Antimicrobial Gageomacrolactins Characterized from the Fermentation of the Marine-Derived Bacterium <i>Bacillus subtilis</i> under Optimum Growth Conditions

Marine bacteria are a potential source of structurally diversified bioactive secondary metabolites that are not found in terrestrial sources. In our continuous effort to search for new antimicrobial agents from marine-derived bacteria, we isolated bacterial strain 109GGC020 from a marine sediment sample collected from Gageocho, Republic of Korea. The strain was identified as <i>Bacillus subtilis</i> based on a 16s rRNA sequence analysis. After a 7-day fermentation of the <i>B. subtilis</i> strain under optimum growth conditions three new and four known secondary metabolites were discovered using chromatographic procedures, and their biological activities were evaluated against both bacteria and crop-devastating fungi. The discovered metabolites were confirmed by extensive 2D NMR and high-resolution ESI-MS data analyses to have the structures of new macrolactin derivatives gageomacrolactins <b>1</b>–<b>3</b> and known macrolactins A (<b>4</b>), B (<b>5</b>), F (<b>6</b>), and W (<b>7</b>). The stereoconfigurations of <b>1</b>–<b>3</b> were assigned based on coupling constant values, chemical derivatization studies, and a literature review. The coupling constants were very crucial to determine the relative geometries of olefins in <b>1</b>–<b>3</b> because of overlap of the ¹H NMR signals. The NMR data of these compounds were recorded in different solvents to overcome this problem and obtain accurate coupling constant values. The new macrolactin derivatives <b>1</b>–<b>3</b> displayed good antibiotic properties against both Gram-positive (<i>S. aureus</i>,<i> B. subtilis</i>, and <i>B. cereus</i>) and Gram-negative (<i>E. coli</i>,<i> S. typhi</i>, and <i>P. aeruginosa</i>) bacteria with minimum inhibitory concentration (MIC) values of 0.02–0.05 μM. Additionally, the antifungal activities of <b>1</b>–<b>7</b> were evaluated against pathogenic fungi and found to inhibit mycelial growth of <i>A. niger</i>,<i> B. cinerea</i>,<i> C. acutatum</i>,<i> C. albicans</i>, and <i>R. solani</i> with MIC values of 0.04–0.3 μM, demonstrating that these compounds were good fungicides