an organism able to live and reproduce only in the presence of free oxygen (e.g., certain bacteria and certain yeasts). Organisms that grow in the absence of free oxygen are termed anaerobes; those that grow only in the absence of oxygen are obligate, or strict, anaerobes. Some species, called facultative anaerobes, are able to grow either with or without free oxygen. Certain others, able to grow best in the presence of low amounts of oxygen, are called microaerophiles.