Classification of microorganisms on the basis of oxygen requirement and tolerance

Two most common terms related to oxygen (O₂) are aerobe and anaerobe. Aerobes are those organisms which grows in the presence of O₂ i.e. they grow in an oxygenated environment. Anaerobes are those organisms that grow in the absence of O₂. Mainly most of the organisms require O₂ for their growth and depending upon their O₂ requirement and tolerance, they are classified into five different classes:

Obligate aerobe

Obligate aerobes are those organisms that require O₂ for their growth; they are completely dependent on O₂. For instance, Mycobacterium tuberculosis, Nocardia asteroids etc. Most of the fungi and algae also belongs to this class. In aerobic respiration, O₂  serves as the terminal electron acceptor in electron transport chain. Also, O₂ is utilized for the synthesis of unsaturated fatty acids and sterols.

Obligate anaerobe

Obligate anaerobes dies in the presence of O_2. For instance bacterial genera like Clostridium, Fusobacterium, Prevotella, Actinomyces etc. Aerobic respiration does not take place in these organisms and they are dependent on fermentation and anaerobic respiration for energy generation. However, fermentation and anaerobic respiration pathway yields less energy as compare to aerobic respiration.

Facultative anaerobes

They do not require O₂ for their growth but they can grow in its presence also. When O₂ is present, they make ATP (Adenosine triphosphate) by aerobic respiration and when O₂ is absent, they switch to fermentation or anaerobic respiration pathway. Examples include Staphylococcus, Escherichia coli, Streptococcus, Shewanella oneidensis etc.

Aerotolerant anaerobes

This group of microorganisms ignore O₂ and they grow equally well in its presence and also in its absence. They does not use O₂  for their growth but they can tolerate the presence of O₂ unlike obligate anaerobe. They use fermentation or anaerobic respiration pathway to produce ATP. They include Clostridium, Actinomyces, Streptococcus etc.

Microaerophiles

Microaerophiles are those organisms that require O₂ for their growth but they are damaged by the normal atmospheric level of O₂ (20%). They require O₂ level below the range of 2 to 10% for their growth.  For example, Campylobacter sp, Helicobacter pylori etc. are microaerophilic.