Sullivan et al. 2021 Science 371: eaba8310

INTRODUCTION
The gastrointestinal (GI) tract is a multikingdom cellular ecosystem that facilitates the procurement of nutrients from the environment. In constant contact with the external world, the small intestine is at once a gateway for life-threatening pathogens and toxins and the site of absorption for life-sustaining nutrients. Consequently, this tissue is tasked with the challenge of balancing its primary functions of nutrient uptake and host defense in response to a complex and constantly changing environment.

This challenge is particularly daunting for omnivores, whose diets change on daily, seasonal, and developmental time scales. The diverse diets of such generalists stand in contrast to those of specialists—animals that consume restricted diets—such as carnivores and herbivores. Whereas these spe…