Bacteria may also be classified according to their metabolic processes. One major division is the difference between aerobic and anaerobic organisms. An aerobe is an organism that uses oxygen in its metabolic processes. An anaerobe is an organism that does not use oxygen in its metabolic processes. Several further distinctions are made based on oxygen sensitivity. A facultative anaerobe is an organism capable of aerobic and anaerobic respiration. An obligate aerobe is an organism that requires oxygen for survival. In contrast, an obligate anaerobe is an organism that dies in the presence of oxygen. An aerotolerant anaerobe is an organism that thrives in the absence of oxygen but is not harmed by the presence of oxygen. A microaerophile is an organism that requires oxygen but can only survive in low-oxygen conditions. Another pair of classifications are aerogenic and anaerogenic. Aerogenic organisms are those that produce gas. Anaerogenic organisms do not produce gas.Simple laboratory procedures are used to determine how to classify bacteria based on their oxygen sensitivity or requirement. To determine whether bacteria are aerobic or anaerobic, a thin wire is used to inoculate the bacteria into a solid culture medium with a "stab" into the medium. If the bacteria grow down the stab, which is away from air, they are anaerobic. If they do not, they are aerobic. To determine whether bacteria are aerogenic, a liquid culture is used, into which a small tube is placed upside down. Bacteria are inoculated into the culture. If a bubble forms in the upside-down tube, the bacteria are aerogenic. If no bubble forms, they are not aerogenic. Frequently, this test is also used to determine fermentation. The liquid medium changes color when sugars in it are fermented. This is an additional way to classify bacteria according to their metabolic processes.
Bacterial Colony Morphology
Nutrition Classification of Bacteria
|Make Their Own Food||Obtain Food Externally|
|Energy Comes from Light||Photoautotroph||Photoheterotroph|
|Energy Comes from Organic Molecules||Chemoautotroph||Chemoheterotroph|