The mouse is the most commonly used model species in biomedical research. Just as human physical and mental health are influenced by the commensal gut bacteria, mouse models of disease are influenced by the fecal microbiome (FM). The source of mice represents one of the strongest influences on the FM and can influence the phenotype of disease models. The FM influences behavior in mice leading to the hypothesis that mice of the same genetic background from different vendors, will have different behavioral phenotypes. To test this hypothesis, colonies of CD-1 mice, rederived via embryo transfer into surrogate dams from four different suppliers, were subjected to phenotyping assays assessing behavior and physiological parameters. Significant differences in behavior, growth rate, metabolism, and hematological parameters were observed. Collectively, these findings show the profound influence of supplier-origin FMs on host behavior and physiology in healthy, genetically similar, wild-type mice maintained in identical environments. Ericsson et al. show that different vendors (suppliers of mouse strains) harbor distinct microbiomes, which drive distinct behavioral phenotypes when the genetics are fixed. They specifically focus on changes relating to exploratory and anxiety-related behavior, physiological phenotypic parameters, glucose metabolism, and blood leukocytes. They conclude by emphasizing that supplier-origin fecal microbiomes represent potential sources of poor experimental reproducibility and suggest means to optimize experimentation with mice and their microbiomes. [ABSTRACT FROM AUTHOR]