Anaerobic

Microorganisms that cannot tolerate oxygen and are killed in its presence are called obligate (or strict) anaerobes. Bacteria of the genus Clostridium, which cause gas gangrene, tetanus, and botulism, belong to this group as well as Bacteroides gingivalis, which thrives in anaerobic crevices between the teeth. Strict anaerobes are killed by oxygen, which is why hydrogen peroxide (which releases oxygen) is frequently applied to wounds. Methane-producing bacteria (methanogens), can be isolated from the anaerobic habitats of swamp sludge sewage and from the guts of certain animals. Methanogens generate the marsh gases of swamps and sewage treatment plants by converting hydrogen and carbon dioxide gases to methane. Some organisms, such as yeast, have adapted to grow in either the presence or absence of oxygen and are termed facultative anaerobes.