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Bacteria are the smallest and most versatile independently living cells. This chapter examines the structural, metabolic, and genetic features that contribute to the ubiquity and diversity of this large group of microorganisms. The discussion which follows focuses on the characteristics of the tiny sliver of the bacterial world which causes disease in humans. The goal is to provide the background and vocabulary fundamental to understanding how bacterial pathogens deploy their structural and metabolic products to confound the immune system and produce injury to the human hosts they invade. These mechanisms will then be explained in the 20 chapters that follow.


As discussed in Chapter 1, in the hierarchy of infectious agents, bacteria are the smallest organisms capable of independent existence. In the wider microbial world, their prokaryotic cell plan is still considered to provide the minimum possible size for an independently reproducing organism. Individuals of different bacterial species that colonize or infect humans range from 0.1 to 10 μm (1 μm = 10–6 m) in their largest dimension. Most spherical bacteria have diameters of 0.5 to 2 μm, and rod-shaped cells are generally 0.2 to 2 μm wide and 1 to 10 μm long. As shown in Figure 1–2, bacteria overlap in at least one dimension with large viruses and some eukaryotic cells, but they are the sole possessors of the 1 μm size.

Bacteria are in the range of 1 to 10 μm

The small size and nearly colorless nature of bacteria require the use of stains for visualization with a light microscope or the use of electron microscopy. The major morphologic forms are spheres, rods, bent or curved rods, and spirals (Figure 21–1A–E). Spherical or oval bacteria are called cocci (singular: coccus) and are typically arranged in clusters or chains. Rods are called bacilli (singular: bacillus) and may be straight or curved. Bacilli that are small and pleomorphic to the point of resembling cocci are often called coccobacilli. Spiral-shaped bacteria may be rigid or flexible and undulating.

FIGURE 21–1.

Shapes of bacteria. A. Staphylococcus aureus, cocci arranged in clusters; scanning electron micrograph (SEM). B. Group B streptococci, cocci arranged in chains; SEM. C. Bacillus species, straight rods; Gram stain. D. Spirochete, phase contrast, SEM. E. Vibrio, curved rods, SEM. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th edition. McGraw-Hill, 2008.)

Bacteria exhibit sphere, rod, and spiral shapes

Whatever the overall shape of the cell, the 1 μm size could not accommodate eukaryotic mitochondria, nucleus, Golgi apparatus, lysosomes, and endoplasmic reticulum in a cell that is itself only as large as an average mitochondrion. The solution is in the unique prokaryotic design of the bacterial cell. A generalized bacterial cell is shown ...

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