What is anaërobic bacteria. They are called facultative anaërobia when able to live either in the presence or absence of free oxygen; obligate, or obligatory, anaërobia when they thrive only in its absence called?

anaerobia — Anaërobic bacteria. They are called facultative anaërobia when able to live either in the presence or absence of free oxygen; obligate, or obligatory, anaërobia when they thrive only in its absence. (Webster's 1913.)

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