The role of melanin in Aspergillus tolerance to biocides and photosensitizers.

M.J Pascoe,J.‐Y Maillard

doi:10.1111/lam.13437

Abstract

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>5·0 log10 reductions in viability were attainable within 5minutes. Wild-type A. brasiliensis conidia were highly tolerant to treatment and 0·4 log10 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 100μgml-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.

Highlights

Photosensitizers are compounds which produce reactive oxygen species (ROS) following excitation by visible light
The three tested biocides were highly effective against suspensions of Candida albicans (Fig. 1a)
As the observed reduction in viability was above the 4 log10 threshold required to substantiate claims of yeasticidal activity under the EN 13624 standard, a combination of Toluidine blue O (TBO) and red light may be suitable for applications in which the microbiological control of yeasts is required

Summary

Introduction

Photosensitizers are compounds which produce reactive oxygen species (ROS) following excitation by visible light. Sub-inhibitory concentrations of antifungal compounds such as kojic acid and tricyclazole have been observed to inhibit melanin biosynthesis in Gaeumannomyces (Elliott 1995) and Aspergillus species (Pal et al 2014). The stark differences in observed efficacy between the two fungal species may be related to the abundance of melanin granules in the cell walls of A. brasiliensis conidia, which are absent in C. albicans.

Results

Conclusion