Describing organisms that are capable of a certain mode of life or function but are not restricted to it. Facultative parasites can live as parasites in or on another organism, but can also live independently; e.g. certain types of ringworm fungus are opportunistic parasites of mammals, living in the skin and hair of their host. Facultative anaerobes are organisms, such as certain bacteria, fungi, and some internal parasites of animals, that are able to alter their metabolism to grow in either the presence or absence of oxygen. The best-known facultative anaerobe is Saccharomyces cerevisiae, the yeast used in brewing. See anaerobic respiration.