External Structures

There are several external structures. These structures are the axial filament, capsule, flagella, glycocalyx, and pili. All external structures perform specific functions, but these structures are not found on bacteria.

Spirochetes possess axial filaments or endoflagella (Figure 3.5).The axial filament runs the length of the spirochete and extends beyond the cell wall. The filament assists provides motility and allows the rigid spirochete to rotate like a corkscrew.

The capsule is a protective structure that is located outside the cell wall. The capsule consists of polysaccharides that are arranged in a loose gel. Very few bacteria produce a capsule.

Nearly half of all bacteria are motile. Except for the axial filament, most bacteria move by means of flagella (Figure 3.6). Each flagellum is a long, helical structure that is made of protein subunits call flagellin. The beating action or whip-like motion of the flagella propels the bacterium.

Glycocalyx is the term used to describe all polysaccharides outside the cell wall. Glycocalyx may be packaged as a thick capsule or loosely collected as a thin slime layer. Glycocalyx enables bacteria to flocculate and form (1) floc particles in the

FIGURE 3.6 Flagellar arrangements in bacteria. The three basic flagellar arrangements in bacteria are monotrichous (a), lophotrichous (b), and peritrichous (c).

activated sludge process and (2) biofilm in fixed film processes such as the trickling filter.

Pili or fibrils are short hollow projections of the cell membrane that extend through the cell wall into the surrounding environment. Each pilus is composed of protein subunits called pilin. The pili are used by some bacteria for attachment to other bacteria for reproduction and by many bacteria for attachment to other bacteria or inert surfaces (floc formation and biofilm development).

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