Within-host evolution and transmission of a human gut symbiont across ecological scales.

Adaptive mutations in Bacteroides thetaiotaomicron rapidly spread across co-housed mice, overcoming colonization resistance and enabling successful transmission among hosts.

• Why it matters: Understanding how gut bacteria evolve and disperse within and between hosts is crucial for grasping their long-term stability and impact on host health, yet this process remains poorly characterized.
• What they did: The study tracked approximately 70,000 genomically barcoded Bt lineages in co-housed mice, analyzing transmission rates, adaptive mutations, and ecological influences through high-resolution lineage tracking and whole-genome sequencing.
• The result: Findings show predictable engraftment of specific lineages driven by shared selection pressures, with diverse adaptation modes involving carbohydrate metabolism, highlighting the complex dynamics of gut microbial evolution across ecological scales.