An organism which is capable of producing energy through aerobic respiration and then switching back to anaerobic respiration depending on the amounts of oxygen and fermentable material in the environment.
In the presence of oxygen, facultative anaerobes use aerobic respiration; without oxygen some of them ferment, some use anaerobic respiration.
Some examples of facultative anaerobic bacteria are the Staphylococci (Gram positive), Escherichia coli (Gram negative), Corynebacterium (Gram positive), and Listeria (Gram positive). Organisms in the Kingdom Fungi can also be facultative anaerobic, such as yeasts.
Word origin: an– from Gk., “not, without,” + Greek āero-, from āēr, air.
Compare: obligate anaerobe.
See also: facultative anaerobe, microaerophile, aerotolerant.