Cross-Feeding Does Not Affect the Evolution of Antibiotic Tolerance

Abstract

Antibiotic tolerance refers to the ability of bacteria to survive short periods of high antibiotic concentration. This is distinct from antibiotic resistance, the ability of bacteria to survive indefinitely under a limited antibiotic concentration. Emerging evidence suggests antibiotic tolerance may be supported by increases in lag time, a delay before growth that can be quantified by the time taken for colonies to become visible. While tolerance is studied less than resistance, ecological influences from between-species interactions upon it are even less frequently studied. Cross-feeding is one such interaction, and a common occurrence in nature. To investigate the impact of cross-feeding on tolerance, the evolution of antibiotic tolerance was induced in both Escherichia coli monoculture and a mutualistic coculture of E. coli and Salmonella enterica. Cyclic exposure to ampicillin followed by growth in antibiotic-free media yielded evolved isolates of E. coli with delayed colony appearance time but no change in growth rate, minimum inhibitory concentration (MIC), or competitive advantage against the ancestral strain. However, the rate of tolerance evolution was unaffected by culture type. Additionally, a computational population model was able to replicate culture-independent tolerance evolution when lag time was the only mutable trait. Antibiotic tolerance appears to be uniquely unaffected by species interactions, demonstrating the need for ecological considerations in the study of tolerance and resistance.