In bacteriology, a fimbria (plural fimbriae), also referred to as an "attachment pilus" by some scientists, is an appendage that can be found on many Gram-negative and some Gram-positive bacteria that is thinner and shorter than a flagellum. This appendage ranges from 3-10 nanometers in diameter and can be up to several micrometers long. Fimbriae are used by bacteria to adhere to one another and to adhere to animal cells and some inanimate objects. A bacterium can have as many as 1,000 fimbriae. Fimbriae are only visible with the use of an electron microscope. They may be straight or flexible.
Fimbriae are a major factor in bacterial virulence (the ability of a bacterium to cause disease), since these structures enable some bacteria to colonize human epithelial cells (cells of mucous membranes).
At the end of each fimbria are special proteins called adhesins. The specific type of adhesin varies by type of bacteria, but regardless of the type, adhesin molecules allow bacteria with fimbriae to adhere to host cells by docking, like a lock and key, with receptor proteins on the surface of host epithelial cells.
This ability of fimbriae to stick to epithilial cells leads to many diseases transmitted via mucous membranes, including gonorrhoeae, bacterial meningitis and infections of internal medical devices and indwelling catheters. The following are descriptions of three species of pathogenic bacteria that rely on their fimbriae to cause disease.
Fimbriae and pili are interchangeable terms used to designate short, hair-like structures on the surfaces of procaryotic cells. Like flagella, they are composed of protein. Fimbriae are shorter and stiffer than flagella, and slightly smaller in diameter. Generally, fimbriae have nothing to do with bacterial movement. Fimbriae are very common in Gram-negative bacteria, but occur in some archaea and Gram-positive bacteria as well. Fimbriae are most often involved in adherence of bacteria to surfaces, substrates and other cells or tissues in nature.