Types: Two general types of Pili are known they are: Sex pili and Common pili. 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.2  Example: Mutants of Neisseria gonorrhoeae that do not form pili are nonpathogen. 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.