Nutrition and Growth of Bacteria

Autotrophic (self-nourishing) bacteria can synthesize all of their organic cell constituents from carbon dioxide, water, and inorganic forms of nitrogen and sulfur. The photoautotrophs extract their energy from sunlight, while the chemoautotrophs obtain energy from inorganic chemical reactions. For example, the hydrogen bacteria oxidize H2 to H2O and sulfur bacteria oxidize H2S to H2SO4. Like the fungi and animals, most bacteria are chemoheterotrophic; they obtain energy from the breakdown of organic compounds. Some of these heterotrophic bacteria are anaerobes which live without O2. Many of them
metabolize complex organic substances such as sugars in the absence of oxygen, a process called fermentation. Others oxidize organic compounds with an inorganic oxidant such as nitrite or sulfate. Members of the genus Clostridium are poisoned by oxygen and are known as obligate anaerobes. Others, including E. coli, are facultative anaerobes, able to grow ither in the presence or in the absence of oxygen. Obligate aerobes depend for energy upon ombustion of organic compounds with oxygen. One of the largest groups of strictly aerobic heterotrophic bacteria, the pseudomonads (Pseudomonas and related genera), are of interest to biochemists because of their ability to oxidize organic compounds, such as alkanes, aromatic hydrocarbons, and steroids, which are not attacked by most other bacteria. Often, the number of oxidative reactions used by any one species of bacteria is limited. For example, the acetic acid bacteria that live in wine and beer obtain all of their energy by oxidation of ethanol to acetic acid:
CH3CH2OH + O2 → CH3COOH + H2O
Bacteria can grow incredibly fast. Under some conditions, it takes a bacterial cell only 10–20 min to double its size and to divide to form two cells. 4 An animal cell may take 24 h for the same process. Equally impressive are the rates at which bacteria transform their foods into other materials. One factor contributing to the high rate of bacterial metabolism may be the large surface to volume ratio. For a small spherical bacterium (coccus) of diameter 0.5 m, the ratio of the surface area to the volume is 12 x 106 m–1, while for an ameba of diameter 150 m the ratio is only 4 x 104 m–1 (the ameba can increase this by sticking out some pseudopods). Thimann33 estimated that for a 90-kg human, the ratio is only 30 m–1. When food is limited, some bacteria such as the Bacillus form spores. These are compact little cells that form inside the vegetative cell and are therefore called endospores. They sometimes have only 1/10 the volume of the parent cell. Their water content is very low, their metabolic rate is near zero, and they are extremely resistant to heat and furthe desiccation. Under suitable conditions, the spores can “germinate” and renew their vegetative growth. Spore formation is one of several examples of the development of specialized cells or differentiation among prokaryotes.