Identification of a black yeast isolated from oak bark as belonging to the genus Phaeococcomyces sp. Analysis of melanin produced by the yeast

Abstract

A black fungus isolated from oak bark was identified as a member of the yeast-like genus Phaeococcomyces. While no sexual reproduction was observed, the isolate showed characteristics associated with basidiomycetous yeasts: it was non-fermentative, produced extracellular urease, was positive with the diazonium blue B colony colour test, and had an enteroblastic form of budding. The isolate produced a black pigment constitutively which was shown to be a melanin. Production of the pigment was inhibited by the incorporation of low levels of the fungicide tricyclazole in growth media, inidicating that it was a pentaketide melanin. Pigmentation mutants were produced. Albino mutants did not produce melanin. Diffusion mutants accumulated the pentaketide melanin pathway oxidation products flaviolin and 2-hydroxyjuglone in culture media. Cross-feeder mutants accumulated scytalone, a pentaketide melanin pathway intermediate, in culture media, and caused albino mutants to melanize when grown in proximity to them on agar plates. A single mutant was isolated which excreted 1,8-dihydroxy naphthalene, the end product of the pathway. Broth cultures of Phaeococcomyces sp. in stationary growth phase released melanin in the form of granules, with an average diameter of 30 nm.Key words: black yeast, melanin production, Phaeococcomyces sp.