Taxonomic characterization and chemical study of the antifungal constituents of Streptomyces sp. KH-F12

An actinomycetes strain designated as KH-F12 showed a broad-spectrum antimicrobial activity against Gram-positive and Gram-negative bacteria and filamentous and unicellular fungi. It was isolated from soil sample collected from Adilam area, Kingdom of Saudi Arabia. Polyphasic taxonomic approaches were applied for the identification. Based on the cultural and biochemical characteristics of the isolate (KH-F12) was classified as a Streptomyces. Comparison with other Streptomyces species using the 16S rRNA gene sequence and the PCR-restriction enzyme Pattern Analysis (PRA) of strain KH-F12 developed a similarity level ranged from 97.2% with Streptomyces albidoflavus. The antifungal activity was stronger than the antibacterial one. An inhibition zone of (12 mm) with B. subtilis ATCC11774, (11 mm) E. coli ATCC8739, (30 mm) with Candida albicans ATCC66027 and Candida albicans ATCC2091, (32 mm) with Saccharomyces cerevisiae ATCC9763, (34 mm) with Aspergillus niger ATTC16404 and a zone of (14 mm) was recorded with Aspergillus fumigatus ATCC1022. The kinetics of antibiotic production, growth and pH were monitored in ISP2 broth culture. The antifungal activity against A niger was stronger than that detected against C. albicans. The antimicrobial activity was found to be correlated with the cell growth, so the biomass increased from the first day to the 8th day. In order to isolate the active secondary metabolites, the lyophilized medium was subjected to various chromatographic purification steps guided by the antifungal assay. An active compound (F-12) with MIC of 0.5 jug mL-1 against the two Candida strains, 1.8 jag mL-1 against A. fumigates and 1.4 jug mL-1 against A niger was isolated. Different spectroscopic techniques were applied to identify the isolated compound. However, the structure of F-12 is still under investigation.