Antibacterial activity, strain identification and DNA transfer by bacterial conjugation in Streptomyces sp. SS200

In the finding of new antimicrobial agents to respond to drug-resistant bacterial pathogens, the genus Streptomyces is still an important source of secondary metabolites associated with antibacterial activity. Strikingly, two-thirds of antibiotics used in human medicine are secondary metabolites from Streptomyces bacteria, among which the biosynthetic pathway of several antibiotics was studied. We report here the isolation of an endophytic bacterial strain from Bougainvillea glabra collected in Vung Tau City, Vietnam. This bacterial strain showed strong antibacterial activity against multiple pathogenic bacteria, including those belonging to the ESKAPE pathogens group. Cultural and molecular characterization identified the isolated strain as Streptomyces sp. SS200. A DNA transfer system by bacterial conjugation was established in Streptomyces sp. SS200 to pave the way for following studies on biosynthetic pathways of secondary metabolites having antibacterial activity in this strain. The key conditions in the conjugal transfer were investigated, including the recipient’s cellular forms, donor Escherichia coli strains, effects of cation2+, conjugation media, and antibiotics overlaying time. The mycelial form of Streptomyces sp. SS200 gave a 5-fold higher conjugation frequency than the spore form. ET12567 strain carrying the mobilizing plasmid pUZ8002 had the highest frequency of conjugation among two E. coli donors, specifically at a double frequency than SM10pir strain. Ca2+ gave more exconjugants than Mg2+, and the highest frequency of conjugation was obtained at 10 mM of this cation. Of four conjugation media, the exconjugants between E. coli and Streptomyces were only found on the Soya Flour Mannitol medium. The overlaying time of antibiotics was optimal at 24 hours of conjugation. These conjugal conditions will be used for the following experiments to find gene candidates involved in the biosynthesis of secondary metabolites with antibacterial activity in Streptomyces sp. SS200.