Shared Environments Can Facilitate Microbial Transmission and Alter Metabolic Outcomes
While dietary changes are often implicated in the "Westernization" of traditional microbiomes, 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 Western and non-Western microbiomes and subsequent health outcomes. We found that both air sharing and physical contact enabled bidirectional transmission: Western mucus-degrading taxa (e.g., Akkermansia) was transferred into Thai microbiomes, while Thai-derived, potentially health-promoting bacteria colonized Western microbiomes. Host's baseline gut microbiome composition emerged as a key factor influencing extent of the shift of microbiomes through air or contact transmission. When exposed to 13 dietary ingredients and food additives prevalent in industrialized diets, the Western microbiome responded differently from its Thai counterpart, with food additives reducing Akkermansia. Corroborating to these changes, Western 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 industrialization, revealing how environmental microbial exchange and diet interplay can shape gut communities and their metabolic consequences.