Heterogeneous efflux pump expression underpins phenotypic resistance to antimicrobial peptides

Antimicrobial resistance threatens the viability of modern medical interventions. It is imperative to develop new strategies to combat resistance mechanisms utilised by bacterial pathogens especially when they enter starved or slow-growing states where they become more refractory to conventional antimicrobial treatment. Antimicrobial peptides have been advocated as a potential therapeutic solution due to the low levels of genetic resistance observed in bacteria against these compounds. Here we show that subpopulations of stationary phase Escherichia coli and Pseudomonas aeruginosa survive tachyplesin treatment without the emergence of genetic mutations. These phenotypic variants selectively induce efflux, secretion of outer membrane vesicles, membrane modifications and proteolytic activity in response to tachyplesin exposure, effectively retaining the peptide only in their membranes where it cannot exert its antimicrobial activity. We discovered that the formation of these phenotypic variants can be prevented by administering tachyplesin in combination with the antidepressant sertraline, suggesting a novel approach for combatting antimicrobial-refractory stationary phase bacteria.