Chemical characterization and antibacterial activity of melanin extracts from selected species of fungi and slime mold

Abstract

The main objectives of this study were to chemically characterize the melanin pigments extracted from potential species of fungi and slime mold and investigate their antibacterial and antibiofilm activities against Cutibacterium acnes (the main pathogenic bacterium causing inflammatory acne vulgaris). The melanin pigment samples were extracted using alkaline solution, then UV-Vis spectroscopy and Fourier transform infrared (FTIR) techniques were used to confirm and initially characterize the melanin nature of the isolated pigments. Antibacterial activities of the samples were evaluated using microdilution method (from these data, minimum inhibitory concentrations [MIC] were calculated) and accompanied with minimum bactericidal concentration (MBC) for confirmation; the antibiofilm activity was evaluated via minimum biofilm eradication assay. Data on UV-Vis and FTIR spectra analysis suggested the extracted pigments were melanin because they had similar features compared with those of previously reported melanins. Besides, the melanins extracted from different samples in this study were quite similar in chemical compositions. In terms of antibacterial activity, among the tested samples, Xylaria sp melanin was found to be the most potential. Melanin from this fungus could inhibit the growth of C. acnes with MIC50 value was smaller than 500ug/ml and MIC90 was about 2000ug/ml, MBC at 2000ug/ml and MBEC50 at 2000 ± 1.7269ug/ml. At the concentration of 2000ug/ml, Xylaria sp displayed a comparable growth inhibition against C. acnes as erythromycin (positive control), whereas, its antibiofilm activity was much lower. This project generated the first set of data on antibacterial activities and initial characteristics of melanin from slime molds and puffballs. The obtained results suggest that melanin from Xylaria sp is worth further investigation for cosmetic applications or acnes treatment