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Microbial fermentation of plant extracts with Lactobacteria is an option to obtain microbiologically stable preparations, which may be applied in complementary medicine. We investigated the metabolic conversion of constituents from <i>Mercurialis perennis</i> L. extracts, which were prepared for such applications. For this purpose, aqueous extracts were inoculated with two Lactobacteria strains, namely <i>Pediococcus sp.</i> (PP1) and <i>Lactobacillus sp</i>. (LP1). Both were isolated from a fermented <i>M. perennis</i> extract and identified by 16S rRNA sequencing. After 1 day of fermentation, an almost complete conversion of the genuine piperidine-2,6-dione alkaloids hermidine quinone (<b>3</b>) and chrysohermidin (<b>4</b>)&#8212;both of them being oxidation products of hermidin (<b>1</b>) &#8212;was observed by GC-MS analysis, while novel metabolites such as methylhermidin (<b>6</b>) and methylhermidin quinone (<b>7</b>) were formed. Surprisingly, a novel compound plicatanin B (bis-(3-methoxy-1<i>N</i>-methylmaleimide); <b>8</b>) was detected after 6 days, obviously being formed by ring contraction of <b>4</b>. An intermediate of a postulated reaction mechanism, isochrysohermidinic acid (<b>14</b>), could be detected by LC-MS. Furthermore, an increase in contents of the metabolite mequinol (4-methoxyphenol; <b>9</b>) upon fermentation points to a precursor glycoside of <b>9</b>, which could be subsequently detected by GC-MS after silylation and identified as methylarbutin (<b>15</b>). <b>15</b> is described here for <i>M. perennis</i> for the first time.