All animals exist in intimate associations with communities of microorganisms that play important roles in the hosts’ normal development and physiology, and under certain circumstances can cause disease. In the Guillemin lab we investigate the molecular dialogues between bacteria and host cells that promote normal tissue development or cause pathology. Toward this goal our laboratory focuses on two experimental models. To understand the mechanisms by which intimate, long-term bacterial-host cell interactions can detrimentally alter host tissue programs of development and homeostasis, we study Helicobacter pylori, a pathogen of the human stomach that is associated with a number of diseases including gastric cancer. In order to understand the benefits that microbial associations confer on animals, we have pioneered a germ-free zebrafish model that allows us to precisely define the contributions of the resident microbiota to development of this model vertebrate.