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Abstract
In Escherichia coli, RpoS is a global gene expression regulator that is up-regulated under environmental stress conditions such as low pH and high osmolarity. Previous studies o have shown that exposure to a high growth temperature of 42 C may have resulted in elevated RpoS levels which then conferred resistance to low pH environment. Likewise, preexposure to sub-lethal low pH has been shown to increase resistance of E. coli to antibiotics, including ceftriaxone. In this study, the role of RpoS in acid-induced cross-protection against ceftriaxone was investigated by subjecting wild-type E. coli strain BW25113 and Δ rpoS strain JW5437-1 to sub-lethal low pH growth environment followed by sub-lethal ceftriaxone treatment. It was observed that acid-stress increased RpoS levels. However, this up-regulation does not confer antibiotic resistance against ceftriaxone by minimum inhibitory concentration (MIC) assays. The Δ rpoS strain grown in low pH MHB had greater resistance against ceftriaxone compared to non acid-stressed control samples whereas the wild-type strain demonstrated no difference in resistance against ceftriaxone between acidstressed and non acid-stressed control samples. This result suggested that other gene regulatory mechanism may be involved in acid-induced antibiotic cross-protection.