# Genetic specialization of key bifidobacterial phylotypes in multiple mother–infant dyad cohorts from geographically isolated populations

## TL;DR

This study explores how different types of bifidobacteria in the gut are passed from mothers to infants and how they vary among ethnic groups in China.

## Contribution

The study reveals genetic specialization and vertical transmission of bifidobacteria in geographically isolated human populations.

## Key findings

- Bifidobacteria strains from the same mother–infant pair were genetically similar, suggesting vertical transmission.
- B. longum subsp. longum and B. breve showed genetic diversity linked to ethnicity, while B. animalis subsp. lactis strains clustered together regardless of ethnicity.
- The findings suggest co-evolution between bifidobacteria and human populations, influencing strain selection for industrial or therapeutic use.

## Abstract

Little has been known about symbiotic relationships and host specificity for symbionts in the human gut microbiome so far. Bifidobacteria are a paragon of the symbiotic bacteria biota in the human gut. In this study, we characterized the population genetic structure of three bifidobacteria species from 58 healthy mother–infant pairs of three ethnic groups in China, geographically isolated, by Rep-PCR, multi-locus sequence analysis (MLSA), and in vitro carbohydrate utilization. One hundred strains tested were incorporated into 50 sequence types (STs), of which 29 STs, 17 STs, and 4 STs belong to B. longum subsp. longum, B. breve, and B. animalis subsp. lactis, respectively. The conspecific strains from the same mother–child pair were genetically very similar, supporting the vertical transmission of Bifidobacterium phylotypes from mother to offspring. In particular, results based on allele profiles and phylogeny showed that B. longum subsp. longum and B. breve exhibited considerable intraspecies genetic heterogeneity across three ethnic groups, and strains were clustered into ethnicity-specific lineages. Yet almost all strains of B. animalis subsp. lactis were incorporated into the same phylogenetic clade, regardless of ethnic origin. Our findings support the hypothesis of co-evolution between human gut symbionts and their respective populations, which is closely linked to the lifestyle of specific bacterial lineages. Hence, the natural and evolutionary history of Bifidobacterium species would be an additional consideration when selecting bifidobacterial strains for industrial and therapeutic applications.