The short chain fatty acid propionic acid activates the Rcs stress response system partially through inhibition of D-alanine racemase

Abstract

The Enterobacterial Rcs stress response system reacts to envelope stresses through a complex two-component phosphorelay system to regulate a variety of environmental response genes such as capsular polysaccharide and flagella biosynthesis. However, beyond Escherichia coli, the stresses that activate Rcs are not well understood. In this study, we used a Rcs system dependent luminescent transcriptional reporter to screen a library of over 240 antimicrobial compounds for those that activated the Rcs system in Serratia marcescens, a Yersiniaceae family bacterium. Using an isogenic rcsB mutant to establish specificity, both new and expected activators were identified including the short chain fatty acid propionic acid found at millimolar levels in the human gut. Propionic acid did not reduce bacterial intracellular pH as hypothesized for its antibacterial mechanism. Rather than reduction of intracellular pH, data suggests that the Rcs-activating mechanism of propionic acid is, in part, due to inactivation of the enzyme alanine racemase. This enzyme is responsible for D-alanine biosynthesis, an amino-acid required for generating bacterial cell walls. These results suggest host gut short chain fatty acids can influence bacterial behavior through activation of the Rcs stress response system.