Facultative anaerobes are microorganisms that can survive in both the presence and absence of oxygen. They can switch between aerobic respiration, which uses oxygen to produce energy, and anaerobic respiration or fermentation when oxygen is not available. This adaptability allows them to thrive in various environments. These organisms are commonly found in diverse habitats, including soil, water, and the human gut. Examples of facultative anaerobes include Escherichia coli and Saccharomyces cerevisiae. Their ability to utilize different metabolic pathways makes them important in ecological systems and various industrial applications, such as brewing and fermentation.