All bacteria, both pathogenic and saprophytic, are unicellular organisms that reproduce by binary fission. Most bacteria are capable of independent metabolic existence and growth, but species of Chlamydia and Rickettsia are obligately intracellular organisms. Bacterial cells are extremely small and are most conveniently measured in microns (10^-6 m). They range in size from large cells such as Bacillus anthracis (1.0 to 1.3 µm X 3 to 10 µm) to very small cells such as Pasteurella tularensis (0.2 X 0.2 to 0.7 µm) Mycoplasmas (atypical pneumonia group) are even smaller, measuring 0.1 to 0.2 µm in diameter. Bacteria therefore have a surface-to-volume ratio that is very high: about 100,000.
Bacteria have characteristic shapes. The common microscopic morphologies are cocci (round or ellipsoidal cells, such as Staphylococcus aureus or Streptococcus, respectively); rods, such as Bacillus and Clostridium species; long, filamentous branched cells, such as Actinomyces species; and comma-shaped and spiral cells, such as Vibrio cholerae and Treponema pallidum, respectively. The arrangement of cells is also typical of various species or groups of bacteria (Fig. 2-1). Some rods or cocci characteristically grow in chains; some, such as Staphylococcus aureus, form grapelike clusters of spherical cells; some round cocci form cubic packets. Bacterial cells of other species grow separately. The microscopic appearance is therefore valuable in classification and diagnosis. The higher resolving power of the electron microscope not only magnifies the typical shape of a bacterial cell but also clearly resolves its prokaryotic organization (Fig. 2-2).

Figure 2-1

Typical shapes and arrangements of bacterial cells.

Figure 2-2

Electron micrograph of a thin section of Neisseria gonorrhoeae showing the organizational features of prokaryotic cells. Note the electron-transparent nuclear region (n) packed with DNA fibrils, the dense distribution of ribosomal particles in the cytoplasm,