Survey of Bacteria

Metabolic Traits of Bacteria

Bacteria are classified based on their metabolic needs for specific organic and inorganic materials.

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.

Bacteria Tests

Simple laboratory tests indicate whether a bacterium is aerobic or anaerobic (left) and aerogenic or anaerogenic (right).
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).

Bacterial Colony Morphology

The overall morphology, or shape, of bacterial colonies varies, as do their margins, or edges, when viewed from above. Elevation describes the way the colony looks when viewed from the side.
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.

Nutrition Classification of Bacteria

  Make Their Own Food Obtain Food Externally
Energy Comes from Light Photoautotroph Photoheterotroph
Energy Comes from Organic Molecules Chemoautotroph Chemoheterotroph

Bacteria can be classified according to their mechanisms of nutrition—their source of both food and energy.

Each field of microbiology may also have classifications specific to that field of study. For example, immunologists often classify bacteria according to their interactions with their host. These may include whether the bacterium is a pathogen, which species it infects, and which body system(s) it targets. Ecologists, on the other hand, may classify bacteria based on their habitat or their benefit to other organisms. For example, many bacteria found in soil help provide nutrients to plants, especially to crops humans rely on for food. One example of habitat classification is the extremophiles, which tend to be mostly archaea and some bacteria. Extremophiles are microorganisms (microbes) that thrive in conditions that do not sustain a wide variety of life. Extremophiles have a number of subcategories. An acidophile is an organism that grows in low pH, or acidic, conditions. A xerophile is a microbe that lives in areas with little or no water. A halophile is a microbe that can tolerate living in high salt concentrations. A thermophile grows best between 50°C and 60°C and can grow at hot temperatures of up to 120°C.