Role of melanin in the black yeast fungi Aureobasidium pullulans and Zalaria obscura in promoting tolerance to environmental stresses and to antimicrobial compounds

Abstract

The role of melanin in Aureobasidium pullulans ATCC 15233 and Zalaria obscura LS31012019, under simulated osmotic, oxidative, and high temperature stress conditions, on the susceptibility to essential oils (EOs) or antifungals and on the resistance to UV-C radiation was investigated. 93.6% of melanized A. pullulans and 92% of Z. obscura survived to 40 °C for 1 h compared to 77% and 76% of the non-melanized ones, while both yeasts tolerated a high concentration of NaCl (up to 30%) and H 2 O 2 (up to 400 mM) regardless of melanin production. Higher EOs antifungal efficacy was observed in non-melanized cells (growth inhibition zone >30 mm) compared to the melanized ones (25 mm). Similarly, the lowest Minimum Inhibitory Concentrations (MIC) and Minimum Fungicidal Concentration (MFC) values were evidenced for Fluconazole, Clotrimazole, Bifonazole and Amphotericin in the non-melanized fungi. Increasing UV-C intensity (up to 2004.5 J/m 2 ) caused total death in the non-melanized strains compared to about 30% growth reduction in the melanized ones. The results of this investigation, the first focused on the biological role of melanin in "black-fungi", are novel and encourage a better understanding of the biochemical features of melanin in the environmental adaptive ability of the new species Z. obscura.