Obligate aerobes require oxygen to live. Microaerophiles require oxygen for growth, but they cannot tolerate normal atmospheric levels of oxygen. Too much oxygen seems to interfere with their metabolic enzymes. Obligate anaerobes do not possess superoxide dismutase or catalase, and therefor cannot cope with the toxic by-products of oxygen metabolism. Also, they possess catalase and superoxide dismutase, and so are not affected by the toxic by-products of oxygen.
Facultative anaerobes utilize oxygen and aerobic respiration whenever possible.
The oxygen requirements of bacteria can be tested by inoculating them into a medium with an oxygen gradient. For this lab, the oxygen gradient will be produced by heating an agar medium deep to drive off any dissolved oxygen. Over time, the top of the deep will become oxygenated, while the bottom will remain anaerobic. Growth just under the surface of the deep indicates a microaerophile. Growth only at the bottom of the deep indicates an obligate anaerobe.