Preparation, Structural Characterization, and <i>In Vitro</i> Modulation of the Gut Microbiota Activity of a Novel α-Glucan in <i>Hericium erinaceus</i>

<i>Hericium erinaceus</i> (<i>H. erinaceus</i>) is a kind of medicinal and edible fungus that contains polysaccharide as the most studied effective component at present. In this paper, a novel α-glucan was successfully isolated from <i>H. erinaceus</i> fruiting bodies by hot water extraction combined with the fractional ethanol precipitation method. To analyze the monosaccharide composition, methylation and nuclear magnetic resonance spectroscopy were performed, and the polysaccharide was identified having α-(1→4)-glycosidic bonds with a single unit α-D-Glc<i>p</i> branch attached at the <i>O</i>-6 position (<i>M</i>_<i>w</i>, 1.43 × 10⁷ g/mol, branching ratio of 9:1). Furthermore, the aqueous solution analysis demonstrated that this α-glucan adopts a rigid spherical conformation. The polysaccharide exhibited resistance to simulated gastrointestinal digestion but was effectively degraded by gut microbiota (GM), leading to significant changes in total polysaccharide content, reducing sugar, and pH value. Notably, the α-glucan was beneficial to increase the contents of acetic and propionic acids in metabolites, stimulated the growth of beneficial bacteria (<i>Lactobacillus</i> and <i>Bifidobacterium</i>), and inhibited the proliferation of harmful bacteria (<i>Fusobacterium</i>). Overall, the study deepened the regulatory effects of α-glucan with different ratios of main and branch chains from <i>H. erinaceus</i> fruiting bodies on GM, which provides theoretical support for developing of GM-targeted foods.