When exposed to a lack of oxygen, facultative anaerobes develop metabolic processes with oxygen, like respiration. But in the absence of oxygen, they can develop processes like fermentation. When in oxygen, they prefer the use of it, because that's what makes them facultative, the fact that they're able to work in both circumstances. Otherwise, tolerants are able to survive but not to work properly, in different environments.
Bacteria can either grow aerobically or anaerobically. Because of their ability to respire and ferment organic substances, these types of bacteria (facultative anaerobes), can continue growing in the presence or the absence of oxygen.
