Understanding the Antimicrobial Mechanism of Functionalised Polyanilines

Abstract

The implantation of medical devices into patients is an essential component of modern medical care; however these devices significantly increase the risk of infection due to biofilm formation on outer and/or inner surface. Impregnating implanted devices with an antimicrobial material could circumvent these infections and the related problems. Polyaniline (PANI) and its functionalised derivatives (fPANI) are a novel class of antimicrobial agents with potential as non-leaching additives to materials and coatings to provide contamination resistant surfaces. A transcriptomic analysis of Escherichia coli has suggested a mechanism of action targeting certain respiratory enzymes, leading to oxidative stress and membrane stress responses in cells challenged with fPANI. E. coli and Staphylococcus aureus mutants lacking elements of the responses to oxidative and membrane stress are more sensitive to fPANI while those lacking respiratory enzymes exhibit decreased sensitivity. To further probe the mechanism of action of fPANI we have used electron paramagnetic resonance (EPR) spectroscopy to demonstrate that an increase in free intracellular iron levels occurs as a result of sublethal fPANI treatment. We believe this is part of the damage occurring that leads to oxidative stress. The generation of reactive oxygen species (ROS) leading to oxidative stress appears to occur within the bacterial cell, as we do not detect ROS in media treated with fPANI, and we cannot protect cells from lethal fPANI challenges by the inclusion ROS scavengers. Damage to bacterial cell membranes by sublethal fPANI treatment is corroborated by substantial reduction in viable cell recovery for fPANI-exposed cells on agar with high salinity. Furthermore, sublethally treated bacterial cells displayed a loss of membrane potential which may be indicative of damage to respiratory enzymes and/or membrane pumps. The work presented suggests that P3ABA kills bacterial cells through three general mechanisms – inducing oxidative stress, mediating membrane damage and causing dissipation of membrane potential – which partially overlap and integrate to mediate loss of cell viability.