Friday, May 15, 2009

Throwing the Clutch (not the Brake) on a Bacterial Flagella

ResearchBlogging.orgFor most my of readers, it is common knowledge that bacteria are more than just singled celled entities; and instead bacteria are complex organisms capable of undergoing large-scale, multicellular activities. Of particular interest to many current microbiologists, is the development of biofilms.

These multicellular structures are likely how many bacteria exist in the environment, and are implemented in a variety of diseases. Taken alone, biofilms are fascinating, but I have a keen interest in understanding exactly how a bacterial cell decides it is time to create a biofilm. (In reality, I have a keen interest in how a bacterial cell decides it is time to do anything, but that is beside the point)

While studying the switch between motility and biofilm development in Bacillus subtilis, Daniel Kearns of Indiana University, wanted to know the status of the flagella during the switch.

Mutants of the biofilm regulator sinR are constitutively in a biofilm state and are nonmotile. However, flagellar genes are still expressed and flagella are still produced. The question then became what is preventing the flagella from moving while in a biofilm.

The first idea would be that the extracellular matrix is physically inhibiting the movement of the flagella. By knocking out a gene required for matrix production (epsH), Kearns saw that the flagella became free, but were unpowered.

Then, as any good geneticist, he screened for mutants that were able to suppress the non-motility phenotype of a sinR, epsH double mutant. All of these mutants mapped to a putative glycosyltransferase (epsE) within the matrix operon. Furthermore, expression of epsE alone was sufficient to inhibit flagellar motion, and known conserved glycosyltransferase residues were not required for inhibition.

This brings up the questions: Where is epsE acting? and Is it a brake (completely stopping all flagellar movement) or a clutch (preventing active rotation)?
By selecting for supressors of redundant epsE, the group found that all suppressors mapped to the fliG gene; a gene known to encode the transduction motor between the proton pump (motAB) and flagellar basal body. So, somehow epsE acts to inhibit the motor.

Furthermore, upon examining flagellar motion (in some awesome movies available here) Kearns showed that the flagella were not braked, that is, the flagella were still capable to rotate freely (and in fact did), however all rotation was due to Brownian motion. This implies that the flagella were disconnected from the motor, rather than stopped completely.

So, epsE is acting as a clutch to disconnect the flagellar basal body from the motor. This actually makes quite a bit of sense. If the cell no longer requires flagellar motion as it went into biofilms, it could do a variety of techniques. One is that it could shut off gene expression for the flagella. However, it would take many generations before its progeny were non-motile and the flagellar apparatus was diluted out. Another is that it could put a brake into the flagella and prevent motion entirely. But, this would cause lots of cell envelope stress while in a biofilm. Brownian motion alone could potentially tear the cell apart.

This leaves the cell with the clutch option. Capable of stopping flagellar motion quickly while keeping the cell envelope intact. Due to its high homology to glycosyltransferases, epsE likely represents an example of a duplicative evolution; whereby a duplication of an enzyme leads to the capability to take one copy of said enzyme and "play with it" to create new functions. In this case a sugar-acting enzyme has become a structural protein in the flagellar apparatus.

Blair, K., Turner, L., Winkelman, J., Berg, H., & Kearns, D. (2008). A Molecular Clutch Disables Flagella in the Bacillus subtilis Biofilm Science, 320 (5883), 1636-1638 DOI: 10.1126/science.1157877

Other Articles of Interst:
Extracellular Membrane Vesicles in Bacteria: Taking Quorum Sensing in New Directions
Altruism in Bacteria: Allowing Yourself to Die for the Good of the Species
Where the Wild Microbes Are: A New Theory in How Pathogens Survive Food Processing

1 comment:

Lab Rat said...

Have you heard of bacterial swarming? It's a little like a sort of moving biofilm; B. subtillis all join up and swarm accross a plate together. Like biofilms, it's an amazing example of bacteria showing almost multicellular activity.