Bacteria and blue-green algae have the simplest pattern of organization. They may be rod-shaped, spherical, or helical; they also may occur singly or in clusters (see M. paratuberculosis on page 2).

The main features of a bacterial cell are diagrammed on the next page. These features are relatively few: cell membrane, wall and capsule, cytoplasm or cell fluid, photosynthetic vesicles or membranes, DNA, ribosomes for protein synthesis, mesosomal infoldings of the cell membrane, and sometimes flagella or pili on the outside (see right).

Bacteria are small; for example, Escherichia coli (E. coli - right) from our intestines is a rounded-end cylinder 10,000 across and 20,000 long, weighing about 2 x 10 gram.