The role of melanin in Aspergillus tolerance to biocides and photosensitizers

Cationic biocides are widely utilized for surface disinfection. Photosensitizers such as toluidine blue O (TBO) produce reactive oxygen species following light excitation and are being investigated as novel biocides for similar applications. Aspergillus brasiliensis conidia contain melanin which protects against environmental stressors. The negative charge and antioxidant properties of melanin may confer resistance to photosensitizers and other biocides. In this study, the yeasticidal and fungicidal activity benzalkonium chloride (BZC), sodium dichloroisocyanurate (NaDCC) and TBO with red light were examined using quantitative suspension tests. All three biocides were highly effective against Candida albicans and > 5 center dot 0 log(10) reductions in viability were attainable within 5 minutes. Wild-type A. brasiliensis conidia were highly tolerant to treatment and 0 center dot 4 log(10) reductions in viability were observed within the same time frame when treated with TBO or BZC. NaDCC was markedly more effective. Inhibition of melanin biosynthesis by culturing with 100 mu g ml(-1) kojic acid resulted in a hypopigmented phenotype with significantly increased sensitivity to all three biocides. These observations indicate that melanin is a significant contributor towards A. brasiliensis tolerance of biocides and photosensitizers and demonstrate that cationic biocides are poorly suited to applications where the control of A. brasiliensis is required.

Automated summary

Cationic biocides are widely used for surface disinfection, while photosensitizers like TBO are being investigated as novel biocides. Melanin in Aspergillus brasiliensis conidia confers resistance to biocides and photosensitizers. Benzalkonium chloride, sodium dichloroisocyanurate, and TBO were highly effective against Candida albicans, but A. brasiliensis conidia showed tolerance to TBO and BZC, with NaDCC being the most effective. Inhibition of melanin biosynthesis increased sensitivity to all three biocides.