| Summary: | Natural products have long been used as a source of antimicrobial agents against various microorganisms. Actinobacteria are a group of bacteria best known to produce a wide variety of bioactive secondary metabolites, including many antimicrobial agents. In this study, four actinobacterial strains found in Singapore terrestrial soil were investigated as potential sources of new antimicrobial compounds. Large-scale cultivation, chemical, and biological investigation led to the isolation of a previously undescribed tetronomycin A (<b>1</b>) that demonstrated inhibitory activities against both Gram-positive bacteria <i>Staphylococcus aureus</i> (SA) and methicillin-resistant <i>Staphylococcus aureus</i> (MRSA) (i.e., MIC<sub>90</sub> of 2–4 μM and MBC<sub>90</sub> of 9–12 μM), and several known antimicrobial compounds, namely nonactin, monactin, dinactin, 4E-deacetylchromomycin A3, chromomycin A2, soyasaponin II, lysolipin I, tetronomycin, and naphthomevalin. Tetronomycin showed a two- to six-fold increase in antibacterial activity (i.e., MIC<sub>90</sub> and MBC<sub>90</sub> of 1–2 μM) as compared to tetronomycin A (<b>1</b>), indicating the presence of an oxy-methyl group at the C-27 position is important for antibacterial activity.
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