Roles of RpoS and Efflux Pumps in Strengthening the Permeability Barrier of Stationary Phase Escherichia coli Cells

Abstract

Many bacteria in our environment, including Escherichia coli live in nutrient-limited conditions. When E. coli cells enter stationary phase in nutrient-limited conditions, the cell envelope undergoes changes that increase resistance to stress. RpoS-dependent gene expression and the export of toxic compounds by the AcrAB-TolC efflux pump are major mechanisms by which the cell envelope is strengthened. The strength of the cell envelope can be measured by testing SDS resistance. A previous study showed that carbon-limited cells mainly rely on RpoS for SDS resistance, while nitrogen-limited cells can use either RpoS or efflux. Based on these findings, we studied the roles of RpoS and the energy-dependent AcrAB-TolC efflux pump in cells that are starved of a sulfur source. Wild-type cells are fully resistant to SDS. acrA and rpoS single mutants are less SDS-resistant than wild-type cells, and acrA rpoS double mutants show severe sensitivity to SDS. These results suggest that in sulfur-limited cells, RpoS and the AcrAB-TolC efflux pump only partially compensate for each other when one is lost. Both are necessary for full SDS resistance. RpoS levels may be insufficient to fully compensate for a loss of efflux, and energy may be insufficient for efflux pumps to fully compensate for RpoS loss. Based on a previous finding that carbon-limited cells primarily use RpoS for SDS resistance, our second aim was to identify RpoS-dependent genes that confer SDS resistance in carbon-limited cells. To do so, we isolated suppressors of SDS sensitivity of an rpoS-deleted strain. Two suppressors were mapped to the lon gene, and one was mapped to the hns gene. H-NS and Lon are global regulators of gene expression and protein stability that help control stress responses. Our findings provide insight into changes that occur in the E. coli cell envelope in nutrient-limited conditions.