Shared environments can facilitate microbial transmission and alter metabolic outcomes

While dietary changes are often implicated in the adaptation of traditional microbiomes to highly industrialized society, it only partially explains the observed microbial shifts post-immigration. Here, we used germ-free mice colonized with human donor stool from the United States and Thailand to investigate how person-to-person transmission of gut commensals - via shared air and physical contact - affects U.S. and Thai microbiomes and subsequent health outcomes. We found that both air sharing and physical contact enabled bidirectional transmission: U.S. mucus-degrading taxa (e.g., Akkermansia) was transferred into Thai microbiomes, while Thai-derived, potentially health-promoting bacteria colonized U.S. microbiomes. The host's baseline gut microbiome composition emerged as a key factor influencing the extent of the microbial remodeling. When exposed to 13 dietary ingredients and food additives prevalent in industrialized diets, the U.S. microbiome responded differently from its Thai counterpart, with food additives reducing Akkermansia. Corroborating these changes, the U.S. microbiome showed a predisposition toward weight gain under industrialized dietary conditions. However, sharing air supply or co-housing mitigated this effect - likely through the transfer of health-promoting bacteria (e.g., Lactobacillus). Our findings show the complexity of microbiome adaptation to industrialization, revealing how environmental microbial exchange and diet interplay can shape gut communities and their metabolic consequences.