A perhaps over-simplistic, but operationally acceptable, description of medically important bacteria in relation to their oxygen requirements is as follows:
• Obligate aerobic bacteria require gaseous oxygen to complete their energy -producing cycle; these organisms cannot grow without a source of oxygen. Examples of obligate aerobic bacteria are Micrococcus spp and Nocardia asteroides.
• Obligate anaerobic bacteria do not require oxygen for metabolic activity, and in fact oxygen is toxic to many of them. Energy is derived from fermentation reactions, which may produce foul-smelling end-products. Examples of such anaerobic bacteria are Bacteroides fragilis and Peptostreptococcus magnus.
• Facultative anaerobic bacteria are those for which there is no absolute requirement for oxygen for growth or energy production; they can either utilize oxygen or grow by anaerobic mechanisms. Such bacteria are most versatile, and are usually able to adapt to their environment, creating energy for growth and multiplication by the most effective mechanism. E. coli and S. aureus are examples of facultative anaerobic organisms.
• There are, in addition to the above, microaerophilic bacteria that grow best at reduced oxygen tensions. Campylobacter jejuni is an example of a microaerophilic bacterium.