Screening for Antimycobacterial Activity of Actinomycetes Collected in Vietnam - Isolation and Activity of Metabolites from Streptomyces alboniger (A121)

Objective: Actinomycetes have been known to be the great natural sources to explore antibiotics for the treatment of tuberculosis (TB). The isolation of actinomycetes from the samples in Vietnam followed by the screening of their antimycobacterial activity was performed in this study. The metabolites isolated from the most active strain were further evaluated for their antimycobacterial, antimicrobial and cytotoxic activity. Methods: Actinomycetes were growth in culture media, isolated and identified by colony, spore chain morphology and 16S rRNA gene sequencing. Agar diffusion assay was used for the screening of the isolated strains against Mycobacterium smegmatis, a safety surrogate for Mycobacterium tuberculosis. The metabolites produced from the most active strain were investigated by actinomycete fermentation, extraction and isolation from biomass and cultures. The structures of the isolated compound were elucidated by spectral data and comparison with the reported literatures. Results: 181 strains were isolated from nine regions along the north to central Vietnam. The five most active strains against Mycobacterium smegmatis were detected. Following the bioassay-guided result, the strain A121 (Streptomyces alboniger) was selected for further isolation of the bioactive metabolites. As a result, obscurolide B-2 beta (1) and chartreusin (2) were obtained and evaluated for their antimycobacterial activity against M. smegmatis. Compound 2 displayed potential antimycobacterial activity, antimicrobial effect against the Gram positive bacteria Staphylococcus aureus, Bacillus subtilis, Lactobacillus fermentum and cytotoxicity against four cancer cell lines KB, HepG-2, Lu-1 and MCF-7. Conclusions: Five strains possessing potential antimycobacterial activity were identified from the samples collected in Vietnam. Two compounds including obscurolide B-2 beta (1) and chartreusin (2) were isolated from the most active strain A121 (Streptomyces alboniger). This is the first time these compounds have been isolated from this strain. Chartreusin (2) exhibited notable antimycobacterial, antimicrobial and cytotoxic activity, making its worthy attention for further drug development, particularly for antituberculosis therapeutic agents.