Motility

Bacterial motility for higher education

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Motility is essential for many bacteria to colonise their hosts. Motility is often facilitated by dynamic surface structures known as flagella. The direction of movement  bacterial cells can be controlled by chemotaxis - a response to external, chemical stimuli.

 

Motility

In order to infect their hosts and cause disease, bacteria must migrate to an appropriate environment for growth and colonisation, be that a particular host cell or an environment with a specific nutrient. This migration of cells is known as motility and is essential for many bacteria to colonise and proliferate. Motility in C.jejuni is mediated by a dynamic filamentous 'tail' known as flagella. Flagellar motility in C. jejuni has been shown to be a key factor in colonisation, with the virulence of the bacterium decreasing severely following its impairment (see diagram and legend below).

fla knockout

 

Above- The figure above taken from Wassenaar et al (1991) shows the ability of C. jejuni to invade human epithelial cells. Fla- is a strain with expression of the flagella flaA gene switched off. R1 and 2 are strains with the flaA gene disrupted with an antibiotic resistance marker whereas R3 has the flaB gene disrupted. We can see that, strains with flaA disrupted/not expressed are far less likely to colonise and invade the human epithelial cell line. This diagram not only shows that flagella motility is essential for virulence, but that flaA and not flaB is the primary gene in this process.

Chemotactic motility

C.jejuni usually express two polar flagella. C. jejuni have evolved mechanisms to move towards beneficial environments for example those with nutrients (chemo-attractants), and away from environments that are damaging to the cells (chemo-repellents). This movement is in response to chemical signals and is known as chemotaxis. Chemo-attractants and chemo-repellents help to establish a chemotactic gradient that governs the direction of Campylobacter motility.

Chemotactic gradient

Above - chemotactic gradients are established by differing concentrations of attractants and repellents.

C. jejuni display two characteristic types of motility:

  • Direct movement in a single direction, known as runs.
  • A periodical, random change in direction known as tumbling.

 

So how do bacteria ever migrate towards favourable environments if they are constantly tumbling and changing their direction? The answer is that movements down the chemotactic gradient will be far greater than movements up a chemotactic gradient. The result is that despite the random changes in direction, the general direction of movement will be towards a favourable environment.


 

Above- You can see the charecteristic tumbling motion of C.jejuni. In this case the chemotactic gradient is moving to the left. Movements to the left of the screen are far more pronounced than movements in other directions.

Chemotactic signal transduction

One key area of C. jejuni research is investigating how these chemotactic signals are translated into a response. The transduction of chemotactic signals to determine direction of flagellar motility in Campylobacter is very complex. You can find an animated PowerPoint explaining the mechanism of chemotaxis in Campylobacter and also chemotactic signal transduction in E.coli on the 'access topic related resources' page. The E. coli system is homologous to the signal transduction system in Campylobacter.

 

Further levels of control

A further level of control over motility in Campylobacter is that expression of the flagella can also be switched on or off. This is known as phase variation and leads to strains of C.jejuni within the population that are incapable of motility. On the phase variation page you can read about the mechanisms underlying this process, and also take our online tutorial investigating the consequences of phase variation in C. jejuni.

Take a look at some of our work on this topic on the 'our research' page.

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