Abstract: Antibiotic resistance is a major global health challenge, and antibiotic-tolerant cells further complicate treatment by contributing to infection relapse and facilitating resistance mutations. While tolerant cells can evolve into resistant strains, their phenotypic and genetic characteristics remain largely unexplored. While bacteria develop various mutational strategies to survive drug treatments, the effects of antibiotic tolerance and resistance on bacterial fitness are not fully understood. Moreover, it remains uncertain whether evolved mutants exhibit similar traits across distinct cell populations exposed to identical conditions. In this study, we utilized
Escherichia coli as a model organism to investigate the evolutionary dynamics of antibiotic mediated adaptation. By applying the fluoroquinolone antibiotic (ofloxacin) under controlled laboratory conditions, we conducted adaptive laboratory evolution (ALE) experiments to explore how bacterial populations respond over time to intermittent antibiotic exposures. Our findings reveal that both ofloxacin tolerance and resistance can arise independently in different
E. coli populations, even when all populations are exposed to identical environmental. This highlights the complexity and variability of bacterial adaptive responses and underscores the potential for diverse evolutionary outcomes in seemingly uniform conditions. Also, we characterized key fitness parameters, including lag score, doubling time, competition score, and various metabolic traits (such as redox activities and intracellular ATP levels), and did not detect strong correlations between the observed fitness trade-offs and the development of antibiotic tolerance or resistance. To gain deeper insight into the molecular mechanisms driving tolerance and resistance, we conducted whole-genome sequencing of the mutant strains. While some common mutations were identified, such as single-nucleotide polymorphisms (SNPs) in the
icd gene, a key component of the citric acid cycle, most cell populations exhibited a wide array of genetic alterations. These mutations appeared highly diverse, with no consistent pattern indicative of a conserved evolutionary trajectory. Our study also reveals distinct phenotypic variations, including strains that exhibit significantly lower minimum inhibitory concentration (MIC) levels than the parental strain while simultaneously demonstrating exceptionally high tolerance to the same antibiotic. Overall, this study explores the phenotypic and mutational landscapes of fluoroquinolone-induced strains, providing valuable insights into the intricate mechanisms of bacterial adaptation. This study is currently under review in the
mSystems journal of the
American Society for Microbiology (ASM) and is also available as a preprint on
bioRxiv (
1).
References
- Mohiuddin, S. G., Kavousi, P., Figueroa, D., Ghosh, S., & Orman, M. A. (2024). The Diverse Phenotypic and Mutational Landscape Induced by Fluoroquinolone Treatment. bioRxiv, 2024-12.
