Final answer:
A facultative anaerobe is an organism that can survive and grow in the presence or absence of oxygen by switching between aerobic respiration and anaerobic respiration or fermentation.
Step-by-step explanation:
A facultative anaerobe is an organism that is flexible as far as its oxygen requirements are concerned. It can survive and grow in both the presence and absence of oxygen.
This means that facultative anaerobes have the capability to switch between aerobic respiration, which uses oxygen to produce ATP, and anaerobic respiration or fermentation, which does not require oxygen. Examples of facultative anaerobes include Escherichia coli and Saccharomyces cerevisiae.