Antifungal Polymeric Materials and Nanocomposites

Abstract

Rising global populations due to medicinal advancements increases the patient population susceptible to superficial and severe fungal infections. Fungi often implicated in these diseases includes the dermatophytes (Microsporum spp., Epidermophtyon spp., Trichophyton spp.) as well as species of the Candida spp., Aspergillosis spp. and Cryptococcus spp. genera. In addition, increasing global populations leads to increasing agricultural demands. Thus, fungal infections of preharvested crops and stored food by plant pathogens such as Magnaporthe oryzae and Fusarium oxysporum can have detrimental socioeconomic effects due to food insecurity. Current antifungal strategies are based mainly on small molecule antifungal drugs. However, these drugs are limited by poor solubility and bioavailability. Furthermore, antifungal resistance against these drugs are on the rise. Thus, antimicrobial polymers offer an alternative antifungal strategy. Antifungal polymers are characterised by cationic and hydrophobic regions where the cationic regions have been shown to interact with microbial phospholipids and membranes. These polymers can be synthetic or natural and demonstrate distinct antifungal mechanisms ranging from fungal cell membrane permeabilisation, cell membrane depolarisation or cell entry. Although the relative importance of such mechanisms is difficult to decipher. Due to the chemical properties of these polymers, they can be combined with other antimicrobial compounds including existing antifungal drugs, charcoals, lipids and metal ions to elicit synergistic effects. In some cases, antifungal polymers and nanocomposites show better antifungal effects or reduced toxicity compared to the widely used small molecule antifungal drugs. This review provides an overview of antimicrobial polymers and nanocomposites with antifungal activity and the current understanding of their antifungal mechanisms.

Keywords: antifungal; antifungal polymers; antimicrobial polymers; fungal infections; nanocomposites.

FIGURE 1

Summary of the proposed targets and mechanisms of antifungal polymers and nanocomposites. (A) Cationic polymers and their derivatives disrupt the fungal cell membrane. Upon entry into the fungal cell, they also disrupt organelle membranes and bind to DNA. Some antifungal nanocomposites with membrane disrupting compounds e.g. Chlorhexidine diacetate salt (CDA) also show a similar effect. (B) Some nanocomposites demonstrate an antifungal effect by forming deposits within grooves of the material surface to minimise fungal adhesion. (C) Inert polymers can be functionalised following the addition of antimicrobial compounds. Some functionalised polymers e.g. charcoal polymers inhibit fungal growth by removing essential ions (e.g. Ca2+) necessary for conidia germination. (D) Some antimicrobial polymers e.g. PQ-1 do not permeabilise the fungal cell membrane. Instead, they disrupt the membrane via intercalation.