Obligate anaerobe

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

Oxygen sensitivity

The oxygen sensitivity of obligate anaerobes has been attributed to a combination of factors including oxidative stress and enzyme production. Oxygen can also damage obligate anaerobes in ways not involving oxidative stress.

Because molecular oxygen contains two unpaired electrons in the highest occupied molecular orbital, it is readily reduced to superoxide (O⁻
₂) and hydrogen peroxide (H
₂O
₂) within cells. A reaction between these two products results in the formation of a free hydroxyl radical (OH^.). Superoxide, hydrogen peroxide, and hydroxyl radicals are a class of compounds known as reactive oxygen species (ROS), highly reactant products that are damaging to microbes, including obligate anaerobes. Aerobic organisms produce superoxide dismutase and catalase 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, methanogens 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, 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 because electrons are exhausted in reducing oxygen.