Anaerobes or anaerobic organisms are those that cannot grow in presence of oxygen. Oxygen is toxic for them so they must rely on other substances as terminal electron acceptor. Their metabolism is of fermentative type in which they reduce available organic compounds to various end products like organic acids and alcohols.

The main reason behind why anaerobic bacteria cannot grow in presence of oxygen can be explained by the following:

Oxygen toxicity:

Anaerobes lack certain enzymes that are essential for bacteria to survive in presence of oxygen. During growth and metabolism, oxygen reduction products are generated within the organism. One oxygen reduction product is superoxide anion produced by univalent reduction of oxygen.

O2e-————> O2

It is generated during the interaction of molecular oxygen with various cellular constituents like reduced riboflavins, flavoproteins, quinines, thiols and iron-sulphur proteins.  It causes intracellular damage by an unknown mechanism but is capable of taking part in a number of destructive reactions potentially lethal to cell. Moreover, products of secondary reactions may amplify toxicity.

One hypothesis is that superoxide anion reacts with hydrogen peroxide generating free hydroxyl radical (most potent biological oxidant) that can attack virtually any organic substance in the cell.

O2 + H2O2 ————> OH – + OH· + O2(Haber-Weiss reaction)

A subsequent reaction between superoxide anion and the hydroxyl radical produce singlet oxygen (O2*) which when excited is very reactive and also damages the cell. 

O2 + OH ————> OH + O2*

Therefore, superoxide anion must be removed from the cell in order to survive in presence of oxygen.

Most facultative and aerobic organism possesses a high concentration of an enzyme called superoxide dismutase. This enzyme converts the superoxide anion into ground-state oxygen and hydrogen peroxide thus preventing the destructive action of superoxide anion.

2O2 +2 H+ —Superoxide dismutase—> O2 + H2O2

The hydrogen peroxide generated in this reaction is an oxidizing agent but it does not damage the cell as much as superoxide anion and tends to diffuse out of the cell. Many organisms possess catalase or peroxidase or both to eliminate hydrogen peroxide. Catalase converts hydrogen peroxide to water and ground-state oxygen molecule. 

2H2O2 —catalase —> H2O + O2

Peroxidase uses a reductant as an electron donor to covert hydrogen peroxide to water and ground-state oxygen molecule.

H2O2 +H2R—peroxidase —> H2O + O2

As anaerobes lack these enzymes that detoxify the toxic oxygen products, they cannot tolerate the toxic effects of oxygen and hence do not survive in presence of oxygen.

The continuous spectrum of oxygen tolerance among bacteria appears to be partly due to activities of enzymes like superoxide dismutase, catalase and peroxidase in the cell and partly to the rate at which the cell takes up oxygen. Other factors that influence oxygen tolerance are the location of protective enzymes in the cell (surface versus cytoplasm),  the rate at which a cell form toxic oxygen products ( hydroxyl radical or singlet oxygen) and the susceptibility of key cellular components to the toxic oxygen products. 

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