Facultative aerobes prefer to generate energy by aerobic respiration when oxygen is available but are capable of switching to anaerobic pathways or fermentation when oxygen becomes limited.
Metabolic flexibility
Facultative aerobes are microorganisms with adaptable metabolic pathways that allow survival in both oxygen-rich and oxygen-poor environments. When oxygen is present, they use it as the terminal electron acceptor in aerobic respiration, producing water and ATP through oxidative phosphorylation. This pathway is highly efficient because it fully oxidizes substrates such as glucose to carbon dioxide. In the absence of oxygen, these organisms shift to anaerobic respiration using alternative acceptors like nitrate or to fermentation pathways that produce organic acids, alcohols or gases as end products. This shift is controlled by regulatory proteins and enzymes that sense oxygen availability and adjust gene expression. Facultative aerobes also produce enzymes such as catalase and superoxide dismutase that detoxify reactive oxygen species generated during aerobic metabolism. Examples include Escherichia coli, Salmonella species and many yeasts. Their metabolic flexibility allows them to inhabit diverse niches from soil and water to animal intestines and to persist during fluctuating oxygen conditions.
Ecological significance and examples
Facultative aerobes play key roles in ecology and medicine. In the human gastrointestinal tract, facultative aerobes consume residual oxygen, creating an anaerobic environment that supports obligate anaerobes that aid digestion. Bacteria such as Staphylococcus aureus and Enterococcus faecalis can grow in wounds or tissues with variable oxygen levels due to their facultative nature. In wastewater treatment, mixed communities of facultative aerobic bacteria degrade organic matter under cycles of aeration and anoxia. Many laboratory model organisms, including E. coli and the yeast Saccharomyces cerevisiae, are facultative aerobes, which has made them valuable for studying metabolic regulation. Their ability to switch between respiratory and fermentative metabolism also underpins industrial processes such as bread making, brewing and biotechnology fermentations.
Facultative aerobes bridge the gap between strict aerobes and strict anaerobes by possessing metabolic pathways for both conditions. This versatility enhances their survival in diverse environments and contributes to their ecological and clinical importance.
Related Terms: Facultative anaerobe, Obligate aerobe, Obligate anaerobe, Fermentation, Aerobic respiration