∞ generated and posted on 2016.08.25 ∞
Organism that is able to metabolize (function) normally despite an absence of oxygen.
Some anaerobes are obligate fermenters while others are obligate respirers (users of electron transport chains to generate ATP). Generally one categorizes anaerobes, however, using the simpler criteria of what is their fate when molecular oxygen is present in combination with what is their ability to utilize that same oxygen. In any case, an anaerobe by definition is able to metabolize, grow, replicate, etc., within anaerobic environments.
If an organism cannot survive in the presence of oxygen then it is referred to as a strict or obligate anaerobe. If the presence of oxygen does not have a negative impact but nonetheless they cannot utilize the oxygen, because they are obligate fermenters, then we describe those organism's in terms of their oxygen requirements as aerotolerant anaerobes. Lastly, if an anaerobe not only can thrive in the presence of molecular oxygen but also utilize that oxygen as a final electron acceptor, then we describe those organisms as facultative anaerobes.
Figure legend: Anaerobes – organisms that can continue to metabolize, replicate, and grow despite an absence of molecular oxygen – come in a variety of types. Strict anaerobes cannot do any of the above in the presence of oxygen. 'Obligate' is often used a synonym for strict. An aerotolerant anaerobe is equivalent to an obligate anaerobe except that these organisms are able to survive and replicate, etc., despite the presence of molecular oxygen. With aerotolerant anaerobes, however, the oxygen is neither helpful nor harmful. Facultative anaerobes, on the other hand, are able to utilize molecular oxygen, switching their metabolisms from generally fermentative absent molecular oxygen to cellular respiration in its presence. Note that even certain obligate aerobes are able to replicate under anaerobic conditions, though displaying strictly anaerobic respiration rather than fermentation.
Because of difficulties in growing obligate anaerobes within the laboratory – due to the need to protect them from air – their exist biases during the enumeration of environmental microorganisms against their detection. Facultative and aerotolerant anaerobes, on the other hand, are as readily under aerobic conditions as obligately aerobic organisms.