Pili and fibriae are present on the cell surface ad they help in attachment of an organism. They are present in bacteria other then the presence of flagella but they are not used for locomotion. The main difference between pili and fibriae is pili are found only in gram negative bacteria but fibriae is found in both, gram negative and gram positive bacteria.
What is Pili?
Pili is an appendage present on cell surface which is used for the purpose of attachment. They are present only in gram negative bacteria and are long, thick, tubular structure which is made up of protein which is known as pilin. Hence the name, pili. Indirectly, pili are useful in reproduction process of a bacteria because they help in attachment of one bacterium to another during the conjugation process, that is why it is also known as sex-pili. Sex-pili are responsible for the sharing of genes. Plasmid genes are responsible for the manufacture of pili and their number is comparatively less then fimbriae. They are thicker, tube like outgrowths which do not play any role in locomotion. It is also known as organ of adhesion due to its quality of sticking to other cells during the process of conjugation. And while doing it, a pipe like tube called conjugation tube is formed which helps in cell to cell attachment.
What is Fimbriae?
They are found in gram negative as well as gram positive bacteria but are shorter in length as compared to pili. Bacterial genes which are present in the nucleoid region are basically responsible for the manufacture of fimbriae. Basic function of fimbriae is cell to surface attachment of the bacteria. They are composed of protein sub-units and are evenly distributed over the entire surface of the cell. It form cluster of cells by sticking to each other and to the surface. They even help some pathogens to adhere tightly to other cells causing infections.
- Pili are found only in gram negative bacteria but fimbriae are found in both, gram negative and positive bacteria.
- Fimbriae are shorter in length as compared to pili.
- Diameter of fimbriae is less than that of pili.
- Pili are more numerous than fimbriae.
- Fimbriae are less rigid than pili.
- Pili are responsible for bacterial conjugation but fimbriae are responsible for cell to surface attachment.
- Formation of pili is governed by plasmid genes but fimbriae is not.