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.
Flagella
Flagella are
filamentous protein
structures
attached to the cell surface that provide the swimming movement for
most
motile procaryotes. Procaryotic flagella are much thinner than
eucaryotic
flagella, and they lack the typical "9 + 2" arrangement of
microtubules.
The diameter of a procaryotic flagellum is about 20 nanometers,
well-below
the resolving power of the light microscope. The flagellar filament is
rotated by a motor apparatus in the plasma membrane allowing the cell
to
swim in fluid environments. Bacterial flagella are powered by proton
motive
force (chemiosmotic potential) established on the bacterial membrane,
rather
than ATP hydrolysis which powers eucaryotic flagella. About half of the
bacilli and all of the spiral and curved bacteria are motile by means
of
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.
coli
is illustrated in Figure 3 below (after Dr. Julius Adler of the
University
of Wisconsin). About 50 genes are required for flagellar synthesis and
function. The flagellar apparatus consists of several distinct
proteins:
a system of rings embedded in the cell envelope (the basal
body),
a
hook-like structure near the cell surface, and the flagellar
filament. The innermost rings, the M and S rings, located in the
plasma
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
which
turns to propel the bacterium.
Figure
3. The ultrastructure
of a bacterial flagellum (after J. Adler). Measurements are in
nanometers.
The flagellum of E. coli consists of three parts, filament,
hook
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
consists
of four ring-shaped proteins stacked like donuts around a central rod
in
the cell envelope. The inner rings, associated with the plasma
membrane,
are the flagellar powerhouse for activating the filament. The outer
rings
in the peptidoglycan and outer membrane are support rings or "bushings"
for the rod. The filament rotates and contracts which propels and
steers
the cell during movement.
Flagella may be
variously distributed over the
surface of bacterial cells in distinguishing patterns, but basically
flagella
are either polar (one or more flagella arising from one or both
poles of the cell) or peritrichous (lateral flagella
distributed
over the entire cell surface). Flagellar distribution is a
genetically-distinct
trait that is occasionally used to characterize or distinguish
bacteria.
For example, among Gram-negative rods, Pseudomonas has polar flagella
to distinguish them from enteric bacteria, which have peritrichous
flagella.
Figure
4. Different
arrangements
of bacterial flagella. Swimming motility, powered by flagella, occurs
in
half the bacilli and most of the spirilla. Flagellar arrangements,
which
can be determined by staining and microscopic observation, may be a
clue
to the identity of a bacterium. See Figure 6 below.
Flagella were proven to
be organelles of
bacterial
motility by shearing them off (by mixing cells in a blender) and
observing
that the cells could no longer swim although they remained viable. As
the
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
response
to environmental stimuli. For example, during chemotaxis a
bacterium
can sense the quality and quantity of certain chemicals in its
environment
and swim towards them (if they are useful nutrients) or away from them
(if they are harmful substances). Other types of tactic response in
procaryotes
include
phototaxis, aerotaxis and magnetotaxis. The
occurrence of tactic behavior provides evidence for the ecological
(survival)
advantage of flagella in bacteria and other procaryotes.
Detecting Bacterial
Motility
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
outline flagella
and
show their pattern of distribution. If a bacterium possesses flagella,
it is presumed to be motile.
Figure
5. Flagellar stains of
three bacteria a. Bacillus cereus b. Vibrio cholerae c.
Bacillus
brevis. (CDC). Since the bacterial flagellum is below the resolving
power of the light microscope, although bacteria can be seen swimming
in
a microscope field, the organelles of movement cannot be detected.
Staining
techniques such as Leifson's method utilize dyes and other components
that
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
Gram-negative
motile rod-shaped bacteria, the enterics have peritrichous flagella
while
the pseudomonads have polar flagella.
2. motility
test
medium demonstrates
if
cells can swim in a semisolid medium. A semisolid medium is inoculated
with the bacteria in a straight-line stab with a needle. After
incubation,
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
to
swim through the medium.
Julius Adler
exploited this observation during his studies of chemotaxis in E.
coli.
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
concentration of
glucose (along the edge of the circle), they swam up the gradient
towards
a higher concentration (the center of the circle), exhibiting their
chemotactic
response to swim towards a useful nutrient. Later, Adler developed a
tracking
microscope that could record and film the track that E. coli
takes
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.
Figure
6. Bacterial
cultures
grown in motility test medium. The tube on left is a non motile
organism;
the tube on right is a motile organism. Motility test medium is a
semi-soft
medium that is inoculated with a straight needle. If the bacteria
are motile,
they will swim away from the line of inoculation in order to find
nutrients,
causing turbidity or cloudiness throughout the medium. If they are non
motile,
they will only grow along the line of inoculation. www.jlindquist.net/
generalmicro/dfmotility.html.
3. direct
microscopic observation of
living
bacteria in a wet mount. One must look for transient movement of
swimming
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
molecules
and bacterial cells. True motility is confirmed by observing the
bacterium
swim from one side of the microscope field to the other side.
Wet
mount of the bacterium Rhodospirillum
rubrum, about
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
bacterium.
Figure 7. A Desulfovibrio
species. TEM. About 15,000X. The bacterium is motile by means of a
single
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
microscope.
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