The Rcs Phosphorelay May Regulate the E. coli Capsule In Response to Sublethal Streptomycin Treatment

The Escherichia coli colanic acid capsule produced by the cps gene confers resistance in the presence of sublethal antibiotic concentrations. However, the exact mechanism of capsule gene regulation in reaction to the presence of antibiotics is unknown. The two main proposed cps regulation pathways both involve the Rcs phosphorelay system, but differ regarding dependence on RpoS for cps transcription. In this study, we explored the changes in capsule formation and antibiotic resistance in response to treatment with sublethal concentrations of streptomycin using deletion mutants of the two proposed regulation pathways. Wild type, Δ rcsB, Δ rpoS, and Δ cps. E. coli strains were incubated in sublethal concentrations of streptomycin. When examined with MIC assays, the deletion mutants showed higher levels of antibiotic resistance compared to wild type strains of E. coli. We also demonstrated that capsule production was induced under sublethal streptomycin conditions with Maneval staining. Furthermore, qPCR detection of cps transcription indicated downregulation in Δ rcsB strains and upregulation in Δ rpoS after streptomycin treatment. Together, these results suggest that E. coli streptomycin resistance is not dependent on capsule production, and that rcsB and rpoS may both play a role in cps regulation when treated with sublethal doses of streptomycin.