Beyond the cell wall

A number of structural features are to be found on the outer surface of the cell wall; these are mainly involved either with locomotion of the cell or its attachment to a suitable surface.

Perhaps the most obvious extracellular structures are flagella (sing: flagellum), thin hair-like structures often much longer than the cell itself, and used for locomotion in many bacteria. There may be a single flagellum, one at each end, or many, depending on the bacteria concerned (Figure 3.10). Each flagellum is a hollow but rigid cylindrical filament made of the protein flagellin, attached via a hook to a basal body, which secures it to the cell wall and plasma membrane (Figure 3.11). The basal body comprises a series of rings, and is more complex in Gram-negative than Gram-positive bacteria. Rotation of the flagellum is an energy-dependent process driven by the basal body, and the direction of rotation determines the nature of the resulting cellular movement. Clockwise rotation of a single flagellum results in a directionless 'tumbling',

Peritrichous

(a) Polar, monotrichous

(b) Polar, amphitrichous

(c) Bipolar, monotrichous

(d) Peritrichous

Figure 3.10 Flagella may be situated at one end (a & b), at both ends (c) or all over the cell surface (d)

(a) Polar, monotrichous

(b) Polar, amphitrichous

(c) Bipolar, monotrichous

(d) Peritrichous

Figure 3.10 Flagella may be situated at one end (a & b), at both ends (c) or all over the cell surface (d)

but if it rotates anticlockwise, the bacterium will 'run' in a straight line (Figure 3.12). Likewise, anticlockwise rotation causes bunched flagella to 'run' by winding around each other and acting as a single structure, whilst spinning in the opposite direction gives rise to multiple independent rotations and tumbling results once more.

Pili (sing: pilus) are structures that superficially resemble short flagella. They differ from flagella, however, in that they do not penetrate to the plasma membrane, and they are not associated with motility. Their function, rather, is to anchor the bacterium to an appropriate surface. Pathogenic (disease-causing) bacteria have proteins called adhesins on their pili, which adhere to specific receptors on host tissues. Attachment pili are sometimes called fimbriae, to distinguish them from another distinct type of pilus, the sex pilus, which as its name suggests, is involved in the transfer of genetic information by conjugation. This is discussed in more detail in Chapter 12.

Outside the cell wall, most bacteria have a polysaccharide layer called a glycocalyx. This may be a diffuse and loosely bound slime layer or a better defined, and generally thicker capsule. The slime layer helps protect against desiccation, and is instrumental in the attachment of certain bacteria to a substratum (the bacteria that stick to your teeth are a good example of this). Capsules offer protection to certain pathogenic bacteria against the phagocytic cells of the immune system. Both capsules and slime layers are key components of biofilms, which form at liquid/solid interfaces, and can be highly significant in such varied settings as wastewater treatment systems, indwelling catheters and the inside of your mouth!

Gram Positive Flagellar Rings

Figure 3.11 Bacterial flagella are anchored in the cell wall and plasma membrane. The filament of the flagellum is anchored by a basal body. In Gram-positive organisms, this comprises two rings inserted in the plasma membrane. In Gram-negative organisms (as shown), there are additional rings associated with the outer membrane and the peptidoglycan layer. Some modern interpretations of flagellar structure view the M and S rings as a single structure. Energy for rotation of the flagellum is derived from the proton motive force generated by the movement of protons across a membrane (see Chapter 6). From Bolsover, SR, Hyams, JS, Jones, S, Shepherd, EA & White, HA: From Genes to Cells, John Wiley & Sons, 1997. Reproduced by permission of the publishers

Figure 3.11 Bacterial flagella are anchored in the cell wall and plasma membrane. The filament of the flagellum is anchored by a basal body. In Gram-positive organisms, this comprises two rings inserted in the plasma membrane. In Gram-negative organisms (as shown), there are additional rings associated with the outer membrane and the peptidoglycan layer. Some modern interpretations of flagellar structure view the M and S rings as a single structure. Energy for rotation of the flagellum is derived from the proton motive force generated by the movement of protons across a membrane (see Chapter 6). From Bolsover, SR, Hyams, JS, Jones, S, Shepherd, EA & White, HA: From Genes to Cells, John Wiley & Sons, 1997. Reproduced by permission of the publishers

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