| Summary: | Siderophores are compounds with high affinity for ferric iron. Bacteria produce these compounds to acquire iron in iron-limiting conditions. Iron is one of the most abundant metals on earth, and its presence is necessary for many vital life processes. Bacteria from the genus <i>Serratia</i> contribute to the iron respiration in their environments, and previously several siderophores have been isolated from this genus. As part of our ongoing search for medicinally relevant compounds produced by marine microbes, a co-culture of a <i>Shewanella</i> sp. isolate and a <i>Serratia</i> sp. isolate, grown in iron-limited conditions, was investigated, and the rare siderophore serratiochelin A (<b>1</b>) was isolated with high yields. Compound <b>1</b> has previously been isolated exclusively from <i>Serratia</i> sp., and to our knowledge, there is no bioactivity data available for this siderophore to date. During the isolation process, we observed the degradation product serratiochelin C (<b>2</b>) after exposure to formic acid. Both <b>1</b> and <b>2</b> were verified by 1-D and 2-D NMR and high-resolution MS/MS. Here, we present the isolation of <b>1</b> from an iron-depleted co-culture of <i>Shewanella</i> sp. and <i>Serratia</i> sp., its proposed mechanism of degradation into <b>2</b>, and the chemical and biological characterization of both compounds. The effects of <b>1</b> and <b>2</b> on eukaryotic and prokaryotic cells were evaluated, as well as their effect on biofilm formation by <i>Staphylococcus epidermidis</i>. While <b>2</b> did not show bioactivity in the given assays, <b>1</b> inhibited the growth of the eukaryotic cells and <i>Staphylococcus aureus</i>.
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