In this article we will discuss about the classification of pili.
Mainly pili are of two types:
(a) Common pili which act to adhere the cell to surfaces, and
(b) Sex pili which join the other bacterial cell for transfer of genome.
Ottow (1975) has classified the pili into the following six groups:
Fimbriae of this group act for adherence to a particular surface including the surface of the other cells too. These are about 300 per cell arranged peritrichously. Pili of Neisseria gonorrhoae, the causal agent of gonorrhea, help the bacterium to colonize the mucous membranes. In the absence of pili on cell surfaces, mucous colonization and disease development cannot occur.
The sex pili of this group have a uniform diameter of about 9 nm and length of about 1-20 M-m. There are about 10 pili per cell. They are filamentous and determined by sex factor. The plasmid carries genes that code for synthesis of sex pili. They make contact between two cells.
Fimbriae of third group are peculiar that are found in Agrobacterium. They are thick and like the hollow tubes.
This group consists of pili which are flexible, rod-shaped and polar. These are found in the species of Pseudomonas and Vibrio.
Fimbriae of group 5 are polarly arranged and contractile in nature. They are found in Agrobacterium spp, Pseudomonas rhodes and Rhizobium lupini. They contract and pull two bacterial cells into close contact and, therefore, promote the conjugation process.
Group 6 is the characteristic bundles of fimbriae found in Gram-positive Coryne- bacterium renale. The filaments function as specific antigens.