Antimicrobial constituents of Hypocrea virens, an endophyte of the mangrove-associate plant Premna serratifolia L.

Emergence of multidrug-resistant pathogenic microorganisms has prompted a worldwide search for new antibiotics from various sources. Endophytic fungi from unique habitats are considered as potential sources of novel bioactive compounds. Sri Lankan mangrove ecosystem is such a distinctive and unexploited resource for the discovery of structurally diverse and biologically active metabolites including antimicrobials. Nine endophytic fungi were isolated from the leaves and twigs of Premna serratifolia L. from a mangrove habitat in the Negombo lagoon and the antimicrobial activities of their laboratory cultures were evaluated. The most promising antimicrobial activity was exhibited by the endophytic fungus Hypocrea virens. Bioassay guided fractionation of the organic extract of this fungus led to the isolation of two known metabolites; the antimicrobial epidithiodioxopiperazine, gliotoxin (1), and the closely related but less active bisdethiobis(methylthio)gliotxin (2). The chemical structures of the two compounds were determined by spectroscopy and confirmed by comparison of mass and nuclear magnetic resonance (NMR) spectral data with the reported values for these molecules. The minimum inhibitory concentration (MIC) values obtained for gliotoxin (1) in the current study are 0.13 μg mL-1 for Bacillus subtilis, 16 μg mL-1 for Staphylococcus aureus, 32 μg mL-1 for Methicillin Resistant Staphylococcus aureus (MRSA) and Escherichia coli, 64 μg mL-1 for Pseudomonas aeruginosa and Candida albicans fungus. This is the first study to report the isolation of endophytic fungi from P. serratifolia and their antimicrobial activities.