Cross‐feeding constrains the evolution of antibiotic resistance in <i>Escherichia coli</i>

The rise of antibiotic‐resistant bacteria poses a critical global health threat. Previous studies on the evolution of bacterial antibiotic resistance have often overlooked the fact that bacteria frequently exist within complex microbial communities, where they engage in mutually beneficial interactions through nutrient exchange. Here, we studied the evolution of antibiotic resistance in an obligate cross‐feeding population. After 15 days of selection under kanamycin, both co‐cultures and mono‐cultures of auxotrophic strains evolved significantly higher resistance. The increase in resistance was significantly lower in the co‐culture consortium than in each mono‐culture consortium. Strains in the co‐cultures showed higher levels of amino acid secretion and a reduced growth rate than those in the mono‐cultures. This suggests a higher cost of amino acid exchange in the co‐cultures. Exogenous amino acids supplementation reduced the cost of metabolic exchange and intensified competition between the two strains due to niche overlap. This further resulted in decreased kanamycin resistance in both the mono‐evolved and the co‐evolved populations under the kanamycin‐free environment. The interaction among the two auxotrophic strains shifted from positive to negative in co‐evolved population. This supports the stress gradient hypothesis, which posits that facilitation is more common in stressful environments, whereas competition dominates when conditions are more benign. Our findings demonstrate that environmental stress and nutrient availability can modulate bacterial antibiotic resistance by altering microbial interactions and fitness dynamics within communities.