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Web Review of Todar's Online Textbook of Bacteriology. "The Good, the Bad, and the Deadly"

Tag words: bacterial structure, flagellum, flagella, pilus, pili, fimbriae, capsule, S-layer, glycocalyx, slime layer, biofilm, outer membrane, LPS, cell wall, peptidoglycan, murein, teichoic acid, plasma membrane, cell membrane, phospholipid bilayer, transport system, proton motive force, pmf, ATPase, DNA, chromosome, nucleoid, ribosome, 30S subunit, 50S subunit, 16S rRNA, inclusion, PHB, glycogen, carboxysome, endospore, parasporal crystal.









Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

Structure and Function of Bacterial Cells (page 3)

(This chapter has 10 pages)

© Kenneth Todar, PhD


Fimbriae and Pili


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 (there are exceptions, e.g. twitching movement on Pseudomonas). 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. In E. coli, a specialized type of pilus, the F or sex pilus, apparently stabilizes mating bacteria during the process of conjugation, but the function of the smaller, more numerous common pili is quite different.

Common pili (almost always called fimbriae) are usually involved in specific adherence (attachment) of procaryotes to surfaces in nature. In medical situations, they are major determinants of bacterial virulence because they allow pathogens to attach to (colonize) tissues and/or to resist attack by phagocytic white blood cells. For example, pathogenic Neisseria gonorrhoeae adheres specifically to the human cervical or urethral epithelium by means of its fimbriae; enterotoxigenic strains of E. coli adhere to the mucosal epithelium of the intestine by means of specific fimbriae; the M-protein and associated fimbriae of Streptococcus pyogenes (See Figure 2) are involved in adherence and to resistance to engulfment by phagocytes.


Figure 8. Fimbriae (common pili) and flagella on the surface of bacterial cells. Left: dividing Shigella enclosed in fimbriae. The structures are probably involved in the bacterium's ability to adhere to the intestinal surface. Right: dividing pair of Salmonella displaying both its peritrichous flagella and its fimbriae. The fimbriae are much shorter and slightly smaller in diameter than flagella. Both Shigella and Salmonella are enteric bacteria that cause different types of intestinal diarrheas. The bacteria can be differentiated by a motility test. Salmonella is motile; Shigella is nonmotile.

Table 3. Some properties of pili and fimbriae
Bacterial species where observed Typical number on cell Distribution on cell surface Function
Escherichia coli (F or sex pilus) 1-4 uniform stabilizes bacteria during transfer of DNA during conjugation
Escherichia coli (common pili or Type 1 fimbriae) 100-200 uniform surface adherence to epithelial cells of the GI tract
Neisseria gonorrhoeae 100-200 uniform surface adherence to epithelial cells of the urogenital tract
Streptococcus pyogenes (fimbriae plus the M-protein) ? uniform adherence, resistance to phagocytosis; antigenic variability
Pseudomonas aeruginosa 10-20 polar surface adherence
Sulfolobus acidocaldarius
(an archaic)
? ? attachment to sulfur particles




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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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