Electrochemical disruption of extracellular electron transfer inhibits Pseudomonas aeruginosa cell survival within biofilms and is synergistic with antibiotic treatment

Abstract

Survival of cells within oxygen-limited regions in Pseudomonas aeruginosa biofilms is enabled by using small redox active molecules as electron shuttles to access distal oxidants. This respiratory versatility makes P. aeruginosa biofilms common in chronic wound infections and recalcitrant to treatment. Here, we show that electrochemically controlling the redox state of these electron shuttles, specifically pyocyanin, can impact cell survival within anaerobic P. aeruginosa biofilms and can act synergistically with antibiotic treatment. We inhibited pyocyanin redox cycling under anoxic conditions by blocking its ability to be re-oxidized and thus serve as an electron shuttle via poising an electrode at a reductive potential that cannot regenerate oxidized pyocyanin (i.e. −400mV vs Ag/AgCl). This resulted in a decrease in CFUs within the biofilm of 100x compared to samples exposed to an electrode poised at an oxidizing potential that permits pyocyanin re-oxidation (i.e. +100mV vs Ag/AgCl). Phenazine-deficient Δphz* biofilms were not affected by the redox potential of the electrode, but were re-sensitized by adding pyocyanin. The effect of EET disruption was exacerbated when biofilms were treated with sub-MICs of a range of antibiotics. Most notably, 4 μg/ml of the aminoglycoside gentamicin in a reductive environment almost completely eradicated wild type biofilms but had no effect on the survival of Δphz* biofilms, suggesting reduced phenazines are toxic, and combined with antibiotic treatment can lead to extensive killing.ImportanceBiofilms provide a protective environment but they also present challenges to the cells living within them, such as overcoming diffusion limitation of nutrients and oxygen. Pseudomonas aeruginosa overcomes oxygen limitation by secreting soluble redox active molecules as electron shuttles to access distal oxygen. Here, we show that electrochemically blocking the redox cycling of one of these electron shuttles, pyocyanin, decreases cell survival within biofilms and acts synergistically with gentamicin to kill cells. Our results highlight the importance of the role that the redox cycling of electron shuttles fulfills within P. aeruginosa biofilms.

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