The Rakoff-Nahoum lab uses ecological and evolutionary approaches to study the mammalian gut microbiome, the collection of microorganisms that call the intestine “home.” We will explore what factors determine which microbes live in the gut: Is it based on our diets, how microbes interact with each other, or whether the host can choose? Is it a combination of all these? Which is most important? While host immune cells are trained to recognize and destroy bacteria, an open question is whether and how our bodies accept and even encourage certain microbes, such as those that may be beneficial, to inhabit the gut. One mechanism could involve the action of lectins, a family of proteins secreted by cells lining the intestine, that bind to sugars—including those that decorate the surface of bacteria to either keep or kill members of the microbiome. Using techniques in physical chemistry, microbiology, nucleotide sequencing, and molecular genetics, we will catalog the lectins produced in the mouse intestine and analyze which microbes they recognize. We will also identify the molecules, produced by the gut bacteria, that alter the profile of lectins to generate a more microbe-friendly environment. By unraveling the “language” through which microbes and host cells communicate, our work could uncover a novel approach to fostering the growth of beneficial bacteria.