Actinomycin Analogs from Soil-Derived Streptomyces sp. PSU-S4-23 with Activity Against MRSA

Genome-based discovery provides a powerful approach for identifying bioactive natural products. In this study, <i>Streptomyces</i> sp. PSU-S4-23 was isolated from soil collected in southern Thailand. Genome analysis revealed a nonribosomal peptide synthetase (NRPS) biosynthetic gene cluster highly similar to the reference actinomycin D cluster, including canonical NRPS genes and a cytochrome P450 associated with oxidative tailoring. Genomic comparison indicated that this strain is distinct from its closest relative <i>S. caeni</i> CGMCC 4.7426^T with ANIb and dDDH values below the species delineation thresholds. In agar diffusion assays, the crude extract exhibited antibacterial activity against <i>Staphylococcus aureus</i> (MSSA and MRSA), <i>Bacillus subtilis</i>, <i>Bacillus cereus</i>, <i>Enterococcus faecalis</i>, <i>Staphylococcus epidermidis</i>, as well as inhibition of <i>Pseudomonas aeruginosa</i> and <i>Acinetobacter baumannii</i>. LC-MS/MS profiling of the crude ethyl-acetate extract was performed. GNPS feature-based molecular networking revealed ions corresponding to actinomycin X₂ (<i>m</i>/<i>z</i> 1269.6), D (<i>m</i>/<i>z</i> 1255.6), and I (<i>m</i>/<i>z</i> 1271.6), confirming production of multiple actinomycin analogs. These findings highlight <i>Streptomyces</i> sp. PSU-S4-23 as a promising actinomycin-producing strain with potential relevance to antibiotic discovery.