Obligate anaerobes are

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s killed by normal

atmospheric An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere ...

concentrations of

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

(20.95% O₂). Oxygen tolerance varies between species, with some species capable of surviving in up to 8% oxygen, while others lose viability in environments with an oxygen concentration greater than 0.5%.

Oxygen sensitivity

The oxygen sensitivity of obligate anaerobes has been attributed to a combination of factors including

oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage. Disturbances in the normal ...

and enzyme production. Oxygen can also damage obligate anaerobes in ways not involving oxidative stress. Because molecular

contains two unpaired

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s in the highest occupied molecular orbital, it is readily reduced to

superoxide In chemistry, a superoxide is a compound that contains the superoxide ion, which has the chemical formula . The systematic name of the anion is dioxide(1−). The reactive oxygen ion superoxide is particularly important as the product of t ...

() and

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

() within cells. A reaction between these two products results in the formation of a free

hydroxyl radical The hydroxyl radical, •HO, is the neutral form of the hydroxide ion (HO–). Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are pr ...

(OH^.). Superoxide, hydrogen peroxide, and hydroxyl radicals are a class of compounds known as

reactive oxygen species In chemistry and biology, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (), water, and hydrogen peroxide. Some prominent ROS are hydroperoxide (H2O2), superoxide (O2−), hydroxyl ...

(ROS), highly reactant products that are damaging to microbes, including obligate anaerobes.

Aerobic organism An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment. The ability to exhibit aerobic respiration may yield benefits to the aerobic organism, as aerobic respiration yields more energy than anaerobic ...

s produce

superoxide dismutase Superoxide dismutase (SOD, ) is an enzyme that alternately catalyzes the dismutation (or partitioning) of the superoxide () anion radical into normal molecular oxygen (O2) and hydrogen peroxide (). Superoxide is produced as a by-product of oxy ...

and

catalase Catalase is a common enzyme found in nearly all living organisms exposed to oxygen (such as bacteria, plants, and animals) which catalyzes the decomposition of hydrogen peroxide to water and oxygen. It is a very important enzyme in protecting ...

to detoxify these products, but obligate anaerobes produce these enzymes in very small quantities, or not at all. The variability in oxygen tolerance of obligate anaerobes (<0.5 to 8% O₂) is thought to reflect the quantity of superoxide dismutase and catalase being produced. In 1986, Carlioz and Touati performed experiments which support the idea that reactive oxygen species may be toxic to anaerobes. '' E. coli,'' a facultative anaerobe, was mutated by a deletion of superoxide dismutase genes. In the presence of oxygen, this mutation resulted in the inability to properly synthesize certain amino acids or use common carbon sources as substrates during metabolism. In the absence of oxygen, the mutated samples grew normally. In 2018, Lu et al. found that in '' Bacteroides thetaiotaomicron,'' an obligate anaerobe found in the mammalian digestive tract, exposure to oxygen results in increased levels of superoxide which inactivated important metabolic enzymes. Dissolved oxygen increases the redox potential of a solution, and high redox potential inhibits the growth of some obligate anaerobes. For example,

methanogen Methanogens are anaerobic archaea that produce methane as a byproduct of their energy metabolism, i.e., catabolism. Methane production, or methanogenesis, is the only biochemical pathway for Adenosine triphosphate, ATP generation in methanogens. A ...

s grow at a redox potential lower than -0.3 V. Sulfide is an essential component of some enzymes, and molecular oxygen oxidizes this to form

disulfide In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

, thus inactivating certain enzymes (e.g. nitrogenase). Organisms may not be able to grow with these essential enzymes deactivated. Growth may also be inhibited due to a lack of reducing equivalents for

biosynthesis Biosynthesis, i.e., chemical synthesis occurring in biological contexts, is a term most often referring to multi-step, enzyme-Catalysis, catalyzed processes where chemical substances absorbed as nutrients (or previously converted through biosynthe ...

because electrons are exhausted in reducing oxygen.

Energy metabolism

Obligate anaerobes convert nutrients into energy through anaerobic respiration or

fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...

. In aerobic respiration, the pyruvate generated from

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...

is converted to

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidation, o ...

. This is then broken down via the TCA cycle and

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

. Anaerobic respiration differs from

aerobic respiration Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cellu ...

in that it uses an

electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. Electron acceptors are oxidizing agents. The electron accepting power of an electron acceptor is measured by its redox potential. In the ...

other than oxygen in the electron transport chain. Examples of alternative electron acceptors include

sulfate The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...

nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

, mercury, and

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

. Fermentation differs from anaerobic respiration in that the pyruvate generated from

is broken down without the involvement of an electron transport chain (i.e. there is no

oxidative phosphorylation Oxidative phosphorylation(UK , US : or electron transport-linked phosphorylation or terminal oxidation, is the metabolic pathway in which Cell (biology), cells use enzymes to Redox, oxidize nutrients, thereby releasing chemical energy in order ...

). Numerous fermentation pathways exist such as

lactic acid fermentation Lactic acid fermentation is a metabolic process by which glucose or other hexose, six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolite lactic acid, lactate, w ...

, mixed acid fermentation, 2-3 butanediol fermentation where organic compounds are reduced to organic acids and alcohol. The energy yield of anaerobic respiration and fermentation (i.e. the number of ATP molecules generated) is less than in aerobic respiration. This is why facultative anaerobes, which can metabolise energy both aerobically and anaerobically, preferentially metabolise energy aerobically. This is observable when facultative anaerobes are cultured in

thioglycolate broth Thioglycolate broth is a multipurpose, enrichment, differential medium used primarily to determine the oxygen requirements of microorganisms. Sodium thioglycolate in the medium consumes oxygen and permits the growth of obligate anaerobes. This, co ...

Ecology and examples

Obligate anaerobes are found in oxygen-free environments such as the intestinal tracts of animals, the deep ocean, still waters, landfills, in deep sediments of soil. Examples of obligately anaerobic

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 ...

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 ...

include '' Actinomyces'', ''

Bacteroides ''Bacteroides'' is a genus of Gram-negative, obligate anaerobic bacteria. ''Bacteroides'' species are non endospore–forming bacilli, and may be either motile or nonmotile, depending on the species. The DNA base composition is 40–48% GC. Un ...

'', '' Clostridium'', ''

Fusobacterium ''Fusobacterium'' is a genus of obligate anaerobic, Gram-negative, non-sporeforming bacteria belonging to Gracilicutes. Individual cells are slender, rod-shaped bacilli with pointed ends. ''Fusobacterium'' was discovered in 1900 by Courmont an ...

'', '' Peptostreptococcus'', '' Porphyromonas'', '' Prevotella'', '' Propionibacterium'', and '' Veillonella''. ''Clostridium'' species are

endospore An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria in the phylum Bacillota. The name "endospore" is suggestive of a spore or seed-like form (''endo'' means 'within'), but it is not a true spore (i.e., not ...

-forming bacteria, and can survive in atmospheric concentrations of oxygen in this dormant form. The remaining bacteria listed do not form endospores. Several species of the ''

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 ...

, Streptomyces, and

Rhodococcus ''Rhodococcus'' is a genus of aerobic, nonsporulating, nonmotile Gram-positive bacteria closely related to ''Mycobacterium'' and ''Corynebacterium''. While a few species are pathogenic, most are benign, and have been found to thrive in a broad ...

'' genera are examples of obligate anaerobe found in soil. Obligate anaerobes are also found in the digestive tracts of humans and other animals as well as in the first stomach of

ruminant Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microb ...

s. Examples of obligately anaerobic

fungal A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as one of the tradit ...

genera include the

rumen The rumen, also known as a paunch, is the largest stomach compartment in ruminants. The rumen and the reticulum make up the reticulorumen in ruminant animals. The diverse microbial communities in the rumen allows it to serve as the primary si ...

fungi '' Neocallimastix'', '' Piromonas'', and '' Sphaeromonas''.

References

{{DEFAULTSORT:Obligate Anaerobe Microbiology

Oxygen sensitivity

Energy metabolism

Ecology and examples

See also

References