[Isolation, classification and biosynthetic potential of endophytic actinomycetes from Stemona]

To isolate endophytic actinomycetes from earthnut samples of Stemona tuberosa Lour. collected from Xishuangbanna, Yunnan Province, and to study the classification, antimicrobial activity and secondary metabolic potential of the isolates.

Samples were treated by a well-established surface-sterilized procedure, and plated on four selective media. The isolates were identified by morphology and 16S rRNA gene sequence analysis. Antimicrobial activity of the isolates was tested using agar diffusion method. Secondary metabolite genes of PKS/NRPS and halogense were detected by PCR amplification. HPLC-UV and VIS-ESI-MS/MS were used to analyze the fermentation product.

We obtained 18 actinomycete isolates from 6 Stemona earthnut samples. The isolates belonged to 4 genera, Streptomyces, Pseudonocardia, Micromonospora and Methylobacterium, and 10 of them were streptomycetes. The isolates also showed distinguish positive rates of antibacterial activity and secondary metabolite genes. Of them 13 strains showed antimicrobial activity against Staphlococcus aureus and/or Pseudomonas aeruginosa. Of them 17 isolates were positive for PKS/NRPS genes, and 8 strains were positive for halogenase gene. Moreover, the representative halogenase gene- positive strain produced metabolites that had typical MS peaks of halogen absorption.

Based on the results of this study, the endophytic actinomycetes from Stemona, dominant by Streptomyces and Micromonospora, have good secondary metabolic potential and could serve as a promising resource for bioactive metabolite discovery in the future.