The Effect of Osmotic Shock on RpoS Expression and Antibiotic Resistance in Escherichia coli

The general stress response regulator σ, commonly known as RpoS, is a sigma subunit of RNA polymerase that is up-regulated when bacteria are exposed to environmental stresses such as osmotic shock, high temperature, UV radiation, and feast-famine conditions. Current knowledge is that a single stress condition can provide stress tolerance and can also lead to cross-protection against unrelated challenges. Exposure to sublethal levels of osmotic stress has been found to increase antibiotic resistance in Escherichia coli. In this study, we sought to elucidate whether this cross-protection is conferred through an RpoSdependent manner. Wild type (WT) and ∆rpoS E. coli strains were subjected to continuous 0.3 M NaCl osmotic shock conditions followed by subsequent exposure to either ampicillin, chloramphenicol, or rifampicin to determine the minimum inhibitory concentration (MIC) of these antibiotics. RpoS levels were determined by Western immunoblotting with a monoclonal RpoS-specific antibody, however no difference was observed between levels of RpoS induction under normal and osmotic shock conditions. RpoS was found to have some effect on antibiotic resistance as the WT strain had greater antibiotic resistance under normal M9 conditions than the ∆rpoS strain. Mechanistic differences between modes of resistance for different antibiotics may also exist. No cross-protection against antibiotics with continuous osmotic shock via RpoS-dependent mechanisms was found as osmotic shock was seen to increase antibiotic susceptibility in both the ∆rpoS and WT strains.