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Efflux-linked Accelerated Evolution of Antibiotic Resistance at a Population Edge

Efflux is a common mechanism of resistance to antibiotics. We show that efflux itself promotes accumulation of antibiotic-resistance mutations (ARM). This phenomenon was initially discovered in a bacterial swarm where the linked phenotypes of high efflux and high mutation frequencies spatially segregated to the swarm edge, driven there by motility. We uncovered and validated a global regulatory network that connects high efflux to downregulation of specific DNA repair pathways even in non-swarming states. The efflux-DNA repair link was corroborated in a clinical ‘resistome’ database: genomes with mutations that increase efflux exhibit a significant increase in ARMs. Accordingly, efflux inhibitors decreased evolvability to antibiotic resistance. Swarms also revealed how bacterial populations serve as reservoir of ARMs even in absence of antibiotic selection pressure. High efflux at the edge births mutants that, despite compromised fitness, survive there because of reduced competition. This finding is relevant to biofilms where efflux activity is high.