Role of Phenolic Acid Metabolism in Enhancing Bioactivity of Mentha Extract Fermented with Plant- Derived Lactobacillus plantarum SN13T

Plant-derived lactic acid bacteria are major fermentation organisms that can grow in medicinal herb extracts enriched with phytochemicals like glycosides, phenolic acids, flavonoids, and tannins. Fermentation with strain-specific Lactobacilli harboring metabolic enzymes can increase the bioactivity and bioavailability of medicinal herbs. Lactobacillus (Lact.) plantarum SN13T and Pediococcus (Ped.) pentosaceus LP28 are plant-derived probiotic strains with clinically reported health effects. In the present study, the bioactivity of the extract of the herb, Mentha, against lipopolysaccharide (LPS)-induced RAW 264.7 macrophage cells were increased when fermented with Lact. plantarum SN13T as compared to the unfermented extract or extract fermented with Ped. pentosaceus LP28. This higher bioactivity was associated with the metabolism of rosmarinic acid (RA) and caffeic acid (CA), the major bioactive phenolic acids reported in Mentha, along with the production of the metabolite dihydrocaffeic acid (DHCA). DHCA was found to be a more potent LPS-induced nitric oxide (NO) inhibitor than its precursor phenolic acids. The metabolism of RA to DHCA via CA could be mediated by the enzymes cinnamoyl ester hydrolase and hydroxycinnamate reductases, encoded by the ceh gene and the hcrRABC gene operon, respectively, which were identified in the complete genome sequence of Lact. plantarum strain SN13T but were absent in Ped. pentosaceus LP28. The genes hcrA , hcrB , and hcrC were significantly and time-dependently overexpressed in Lact. plantarum SN13T when grown in the Mentha extract, suggesting the role of phenolic acid metabolism in enhancing its bioactivity.