Recent studies suggest that antibiotic exposure can trigger resilience in a significant fraction of the bacterial population. Resilience promotes tolerance, enabling prolonged survival at lethal concentrations of antibiotics despite the lack of genetic resistance. Mechanisms underlying this phenomenon remain poorly understood. In this work, we have investigated increased resilience in Escherichia coli following a brief exposure to rifampicin. Rifampicin-pretreatment induces a persister-like phenotype, with ∼1% of the total population surviving subsequent exposure to lethal doses of another antibiotic, ampicillin. Interestingly, these cells retain motility, indicating that they were not metabolically dormant. Using flow cytometry and ethidium bromide (EtBr) uptake assays, we observed a rapid decrease in intracellular EtBr accumulation within minutes of exposure to rifampicin. Because swimming speeds did not change, efflux amplification is unlikely. Furthermore, rifampicin-induced dye accumulation was decreased even in ΔacrA mutant lacking a major efflux pump. Therefore, we hypothesized that the dye accumulation decreased because of decreased outer-membrane permeability of the cells. Brief exposures to other antibiotics from the rifamycin class produced a similar effect. However, several other antibiotics failed to trigger the effect, suggesting specificity in response. Together, these results support a model in which rifampicin-exposure inhibits porin-mediated passive uptake. Our measurements with porin mutants were consistent with this hypothesis. If valid, this could explain how a large fraction of Gram-negative bacterial populations becomes resilient under antibiotic stress, suggesting a novel mechanism for antibiotic tolerance. Although rifamycin-class antibiotics are first-line treatments against Mycobacterium tuberculosis infections, our findings suggest they induce an unintended biophysical effect that potentially increases the odds of antibiotic resistance to a wide class of antibiotics in other species. Future insights into the mechanism of dynamic porin-inhibition may open new avenues for combating the rising threat of tolerance and antibiotic resistance.
Romit Bishayi (Sun,) studied this question.