Enhancing the production of secreted melanin by Streptomyces spp. using common tyrosine-containing media

Soil-isolated bacterial strains producing black pigment, identified as Streptomyces spp. strains AQ2 and CQ2 through 16S rRNA gene analysis, exhibited distinct morphological characteristics. Among the tested culture media, potato dextrose agar (PDA), which contains a high concentration of the tyrosine precursor, induced the highest production of black pigment compared to malt extract agar (MEA), yeast malt extract agar (YMA), oatmeal agar (ISP3), and glycerol asparagine agar (ISP5). FTIR analysis confirmed that the black pigment closely resembles melanin, with its production significantly increasing in response to higher tyrosine concentrations. Additionally, melanin biosynthesis was investigated through the induction of secondary metabolite gene expression using chloramphenicol-supplemented media at subinhibitory concentrations (0-40 mu g/ml). The results indicated that antibiotic-supplemented ISP1 (tryptone yeast extract agar), ISP6 (peptone yeast extract iron agar), and ISP7 (tyrosine agar; L-tyrosine 0.5 g/l) were less effective at promoting black pigment production than PDA. Furthermore, amplification of the tyrosinase gene melC1 confirmed its presence in both strains, with the expected product size obtained from CQ2, while AQ2 yielded a larger amplicon. These findings suggest that Streptomyces spp. strains AQ2 and CQ2 utilize the tyrosinase pathway for melanin biosynthesis, which is significantly enhanced in the presence of tyrosine. Although preliminary tests demonstrated fungicidal activity of the melanin-producing bacteria against selected fungal strains, it remains unclear whether melanin alone is responsible for this activity. Further studies are required to confirm the antifungal properties of the extracted melanin. In summary, this study highlights the potential of actinomycete-derived metabolites for future antifungal applications.