Bacteroides uniformis ATCC 8492 Utilizes Lotus Seed Resistant Starch via a PUL18-Driven Exterior-to-Interior Degradation Strategy

Bacteroides uniformis (Bu) is a promising probiotic, yet its mechanism for utilizing lotus seed resistant starch (LRS) remains unclear. Here, growth analysis, starch structural characterization, transcriptomics, and molecular docking were integrated to elucidate the LRS utilization strategy of Bu. Bu efficiently utilized LRS and released abundant extracellular maltooligosaccharides during fermentation. Structural analyses revealed an exterior-to-interior degradation pattern characterized by the preferential removal of peripheral short chains and progressive surface erosion of starch granules. Transcriptomic profiling identified polysaccharide utilization locus 18 (PUL18) as the core genetic locus induced by LRS. Two outer-membrane glycoside hydrolases, GH13_47 and GH13_48, initiated extracellular starch depolymerization, while periplasmic GH97 and GH13 further hydrolyzed the imported oligosaccharides. Cytoplasmic GH65 completed intracellular saccharide metabolism, directing the carbon flux toward acetate and propionate production. These findings reveal a PUL18-driven mechanism underlying LRS utilization by Bu and provide new insights into the resistant starch metabolism by gut Bacteroides.