Treatment of Escherichia coli K-12 with sub-inhibitory concentrations of antimicrobial agents does not induce RpoSmediated cross-protection to T7 bacteriophage infection

SUMMARY The Rcs-phosphorelay pathway is used by Escherichia coli to express the alternative sigma factor RpoS in response to extracellular stressors. It has been proposed that RpoS may mediate cross-protection from both antibiotics and bacteriophage infection. Here, we hypothesize that, upon pre-treatment with sub-inhibitory antibiotic concentrations, RpoS induction in E. coli can confer additional protection from T7 bacteriophage-mediated lysis. Using Minimum Inhibitory Concentration (MIC) assays, we established the sub-lethal antibiotic concentrations for wild-type (WT), RpoS- and Rcs-deficient strains of E. coli K-12. Then, bacteriophage-mediated lysis assays were conducted in E. coli K-12, pre-treated with sub-inhibitory antibiotic concentrations. However, we observed no significant delay in bacteriophage T7-mediated cell lysis. Further, using the MIC assay method, we also observed that RpoS deletion results in impaired growth in presence of antibiotics, suggesting that this protein is partially responsible for the intrinsic antibiotic resistance mechanisms.
Additionally, we found that prolonged treatment with beta-lactam antibiotics restored cell growth, indicating utilization of beta-lactam-specific resistance mechanisms by E. coli K-12. Altogether, our findings suggest that while RpoS is partially responsible for intrinsic resistance to antibiotics in E. coli K-12, defence pathways regulated by this protein are not required to induce cross-protection from bacteriophage-mediated cell lysis.