COMPANION ANIMALS SYMPOSIUM: Humanized animal models of the microbiome

Humans and other mammals are col- onized by trillions of microorganisms, most of which reside in the gastrointestinal tract, that provide key metabolic capabilities, such as the biosynthesis of vi- tamins and AA, the degradation of dietary plant poly- saccharides, and the metabolism of orally administered therapeutics. Although much progress has been made by studying the human microbiome directly, compar- ing the human microbiome with that of other animals, and constructing in vitro models of the human gut, there remains a need to develop in vivo models where host, microbial, and environmental parameters can be manipulated. Here, we discuss some of the initial re- sults from a promising method that enables the direct manipulation of microbial community structure, envi- ronmental exposures, host genotype, and other factors: the colonization of germ-free animals with complex microbial communities, including those from humans or other animal donors. Analyses of these resulting "humanized" gut microbiomes have begun to reveal 1) that key microbial activities can be transferred from the donor to the recipient animal (e.g., microbial re- duction of cholesterol and production of equol), 2) that dietary shifts can affect the composition, gene abun- dance, and gene expression of the gut microbiome, 3) the succession of the microbial community in infants and ex-germ-free adult animals, and 4) the biogeogra- phy of these microbes across the length of gastrointes- tinal tract. Continued studies of humanized and other intentionally colonized animal models stand to provide new insight into not only the human microbiome, but also the microbiomes of our animal companions.