Probiotic and immune-modulatory capacities of three human gut-derived strains of Parabacteroides distasonis

The human gut microbiome harbors diverse beneficial bacteria with potential roles in supporting host health. Parabacteroides distasonis has recently attracted interest as a next-generation probiotic (NGP) candidate; however, functional evidence for human-derived strains remains limited. Here, three human gut-derived P. distasonis strains (B2-S-102, B2-Q-110, and Y3-G-102) were isolated from healthy individuals and characterized using comparative genomics and in vitro functional assays. Species-level identity was supported by 16S rRNA gene analysis and whole-genome relatedness metrics (ANI > 97% and dDDH > 70%), consistent with established species delineation thresholds. Under controlled laboratory conditions, the strains showed tolerance to acidic pH (pH 2.0), bile salts (0.3%), and simulated gastric and intestinal fluids. Functionally, the strains exhibited measurable antioxidant activity (35.03 ± 7.76% to 51.22 ± 5.60% DPPH inhibition) and α-amylase inhibitory activity (51.03 ± 32.12% to 69.23 ± 4.26%) in vitro. Cell-free supernatants inhibited albumin denaturation (47.65 ± 3.56% to 65.26 ± 4.15%), while live bacteria reduced nitric oxide production and pro-inflammatory cytokines (IL-6, TNF-α, IFN-γ, and IL-1β) in LPS-stimulated RAW 264.7 macrophages (p < 0.05). The strains also displayed in vitro growth-inhibitory activity against Escherichia coli, Acinetobacter baumannii, and Salmonella enteritidis. Genome mining identified multiple biosynthetic gene clusters, indicating genetic potential for secondary metabolite production; however, expression and metabolite identity were not experimentally validated. No haemolytic activity was observed, supporting a favorable preliminary safety profile. Overall, these findings provide preliminary in vitro evidence supporting the potential of human-derived P. distasonis strains as NGP candidates for further evaluation.