The Diverse Phenotypic and Mutational Landscape Induced by Fluoroquinolone Treatment

Despite extensive research on antibiotic resistance, the potential effects of antibiotic treatments on bacterial tolerance and resistance remain a significant concern. Although bacterial cells adopt a variety of mutational strategies to resist unfavorable circumstances, it is still unclear how antibiotic tolerance and resistance mechanisms affect bacterial fitness characteristics and whether evolved mutants exhibit similar properties across different cell populations subjected to the same conditions. Here, we used Escherichia coli, a fluoroquinolone antibiotic (ofloxacin), and adaptive laboratory evolutionary experiments to demonstrate that ofloxacin tolerance and resistance can evolve independently across different cell populations exposed to identical conditions. Fitness attributes, such as lag score, doubling time, competition score, and other metabolic features, were variably affected by antibiotic tolerance and resistance mechanisms. However, we did not observe strong and apparent correlations between fitness trade-offs and antibiotic tolerance and resistance. While our whole-genome sequencing identified some shared mutations, such as single nucleotide polymorphisms in the icd gene (a crucial citric acid cycle gene), evolved cell populations exhibited diverse genetic mutations without a clear pattern of a conserved evolutionary pathway. Our study also identifies unique phenotypes, such as those displaying significantly lower minimum inhibitory concentration levels compared to the parental strain yet showing remarkably high tolerance to the same antibiotic. Altogether, our study, examining the phenotypic and mutational landscapes of fluoroquinolone-induced strains, contributes to our understanding of complex bacterial adaptation mechanisms.