Fungal cell barriers and organelles are disrupted by polyhexamethylene biguanide (PHMB)

The similarities between fungal and mammalian cells pose inherent challenges for the development of treatments for fungal infections, due to drug crossover recognition of host drug targets by antifungal agents. Thus, there are a limited number of drug classes available for treatment. Treatment is further limited by the acquisition and dissemination of antifungal resistance which contributes to the urgent need of new therapies. Polyhexamethylene biguanide (PHMB) is a cationic antimicrobial polymer with bactericidal, parasiticidal and fungicidal activities. The antifungal mechanism of action appears to involve preferential mechanical disruption of microbial cell structures, offering an alternative to conventional antifungals. However, the antifungal mechanisms have been little studied. The aim of this study was to characterise PHMB's activities on selected yeast (Saccharomyces cerevisiae, Candida albicans) and filamentous fungal species (Fusarium oxysporum, Penicillium glabrum). Fungal membrane disruption, cell entry and intracellular localisation activities of PHMB were evaluated using viability probe entry and polymer localisation studies. We observed that PHMB initially permeabilises fungal cell membranes and then accumulates within the cytosol. Once in the cytosol, it disrupts the nuclear membrane, leading to DNA binding and fragmentation. The electrostatic interaction of PHMB with membranes suggests other intracellular organelles could be potential targets of its action. Overall, the results indicate multiple antifungal mechanisms, which may help to explain its broad-spectrum efficacy. A better understanding of PHMB's mechanism(s) of action may aid the development of improved antifungal treatment strategies.