To determine the dosage at which antibiotic resistance evolution is most rapid, we treated Escherichia coli
in vitro, deploying the antibiotic erythromycin at dosages ranging from zero to high. Adaptation was fastest
just below erythromycin’s minimal inhibitory concentration (MIC) and genotype-phenotype correlations
determined ...
To determine the dosage at which antibiotic resistance evolution is most rapid, we treated Escherichia coli
in vitro, deploying the antibiotic erythromycin at dosages ranging from zero to high. Adaptation was fastest
just below erythromycin’s minimal inhibitory concentration (MIC) and genotype-phenotype correlations
determined from whole genome sequencing revealed the molecular basis: simultaneous selection for copy
number variation in 3 resistance mechanisms which exhibited an ‘inverted-U’ pattern of dose-dependence,
as did several insertion sequences and an integron. Many genes did not conform to this pattern, however,
reflecting changes in selection as dose increased: putative media adaptation polymorphisms at zero antibiotic
dosage gave way to drug target (ribosomal RNA operon) amplification at mid dosages whereas prophage-mediated drug efflux amplifications dominated at the highest dosages. All treatments exhibited E. coli
increases in the copy number of efflux operons acrAB and emrE at rates that correlated with increases
in population density. For strains where the inverted-U was no longer observed following the genetic
manipulation of acrAB, it could be recovered by prolonging the antibiotic treatment at sub-MIC dosages.