Fimbriae & Pili

Fimbriae

Present in many Gram negative and some Gram positive bacteria, fimbriae (singular fimbria) are thin, short hair like appendages on the surface of bacteria. They are thinner than flagella and are not involved in motility. They help in the attachment of bacteria to host tissues and some solid surfaces like rock in streams.  They are made up of helically arranged protein subunits and are about 3 to 10 nm in diameter and several micrometers in length. They can either occur at the pole of bacterial cell or over the entire surface of bacterial cell and also they vary in number from a few to several hundred per cell.

Pili

As fimbriae, pili (singular pilus) are thin, hairlike appendages on the surface of bacteria. Pili are 9 to 10 nm in diameter; slightly larger than fimbriae. They are present mostly in Gram negative bacteria with a very few Gram positive one like Corynbacterium renale. They are made up of oligomeric protein called pilins. They helps in the process of conjugation (transfer of genetic material) and hence they are also called sex pili. Pili also acts as a receptor for donor specific phages and provides a site for virus attachment at the start of their reproductive cycle. 

Note: Fimbria is basically a short pilus that helps in the attachment of bacteria to a surface. Hence, fimbriae are sometime known as attachment pili.