Antimicrobial activity of secondary metabolites produced by Colletotrichum species, an endophytic fungus on Vernonia amygdalina Del (fam. Asteraceae)

Abstract

The emergence of antibiotic-resistant strains of microorganisms calls for research and the development of strategies to curb this menace. The screening of antimicrobial compounds from endophytic fungi is a promising way to meet the increasing threat of drug-resistant. This study aims at evaluating the antimicrobial activity of the endophytic fungal extract of Colletotrichum sp. from the leaves of Vernonia amygdalina (Del). Endophytic fungal isolation, cultivation, and identification were carried out using standard methods. Subsequently, fermentation and extraction of the secondary metabolites were done on a rice medium. About 1.2g of the fungal extract was subjected to vacuum liquid chromatography (VLC) using gradient-based elution; n-hexane and ethyl acetate followed by dichloromethane and methanol. The high-performance liquid chromatography (HPLC) analysis of the crude extract and VLC fractions were also carried out. Gel permeation chromatography using Sephadex LH-20 was carried out on Fraction JT2. Antimicrobial Assays for fraction JT4 and the Sephadex LH-20 fractions (JT2C – JT2E) were also carried out. The results were analyzed using Statistical Package for Social Sciences. The high-performance liquid chromatography (HPLC) analysis of the crude extract and VLC fractions revealed the presence of the five bioactive metabolites (acropyrone, beauvericin, indole-3-carbaldehyde, indolyl-3-acetic acid, and rocaglamid A). The result of the antimicrobial activity of the VLC fraction, JT4, showed that the fraction exhibited a significant inhibitory effect against Bacillus subtilis, Pseudomonas aeruginosa at 0.25 – 1 mg/ml (IZD) and antifungal activity against Aspergillus niger only at 0.0625 – 1 mg/ml (IZD). The result of the antimicrobial activity of the gel chromatographic fractions revealed a significant effect against Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli at 0.0625 – 1 mg/ml and had no inhibitory effect against Bacillus subtilis, Aspergillus niger and Candida albicans. The results of this study show that endophytic fungi on V. amygdalina could be a potential source of bioactive compounds for the development of potential and effective antibiotics.