De novo acquisition of antimicrobial resistance in six bacterial species from the food chain

Antimicrobial resistance evolves through diverse genetic routes, yet often yields similar phenotypic outcomes. Using experimental evolution across six bacterial species and multiple antibiotics, we show that resistance arises through distinct, species-specific trajectories with limited mutational convergence. We further demonstrate that regulatory mutations and copy number variations can accelerate resistance evolution but are frequently reversible when selection is removed. Analysis of collateral sensitivity and cross-resistance networks across evolved populations reveals little conservation between species, underscoring the context dependence of resistance pathways. Overall, our findings highlight the diversity, reversibility, and evolutionary instability of genetic changes underlying antimicrobial resistance, offering principles that may guide strategies to slow resistance emergence in clinical and veterinary settings.