FIG 5.

Antibiotic pretreatment induces both relA-dependent and relA-independent ampicillin tolerance of BW25113 E. coli. The antibiotic pretreatment was performed for 30 min at 37°C in MOPS medium supplemented with 0.4% glucose and amino acids at 25 μg/ml using BW25113 E. coli wild-type strain (A to C) and an isogenic relA knockout (ΔrelA; D to F), followed by the addition of ampicillin to a final concentration of 200 μg/ml. The surviving fraction was determined by LB plating and colony counting. The bacteriostatic antibiotics were used at concentrations reducing the growth rate by half, and concentrations are indicated in μM on the figures, e.g., mup₇₀ indicates pretreatment with 70 μM mupirocin. Error bars indicate the standard errors of the mean (three to five biological replicates). Where indicated by brackets, the P values were calculated by using a two-tailed Welch's t test relative to the killing time course of an untreated culture.