FIG 8.

Model of interactions between efflux and envelope permeability in SDS resistance. (A) In actively growing cells, SDS is able to penetrate the OM and is then removed by the AcrAB-TolC efflux pump. This energy-intensive process, which relies on the proton motive force, is made efficient by the abundance of nutrients available to the cell. (B) In carbon-limited cells, an RpoS-dependent decrease in OM permeability through pathways that involve SanA, PBP5, and YhdP prevents SDS from entering the cell. Early in treatment, any SDS that does penetrate the OM is removed by the AcrAB-TolC efflux pump; however, lack of energy in carbon-limited cells eventually leads the efflux pump to fail and causes a low level of cell death. When rpoS is removed, SDS penetrates the OM and inefficiency in the efflux pumps due to lack of energy causes the cells to die quickly. (C) In nitrogen-limited cells, an RpoS-dependent mechanism that is different from that employed in carbon-limited cells prevents SDS entry into the cell; however, the loss of rpoS can be compensated for by the activity of the AcrAB-TolC efflux pump, suggesting that although there is a lack of available nitrogen for making new proteins, nitrogen-limited cells have sufficient energy available in the form of the proton motive force for efficient efflux over the time scale of the experiment.