Interspecies interaction alters the trajectory of antibiotic resistance evolution by amplifying negative fitness epistasis.

Interspecies interactions can influence the physiology of competing species, shaping their long-term evolutionary trajectories. Although interspecific competition's role in community dynamics is well-documented, its impact on evolutionary outcomes and mechanisms is less explored. Here, we investigate how interspecies competition affects antibiotic resistance evolution in the gut pathogen Salmonella enterica within synthetic microbial communities. Specifically, we examine how the presence of an interspecific competitor, Escherichia coli, modulates resistance evolution at low streptomycin concentrations. Our findings reveal that interspecies competition results in the selection of S. enterica mutants with higher resistance levels by increasing the likelihood of accumulating resistance mutations that follow a trajectory of negative fitness epistasis. We show that this effect is driven by the enhanced expression of the cryptic aminoglycoside transferase gene (aadA). Our study thus links antibiotic resistance evolution to competition-induced physiological changes, emphasizing the interplay between interspecies interaction and adaptation to environmental conditions.