Voluntary; possessing the power to do or not to do a thing.
Capable of existing under different conditions, as a microorganism that can exist in either aerobic or anaerobic conditions.
(Of life form or habitat requirement) occasional or incidental, as opposed to obligatory or necessary.
Able to adjust to different environments or conditions (e.g., facultative anaerobe, organism that normally lives without air but can survive in air).
Describing an organism that is not restricted to one way of life. A facultative parasite can live either as a parasite or, in different conditions, as a non-parasite able to survive without a host.
Having the ability to do something that is not compulsory.
Refers to bacteria that can survive both with and without oxygen.