*Obligate aerobes need oxygen to grow. In a process known as cellular respiration, these organisms use oxygen to oxidize substrates (for example sugars and fats) and generate energy.
*Facultative anaerobes use oxygen if it is available, but also have anaerobic methods of energy production.
*Microaerophiles require oxygen for energy production, but are harmed by atmospheric concentrations of oxygen (21% O).
*Aerotolerant anaerobes do not use oxygen but are not harmed by it.
When an organism is able to survive in both oxygen and anaerobic environments, the use of the Pasteur effect can distinguish between facultative anaerobes and aerotolerant organisms. If the organism is using fermentation in an anaerobic environment, the addition of oxygen will cause facultative anaerobes to suspend fermentation and begin using oxygen for respiration. Aerotolerant organisms must continue fermentation in the presence of oxygen.
Facultative organisms grow in both oxygen rich media and oxygen free media.