Nutritional intervention trial of yeast mannans derived from baker's yeast enriches Bacteroides and Parabacteroides with concurrent reduction of Klebsiella pneumoniae

Introduction Supplementation with a yeast cell wall fraction rich in mannans is well tolerated in adults without gastrointestinal disease; however, limited research has examined its selective utilization by the gut microbiota and associated health benefits. The aim of this analysis was to investigate the specific effects of yeast mannans on the composition and metabolic function of the gut microbiota. Methods Fecal samples from an open-label study of adults consuming 15 g of yeast mannans per day were profiled for fecal microbiota composition using whole metagenome sequencing, and fecal and urine metabolomics were analyzed by direct infusion and liquid chromatography-tandem mass spectrometry (DI-MS/MS and LC-MS/MS). Results Differential abundance testing identified 46 species whose relative abundances increased with the yeast mannan intervention, primarily in the genera Bacteroides and Parabacteroides . Thomasclavelia spiroformis showed the largest fold increase. Four species decreased in relative abundance, with a greater-than-2-fold decrease in Klebsiella pneumoniae , with 45% of participants showing no detection following the yeast mannan intervention. Minor metabolomic changes were noted, including increased urinary excretion of N -acetylalanine, alanine, and caproic acid, and decreases in fecal concentrations of ether-linked phosphatidylcholines C36:4 and C40:1. Discussion Yeast mannan supplementation selectively enriched several Bacteroides and Parabacteroides , saccharolytic commensals that may outcompete Klebsiella pneumoniae , an opportunistic pathobiont capable of causing extraintestinal infections.