The “Unité de Pathogénie Microbienne Moléculaire” or “PMM Unit” is located at Institut Pasteur. It was founded in 1989 and has been associated with INSERM since its onset. Its central research topic has historically been the molecular and cellular analysis of bacterial pathogenesis. Our studies have been marked by a multidisciplinary approach encompassing genetics/genomics, cell biology, immunology and experimental medicine, with the permanent concern to validate in vitro data in in vivo models of infection. Our microorganism of interest has largely been Shigella, a model pathogen for the study of the rupture, invasion, subversion and inflammatory destruction of the intestinal epithelial barrier. Over the past 5 years, we have started to study the molecular interactions established between the intestinal microbiota and the gut epithelium. Our current aim is to establish a balance between the study of gut bacterial symbiosis leading to homeostasis, and its rupture by pathogens or dysbiotic conditions.