Obligate anaerobe
microorganism
Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (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%.
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 (H2O2) 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.
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