Characterization and Antibacterial Activity of Melanin Pigment from Marine Bacterium Actinoalloteichus cyanogriseus

Introduction: Marine microbes are known to produce novel bioactive compounds. In this work, the bacterium Actinoalloteichus cyanogriseus, AU-RM-4, isolated from marine soil sediment, capable of producing high amounts of black extracellular melanin pigment when grown on anaerobic actinomycetes agar is reported. The maximum melanin production was approximately 2.5 mg/mL. Methods: The melanin pigment was recovered using acid treatment and the characteristic melanin absorption peaks at 253 nm and 291 nm were observed in the UV range. The antibacterial activity of the melanin pigment against a range of clinically infectious bacteria was assayed in vitro by disc diffusion and serial dilution methods. Results: The A. cyanogriseus melanin showed antimicrobial activity against Staphylococcus aureus and Klebsiella pneumoniae (MIC value -500 μg/mL). The effects of the AU-RM-4 melanin on the morphology of pathogenic bacteria were observed under scanning (SEM) and transmission electron microscopy (TEM). It is concluded that A. cyanogriseus isolate AU-RM-4 is a promising melanin-producing marine bacterium, whose melanin pigment has antibacterial activity against certain pathogenic bacteria (S. aureus and K. pneumoniae). Conclusion: Melanin produced by A. cyanogriseus AU-RM-4 could have commercial potential in the pharmaceutical and cosmetic industries for the production of pharmaceutical formulations, and also in the food industries as a colouring and antibacterial agent.