Obligate anaerobes aremicroorganisms killed by normalatmospheric concentrations ofoxygen (20.95% O₂).^[1][2] 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%.^[3]

Obligate anaerobes, which die when normal amounts of oxygen are present, are contrasted withobligate aerobes, which die without oxygen. Bacteria that fall in between these two extremes may be classified as eitherfacultative anaerobes, which can use oxygen but also survive without it, ormicroaerophiles, which need lower levels of oxygen.Aerotolerant organisms are indifferent to the presence or absence of oxygen.

The oxygen sensitivity of obligate anaerobes has been attributed to a combination of factors includingoxidative stress and enzyme production. Oxygen can also damage obligate anaerobes in ways not involving oxidative stress.^{[citation needed]}

Because molecularoxygen contains two unpairedelectrons in thehighest occupied molecular orbital, it is readily reduced tosuperoxide (O⁻
₂) andhydrogen peroxide (H
₂O
₂) within cells.^[1] A reaction between these two products results in the formation of a freehydroxyl radical (OH^.).^[4] Superoxide, hydrogen peroxide, and hydroxyl radicals are a class of compounds known asreactive oxygen species (ROS), highly reactant products that are damaging to microbes, including obligate anaerobes.^[4]Aerobic organisms producesuperoxide dismutase andcatalase to detoxify these products, but obligate anaerobes produce these enzymes in very small quantities, or not at all.^[1][2][3][5] 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.^[2][3]

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.^[6] In the absence of oxygen, the mutated samples grew normally.^[6] In 2018, Lu et al. found that inBacteroides thetaiotaomicron, an obligate anaerobe found in the mammalian digestive tract, exposure to oxygen results in increased levels of superoxide which inactivated important metabolic enzymes.^[6]

Dissolved oxygen increases theredox potential of a solution, and high redox potential inhibits the growth of some obligate anaerobes.^[3][5][7] For example,methanogens grow at a redox potential lower than -0.3 V.^[7] Sulfide is an essential component of some enzymes, and molecular oxygen oxidizes this to formdisulfide, thus inactivating certain enzymes (e.g.nitrogenase). Organisms may not be able to grow with these essential enzymes deactivated.^[1][5][7] Growth may also be inhibited due to a lack ofreducing equivalents forbiosynthesis because electrons are exhausted in reducing oxygen.^[7]

Obligate anaerobes convert nutrients into energy throughanaerobic respiration orfermentation. In aerobic respiration, the pyruvate generated fromglycolysis is converted toacetyl-CoA. This is then broken down via theTCA cycle andelectron transport chain. Anaerobic respiration differs fromaerobic respiration in that it uses anelectron acceptor other than oxygen in the electron transport chain. Examples of alternative electron acceptors includesulfate,nitrate,iron,manganese,mercury, andcarbon monoxide.^[8]

Fermentation differs from anaerobic respiration in that the pyruvate generated fromglycolysis is broken down without the involvement of an electron transport chain (i.e. there is nooxidative phosphorylation). Numerous fermentation pathways exist such aslactic acid fermentation,mixed acid fermentation,2-3 butanediol fermentation where organic compounds are reduced to organic acids and alcohol.^[8][4]

The energy yield of anaerobic respiration and fermentation (i.e. the number ofATP molecules generated) is less than in aerobic respiration.^[8] This is whyfacultative anaerobes, which can metabolise energy both aerobically and anaerobically, preferentially metabolise energy aerobically. This is observable when facultative anaerobes are cultured inthioglycolate broth.^[1]

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.^[9] Examples of obligately anaerobicbacterialgenera includeActinomyces,Bacteroides,Clostridium,Fusobacterium,Peptostreptococcus,Porphyromonas,Prevotella,Propionibacterium, andVeillonella.Clostridium species areendospore-forming bacteria, and can survive in atmospheric concentrations of oxygen in this dormant form. The remaining bacteria listed do not form endospores.^[5]

Several species of theMycobacterium,Streptomyces, andRhodococcus genera are examples of obligate anaerobe found in soil.^[10] Obligate anaerobes are also found in the digestive tracts of humans and other animals as well as in the first stomach ofruminants.^[11]

Examples of obligately anaerobicfungal genera include therumen fungiNeocallimastix,Piromonas, andSphaeromonas.^[12]

• Aerobic respiration
• Fermentation

• Microbiology

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