Pili vs Fimbriae
Pili and fimbriae are known as filamentous appendages, which are used mainly for adhesion. These structures are extremely fine appendages arising from the surface of bacteria and are firstly described by hoodwink and van Iterson. These are thinner than flagella and are not used in motility. The terms Pili and fimbriae are used interchangeably, but they can be distinguished. The term pili is used mainly for longer and sparser appendages, whereas fimbriae are used for shorter and numerous appendages.
Pili
Pili are hair-like microfibers with approximately 0.5 to 2 µm in length and 5 to 7 nm in diameter. These structures are thinner, shorter and more numerous than flagella and are found only on Gram-negative cells. Pili help bacteria cells to attach to a certain surface; therefore, called an organ of adhesion. Unlike the flagella, pili are not used in motility. Special type of pili called ‘sex pili’ is required in the process of bacterial conjugation. A pilus makes a cytoplasmic concoction called ‘conjugation tube’ with the host cell. This tube is then used to transfer genetic materials from the donor cell to recipient cell. The production of sex pili is genetically controlled by episomes.
Fimbriae
Fimbriae are tiny, bristle- like fibers arising from the surface of bacterial cells. Fimbriae have slender tube-like structure, which are composed of helically arranged protein sub- units. Normally, single bacterial cell can be covered with approximately 1000 fimbriae. They are normally distributed evenly over the entire surface of a cell, or they occur at the poles of cells. Fimbriae help to form thick aggregates of cells by sticking to each cell and to surface. This helps some pathogens to adhere tightly to the epithelial cells of host tissues so that they can cause infection very easily. For example, bacteria like gonococcus and E-coli use fimbriae to invade the urinary tract and the intestine respectively. Strains of these pathogens without fimbriae are not able to cause infections. Fimbriae are made up of proteins, and they have a molecular weight of 18,000 daltons. They can be observed only under the electron microscope.
What is the difference between Pili and Fimbriae?
• Fimbriae are shorter than pili.
• The diameter of pili is higher than that of fimbriae.
• A cell can have 1 to 10 pili and about 200 to 300 fimbriae.
• Pili are more rigid than fimbriae.
• Pili are made up of pilin protein, whereas fimbriae are made up of fimbrillin.
• Fimbriae are specialized for attachment of bacteria cell to a host, whereas pili are responsible for bacterial conjugation.