Bacterial Fimbriae (pili)

Bacteria use adherence fimbriae (pili) to overcome the body’s defense mechanism and cause disease. Pili are small hairs that enable some pathogens to attach and adhere easily to cell surface particularly mucous membranes. Bacteria possessing pili include Neisseria gonorrhoeae and some strains of Escherichia coli, Salmonella and Shigella species. Fimbriae (pili) are shorter, straighter and more numerous than bacterial flagella and are composed by subunits of protein called pilin.
Found mainly in Gram negative organisms, Fimbriae or pili (singlular: pilus) are hair like filaments (tiny hollow projections) that extend from the cell membrane into the external environment. 
Length: up to 2 µm
Types: Two general types of Pili are known they are:

1. Sex pili (long conjugation pili or F pili) and
2. Common pili (short attachment pili also called fimbriae).

Medical Importance of Fimbriae or Pili

1. Common pili (Adhesins): They mediate the attachment of bacteria to specific receptors on the human cell surface, which is the first step in establishing infection in some organisms.  They contribute to the pathogenicity of certain bacteria—their ability to produce disease—by enhancing colonization on the surfaces of the cells of other organisms.
   Example: Mutants of Neisseria gonorrhoeae that do not form pili are nonpathogen.
2. Sex pili (conjugation tube): It is a specialized kind of pili that forms the attachment between male (donor) and the female (recipient) bacteria during conjugation and acts as a conduit for the passage of DNA. This process is well characterized in the gram negative bacillus Escherichia coli.
3. Some pili are also involved in biofilm formation, phage transduction, DNA uptake and a special form of bacterial cell movement, known as ‘twitching motility’