The Rcs Phosphorelay System and RcsB Regulated rprA Contribute to Intrinsic Antibiotic Resistance in Escherichia coli Exposed to Antibiotics Targeting the Cell Wall

Bacteria orchestrate gene expression to cope with stressful conditions. Physical and antibiotic stressors induce regulators of stress response pathways to allow enhanced bacterial survival. The Rcs phosphorelay system regulates rprA, a small non-coding RNA which in turn regulates translation of RpoS, in addition to genes important for the production of capsule. RpoS is a central regulator of the general stress response. Prior studies have shown that Escherichia coli exhibits enhanced bacterial survival upon exposure to β-lactam antibiotics with the activation of the Rcs pathway using a mechanism independent of capsule synthesis. This mechanism of Rcs-mediated antibiotic resistance is not well understood. It is also not known whether this resistance is linked to specific classes of antibiotics. The objective of this study was to investigate the role of the Rcs regulon in response to treatment with antibiotics targeting either peptidoglycan synthesis or protein synthesis. We hypothesized that an rcsB deletion mutant E. coli K-12 strain would exhibit an increased antibiotic sensitivity phenotype when exposed to antibiotics inhibiting cell wall synthesis. Antibiotics inhibiting protein synthesis were not expected to elicit different antibiotic sensitivity phenotypes in wild-type or ΔrcsB strains because they were not anticipated to induce the Rcs phosphorelay response. We show that the presence of RcsB increases antibiotic tolerance to penicillin and phosphomycin but not to tetracycline or streptomycin. The wild-type and ΔrcsB strains showed no difference in susceptibility to classes of antibiotics inhibiting protein synthesis, yet exhibited different antibiotic sensitivity phenotypes to classes of antibiotics inhibiting cell wall synthesis. The ΔrcsB mutant showed a 4-fold decrease in penicillin resistance and a 2-fold decrease in phosphomycin resistance compared to the wild-type strain. We postulate that rprA functions downstream of RcsB, since RcsB is necessary to achieve full expression of rprA in the presence of penicillin. Taken together, this study suggests that RcsB contributes to intrinsic resistance of E. coli to antibiotic stressors perturbing the peptidoglycan synthesis. Moreover, RcsB appears to regulate rprA expression in response to treatment with penicillin. This study provides insight into intrinsic stress response mechanisms contributing to bacterial multidrug resistance.