| Summary: | Bacteria of the genus <i>Streptomyces</i> produce various specialized metabolites. Single biosynthetic gene clusters (BGCs) can give rise to different products that can vary in terms of their biological activities. For example, for alnumycin and the shunt product K115, antimicrobial activity was described, while no antimicrobial activity was detected for the shunt product 1,6-dihydro 8-propylanthraquinone. To investigate the antibacterial activity of 1,6-dihydro 8-propylanthraquinone, we produced alnumycin and 1,6-dihydro 8-propylanthraquinone from a <i>Streptomyces</i> isolate containing the alnumycin BGC. The strain was cultivated in liquid glycerol–nitrate–casein medium (GN), and both compounds were isolated using an activity and mass spectrometry-guided purification. The structures were validated via nuclear magnetic resonance (NMR) spectroscopy. A minimal inhibitory concentration (MIC) test revealed that 1,6-dihydro 8-propylanthraquinone exhibits antimicrobial activity against <i>E. coli</i> Δ<i>tolC</i>, <i>B. subtilis</i>, an <i>S. aureus</i> type strain, and a vancomycin intermediate-resistance <i>S. aureus</i> strain (VISA). Activity of 1,6-dihydro 8-propylanthraquinone against <i>E. coli</i> Δ<i>tolC</i> was approximately 10-fold higher than that of alnumycin. We were unable to confirm gyrase inhibition for either compound and believe that the modes of action of both compounds are worth reinvestigating.
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