Anaerobic bacteria are bacteria that cannnot use oxygen in their metabolism, but are poisoned and killed by this molecule. These bacteria are also said to be strictly anaerobic (= obligate anaerobic) to distinguish them from the oxygen tolerant bacteria and the facultatively anaerobic (= facultatively aerobic) bacteria. Oxygen tolerant bacteria does not use oxygen in their metabolism. However, they are not poisoned by oxygen, but can live and multiply in the presence of oxygen, at least for a certain time. Facultatively anaerobic bacteria are not poisoned by oxygen and can switch their metabolism, so that in the presence of oxygen they utilize oxygen in metabolism, but in the absence of oxygen, they can extract energy in other ways (e.g. by fermentation or anaerobic respiration).
The oxygen molecule consists of two atoms of oxygen, making the molecule stable because the two atoms can then share a pair of electrons in the outer electron shell (see Fig. A). In the presence of oxygen (O2) in an aqueous solution (e.g. in a cell) small amounts of hydrogen peroxide (H2O2) and superoxide radicals are always formed by an equilibrium reaction. The superoxide radical (see Fig. B) is usually designated O2·-, to show that it consists of an oxygen molecule, which has taken up an extra electron (·), making it negatively charged. Hydrogen peroxide and the superoxide radical in particular are highly toxic to cells because they are very reactive and can affect a variety of substances, e.g. oxidize unsaturated fatty acids, leading to the so-called oxidative stress. In order to survive in an oxygen-containing environment, the cells, therefore, have enzymes that can metabolize (detoxify) hydrogen peroxide and the superoxide radical.
Strictly anaerobic bacteria lacks all enzymes which metabolize hydrogen peroxide and superoxide radicals. Other bacteria have superoxide dismutase (SODM), which converts superoxide radicals into hydrogen peroxide and oxygen (see Fig. C). Aerobic and most facultatively anaerobic bacteria have catalase, which converts hydrogen peroxide to water and oxygen (see Fig. C). Many oxygen-tolerant anaerobic bacteria have a peroxidase, which converts hydrogen peroxide to water by utilizing NADH2 (see Fig. C; click on it to enlarge it).