-31 Pili and fimbriae are involved in adhesion, motility and DNA exchange

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  • Pili and fimbriae are smaller than flagella, but have a similar structure
  • Pili can serve in DNA exchange.
  • Pili and fimbriae are often involved in attachment to surfaces and are important for biofilm formation.

Pili and fimbriae are structurally similar to flagella and are composed of one or a few proteins arranged in a helical fashion. Figure 2-49 shows pili isolated from Neisseria gonorrhea. Each protein subunit assembles on the growing structure at the tip, as is the case with flagella. There are a number of genes necessary for the successful construction of pili and their products might perform functions such as moving the structural proteins across the membrane, methylating the structural proteins or retracting the pilus. The same is generally true for fimbriae.

Pili and fimbriae

Figure 2.49. Pili and fimbriae. The pili shown in this micrograph are those of Neisseria gonorrhea with Tobacco Mosaic virus (the thicker structures) added as a size reference. The width of the Tobacco Mosaic virus is 0.018 µm. (Source: Katrina Forest, University of Wisconsin-Madison)

Fimbriae are found on many bacteria and 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. Some microbes attach to hosts by fimbriae, and successful colonization of many surfaces is totally dependent upon the ability to make fimbriae. Swarming microbes such as Myxococcus use them to sense the presence of similar microbes, which helps keep their "hunting packs" together.

Pili are longer than fimbriae and there are only a few per cell. They are known to be receptors for certain bacterial viruses, but certainly the bacterium makes them for another purpose. There are two basic functions for pili: gene transfer and attachment to surfaces. In genetic transfer in a broad variety of bacteria, a donor bacterium attaches to a recipient by the sex pilus. Then the donor cell depolymerizes the pilus at the end that is attached to itself, which draws the cells together and eventually a small pore is created between the two cells. DNA is then transferred through that pore from the donor to the recipient and the cells separate. For a long time, it was thought that the donor bacterium's genome passed through the sex pilus into the recipient, but this is certainly not the case. Transfer of genes this way is not restricted to related species, which implies that a pilus from one organism can attach to a variety of others. Conjugation, as this transfer process is known, is one explanation for the rapid spread of drug resistance in many different species of bacteria and is covered in depth in the Chapter on Genetics and Genomics.

Pili have also been show to be important for the attachment of many types of microorganisms to surfaces. For example, Neisseria gonorrhoeae, the causative agent of gonorrhea, has a special pilus that helps it adhere to the urogenital tract of its host. The microbe is much more virulent when able to synthesize pili.

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