PILI AND FIMBRIAE

 PILI

·       Pili are long hair like tubular microfibers like structure.

·       Pili are present on some Gram negative bacteria only.

·       Based on Hemagglutinating ability and Mannose sensitivity, Pili are classified into four types. They are

i.       Type I - Occur in Escherichia coli, Klebsiella, Shigella and Salmonella. They are mannose sensitive.

ii.     Type II - Present in Salmonella gallinarum and Salmonella pullorum, devoid of any hemagglutinating or adhesive properties.

iii.   Type III - Present in some strains of Klebsiella, Serratia, etc. They agglutinate RBC only after heating and are mannose resistant.

iv.   Type IV - Mannose resistant and occur in Proteus.

·       Pili are made up of protein called Pilin.

·       Comparatively, Pili are longer than Fimbriae and shorter than Flagella.

·       Approximate length of Pili is 0.5 µm – 2.0 µm.

·       Number of Pili per cell is very less. It is usually 1 to 10 per cell. They are randomly arranged on the surface of the bacterial cell.

·       Pili are more rigid than Fimbriae.

·       The formation of Pili is controlled by the gene present in the Plasmids.

·       Sex pili is a specialized kind of Pili which is responsible for the attachment of donor and recipient cells in bacterial conjugation. These pili are 6 - 7 nm in dm and vary 1 - 4 in number. The sex pili are of two types:

i.       F pili - Specifically adsorb male specific RNA and DNA Bacteriophages. They are encoded by sex factor F and fertility inhibition - positive resistance factors (fi 1 R factors).

ii.     I pili - Adsorb male specific filamentous DNA phages, encoded by col factor and fi 2 R factor.

·       The main function of Pili is

a)     Gene transfer mechanism (Conjugation – the process of gene transfer through sex pili)

b)     Attachment (Bacterial virulence factor because of its Antigenic nature)

c)     The majority of the Pili can act as a receptor for Viruses.  

FIMBRIAE

·       Fimbriae are bristle like short appendages evenly distributed on the surface of the bacterial cell.

·       The total number of Fimbriae per cell is ranging from 200 – 400.

·       Approximate length of Fimbriae is 0.03 µm to 0.14 µm.

·       Fimbriae are present in both Gram positive bacteria and Gram negative bacteria.

·       Fimbriae are made up of Fimbrillin protein.

·       Comparatively, Fimbriae are shorter, fragile and thinner than Flagella and Pili.

·       Similar to Flagella, the formation of Fimbriae is controlled by the gene present in the Nucleoid region.

·       The main function of Fimbriae is surface attachment on host cells only.

·       Fimbriae also help in the formation of Pellicle and Biofilms.