Hotspot Dosages of Most Rapid Antibiotic Resistance Evolution

We treated Escherichia coli with the antibiotic erythromycin from zero to high dosages to determine how the evolutionary dynamics of antibiotic resistant phenotypes and genotypes depend on dose. The most rapid increase in resistance was observed just below erythromycin’s minimal inhibitory concentration (MIC) and genotype-phenotype correlations determined from whole genome sequencing revealed the molecular basis of this: simultaneous selection for copy number variation in 3 resistance mechanisms which shared an ‘inverted-U’ pattern of dose-dependent selection with several insertion sequences and an integron. Many genes did not conform to this pattern, however, because of changes in selection as dose increased: media adaptation at zero-to-low dosages gave way to drug target (ribosomal RNA operon) amplification at mid dosages whereas prophage-mediated drug efflux dominated at higher dosages where population densities were lowest. All dosages saw E. coli amplify the efflux operons acr and emrE at rates that correlated strongly with changes in population density that exhibited an inverted-U geometry too. However, we show by example that inverted-U geometries are not a universal feature of dose-resistance relationships.