Lactiplantibacillus plantarum Fermentation Alters the Physicochemical Characteristics and Enhances the Antioxidant and Immunomodulatory Activities of Eucommia ulmoides Leaf Polysaccharides

Eucommia ulmoides leaf polysaccharides (EUPSs) are plant-derived macromolecules with antioxidant and immunomodulatory potential. This study investigated whether solid-state fermentation using Lactiplantibacillus plantarum modifies the physicochemical characteristics and bioactivity of EUPS. Unfermented and fermented polysaccharide fractions (EUPS and FEUPS, respectively) were compared for yield, monosaccharide composition, molecular weight, infrared characteristics, morphology, and radical-scavenging activity, and were further evaluated in cyclophosphamide-induced immunosuppressed mice. Fermentation increased the recovery yield of the dried polysaccharide fraction to 19.45%, representing a 55.1% increase compared with the unfermented extraction, and altered the monosaccharide composition and molecular weight distribution while largely preserving the major functional groups. FEUPS exhibited a looser, more porous morphology, and stronger DPPH•, ABTS•+, and hydroxyl radical-scavenging activities than EUPS. In vivo, FEUPS improved body weight gain, histopathological features, serum immunoglobulin levels, hepatic antioxidant status, and splenic gene-expression profiles. Compared with the corresponding EUPS groups, FEUPS showed significant differences in selected immune- and antioxidant-related parameters, particularly at the medium dose. FEUPS was also associated with changes in cecal microbiota composition and correlations between bacterial taxa and host immune and antioxidant markers. Overall, fermentation improved the yield and functional properties of EUPS, highlighting the potential of FEUPS for incorporation into functional food products.