Reduced Tetracycline Resistance when RpoS is Over-produced through Heat Shock in Escherichia coli

Abstract

The stationary phase sigma factor, RpoS, present in Escherichia coli is responsible for the phenomenon known as cross-protection where the cell acquires secondary resistance to a stress factor once exposed to an unrelated primary stress. Previous studies have shown contradictory observations based on the effects of RpoS overproduction where one study observed a positive outcome and another study observed an inhibitory outcome. Our study tested the possibility that low levels of RpoS contributed to positive cellular protection and higher levels contributed to inhibition of cell protection. RpoS was overexpressed in both a wild type strain, BW28357, and Δ rpoS, BW28465, E. coli strain though heat shock at 45 °C for various lengths of time. Minimum inhibitory concentration (MIC) assays with tetracycline were performed in order to determine conferred levels of protection. RpoS was then isolated and quantified using Western blot. Western blot results revealed an increase in RpoS concentration with prolonged heat shock. Western blot analysis coupled with MIC results revealed that protection against tetracycline was diminished as RpoS expression was upregulated beyond a certain level.