FIG 6.

RelA functionality does not determine the norfloxacin tolerance of BW25113 E. coli induced by antibiotic pretreatment. The antibiotic pretreatment was performed for 30 min at 37°C in MOPS media supplemented with 0.4% glucose and amino acids at 25 μg/ml using BW25113 E. coli wild-type strain (A and B) and an isogenic relA knockout (ΔrelA; C and D), followed by the addition of norfloxacin to a final concentration of 5 μg/ml, and the surviving fraction was determined by LB plating and colony counting. 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). P values were calculated using a two-tailed Welch's t test between wild-type and relaxed strains.