Driving microbes round the bend: Chemotaxis and sodium-ion and proton motors

Many microbial assemblages utilise chemotactic capability to detect nutrients across a gradient in tandem with a ‘run and tumble’ style of moving, whereby the bacteria will typically use a flagellum to move it around it’s medium in straight lines, a ‘run’. It will periodically ‘tumble’ by reversing the direction of the flagellum which will change its direction. When a microbe senses it is following a nutrient gradient it will perform fewer ‘tumbles’.

Within the marine environment, however, nutrients are generally patchy and being able to chemotactically find them is much more difficult, as a result, microbes have to move in excess of 200µm s^-1. To help with this many marine microbes have modified the usual ‘run and tumble’ mechanism to include a reversal where by the microbe can reverse its direction so that it can move back into a nutrient patch if it otherwise would have drifted away.

The flagella of flagellate bacteria have embedded within their cell wall and cell membrane a rotary molecular motor. There are 2 mechanisms which power these motors, sodium-ion motors and proton motors. They drive the flagella using an electrochemical gradient involving either sodium ions or protons respectively.

The research of the authors of this particular paper focused on how much of a contribution the 2 main motor mechanisms for driving the flagella in marine flagellate microbes made in terms of high speed motility. In order to do this, inhibitors of the mechanisms were introduced to microbes collected from the water column and microbes from recent isolates. This was done for realism and future applicability which allows this study to be applied in a wider context although I would be wary of doing so as comparing a natural sample of microbes to an isolate does not really give a good view of how the motor mechanisms are affected as an isolate sample will not necessarily react in the same way as a natural sample.

The inclusion of an amiloride tolerant, unidentified isolate just to provide a link and comparison to the amiloride literature seemed unnecessary as they had already identified that amiloride was not being used in preference of the sodium-ion uncoupler monensin, mainly on the grounds that the monensin was less toxic to the cells and would work at a thousandth the concentration that amiloride would. This inclusion would only really serve to show that monensin was less toxic than amiloride which had already been identified beforehand. With the isolate being tolerant to amiloride anyway i’m not entirely certain what the authors were trying to accomplish by it. However, later on in the paper it was mentioned that amiloride had indeed been used to test whether or not the sodium-ions present were responsible for the motility of the community.

Although this paper does have very good applications in the research of chemotactic ability in flagellate bacteria, and in showing that many bacteria can maintain the speeds required for chemotaxis within the marine environment by use of 2 particular motors, I feel that this paper should not have drawn inferences or conclusion from inconclusive data concerning the toxicity of the uncouplers for the sodium-ion motors and its effects.

Overall this paper has highlighted some interesting areas for future research in areas such as microbial physiology and the interactions it has with the marine environment and exactly how marine microbe assemblages manage to maintain the high speeds they do within the viscous media that they inhabit.

Mitchell J.G., Barbara G.M., 1999, High speed marine bacteria use sodium-ion and proton driven motors, Aquatic Microbial Ecology, 18, 227-233