Facultative aerobes are microorganisms that can survive in both the presence and absence of oxygen. They prefer to use oxygen for respiration when it is available, as this process generates more energy. However, if oxygen is not present, they can switch to anaerobic respiration or fermentation to produce energy. These organisms are commonly found in various environments, including soil and the human gut. Examples of facultative aerobes include certain species of Escherichia coli and Staphylococcus aureus. Their ability to adapt to different oxygen levels allows them to thrive in diverse conditions.