Fig 5.

Emergence of high-level drug resistance follows low-level resistance mediated by efflux pumps. (A) On day 7, a low-level drug-resistant subpopulation was encountered. The subpopulation is a phenotype that grows on agar supplemented with 3× MIC (96 mg/liter) but not on agar supplemented with 256 mg/liter azithromycin. Since some of the population did not grow on plates supplemented with the efflux pump inhibitor thioridazine at 1 mg/liter (which has no effect on the microbial kill on its own), this means that efflux pumps accounted for a substantial portion of the resistance. The scale for the bacterial burden is a log scale, so what looks like small differences on this scale are relatively large on a linear scale. (B) By day 28, this low-level-resistant subpopulation had increased substantially, as had the proportion that could be inhibited by the efflux pump inhibitor. (C) A high-level-resistant subpopulation (which grew on 256 mg/liter azithromycin) had emerged by day 28 in some HFSs. Thioridazine had no effect on the size of that population. Neither a low-level- nor a high-level-resistant azithromycin subpopulation was encountered in untreated controls (not shown). The error bars indicate SD.