An alkaline-extracted low-molecular-weight heteroglucan from Ganoderma lucidum enhances immunity by modulating the gut microbiota.

Ganoderma lucidum polysaccharides exhibit various biological activities, but conventional extraction is biased toward high-molecular-weight (Mw) polymers. This study aimed to explore the structure and bioactivity of low-Mw polysaccharides from G. lucidum (AGLP1). A low-Mw heteroglucan from G. lucidum was purified and identified as a heteroglucan mainly composed of a (1 → 3)-linked glucopyranosyl backbone with (1 → 6)-linked glucopyranosyl side chains. In vivo investigations revealed that AGLP1 significantly enhanced the spleen index, macrophage phagocytic activity and proliferation of T and B lymphocytes. These immunomodulatory effects were gut microbiota-dependent, as evidenced by their abolition in microbiota depleted mice via antibiotic (ABX) treatment. Mechanistically, AGLP1 reshaped the gut microbiota and serum metabolite profile by enriching potential beneficial bacteria such as Ruminococcaceae and Eubacterium brachy group, as well as associated metabolites including decanoic acid, stearic acid and pimelic acid. In vitro fermentation using a human fecal model showed that AGLP1 also consistently increased the abundances of Ruminococcaceae and Eubacterium brachy group. These findings demonstrate that the immunomodulatory and microbiota-reshaping effects of AGLP1, observed in animal models, were also replicated in a human gut simulation. This underscores its potential as a functional food ingredient.