adj. describing microorganisms that grow best at very low oxygen concentrations (i.e. below the atmospheric level).
n. any organism, especially a microbe, that is able to live and grow in the absence of free oxygen. A facultative anaerobe is a microorganism that grows best in the presence of oxygen but is capable of some growth in its absence. An obligate anaerobe can grow only in the absence of free oxygen. Compare aerobe; microaerophilic.... anaerobe
Health Encyclopedia