Facultative anaerobes are microorganisms that can thrive in both the presence and absence of oxygen. They possess the ability to switch between aerobic respiration, which uses oxygen to produce energy, and anaerobic respiration or fermentation, which does not require oxygen. This adaptability allows them to survive in various environments, from oxygen-rich to oxygen-poor conditions. These organisms play a crucial role in many ecosystems and are commonly found in soil, water, and the human gut. Examples of facultative anaerobes include Escherichia coli and Staphylococcus aureus, which can impact health and disease depending on their environment.