Found mainly in Gram-negative organisms, fimbriae or pili (singular: pilus) are hair-like filaments (tiny hollow projections) that extend from the cell membrane into the external environment. A pilus is composed of subunits of the protein pilin. Fimbriae (pili) are shorter, straighter, and more numerous than bacterial flagella.
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 surfaces, particularly mucous membranes. Bacteria possessing pili include Neisseria gonorrhoeae and some strains of Escherichia coli, Salmonella, and Shigella species.
For example, the pili of Neisseria gonorrhoeae and E. coli mediate the attachment of the organisms to the urinary tract epithelium. Uropathic pili of E. coli attach to Gal-Gal receptors on bladder epithelium. These pili are also called P fimbria or pyelonephritis-associated pili (PAP).
Pili is one of the most important virulence factors of Neisseria gonorrhoeae (piliated gonococci are usually virulent, whereas nonpiliated strains are avirulent). They mediate attachment to mucosal cell surfaces and are antiphagocytic. Neisseria gonorrhoeae has multiple serotypes based on the antigenicity of its pilus protein; more than 100 serotypes are known. Repeated gonococcal infections are common primarily due to pili and the outer membrane proteins antigenic changes.
Length: up to 2 µm
Types: Two general types of pili are known they are:
- Sex pili (long conjugation pili or F pili) and
- Common pili (short attachment pili, also called fimbriae).
Medical Importance of Fimbriae or Pili
- 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.
For example, mutants of Neisseria gonorrhoeae that do not form pili are nonpathogens.
- 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.
- Some pili are also involved in biofilm formation, phage transduction, DNA uptake, and a special form of bacterial cell movement known as ‘twitching motility.