Structure and Function of Bacterial Cells (page 2)
(This chapter has 10 pages)
© Kenneth Todar, PhD
Appendages: flagella, fimbriae and pili
Salmonella is an enteric bacterium related to E.
coli. The enterics are motile by means of peritrichous flagella.
attached to the cell surface that provide the swimming movement for
motile procaryotes. Procaryotic flagella are much thinner than
flagella, and they lack the typical "9 + 2" arrangement of
The diameter of a procaryotic flagellum is about 20 nanometers,
the resolving power of the light microscope. The flagellar filament is
rotated by a motor apparatus in the plasma membrane allowing the cell
swim in fluid environments. Bacterial flagella are powered by proton
force (chemiosmotic potential) established on the bacterial membrane,
than ATP hydrolysis which powers eucaryotic flagella. About half of the
bacilli and all of the spiral and curved bacteria are motile by means
flagella. Very few cocci are motile, which reflects their adaptation to
dry environments and their lack of hydrodynamic design.
The ultrastructure of
the flagellum of E.
is illustrated in Figure 3 below (after Dr. Julius Adler of the
of Wisconsin). About 50 genes are required for flagellar synthesis and
function. The flagellar apparatus consists of several distinct
a system of rings embedded in the cell envelope (the basal
hook-like structure near the cell surface, and the flagellar
filament. The innermost rings, the M and S rings, located in the
membrane, comprise the motor apparatus. The outermost rings, the P and
L rings, located in the periplasm and the outer membrane respectively,
function as bushings to support the rod where it is joined to the hook
of the filament on the cell surface. As the M ring turns, powered by an
influx of protons, the rotary motion is transferred to the filament
turns to propel the bacterium.
3. The ultrastructure
of a bacterial flagellum (after J. Adler). Measurements are in
The flagellum of E. coli consists of three parts, filament,
and basal body, all composed of different proteins. The basal body and
hook anchor the whip-like filament to the cell surface. The basal body
of four ring-shaped proteins stacked like donuts around a central rod
the cell envelope. The inner rings, associated with the plasma
are the flagellar powerhouse for activating the filament. The outer
in the peptidoglycan and outer membrane are support rings or "bushings"
for the rod. The filament rotates and contracts which propels and
the cell during movement.
Flagella may be
variously distributed over the
surface of bacterial cells in distinguishing patterns, but basically
are either polar (one or more flagella arising from one or both
poles of the cell) or peritrichous (lateral flagella
over the entire cell surface). Flagellar distribution is a
trait that is occasionally used to characterize or distinguish
For example, among Gram-negative rods, Pseudomonas has polar flagella
to distinguish them from enteric bacteria, which have peritrichous
of bacterial flagella. Swimming motility, powered by flagella, occurs
half the bacilli and most of the spirilla. Flagellar arrangements,
can be determined by staining and microscopic observation, may be a
to the identity of a bacterium. See Figure 6 below.
Flagella were proven to
be organelles of
motility by shearing them off (by mixing cells in a blender) and
that the cells could no longer swim although they remained viable. As
flagella were re-grown and reached a critical length, swimming movement
was restored to the cells. The flagellar filament grows at its tip (by
the deposition of new protein subunits) not at its base (like a hair).
Procaryotes are known to
exhibit a variety of
types of tactic behavior, i.e., the ability to move (swim) in
to environmental stimuli. For example, during chemotaxis a
can sense the quality and quantity of certain chemicals in its
and swim towards them (if they are useful nutrients) or away from them
(if they are harmful substances). Other types of tactic response in
phototaxis, aerotaxis and magnetotaxis. The
occurrence of tactic behavior provides evidence for the ecological
advantage of flagella in bacteria and other procaryotes.
Since motility is a
primary criterion for the
diagnosis and identification of bacteria, several techniques have been
developed to demonstrate bacterial motility, directly or indirectly.
1. flagellar stains
show their pattern of distribution. If a bacterium possesses flagella,
it is presumed to be motile.
5. Flagellar stains of
three bacteria a. Bacillus cereus b. Vibrio cholerae c.
brevis. (CDC). Since the bacterial flagellum is below the resolving
power of the light microscope, although bacteria can be seen swimming
a microscope field, the organelles of movement cannot be detected.
techniques such as Leifson's method utilize dyes and other components
precipitate along the protein filament and hence increase its effective
diameter. Flagellar distribution is occasionally used to differentiate
between morphologically related bacteria. For example, among the
motile rod-shaped bacteria, the enterics have peritrichous flagella
the pseudomonads have polar flagella.
cells can swim in a semisolid medium. A semisolid medium is inoculated
with the bacteria in a straight-line stab with a needle. After
if turbidity (cloudiness) due to bacterial growth can be observed away
from the line of the stab, it is evidence that the bacteria were able
swim through the medium.
exploited this observation during his studies of chemotaxis in E.
He prepared a gradient of glucose by allowing the sugar to diffuse into
a semisolid medium from a central point in the medium. This established
a concentration gradient of glucose along the radius of diffusion. When
E. coli cells were seeded in the medium at the lowest
glucose (along the edge of the circle), they swam up the gradient
a higher concentration (the center of the circle), exhibiting their
response to swim towards a useful nutrient. Later, Adler developed a
microscope that could record and film the track that E. coli
as it swims towards a chemotactic attractant or away from a chemotactic
repellent. This led to an understanding of the mechanisms of bacterial
chemotaxis, first at a structural level, then at a biomolecular level.
grown in motility test medium. The tube on left is a non motile
the tube on right is a motile organism. Motility test medium is a
medium that is inoculated with a straight needle. If the bacteria
they will swim away from the line of inoculation in order to find
causing turbidity or cloudiness throughout the medium. If they are non
they will only grow along the line of inoculation. www.jlindquist.net/
microscopic observation of
bacteria in a wet mount. One must look for transient movement of
bacteria. Most unicellular bacteria, because of their small size, will
shake back and forth in a wet mount observed at 400X or 1000X. This is
Brownian movement, due to random collisions between water
and bacterial cells. True motility is confirmed by observing the
swim from one side of the microscope field to the other side.
mount of the bacterium Rhodospirillum
1500X mag. Click here or on the image for a short video from the
Department of Microbiology and Immunology, University of Leicester,
that illustrates swimming motility of this photosynthetic purple
Figure 7. A Desulfovibrio
species. TEM. About 15,000X. The bacterium is motile by means of a
polar flagellum. Of course, one can determine the presence of flagella
by means of electron microscopy. Perhaps this is an alternative way to
determine bacterial motility, if you happen to have an electron