Abstract / Summary
Introduction: Bifidobacterium adolescentis is a key beneficial member of the human gut microbiota, but its genomic diversity and evolutionary drivers across human populations remain poorly characterized. Objectives: Understanding genomic functional heterogeneity and evolutionary patterns in human gut-derived B. adolescentis . Methods: We performed a comparative genomic analysis of 395 B. adolescentis , mainly from China ( n = 169) and the United States ( n = 146), with smaller sets from Australia, Italy, and the United Kingdom, to investigate functional heterogeneity and evolutionary mechanisms. Our analysis integrated core and pan-genome architecture, phylogenomics, single nucleotide polymorphism (SNP)-based population structure, carbohydrate-active enzyme profiles, CRISPR-Cas systems, antibiotic resistance genes, and recombination dynamics. Results: The pan-genome was open and highly plastic. Phylogenetic reconstruction identified two major clades with strong geographic stratification: Chinese isolates predominantly clustered in Clade B, while U.S. isolates grouped in Clade A. Functional annotation showed regional specialization in carbohydrate-active enzymes, with Chinese isolates enriched in glycosyltransferase families and U.S. isolates in carbohydrate-binding module and carboxylesterase families, likely reflecting dietary adaptations. Genomic islands were hotspots for horizontal gene transfer, harboring region-specific carbohydrate-active enzymes and antibiotic resistance genes such as tet(W/32/O) and ermX , which were frequently located in Chinese isolates. Recombination was found to be the primary driver of genetic diversity, with recombination-to-mutation ratios approaching and exceeding 3.0 in Chinese and U.S. isolates. Linkage disequilibrium decay further supported higher recombination rates in these populations. Conclusion: B. adolescentis has diverged into two major genomic lineages, primarily associated with isolates from China and the U.S. This divergence reflects adaptation to distinct host-associated ecological factors, such as diet, antibiotic exposure, and lifestyle, and is predominantly driven by extensive homologous recombination rather than point mutations. These findings highlight how regional selective pressures shape the genomic and functional landscape of this key gut symbiont.