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More Cell Wall
Surface Structures (cont.)
©2001 Timothy Paustian, University of Wisconsin-Madison
Fimbriae are shorter and straighter than flagella and are more numerous. Not all bacteria synthesize them. Fimbriae do not function in motility, but are thought to be important in attachment to surfaces or at least in cells sticking together.
Figure 1 - Transmission electron micrograph of Aquaspirillum hydrophila showing flagella (thick, long structures) and pili (thinner fibers)
Pili are longer than fimbriae and there are only a few per cell. They are known to be receptors for certain bacterial viruses, but I doubt the bacteria makes them for that purpose. There are two basic functions for pili, gene transfer and attachment
The sex pilus (or F-pilus) is involved in sexual reproduction of certain bacteria. A donor bacteria will attach to a recipient via the sex pilus. Then a copy of part of the donor bacterium's genome passes through the sex pilus into the recipient. This is a mechanism of genetic exchange between bacteria. Interestingly, transfer of genes this way is not restricted to species. It is possible for E. coli to transfer information to many different Gram negative species. Conjugation, as it is called, is one explanation for the rapid spread of drug resistance in many different species of bacteria.
Pili have also been show to be important for the attachment of some pathogenic species to their host. Neisseria gonorrheae, the causative agent of gonorrhea, has a special pili that helps it adhere to the urogenital tract of its host. The microbe is much more virulent when able to synthesize pili.
Many, but not all bacterial cells have an external coating excreted onto the outside of the cell. There are two types of glycocalyx, capsules and slime layers, but the difference between the two is somewhat arbitrary.
A glycocalyx is a general term for any network of polysaccharide or protein containing material extending outside of the cell. A capsule is closely associated with cells and does not wash off easily. A slime layer is more diffuse and is easily washed away.
Figure 2 - Capsule surrounding cells of Streptococcus species. The capsule is about the diameter of the cell.
Glycocalyx contains polysaccharide, although some may also contain protein(s), typically glycoproteins. There are many different types of polysaccharides plus some polyalcohols and amino sugars in glycocalyx and the exact makeup is species specific. The structure can be thick or thin, rigid or flexible. Glycocalyx is identified by staining cells with India ink, which does not penetrate the structure. When observed in the microscope the cells appear dark with an outline around them. This outline is the capsule or slime layer.
This brings us to the outside of the microbe and to an end of this chapter on bacterial structure. You should now have a better understanding of the molecular make-up of the typical bacteria. Some ideas that you should take away from this chapter are...
In subsequent chapters we will learn what all these various parts are doing.
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