able to grow and replicate inside other cells. Infections due to obligate intracellular bacteria may be
asymptomatic
Asymptomatic (or clinically silent) is an adjective categorising the medical conditions (i.e., injuries or diseases) that patients carry but without experiencing their symptoms, despite an explicit diagnosis (e.g., a positive medical test).
P ...
, requiring an
incubation period
Incubation period (also known as the latent period or latency period) is the time elapsed between exposure to a pathogenic organism, a chemical, or ionizing radiation, radiation, and when symptoms and signs are first apparent. In a typical infect ...
. Examples of obligate intracellular bacteria include ''
Rickettsia prowazekii
''Rickettsia prowazekii'' is a species of gram-negative, obligate intracellular parasitic, aerobic bacilliform bacteria of class Alphaproteobacteria that is the etiologic agent of epidemic typhus, transmitted in the feces of lice. In North Am ...
'' (
typhus
Typhus, also known as typhus fever, is a group of infectious diseases that include epidemic typhus, scrub typhus, and murine typhus. Common symptoms include fever, headache, and a rash. Typically these begin one to two weeks after exposu ...
) and
Rickettsia rickettsii
''Rickettsia rickettsii'' is a Gram-negative, intracellular, cocco-bacillus bacterium that was first discovered in 1902. Having a reduced genome, the bacterium harvests nutrients from its host cell to carry out respiration, making it an organo ...
, (
Rocky Mountain spotted fever
Rocky Mountain spotted fever (RMSF) is a bacterial disease spread by ticks. It typically begins with a fever and headache, which is followed a few days later with the development of a rash. The rash is generally Petechial rash, made up of small s ...
).
Chlamydia
Chlamydia, or more specifically a chlamydia infection, is a sexually transmitted infection caused by the bacterium ''Chlamydia trachomatis''. Most people who are infected have no symptoms. When symptoms do appear, they may occur only several w ...
are intracellular parasites. These pathogens can cause
pneumonia
Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...
or
urinary tract infection
A urinary tract infection (UTI) is an infection that affects a part of the urinary tract. Lower urinary tract infections may involve the bladder (cystitis) or urethra (urethritis) while upper urinary tract infections affect the kidney (pyel ...
and may be involved in
coronary heart disease
Coronary artery disease (CAD), also called coronary heart disease (CHD), or ischemic heart disease (IHD), is a type of cardiovascular disease, heart disease involving Ischemia, the reduction of blood flow to the cardiac muscle due to a build-up ...
.
Other groups of intracellular bacterial pathogens include ''
Salmonella
''Salmonella'' is a genus of bacillus (shape), rod-shaped, (bacillus) Gram-negative bacteria of the family Enterobacteriaceae. The two known species of ''Salmonella'' are ''Salmonella enterica'' and ''Salmonella bongori''. ''S. enterica'' ...
'', ''
Neisseria
''Neisseria'' is a large genus of bacteria that colonize the mucous membranes of many animals. Of the 11 species that colonize humans, only two are pathogens: '' N. meningitidis'' and '' N. gonorrhoeae''.
''Neisseria'' species are Gram-negative ...
'', ''
Brucella
''Brucella'' is a genus of Gram-negative bacterium, bacteria, named after David Bruce (microbiologist), David Bruce (1855–1931). They are small (0.5 to 0.7 by 0.6 to 1.5 μm), non-Bacterial capsule, encapsulated, non-motile, facultatively ...
'', ''
Mycobacterium
''Mycobacterium'' is a genus of over 190 species in the phylum Actinomycetota, assigned its own family, Mycobacteriaceae. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis (''Mycobacterium tuberculo ...
'', ''
Nocardia
''Nocardia'' is a genus of weakly staining Gram-positive, catalase, catalase-positive, rod-shaped bacteria. It forms partially acid-fast beaded branching filaments (appearing similar to fungi, but being truly bacteria). It contains a total of 8 ...
,'' ''
Listeria
''Listeria'' is a genus of bacteria that acts as an intracellular parasite in mammals. As of 2024, 28 species have been identified. The genus is named in honour of the British pioneer of sterile surgery Joseph Lister. ''Listeria'' species ...
'', ''
Francisella'', ''
Legionella
''Legionella'' is a genus of gram-negative bacteria, gram-negative bacteria that can be seen using a silver stain or grown in a special media that contains cysteine, an amino acid. It is known to cause legionellosis (all illnesses caused by ''Legi ...
'', and ''
Yersinia pestis
''Yersinia pestis'' (''Y. pestis''; formerly ''Pasteurella pestis'') is a Gram-negative bacteria, gram-negative, non-motile bacteria, non-motile, coccobacillus Bacteria, bacterium without Endospore, spores. It is related to pathogens ''Yer ...
''. These can exist intracellularly, but can exist outside host cells.
Infections in specific tissue
Bacterial pathogens often cause infection in specific areas of the body. Others are generalists.
*
Bacterial vaginosis
Bacterial vaginosis (BV) is an infection of the vagina caused by excessive growth of bacteria. Common symptoms include increased vaginal discharge that often smells like fish. The discharge is usually white or gray in color. Burning with urina ...
is a condition of the
vaginal microbiota in which an excessive growth of ''
Gardnerella vaginalis
''Gardnerella vaginalis'' is a species of Gram-variable-staining facultative anaerobic bacteria. The organisms are small (1.0–1.5 μm in diameter) non-spore-forming, nonmotile coccobacilli.
Once classified as ''Haemophilus vaginalis'' ...
'' and other mostly
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 ...
displace the beneficial ''
Lactobacilli
The ''Lactobacillaceae'' are a family of lactic acid bacteria. It is the only family in the lactic acid bacteria which includes homofermentative and heterofermentative organisms; in the ''Lactobacillaceae,'' the pathway used for hexose fermentati ...
'' species that maintain healthy vaginal microbial populations.
*
Bacterial meningitis
Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, intense headache, vomiting and neck stiffness and occasionall ...
is a bacterial
inflammation
Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...
of the
meninges
In anatomy, the meninges (; meninx ; ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in the subarachnoid spac ...
, which are the protective membranes covering the
brain
The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...
and
spinal cord
The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...
.
*
Bacterial pneumonia
Bacterial pneumonia is a type of pneumonia caused by bacterial infection.
Types
Gram-positive
'' Streptococcus pneumoniae'' () is the most common bacterial cause of pneumonia in all age groups except newborn infants. ''Streptococcus pneumoniae ...
is a bacterial infection of the
lung
The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...
s.
*
Urinary tract infection
A urinary tract infection (UTI) is an infection that affects a part of the urinary tract. Lower urinary tract infections may involve the bladder (cystitis) or urethra (urethritis) while upper urinary tract infections affect the kidney (pyel ...
is predominantly caused by bacteria. Symptoms include the strong and frequent sensation or urge to urinate, pain during
urination
Urination is the release of urine from the bladder through the urethra in Placentalia, placental mammals, or through the cloaca in other vertebrates. It is the urinary system's form of excretion. It is also known medically as micturition, v ...
, and urine that is cloudy. The most frequent cause is ''
Escherichia coli
''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
''.
Urine
Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...
is typically sterile but contains a variety of salts and waste products. Bacteria can ascend into the
bladder
The bladder () is a hollow organ in humans and other vertebrates that stores urine from the kidneys. In placental mammals, urine enters the bladder via the ureters and exits via the urethra during urination. In humans, the bladder is a distens ...
or
kidney
In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...
and causing
cystitis
A urinary tract infection (UTI) is an infection that affects a part of the urinary tract. Lower urinary tract infections may involve the bladder (cystitis) or urethra ( urethritis) while upper urinary tract infections affect the kidney (py ...
and
nephritis
Nephritis is inflammation of the kidneys and may involve the glomeruli, tubules, or interstitial tissue surrounding the glomeruli and tubules. It is one of several different types of nephropathy.
Types
* Glomerulonephritis is inflammation ...
.
*
Bacterial gastroenteritis is caused by enteric, pathogenic bacteria. These pathogenic species are usually distinct from the usually harmless bacteria of the normal
gut flora
Gut microbiota, gut microbiome, or gut flora are the microorganisms, including bacteria, archaea, fungi, and viruses, that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the g ...
. But a different strain of the same species may be pathogenic. The distinction is sometimes difficult as in the case of ''
Escherichia
''Escherichia'' ( ) is a genus of Gram-negative, non-Endospore, spore-forming, Facultative anaerobic organism, facultatively anaerobic, rod-shaped bacteria from the family Enterobacteriaceae. In those species which are inhabitants of the gastroin ...
''.
* Bacterial
skin infection A skin infection is an infection of the skin in humans and other animals, that can also affect the associated soft tissues such as loose connective tissue and mucous membranes. They comprise a category of infections termed skin and skin structure i ...
s include:
**
Impetigo
Impetigo is a contagious bacterial infection that involves the superficial skin. The most common presentation is yellowish crusts on the face, arms, or legs. Less commonly there may be large blisters which affect the groin or armpits. The les ...
is a highly contagious
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
l
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
infection
An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmis ...
commonly seen in children. It is caused by ''
Staphylococcus aureus
''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posi ...
'', and ''
Streptococcus pyogenes
''Streptococcus pyogenes'' is a species of Gram-positive, aerotolerant bacteria in the genus '' Streptococcus''. These bacteria are extracellular, and made up of non-motile and non-sporing cocci (round cells) that tend to link in chains. They ...
''.
[Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson; & Mitchell, Richard N. (2007). ''Robbins Basic Pathology'' (8th ed.). Saunders Elsevier. pp. 843 ]
**
Erysipelas
Erysipelas () is a relatively common bacterial infection of the superficial layer of the skin ( upper dermis), extending to the superficial lymphatic vessels within the skin, characterized by a raised, well-defined, tender, bright- red rash, ...
is an acute
streptococcus
''Streptococcus'' is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a sing ...
bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
l infection of the deeper skin layers that spreads via with lymphatic system.
**
Cellulitis
Cellulitis is usually a bacterial infection involving the inner layers of the skin. It specifically affects the dermis and subcutaneous fat. Signs and symptoms include an area of redness which increases in size over a few days. The borders of ...
is a diffuse
inflammation
Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...
of
connective tissue
Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...
with severe inflammation of dermal and subcutaneous layers of the
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
. Cellulitis can be caused by normal
skin flora
Skin flora, also called skin microbiota, refers to microbiota (community (ecology), communities of microorganisms) that reside on the skin, typically human skin.
Many of them are bacterium, bacteria of which there are around 1,000 species upon hu ...
or by
contagious contact, and usually occurs through open skin, cuts,
blister
A blister is a small pocket of body fluid (lymph, serum, plasma, blood, or pus) within the upper layers of the skin, usually caused by forceful rubbing (friction), burning, freezing, chemical exposure or infection. Most blisters are filled ...
s, cracks in the skin,
insect bite
Insects (from Latin ') are hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed ...
s,
animal bites,
burns
Burns may refer to:
Astronomy
* 2708 Burns, an asteroid
* Burns (crater), on Mercury
People
* Burns (surname), list of people and characters named Burns
** Burns (musician), Scottish record producer
Places in the United States
* Burns, ...
,
surgical wound
A surgical incision is a cut made through the skin and soft tissue to facilitate an operation or procedure. Often, multiple incisions are possible for an operation. In general, a surgical incision is made as small and unobtrusive as possible t ...
s, intravenous
drug injection
Drug injection is a method of introducing a drug into the bloodstream via a hollow hypodermic needle, which is pierced through the skin into the body (usually intravenously, but also at an intramuscular or subcutaneous, location). Intravenous ...
, or sites of
intravenous
Intravenous therapy (abbreviated as IV therapy) is a medical technique that administers fluids, medications and nutrients directly into a person's vein. The intravenous route of administration is commonly used for rehydration or to provide nutr ...
catheter
In medicine, a catheter ( ) is a thin tubing (material), tube made from medical grade materials serving a broad range of functions. Catheters are medical devices that can be inserted in the body to treat diseases or perform a surgical procedure. ...
insertion. In most cases it is the skin on the face or lower legs that is affected, though cellulitis can occur in other tissues.
Mechanisms of damage
The symptoms of disease appear as pathogenic bacteria damage host tissues or interfere with their function. The bacteria can damage host cells directly or indirectly by provoking an immune response that inadvertently damages host cells,
or by releasing
toxins
A toxin is a naturally occurring poison produced by metabolic activities of living cells or organisms. They occur especially as proteins, often conjugated. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived ...
.
Direct
Once pathogens attach to host cells, they can cause direct damage as the pathogens use the host cell for nutrients and produce waste products.
For example, ''
Streptococcus mutans
''Streptococcus mutans'' is a Facultative anaerobic organism, facultatively anaerobic, gram-positive coccus (round bacteria, bacterium) commonly found in the human oral cavity and is a significant contributor to dental caries, tooth decay.
The m ...
'', a component of
dental plaque
Dental plaque is a biofilm of microorganisms (mostly bacteria, but also fungi) that grows on surfaces within the mouth. It is a sticky colorless deposit at first, but when it forms Calculus (dental), tartar, it is often brown or pale yellow. It is ...
, metabolizes dietary sugar and produces acid as a waste product. The acid decalcifies the tooth surface to cause
dental caries
Tooth decay, also known as caries,The word 'caries' is a mass noun, and is not a plural of 'carie'.'' is the breakdown of teeth due to acids produced by bacteria. The resulting cavities may be a number of different colors, from yellow to black ...
.
Toxin production
Endotoxins
Lipopolysaccharide (LPS), now more commonly known as endotoxin, is a collective term for components of the outermost membrane of the cell envelope of gram-negative bacteria, such as '' E. coli'' and ''Salmonella'' with a common structural archit ...
are the lipid portions of lipopolysaccharides that are part of the outer membrane of the cell wall of
gram-negative bacteria
Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...
. Endotoxins are released when the bacteria
lyses, which is why after antibiotic treatment, symptoms can worsen at first as the bacteria are killed and they release their endotoxins.
Exotoxins
An exotoxin is a toxin secreted by bacteria. An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. They are highly potent and can cause major damage to the host. Exotoxins may be secreted, or, sim ...
are secreted into the surrounding medium or released when the bacteria die and the cell wall breaks apart.
Indirect
An excessive or inappropriate immune response triggered by an infection may damage host cells.
[
]
Survival in host
Nutrients
Iron is required for humans, as well as the growth of most bacteria. To obtain free iron, some pathogens secrete proteins called siderophores
Siderophores (Greek: "iron carrier") are small, high-affinity iron-Chelation, chelating compounds that are secreted by microorganisms such as bacteria and fungi. They help the organism accumulate iron. Although a widening range of siderophore func ...
, which take the iron away from iron-transport proteins by binding to the iron even more tightly. Once the iron-siderophore complex is formed, it is taken up by siderophore receptors on the bacterial surface and then that iron is brought into the bacterium.
Bacterial pathogens also require access to carbon and energy sources for growth. To avoid competition with host cells for glucose which is the main energy source used by human cells, many pathogens including the respiratory
The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gr ...
pathogen ''Haemophilus influenzae
''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, Motility, non-motile, Coccobacillus, coccobacillary, facultative anaerobic organism, facultatively anaerobic, Capnophile, capnophili ...
'' specialise in using other carbon sources such as lactate that are abundant in the human body
Identification
Typically identification is done by growing the organism in a wide range of cultures which can take up to 48 hours. The growth is then visually or genomically identified. The cultured organism is then subjected to various assays to observe reactions to help further identify species and strain.
Treatment
Bacterial infections may be treated with antibiotic
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...
s, 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 p ...
if they kill bacteria or bacteriostatic
A bacteriostatic agent or bacteriostat, abbreviated Bstatic, is a biological or chemical agent that stops bacteria from reproducing, while not necessarily killing them otherwise. Depending on their application, bacteriostatic antibiotics, disinfec ...
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. For example, the antibiotics chloramphenicol
Chloramphenicol is an antibiotic useful for the treatment of a number of bacterial infections. This includes use as an eye ointment to treat conjunctivitis. By mouth or by intravenous, injection into a vein, it is used to treat meningitis, pl ...
and tetracyclin
Tetracycline, sold under various brand names, is an antibiotic in the tetracyclines family of medications, used to treat a number of infections, including acne, cholera, brucellosis, plague, malaria, and syphilis. It is available in oral a ...
inhibit the bacterial ribosome
Ribosomes () are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order s ...
but not the structurally different eukaryotic ribosome, so they exhibit selective toxicity. Antibiotics are used both in treating human disease and in intensive farming
Intensive agriculture, also known as intensive farming (as opposed to extensive farming), conventional, or industrial agriculture, is a type of agriculture, both of arable farming, crop plants and of Animal husbandry, animals, with higher levels ...
to promote animal growth. Both uses may be contributing to the rapid development of antibiotic resistance
Antimicrobial resistance (AMR or AR) occurs when microbes evolve mechanisms that protect them from antimicrobials, which are drugs used to treat infections. This resistance affects all classes of microbes, including bacteria (antibiotic resis ...
in bacterial populations. Phage therapy
Phage therapy, viral phage therapy, or phagotherapy is the therapeutic use of bacteriophages for the treatment of pathogenic bacterial infections. This therapeutic approach emerged at the beginning of the 20th century but was progressively r ...
, using bacteriophage
A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...
s can also be used to treat certain bacterial infections.
Prevention
Infections can be prevented by antiseptic
An antiseptic ( and ) is an antimicrobial substance or compound that is applied to living tissue to reduce the possibility of sepsis, infection, or putrefaction. Antiseptics are generally distinguished from ''antibiotics'' by the latter's abil ...
measures such as sterilizing 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 sterilized to prevent infection by bacteria. Disinfectants
A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than ...
such as bleach
Bleach is the generic name for any chemical product that is used industrially or domestically to remove color from (i.e. to whiten) fabric or fiber (in a process called bleaching) or to disinfect after cleaning. It often refers specifically t ...
are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection. Bacteria in food are killed by cooking to temperatures above 73 °C (163 °F).
List of genera and microscopy features
Many genera
Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...
contain pathogenic bacterial species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
. They often possess characteristics that help to classify and organize them into groups. The following is a partial listing.
List of species and clinical characteristics
File:Overall age-standardised mortality rate per 100 000 population for 33 pathogens investigated, 2019.jpg, Overall age-standardised mortality rate per 100 000 population for 33 pathogens investigated, 2019
File:Global number of deaths (A) and YLLs (B), by pathogen and infectious syndrome, 2019.jpg, Global number of deaths (A) and YLLs (B), by pathogen and infectious syndrome, 2019
File:Global number of deaths, by pathogen, age, and sex groups, 2019.jpg, Global number of deaths, by pathogen, age, and sex groups, 2019
This is description of the more common genera and species presented with their clinical characteristics and treatments.
Genetic transformation
Of the 59 species listed in the table with their clinical characteristics, 11 species (or 19%) are known to be capable of natural genetic transformation
In molecular biology and genetics, transformation is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous genetic material from its surroundings through the cell membrane(s). For transformation to t ...
.[Bernstein H, Bernstein C, Michod RE (2018). Sex in microbial pathogens. Infection, Genetics and Evolution volume 57, pages 8-25. https://doi.org/10.1016/j.meegid.2017.10.024] Natural transformation is a bacterial adaptation for transferring DNA
Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
from one cell to another. This process includes the uptake of exogenous DNA from a donor cell by a recipient cell and its incorporation into the recipient cell's genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
by recombination. Transformation appears to be an adaptation for repairing damage
Damage is any change in a thing, often a physical object, that degrades it away from its initial state. It can broadly be defined as "changes introduced into a system that adversely affect its current or future performance".Farrar, C.R., Sohn, H., ...
in the recipient cell's DNA. Among pathogenic bacteria, transformation capability likely serves as an adaptation that facilitates survival and infectivity. The pathogenic bacteria able to carry out natural genetic transformation (of those listed in the table) are ''Campylobacter jejuni
''Campylobacter jejuni'' is a species of pathogenic bacteria that is commonly associated with poultry, and is also often found in animal feces. This species of microbe is one of the most common causes of food poisoning in Europe and in the US, w ...
'', ''Enterococcus faecalis
''Enterococcus faecalis'' – formerly classified as part of the group D '' Streptococcus,'' is a Gram-positive, commensal bacterium naturally inhabiting the gastrointestinal tracts of humans. Like other species in the genus '' Enterococcus'', ' ...
'', ''Haemophilus influenzae
''Haemophilus influenzae'' (formerly called Pfeiffer's bacillus or ''Bacillus influenzae'') is a Gram-negative, Motility, non-motile, Coccobacillus, coccobacillary, facultative anaerobic organism, facultatively anaerobic, Capnophile, capnophili ...
'', ''Helicobacter pylori
''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, Flagellum#bacterial, flagellated, Bacterial cellular morphologies#Helical, helical bacterium. Mutants can have a rod or curved rod shape that exhibits l ...
'', ''Klebsiella pneumoniae
''Klebsiella pneumoniae'' is a Gram-negative, non-motile, encapsulated, lactose- fermenting, facultative anaerobic, rod-shaped bacterium. It appears as a mucoid lactose fermenter on MacConkey agar.
Although found in the normal flora of the mo ...
'', ''Legionella pneumophila
''Legionella pneumophila'', the primary causative agent for Legionnaires' disease, Legionnaire's disease, is an Aerobic organism, aerobic, pleomorphic, Flagellum, flagellated, non-spore-forming, Gram-negative bacteria, Gram-negative bacterium. ' ...
'', ''Neisseria gonorrhoeae
''Neisseria gonorrhoeae'', also known as ''gonococcus'' (singular) or ''gonococci'' (plural), is a species of Gram-negative diplococci bacteria first isolated by Albert Ludwig Sigesmund Neisser, Albert Neisser in 1879. An obligate human pathog ...
'', ''Neisseria meningitidis
''Neisseria meningitidis'', often referred to as the meningococcus, is a Gram-negative bacterium that can cause meningitis and other forms of meningococcal disease such as meningococcemia, a life-threatening sepsis. The bacterium is referred to a ...
'', ''Staphylococcus aureus
''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posi ...
'', ''Streptococcus pneumoniae
''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, hemolysis (microbiology), alpha-hemolytic member of the genus ''Streptococcus''. ''S. pneumoniae'' cells are usually found in pairs (diplococci) and do not f ...
'' and ''Vibrio cholerae
''Vibrio cholerae'' is a species of Gram-negative bacteria, Gram-negative, Facultative anaerobic organism, facultative anaerobe and Vibrio, comma-shaped bacteria. The bacteria naturally live in Brackish water, brackish or saltwater where they att ...
''.
See also
* Human microbiome project
The Human Microbiome Project (HMP) was a United States National Institutes of Health (NIH) research initiative to improve understanding of the microbiota involved in human health and disease. Launched in 2007, the first phase (HMP1) focused on i ...
* List of antibiotics
The following is a list of antibiotics. The highest division between antibiotics is bactericidal and bacteriostatic. Bactericidals kill bacteria directly, whereas bacteriostatics prevent them from dividing. However, these classifications are base ...
* Pathogenic viruses
A viral disease (or viral infection) occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions) attach to and enter susceptible cells.
Examples include the common cold, gastroenteritis, COVID-19, ...
Notes
References
External links
Bacterial Pathogen Pronunciation
by Neal R. Chamberlain, Ph.D. at A.T. Still University
Pathogenic bacteria
genomes and related information a
PATRIC
a Bioinformatics Resource Center funded b
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