| Summary: | A marine-derived actinomycete (<i>Streptomyces</i> sp. MBTI36) exhibiting antibacterial activities was investigated in the present study. The strain was identified using genetic techniques. The 16S rDNA sequence of the isolate indicated that it was most closely related to <i>Streptomyces microflavus</i>. Furthermore, a new chromomycin A<sub>9</sub> (<b>1</b>), along with chromomycin Ap (<b>2</b>), chromomycin A<sub>2</sub> (<b>3</b>), and chromomycin A<sub>3</sub> (<b>4</b>), were isolated from the ethyl acetate extract. Their structures were determined using extensive spectroscopic methods including 1D and 2D NMR, and HRMS, as well as comparisons with previously reported data. Compounds <b>1</b>–<b>4 </b>showed potent antibacterial activities against Gram-positive bacteria including methicillin-resistant <i>Staphylococcus aureus</i> (MRSA). During a passage experiment, minimum inhibitory concentration (MIC) values for compounds <b>1</b>–<b>4 </b>showed no more than a 4-fold increase from the starting MIC value, indicating that no resistance was detected over the 21 passages.
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