Pluramycinone Metabolites from Streptomyces sp. SA0215, Isolated from the Saudi Red Sea Sediments with Antimicrobial and Cytotoxic Activity

A derived actinomycete, Streptomyces sp. SA0215 was isolated from Saudi Red Sea sediments; the strain was identified by morphology and by phylogenetic analysis of the 16S rRNA gene sequence. The chemical analysis of its culture broth yielded 9 secondary metabolites. The compounds were isolated by a series of chromatographic steps, and their structures established by detailed spectroscopic analysis of the NMR and MS data. Pluramycinone metabolites 1-6 with the structurally related indomycinone analogous as saptomycin-A (1), saptomycin-F (2), α-indomycinone (3), β-indomycinone (4), γ-indomycinone (5), kidamycin (6) and furthermore, resistomycin (7), 1-acetyl-β-carboline (8) and ergosterol peroxide (9) were identified from the culture broth extract. Bioactivity of the indomycinones 1–6 was evaluated in antimicrobial and cytotoxicity assays. β-Indomycinone (4) was found to exhibit potent broad spectrum antibacterial activities against the tested pathogens Klebsiella pneumoniae, Escherichia coli and Staphylococcus aureus with inhibition zones diameters of 18, 20 and 24 mm, respectively, compared to the inhibition zones of the antibiotic reference oxytetracycline. The MIC values against the same pathogens were determined 7.2, 11.5, and 13.7 µg/mL, respectively. While γ-indomycinone (5) displayed high activity against the Gram-positive S. aureus with MIC value 10.3 µg/mL, compared to the potent activity of the crude extract (MIC = 9.6 µg/mL). Moreover, saptomycin-A (1) displayed the highest cytotoxic activity against human HepG2 cells with an IC₅₀ value of 19.3 μM, followed by β-indomycinone with an IC₅₀ value of 23.5 μM, while the extract showed the highest activity against human MCF7 cells with an IC₅₀ value of 30.5 µg/mL.