The Mechanism of Bidirectional pH Taxis in Bacillus subtilis

We investigated pH taxis in Bacillus subtilis. This bacterium was found to perform bidirectional taxis in response to external pH gradients, enabling it to preferentially migrate to neutral environments. We next investigated the chemoreceptors involved in sensing pH gradients. We identified four chemoreceptors involved in sensing pH: McpA and TIpA for sensing acidic environments and McpB and TIpB for sensing alkaline ones. In addition, TIpA was found to also weakly sense alkaline environments. By analyzing chimeras between McpA and TIpB, the principal acid- and base-sensing chemoreceptors, we identified four critical amino acid residues-Thr(199), GIn(200), His(273), and Glu(274) on McpA and Lys(199), Glu(200), GIn(273), and Asp(274) on TIpB-involved in sensing pH. Swapping these four residues between McpA and TIpB converted the former into a base receptor and the latter into an acid receptor. Based on the results, we propose that disruption of hydrogen bonding between the adjacent residues upon pH changes induces signaling. Collectively, our results further our understanding of chemotaxis in B. subtilis and provide a new model for pH sensing in bacteria. IMPORTANCE Many bacteria can sense the pH in their environment and then use this information to direct their movement toward more favorable locations. In this study, we investigated the pH sensing mechanism in Bacillus subtilis. This bacterium preferentially migrates to neutral environments. It employs four chemoreceptors to sense pH. Two are involved in sensing acidic environments, and two are involved in sensing alkaline ones. To identify the mechanism for pH sensing, we constructed receptor chimeras of acid- and base-sensing chemoreceptors. By analyzing the responses of these chimeric receptors, we were able to identify four critical amino acid residues involved in pH sensing and propose a model for the pH sensing mechanism in B. subtilis.