Introductory Microbiology

Fimbriae and pili

• Fimbriae and pili are short filamentous structures composed of protein that extend from the surface of a cell. Fimbriae enable organisms to stick to surfaces, including animal tissues in the case of some pathogenic bacteria or to form pellicles or biofilms on surfaces. Notorius among these pathogens include Salmonella typhimurium (salmonellosis), Neisseria gonorrhoeae (gonorrhoea), and Bordetella pertussis (whooping cough).
• Pili are structurally similar to fimbriae but are typically longer and only one or a few pili are present on the surface. Because they serve as receptors for certain types of viruses, pili can be seen under the electron microscope when they become coated with virus particles. Although possibly involved in attachment as for fimbriae, pili are clearly involved in the process of conjugation (a form of genetic exchange) in prokaryotes.
• Many classes of fimbriae/pili are known, distinguished by their structure and function. One class called type IV fimbriae/pili is involved in an unusual form of motility in certain bacteria called twitching motility. Twitching motility is a type of movement on solid surfaces, where it is thought that rapid and reversible extension and retraction of the fimbriae allow the cell to crawl along the surfaces. Unlike other fimbriae, type IV fimbriae are found only at the poles of the cells and besides motility have been implicated as key host colonization factors in a variety of pathogens including Vibrio cholerae (cholera), and Neisseria menigitidis. Type IV fimbriae are also thought to mediate genetic transfer by the process of transformation in a wide variety of bacteria.
• Pili are hollow, non-helical filamentous appendages that are thinner, shorter, and more numerous than flagella. They do not function in motility since they are found on non-motile as well as motile species. There are, however, several functions associated with different types of pili. One type known as the F. pilus (or sex pilus), serves as the port of entry of genetic material during bacterial mating. Some pili plays a major role in human infection by allowing pathogenic bacteria to attach to epithelial cells lining the respiratory, intestinal, or genitourinary tracts. This attachment prevents the bacteria from being washed away by the flow of mucous or body fluids and permits the infection to be established.