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Surface Structures (cont.)

©2001 Timothy Paustian, University of Wisconsin-Madison

Fimbriae

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

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.

Glycocalyx

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.

Structure

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.

Function

• Attachment

  These structures are thought to help cells attach to their target environment. Streptococcus mutans produces a slime layer in the presence of sucrose. This results in dental plaque and many bacteria can stick to tooth surfaces and cause decay once S. mutans forms a slime layer. Vibrio cholerae, the cause of cholera, also produces a glycocalyx which helps it attach to the intestinal villi of the host.

• Protection from phagocytic engulfment. Bacterial pathogens are always in danger of being "eaten" by phagocytes. (Host cells that protect you from invaders.) Streptococcus pneumoniae, when encapsulated is able to kill 90% of infected animals, when non-encapsulated no animals die. The capsule has been found to protect the bacteria by making it difficult for the phagocyte to engulf the microbe.

• Resistance to drying. Capsules and slime layers inhibit water from escaping into the environment.

• Reservoir for certain nutrients. Glycocalyx will bind certain ions and molecules. These can then be made available to the cell.

• Depot for waste products. Waste products of metabolism are excreted from the cell, and will accumulate in the capsule. This binds them up, and prevents the waste from interfering with cell metabolism.

Summary

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...

• Although bcateria are small, they have a fair amount of molecular complexityl.

• Many bacterial structures are made of polymers

• The sequence of the polymers dictate their structure and subsequently their funciton.

• The building blocks that make up the average bacteria are also found in all living things, at least on this planet. We share more things in common with microbes that we have differences.

In subsequent chapters we will learn what all these various parts are doing.

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