My Vibrio is faster than yours

Knowing how chemotaxis works is fundamental to understanding nutrient acquisition in bacteria. Our understanding of bacterial motility mainly comes from research on Escherichia coli, a gammaproteobacterium with multiple flagella. The majority of marine bacteria have a single polar flagellum, and the typical ‘run and tumble’ motility that we usually assign to chemotactic movement is not applicable as a result. The peritrichous arrangement of E. coli’s flagella means that when rotated in one direction, the flagella bundle together and propel the bacterium in a straight line. Rapid reversal of the rotation causes the bundle to splay out which gives the ‘tumble’ and a subsequent change in direction. Marine bacteria on the other hand, rarely display the classic ‘tumble’ and instead alternate between forwards and backwards runs, with 180^o reversals or 90^o flicks. The flicks are caused by the deformation of the hook (a structure that connects the flagellar filament to the rotating motor) and the likelihood of this deformation is speed dependant. It is unknown how this speed dependant motility affects chemotaxis, and therefore how efficiently marine bacteria can move towards a nutrient.

Son et al investigated how the dependence on speed effects chemotaxis, and how chemokinesis (the ability to alter swimming speed in response to the concentration of a stimulus) can enhance the movement towards a nutrient. The chemotaxis of over 50,000 Vibrio alginolyticus individuals was quantified by tracking the movement of the cells along a gradient of the amino-acid serine, via video microscopy. The swimming speeds of individual cells was measured in order to quantify the natural variation in speed within the population, and each cell was assigned to one of twelve ‘speed bins’. The results very clearly demonstrate that faster cells had higher chemotactic precision – i.e. they accumulated more tightly at the stimulus. This trend was also backed up by the chemotactic migration coefficient (CMC) which measures displacement of a populations centre of mass from a central point in the gradient, where 0 is no chemotactic response, 1 is maximum attraction, and -1 is maximum repulsion. The CMC value increased from 0.44 to 1, from the slowest speed bin to the fastest, which confirms the speed dependence of chemotaxis in this species. The authors also conducted this experiment with E. coli (using a gradient of a-methylaspartate, and 5 speed bins due to the smaller range of speed variation between cells) and found that chemotactic precision was not speed dependant.

The authors used their data to create a marine bacteria chemotactic model, which takes into account swimming speed, along with reorientation and flick frequency. They show that at their normal swimming speeds, the motility of V. alginolyticus is less random than that of E. coli, and randomness increases with speed (i.e flicks and reversals increase). This helps to explain why chemotactic precision also increases with speed.

The speed with which a cell can move towards a nutrient has obvious survival benefits, yet faster swimming is likely to be more energetically costly. The huge difference in chemotactic motility and precision between V. alginolyticus and E. coli demonstrates the importance of studying more than one species in order to understand how microbes acquire nutrients. I think this paper is really interesting and builds upon our current understanding of marine bacterial motility, as well as linking nicely with one of the authors earlier papers (link to my review) which looks at chemotaxis towards lysed phytoplankton. The information about the speed dependant precision from this study may affect how models of resource utilisation and microscale interaction are designed and used to predict the wider ecological consequences this could have.

Son. K., Menolascina. F., Stocker. R. (2016) Speed-dependant chemotactic precision in marine bacteria. PNAS. 113 (31). 8624-8629