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. Fermentation of extracts of Artemisia princeps and Paeonia lactiflora has been previously found to increase their bioactivities. Therefore, this study explores the possibility of increasing the bioactivity of Mentha arvensis (Mentha) extract against lipopolysaccharide (LPS)-induced RAW 264.7 macrophage cells by fermenting with plant-derived probiotic strains Lactobacillus (Lact.) plantarum SN13T and Pediococcus (Ped.) pentosaceus LP28. As a result, fermentation with SN13T significantly increased the bioactivity of Mentha extract as compared to unfermented or LP28-fermented extracts. 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 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.