Role of a single MCP in evolutionary adaptation of Shewanella putrefaciens for swimming in planktonic and structured environments

Bacteria can adapt to their environments by changing phenotypic traits by mutations. However, improving one trait often results in deterioration of another one, a trade-off which limits the degree of adaption. The gammaproteobacterium Shewanella putrefaciens CN-32 has an elaborate motility machinery comprising two distinct flagellar systems and an extensive chemotaxis array with 36 methyl-accepting chemotaxis sensor proteins (MCPs). In this study we performed experimental selection on S. putrefaciens for increased spreading through a porous environment. We readily obtained a mutant that showed a pronounced increase in covered distance. This phenotype was almost completely caused by a deletion of 24 bp from the chromosome, which leads to a moderately enhanced production of a single MCP. Accordingly, chemotaxis assays under planktonic conditions and cell tracking in soft agar showed that the mutation improved navigation through nutritional gradients. The study demonstrates how differences in the abundance of a single MCP can lead to an efficient upgrade of directed flagella-mediated motility in specific environments at a low expense of cellular resources. ImportanceExperimental evolution experiments have been used to determine the trade-offs occurring in specific environments. Several studies that have used the spreading behavior of bacteria in structured environments identified regulatory mutants that increase the swimming speed of the cells. While this results in a higher chemotaxis drift, the growth fitness decreases as the higher swimming speed requires substantial cellular resources. Here we show that rapid chemotaxis adaptation can also be achieved through modification of the chemotaxis signal input at a low metabolic cost for the cell.