Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation. In contrast, obligate anaerobes die in the presence of oxygen. Some examples of facultative anaerobic bacteria are Staphylococcus, Escherichia coli, and Listeria. Certain eukaryote phyla are also facultative anaerobes, including fungi such as yeasts and many aquatic invertebrates such as Nereid polychaetes, for example. There are also circulating white blood cells that are classified as facultative anaerobes. These include neutrophils, monocytes and tissue macrophages.