facultative

Oxygen requirement classifies bacteria in a way that predicts culture conditions and disease sites. Obligate aerobes — Nocardia, Pseudomonas, Mycobacterium tuberculosis, Bacillus, and Brucella — need oxygen and explain, for instance, why reactivation tuberculosis favours the well-oxygenated lung apices. Obligate anaerobes — Clostridium, Bacteroides, Fusobacterium, Actinomyces — lack catalase and superoxide dismutase, are killed by oxygen, and flourish in devitalized tissue and abscesses; they are also intrinsically resistant to aminoglycosides, which require oxygen-dependent uptake. Facultative anaerobes include most Enterobacteriaceae, staphylococci, and streptococci, and this metabolic flexibility lets them colonize both the gut lumen and oxygenated tissue. Microaerophiles such as Campylobacter and Helicobacter need reduced oxygen tension. The word is used the same way in 'facultative intracellular', meaning an organism that can replicate inside host cells but does not require it, in contrast to obligate intracellular Rickettsia and Chlamydia.