Facultative anaerobic organism
A facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation under anaerobic conditions.
Organisms in the Kingdom Fungi can also be facultative anaerobic, such as yeasts.
Factors influencing the switch are the concentrations of oxygen and fermentable material in the environment. In brewer's yeast, the Pasteur shift is the observed cessation of oxygen consumption when fermentable sugar is supplied. In a growing culture, the energy "economics" disfavors respiration due to the "overhead cost" of producing the apparatus, as long as sufficient fermentable substrate is available, even though the energy output per mole of fermented material is far less than from respiration's complete oxidation of the same substrate. The switching off of respiration is far faster than the reverse, because in a culture which has grown for a few generations via fermentation, it will take time for mitochondria, which are depleted, to proliferate.