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Unformatted text preview: Structures and Functions of Microbial Motility
The driving force for the evolution of motility appears to be acquisition of food. Thus, motility
has also evolved with a mechanism to sense the environment and control motility in response.
Almost all microorganisms have mechanisms for motility, both prokaryotic and eukaryotic.
Despite the widespread nature of motility, the mechanisms for motility have independently
evolved several times. Some forms of motility are very well understood (e.g. the bacterial
flagella), but scientists continue to identify microorganisms that exhibit motility through
unknown mechanisms. The study of motility, specifically the bacterial flagella, has made
fundamental breakthroughs in how motors, environmental sensing, and protein secretion
function. Many microbial pathogens are motile, and this motility plays a major role in their
ability to cause disease. Reading. 11.5 Structure and Function of Cell Appendages (pg. 307-314) E.coli and Experiments that Lead to Our Understanding of Bacterial
Here are presented some of the key experimental methods and observations that lead to our
understanding of chemotaxis in E. coli. The point here is not to memorize the particular
experiments. These are presented to elucidate the scientific method. Quantification of Chemotaxis
Chemotaxis=movement in response to a chemical. This chemical could be either an
attractant or repellent. The experiment described in the textbook demonstrated that bacteria
exhibited chemotaxis. With an assay to study chemotaxis, scientists could then make
perturbations to the system and then quantify what effect those perturbations had on the
1. Understand how to measure chemotaxis.
2. Know that the concentration of bacteria in the capillary tube relative
to the exterior of the tube will change whether there is an attractant or
repellent present in the tube or whether the solution in the tube is the
same as the solution in the beaker.
3. Be able to explain why these different results would be obtained. Microscopic Analysis of Chemotaxis
Models for how detecting the presence of an attractant or repellent affected the motility of E.
coli was developed through the use of time-lapsed photography of microscopic observations.
This analysis revealed that E. coli moved in a series of runs and tumbles (see movie of the
course web site). This movement did not change as long as E. coli was in a homogeneous
solution, whether or not an attractant or repellent was present. But remember, in the tube
experiment (above), a gradient of attractant or repellent would have been set up as the 1 chemical diffused from the tube. Thus, when a gradient is established under the microscope
a change in the motility of E. coli was observed. This behavior is described in the textbook.
1. Be able to explain the difference between a spatial and temporal
gradient. Know that bacteria respond to a temporal gradient. For an
example of response to a spatial gradient, see the movie on the course
website of the eukaryotic microbe, Dictyostelium.
2. Be able to explain how bacteria can, using only random movement,
effectively swim towards an attractant. Isolation of Chemotaxis or Motility Defective Mutants
Isolation and characterization of mutants defective for a process of interest is a powerful
method to reveal the molecular basis of a process. Thus, to reveal the molecular mechanism
of bacterial motility, scientist isolated mutants of E. coli that exhibited a defect in motility or
chemotaxis, called Mot- and Che-, respectively. The key step in the isolation of mutants is
the development of conditions that allow the screening of thousand of mutant cells for the
desired, altered phenotype.
To isolate Mot- and Che- E. coli mutants, an agar plate-based assay was developed. The use
of agar plates allows thousands of cells to be analyzed simultaneously. The agar plate used
to isolate the Mot- and Che- mutants contained soft agar (i.e. semi-solid). E. coli can swim in
this soft agar. To generate a gradient for chemotaxis, the plates also contained a very small
amount of a carbon source. E.coli cells are inoculated in a spot on the plate. These bacteria
use the carbon compound present near the spot and grow and divide. As they exhaust the
carbon compound at the site of inoculation, they create a gradient, with the highest
concentration always being outside the zone of the bacteria. Thus, E. coli will chemotax
outward, to the higher concentration of carbon compound, use that compound to grow and
divide, starting the cycle again.
To generate mutants, a culture of wild-type bacteria is mutagenized (do not worry about
how). This creates a culture of cells that each have different mutations. The culture of
mutagenized cells can then be spread on the plates to separate the cells, so that each cell will
give rise to a colony. Mot- mutants
are unable to move from the spot of
inoculation. These mutants form
pinpoint colonies on the plates as
compared to the wild-type cells that
spread outward to form large
colonies. Che- mutants are able to
move, but are not able to sense the
gradient of the carbon compound.
These Che- mutants have
undirected, random motility. By
this random movement, the Che2 mutants move away from point of inoculation, but not to the extent that wild-type cells move.
Thus, Che- mutants form colonies that are larger than the Mot- mutants, but not as large as
those formed by the wild-type cells (see above figure). Characterization of Mot- Mutants
To learn what mutant bacteria reveal about the molecular mechanism of a process, scientists
have to characterize them further. In what gene is the mutation? We will not go into how to
do this, just suffice it to know that this is an essential part of characterizing mutants. If the
mutation is in a gene of known function, then this information will provide obvious models
to test further. But, if the mutation is in a gene that belongs to a family of genes that has
never been characterized, then scientists need to design experiments to gain a more finedetailed knowledge of the phenotype of the mutant strain. This additional information should
help to generate testable models regarding the role of the mutated in gene in the process
under study. An example of this is how Mot- mutants were characterized. The Mot- mutants
were analyzed for changes in structure by electron microscopy. Figure 4.54 on page 92
shows examples of flagellated bacterial cells observed by electron microscopy. Many of the
Mot- E. coli cells lacked observable flagella. This lead to the hypothesis that the gene that
were mutated in the Mot- mutants encoded proteins that were either part of the flagellum or
involved in the assembly of the flagellum. Through these types of experiments we have gain
a detailed knowledge of the bacterial flagella. Motility Structures
Bacterial Extracellular Flagella
“The mucus layer associated with epithelium forms inevitable physical and chemical
obstacles for pathogens. Similarly, dynamic processes, such as the upward flow of mucus
from the bronchial epithelia or peristalsis in the intestine, have to be counterbalanced by
pathogens if colonization is to be achieved. In the host, motility combined with
chemotaxis enables fine-tuned access of pathogens to the target mucosal tissues. Motility
functions of Helicobacter pylori (cause of stomach ulcers) and Pseudomonas aeruginosa
(cause of mortality of cystic fibrosis patients) are crucial for infection of the stomach and
lungs, respectively. Colonization of intestinal mucosa by Vibrio cholerae (cause of
cholera) strictly requires motility. Because motility increases the occurrence of host–
pathogen interactions, this feature contributes to the main role of the flagellum in
Adapted from Ramos HC, Rumbo M, and Sirard JC (2004) Trends Microbiol.
1. Understand that the flagellum is a rigid helix.
2. A motor in the cytoplasmic membrane turns this rigid helix to propel
the bacterium forward.
3. The energy for the motor is derived from pumping protons into the
cell. 3 4. Know that there are many different described arrangements for
flagella. Periplasmic Flagella
Spirochaetes have periplasmic flagella, and these flagella are believed to contribute to the
corkscrew shape of these organisms (for a picture, see page 408). These periplasmic
flagella also contribute to motility. Spirochaete species include those that cause Lyme
disease and syphilis. During the course of these diseases, these bacteria move from the
bloodstream into tissue. Their particular type of motility may contribute to their ability to
make this transfer. Type IV Pili
Twitching motility is a form of surface motility mediated by type IV pili. Pili work
through an extrusion, adhesion, and then retraction mechanism. For movies
demonstrating the use of type IV pili in movement, see the course web site. Many
questions remain regarding the mechanism of twitching motility. How does sensing the
presence of an attractant or repellent control pili movement? What is the energy source
for this motility? Slime Nozzles
Slime nozzles are used for surface movement. Hoe they work is unclear, but it has been
proposed that as the extruded slime hydrates, the bacterium is pushed in the opposite
direction. Actin Tails
Intracellular pathogenic bacteria use this form of motility. Many of these bacteria do not
appear motile outside of a host cell. In the host cell, a tail of host-derived actin forms,
which appears to constantly assemble and disassemble as the bacterium move forward.
The purpose of this motility appears to be to move from one cell to another. See some
fun movies on the course web site to gain a better appreciation of this form of motility. Miscellaneous
There are many bacteria that are motile, but do not appear to use any of the above
described structures. Some example that are being researched are the Ratchet structure
used by members of the Cytophaga-Flavobacterium group of bacteria, the contractile
cytoskeleton of Sprioplasma, and the cili-like structures of Synechococcus. Archea
Archea have structures on their surface that resemble flagella and that appear to mediate
swimming motility. Despite this similarity, the genes that encode the flagella components are
similar to those for Type IV pili. What modifications to these genes has occurred to create
swimming flagella in the Archea and to create surface-moving pili in the Bacteria? 4 Eukaryotes
Movies of eukaryotic microbial motility are available on the course website. Three types of
motility are known for eukaryotic microbes: flagella and cili for swimming and amoeboid
motion for surface translocation. These forms of motility are not related to the prokaryotic forms
at the molecular level. Mechanisms for motility seem to have independently evolved many
times. 5 ...
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This note was uploaded on 03/06/2012 for the course MIMG 100 taught by Professor Lazazzera during the Summer '10 term at UCLA.
- Summer '10