<Li> Aerotolerant organisms, which cannot use oxygen for growth, but tolerate its presence . </Li> <Li> Facultative anaerobes, which can grow without oxygen but use oxygen if it is present . </Li> <P> Some obligate anaerobes use fermentation, while others use anaerobic respiration . Aerotolerant organisms are strictly fermentative . In the presence of oxygen, facultative anaerobes use aerobic respiration; without oxygen, some of them ferment; some use anaerobic respiration . </P> <P> There are many anaerobic fermentative reactions . </P>

Bacteria which cannot exist in the presence of free oxygen are