Evaluating cytotoxicity and antibacterial activities of melanin extracted from different microbial groups

Abstract

This study was carried out to investigate the antibacterial activity and evaluate the cytotoxicity of melanin and melanin derivatives toward cancer cell lines. Melanin was extracted from two medicinal fungal species including Ganoderma applanatum and Daedaleopsis tricolor in alkaline solution. The extracted melanin was then modified with arginine (to form melanin derivative) to enhance their water solubility. Antibacterial and cytotoxicity evaluation of the samples was determined by the microdilution method and SRB assay, respectively. Data on UV-vis and Fourier-transform infrared spectroscopy (FTIR) analysis confirmed the pigments extracted from D. tricolor and G. applanatum were melanin and the attachment of L- arginine to the melanin samples was successful. The obtained data suggested that melanin derivatives of both fungi displayed higher antibacterial activities compared to their original melanin samples and the activities are dose dependent. The melanin and melanin derivatives from D. tricolor show higher antibacterial ability than those of G. applanatum. Especially, at a concentration of < 4000 (µg/mL), melanin derivative from D. tricolor could inhibit more than 90% Staphylococcus aureus growth, which is similar to that of the erythromycin (positive control) at the same concentration, Thus, it could be considered an effective antibacterial agent against S. aureus. It should be noted that arginine alone did not inhibit S. aureus. In term of cytotoxicity, melanin extracted from D. tricolor and G. applanatum showed a slightly higher percentage of growth inhibition toward lung cancer cell (NCl-H460) than that of melanin derivative at the concentration of 100 (µg/mL). However, their cytotoxicity was significantly lower compared to the positive control (camptothecin) at a concentration of only 0.007 (µg/mL). Notably, both the melanin samples were non-toxic to the normal cell line. This research found that melanin modification could significantly enhance the antibacterial activities since it improved the solubility of melanin in water. However, it might be due to the low concentration used or just the nature of the samples, the 10 cytotoxicity of melanin and melanin derivative toward the lung cancer cell line was not significant.