A facultative anaerobe can grow in both oxygen-rich and oxygen-poor environments, using aerobic respiration when oxygen is available and switching to anaerobic respiration or fermentation when it is not.
Energy strategies
Facultative anaerobes possess metabolic pathways that allow them to adapt to changing oxygen levels. In the presence of oxygen, they perform aerobic respiration, using oxygen as the terminal electron acceptor to oxidize substrates completely and produce ATP efficiently through oxidative phosphorylation. When oxygen becomes scarce, these organisms shift to anaerobic respiration, using alternative electron acceptors such as nitrate, fumarate or sulfate, or to fermentation pathways that produce organic acids, alcohols or gases. This flexibility is mediated by regulatory systems that sense oxygen tension and modulate gene expression and enzyme activity. Facultative anaerobes also produce enzymes like superoxide dismutase and catalase to detoxify reactive oxygen species formed during aerobic metabolism, enabling them to tolerate oxidative stress. Examples include many enteric bacteria such as Escherichia coli, Salmonella and Vibrio species, as well as yeasts like Saccharomyces cerevisiae. Their dual energy strategies support survival in diverse habitats, from soil and water columns to animal intestines and laboratory media.
Ecological roles and examples
Facultative anaerobes have important ecological and medical roles. In the human gut, facultative anaerobes consume oxygen diffusing from the mucosa, thereby creating an anaerobic environment that supports obligate anaerobic bacteria involved in fermentation of dietary fibres. Pathogens such as Staphylococcus aureus and Listeria monocytogenes are facultative anaerobes, allowing them to colonize tissues with variable oxygen availability. In food production, the facultative anaerobic yeast S. cerevisiae ferments sugars to ethanol and carbon dioxide under anaerobic conditions but respires aerobically when oxygen is supplied, a phenomenon known as the Pasteur effect. Facultative anaerobes also contribute to biogeochemical cycles by reducing nitrate to nitrite or nitrogen gas in environments where oxygen fluctuates. Their ability to switch between metabolic modes makes them resilient to environmental changes and useful in industrial and research applications.
Facultative anaerobes occupy a metabolic middle ground, capable of generating energy through aerobic or anaerobic mechanisms. This versatility enables them to colonize many ecological niches and contributes to their significance in health, industry and environmental processes.
Related Terms: Facultative aerobe, Obligate anaerobe, Obligate aerobe, Fermentation, Anaerobic respiration