Structure and Function of Bacterial Cells (page 3)
(This chapter has 10 pages)
© 2009 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 |
chapter continued
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