Metabolic Traits of Bacteria

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. Other metabolic tests may indicate whether organisms contain specific enzymes. These enzymes include proteases, oxidases, lactases, nitrate reductases, and urease. In addition, some simple tests classify bacteria based on their appearance when cultured on solid medium. Masses of microbial cells when grown on solid media are called colonies and they may vary in color, in shape, in the appearance of their margin, and in the shape of the colony's elevation. Here, margin describes the edges of the colony, while elevation describes the way the colony appears when viewed from the side. Colonies may have an overall circular pattern of growth outward form the initial colony forming unit, or they may take on a range of other shapes. The margins of the colony may be entire (smooth), curled (wavy), lobate (deeply wavy), filiform (long extensions from the central colony), or undulate (heterogeneously lobed). The elevation of colonies may be flat (uniform surface), raised (gently curving from the margin inward), convex (steeply curving from the margin inward), umbonate (with a raised bump in the middle), or crateriform (with a dimple in the center). Bacteria may also be classified based on their mechanism of nutrition. The major divisions are autotroph and heterotroph. An autotroph is an organism that can make its own food. A heterotroph is an organism that obtains energy and carbon from consuming other organisms. Autotrophic bacteria get their energy from light (photoautotroph), such as Rhodobacter sphaeroides, or from the oxidation of inorganic compounds (chemoautotroph), such as Thiobacillus ferrooxidans. Heterotrophic bacteria, similarly, get their energy from light (photoheterotroph), such as Heliobacterium modesticaldum, or from organic molecules (chemoheterotroph), such as Pseudomonas aeruginosa. There are also photoheterotrophs that may synthesize ATP using light energy under certain conditions, but do not perform photosynthesis or use carbon dioxide as a sole source of carbon.