Fig. 6. Proposed model of PMF-mediated development of starvation-induced tolerance.

a Maintaining PMF is essential for the prolonged survival of starvation-induced tolerant cells. Although bacteria may not be able to support a strong PMF when entering the tolerance phase, they nevertheless maintain a basic PMF level upon encountering starvation for a prolonged period. Efflux activities driven by PMF extrude β-lactams to facilitate tolerance formation; other membrane protein activities which presumably involve the import/export of specific metabolites/nutrients are supported by PMF and are also important for maintaining a tolerance phenotype. b Effect of PMF dissipators such as CCCP on tolerant cell killing. PMF dissipators cause rapid dissipation of bacterial membrane PMF and hence inhibition of ATP production, which in turn affects a series of cellular functions that are involved in maintaining the tolerance phenotype, leading to the killing of the tolerant cells. c Effect of PMF dissipator and ampicillin on tolerant cell killing. Tolerant cells are eradicated more effectively in the presence of β-lactam if PMF cannot be maintained under starvation stress. Dissipation of bacterial membrane PMF suppresses antibiotic efflux activities, leading to accumulation of antibiotics in the periplasm of tolerant cells. The accumulation of antibiotics and arrest of other cellular functions lead to the more effective killing of tolerant cells. d Complete dissipation of PMF with or without the presence of antibiotics results in the death of tolerant cells.