Understanding the Antimicrobial Mechanism for Applications of Polyaniline and Functionalised Polyanilines

Abstract

The discovery that the conducting polymer polyaniline (PANI) and its functionalised derivative poly(3-aminobenzoic acid), P3ABA, are antimicrobial has presented new possibilities for their application. An understanding of the antimicrobial mechanism will support the pursuit of these new applications. The mode of action of PANI and P3ABA was first investigated using E. coli single gene deletion mutants. This investigation was based on the idea that bacteria carrying mutations in genes encoding antimicrobial targets will have reduced sensitivity to treatment, while the opposite occurs for bacteria carrying mutations in genes encoding antimicrobial stress-response mediators. Following this, the involvement of reactive oxygen species, and targeting of metabolic and respiratory machinery, in the antimicrobial action of PANI and P3ABA were explored by determining activity against target bacteria in aerobic and anaerobic conditions, and in rich and minimal media. The antimicrobial mechanism of polyaniline is hypothesised to involve production of hydrogen peroxide and dysregulation of iron homeostasis, a notion that is supported by the supersensitivity of a hydrogen peroxide scavenger mutant to PANI treatment; the insensitivity of an iron import mutant to PANI treatment; and protection against PANI killing of E. coli and S. aureus in anaerobic conditions. The antimicrobial activity of P3ABA is different and is postulated to involve targeting of ATP synthase, uncoupling electron transport from ATP synthesis, resulting in a futile proton cycle and loss of proton motive force. The insensitivity of an ATP synthase deletion mutant to P3ABA treatment supported this hypothesis, along with the greater activity of P3ABA against E. coli in rich media (with increased respiration rates) compared to minimal media. P3ABA antimicrobial action is also hypothesised to involve perturbation of iron homeostasis, supported by the increased sensitivity of a deletion mutant missing a gene involved in regulation of iron levels, and acid stress, supported by the increased sensitivity of a deletion mutant missing an acid stress response mediator. The protection of surfaces from microbial contamination as part of measures to reduce transmission of pathogens in hospitals and during food processing is one potential application. The incorporation of PANI and P3ABA into model absorbent and non-absorbent materials gave a profile whereby PANI was effective at protecting absorbent surfaces from E. coli, S. aureus and M. smegmatis, while non-absorbent surfaces offered no protection against these organisms. P3ABA was able to reduce contamination of E. coli, S. aureus, M. smegmatis and M. tuberculosis on absorbent surfaces while non-absorbent surfaces containing P3ABA were effective against E. coli, M. smegmatis and M. tuberculosis. The bactericidal activity of PANI and P3ABA containing non-absorbent surfaces was most active against low bacterial loads and in the absence of organic matter.