Bacterial physiology: Chilling out away from the crowds

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Escherichia coli has a unique density-dependent mechanism for switching its response to temperature that is mediated through the methylation of receptors for aspartate, serine and glycine.

Salman and Libchaber report in Nature Cell Biology that Escherichia coli has a unique density-dependent mechanism for switching its response to temperature, which is mediated through the methylation of receptors for aspartate, serine and glycine.

Thermotaxis and chemotaxis, the sensing and responding to temperature and chemical gradients, respectively, are closely linked and are essential processes that can be found in all cells. Interestingly, E. coli switches its preference for warmth to cold depending on the chemical environment. The authors examined the response of E. coli grown at different densities to temperature gradients.

Bacteria that were grown below a crucial threshold density and then subjected to a temperature gradient between 18°C and 30°C swam up the gradient towards 30°C, but bacteria grown above the threshold density swam down the gradient towards 18°C. The responses of the bacterial populations that were grown above and below the threshold density were not affected when mixed together, suggesting that this behaviour is dependent on the physiological state of bacteria and is controlled inside the cells.

E. coli expresses five chemotaxis receptors, including two high-abundance receptors for sensing aspartate (Tar), and serine and glycine (Tsr). Deletion of these receptors showed that Tsr-deficient bacteria accumulate at cooler temperatures, whereas Tar-deficient bacteria accumulate at warmer temperatures at all growth densities. The authors proposed that as Tsr is more abundant than Tar under normal conditions, bacteria are attracted to warmth. When the bacteria grow, however, they secrete glycine, which causes increased methylation of Tsr, making the receptor insensitive to temperature changes; Tar takes over the control of such changes, and the bacteria will seek colder temperatures. By methylating the different mutant strains, the authors showed that their hypothesis was correct. Further, they showed that the thermosensing switch is reinforced by an increased Tar:Tsr expression ratio as the bacteria grow to higher densities.

So, as the nutrient supply is depleted by bacterial growth, the bacteria move towards cooler areas, which reduces the rates of growth and metabolism — this is a simple but clever mechanism to slow growth if food is in short supply.

Sharon Ahmad

References

Salman, H. & Libchaber, A. A concentration-dependent switch in the bacterial response to temperature. Nature Cell Biol. 9, 1098–1100 (2007) | Article | PubMed |