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Bacteria that lack the ability to make superoxide dismutase and catalase are exquisitely sensitive to the presence of molecular oxygen and, in general, must grow anaerobically using fermentation. Bacteria that possess these protective enzymes can grow in the presence of oxygen, but whether they use the oxygen in metabolism or not depends on their ability to respire. Whether these oxygen-resistant bacteria can grow anaerobically depends on their ability to ferment. Various combinations of these two characteristics (oxygen resistance and the ability to use molecular oxygen as a final acceptor) are represented in different species of bacteria, resulting in the five general classes shown in Table 3 – 1.

Unlike the capsule, which is dispensable for survival outside the body of the host, the wall has vital functions in all environments. It protects the cell from mechanical disruption and from bursting caused by the turgor pressure resulting from the hypertonicity of the cell interior relative to the environment. The wall provides a barrier against certain toxic chemical and biological agents. In some bacterial species, such as Streptococcus (see Chapter 17), it provides a protection from phagocytosis and helps in the binding to eukaryotic cell hosts.

Cytosol The dense cytosol is bounded by the cell membrane. It appears granular because it is densely packed with ribosomes, which are much more abundant than in the cytoplasm of eukaryotic cells. This is a reflection of the higher growth rate of bacteria. Each ribosome is a ribonucleoprotein particle consisting of three species of rRNA (5 S, 16 S, and 23 S) and about 56 proteins. The overall subunit structure (one 50 S plus one 30 S particle) of the 70 S bacterial ribosome resembles that of eukaryotic ribosomes (which are 80 S, composed of one 60 S and one 40 S particle), but is smaller and differs sufficiently in function that a very large number of antimicrobics have the prokaryotic ribosome as their target.