Isolation and characterization of an endophytic Streptomyces sp. YBRX0 from Croton tonkinensis and assessment of its bioactive potential.

The rise of antimicrobial resistance and emerging infectious diseases necessitates the discovery of novel antibacterial agents. Endophytic actinomycetes, especially Streptomyces species colonizing medicinal plant tissues, constitute a reservoir of structurally diverse bioactive metabolites that remains insufficiently characterized. This study reports the isolation and comprehensive characterization of an endophytic actinomycete, designated strain YBRX0, recovered from the root tissues of the Vietnamese medicinal plant Croton tonkinensis Gagnep. Strain identity was established through an integrated approach combining colony morphology, scanning electron microscopy, physiological and biochemical profiling, and 16S rRNA gene phylogenetic analysis. Bioactivity screening of the ethyl acetate fermentation extract employed agar well diffusion to assess antibacterial spectrum, with inhibitory potency quantified by broth microdilution. Antioxidant capacity was measured via DPPH radical scavenging, and secondary metabolite composition was profiled by GC-MS. The integrated characterization placed strain YBRX0 unambiguously within the genus. At 1 mg/mL, the ethyl acetate extract demonstrated broad-spectrum activity against both Gram-positive and Gram-negative pathogens, with MIC values ranging from 8 to 16 μg/mL. Antioxidant screening yielded an IC₅₀ of 197.26 ± 8.87 μg/mL. GC-MS tentatively identified Pyrrolopyrazine diketopiperazines and Benzeneacetamide as principal constituents. Collectively, these findings indicate that the crude extract of Streptomyces sp. YBRX0 exhibited notable antibacterial activity despite being an unfractionated mixture, supporting its potential as a promising source of bioactive metabolites for further bioprospecting and compound isolation efforts targeting drug-resistant pathogens.