The Rcs stress response system modulates Serratia marcescens induced inflammation and bacterial proliferation in a rabbit keratitis model

In this study, we tested the hypothesis that the conserved bacterial Rcs stress response system mediates corneal pathogenesis associated with Serratia marcescens ocular infections. This was accomplished by modifying Rcs activity using mutant strains. These include a mutant that has a hyper-active Rcs system due to deletion of the IgaA family gene, gumB, and a gumB rcsC double mutant that is defective for Rcs signaling. The role of the Rcs system and bacterial stress response systems for microbial keratitis is not known. Here we observed that the Rcs-activated gumB mutant had a >50-fold reduction in proliferation compared to the wild type within rabbit corneas at 48 h, and demonstrated a notable reduction in inflammation based on inflammatory signs and proinflammatory markers measured at the RNA and protein levels. The gumB mutant phenotypes could be complemented by wild-type gumB on a plasmid and partially complemented by restoration of shlA cytolysin expression and elimination of capsular polysaccharide production. We observed that inactivation of the Rcs stress response system completely restored corneal virulence to the gumB mutant. NanoString transcriptional analysis of bacterial genes expressed during microbial keratitis demonstrated expression of gumB, rcsB, shlA, and three metalloprotease genes. Data suggest that the bacterial capsular polysaccharide is not necessary for infection, but capsule overexpression reduces inflammation. Together, these data indicate that GumB regulates virulence factor production through the Rcs system and this overall stress response system is a key mediator of a bacteriums ability to induce vision-threatening keratitis.