Discovery and Biosynthesis of Antimicrobial Phenethylamine Alkaloids from the Marine Flavobacterium <i>Tenacibaculum discolor</i> sv11

The bacterial genus <i>Tenacibaculum</i> has been associated with various ecological roles in marine environments. Members of this genus can act, for example, as pathogens, predators, or episymbionts. However, natural products produced by these bacteria are still unknown. In the present work, we investigated a <i>Tenacibaculum</i> strain for the production of antimicrobial metabolites. Six new phenethylamine (PEA)-containing alkaloids, discolins A and B (<b>1</b> and <b>2</b>), dispyridine (<b>3</b>), dispyrrolopyridine A and B (<b>4</b> and <b>5</b>), and dispyrrole (<b>6</b>), were isolated from media produced by the predatory bacterium <i>Tenacibaculum discolor</i> sv11. Chemical structures were elucidated by analysis of spectroscopic data. Alkaloids <b>4</b> and <b>5</b> exhibited strong activity against Gram-positive <i>Bacillus subtilis</i> DSM10, <i>Mycobacterium smegmatis</i> ATCC607, <i>Listeria monocytogenes</i> DSM20600, and <i>Staphylococcus aureus</i> ATCC25923, with minimum inhibitory concentration (MIC) values ranging from 0.5 to 4 μg/mL, and moderate activity against <i>Candida albicans</i> FH2173 and <i>Aspergillus flavus</i> ATCC9170. Compound <b>6</b> displayed moderate antibacterial activities against Gram-positive bacteria. Dispyrrolopyridine A (<b>4</b>) was active against efflux pump deficient <i>Escherichia coli</i> ATCC25922 Δ<i>tolC</i>, with an MIC value of 8 μg/mL, as well as against <i>Caenorhabditis elegans</i> N2 with an MIC value of 32 μg/mL. Other compounds were inactive against these microorganisms. The biosynthetic route toward discolins A and B (<b>1</b> and <b>2</b>) was investigated using <i>in vivo</i> and <i>in vitro</i> experiments. It comprises an enzymatic decarboxylation of phenylalanine to PEA catalyzed by DisA, followed by a nonenzymatic condensation to form the central imidazolium ring. This spontaneous formation of the imidazolium core was verified by means of a synthetic one-pot reaction using the respective building blocks. Six additional strains belonging to three <i>Tenacibaculum</i> species were able to produce discolins, and several DisA analogues were identified in various marine flavobacterial genera, suggesting the widespread presence of PEA-derived compounds in marine ecosystems.