(154a) Biofilm Growth Drives the Selective Targets and Trajectories during the Evolution of Antimicrobial Resistance

Acinetobacterbaumanniiis a persistent nosocomial pathogen categorized as one of the highest threats to patient safety. Part of its persistence is due to the ability to live in protected surface communities (biofilms) and evolve resistance to antibiotics, making the understanding of these evolutionary strategies a top public health priority. Using the susceptible A. baumanniiATCC 17978 as a model strain, we combined experimental evolution, antibiotic susceptibility tests, and whole genome sequencing to investigate different antibiotic-specific evolutionary pathways. We exposed both a biofilm and a well-mixed planktonic population to two different environments: i) sub-inhibitory concentrations of ciprofloxacin and ii) steadily increasing concentrations of ciprofloxacin. Our work demonstrates that bacteria evolve along distinct evolutionary pathways depending upon the mode of growth. Biofilm adapted lineages acquire mutations in three RND efflux pump regulators to mitigate antibiotic exposure whereas planktonic adapted lineages also acquire mutations in the target of ciprofloxacin, DNA gyrase. The biofilm-adapted lineages are more diverse, indicating spatial structure reduces clonal interference and can lead to greater odds of surviving fluctuating environmental stressors. Additionally, there is a trade-off between fitness cost and resistance level conferred by the adaptations in the different lifestyles and those lifestyle-dependent resistance pathways are related with cross-resistance and collateral sensitivity interactions between different antibiotics. We argue that the mode of growth (biofilm or planktonic) is an underappreciated yet key factor in the evolution of antibiotic resistance.