Fimbria (bacteriology)

In bacteriology, a fimbria (Latin for 'fringe', plural fimbriae), also referred to as an "attachment pilus" by some scientists, is a short appendage found on many Gram-negative and some Gram-positive bacteria, and that is thinner and shorter than a flagellum. This appendage ranges from 3–10 nanometers in diameter and can be as much as 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 possess adhesins which attach them to some sort of substratum so that the bacteria can withstand shear forces and obtain nutrients. For example, "E. coli" uses them to attach to mannose receptors. 
Some aerobic bacteria form a very thin layer at the surface of a broth culture. This layer, called a pellicle, consists of many aerobic bacteria that adhere to the surface by their fimbriae. Thus, fimbriae allow the aerobic bacteria to remain both on the broth, from which they take nutrients, and near the air.
All fimbriae are pili; they are only called fimbriae because of their purpose. The term "fimbria" can refer to many different (structural) types of pilus, as many different types of pili have been used for adhesion, a case of convergent evolution. The Gene Ontology system does not treat fimbriae as a distinct type of appendage, using the generic pilus (GO:0009289) type instead.
Examples.
"Gram-negative bacteria assemble functional amyloid surface fibers called curli." Curli are a type of fimbriae. Curli are composed of proteins called curlins. Some of the genes involved are "CsgA", "CsgB", "CsgC", "CsgD", "CsgE", "CsgF", and "CsgG".
Another type are called type 1 fimbriae. They contain FimH adhesins at the "tips". The chaperone-usher pathway is responsible for moving many types of fimbriae out of the cell, including type 1 fimbriae and the P fimbriae.
Virulence.
Fimbriae are one of the primary mechanisms of virulence for "E. coli", "Bordetella pertussis", "Staphylococcus" and "Streptococcus" bacteria. Their presence greatly enhances the bacteria's ability to attach to the host and cause disease.