GBS (Streptococcus agalactiae) is a Gram-positive encapsulated commensal bacterium of the human intestinal flora which is also present in the vaginal flora of 30% of healthy women. However, when neonates are infected through their mother during delivery, it can turn into a highly pathogenic bacterium, being the leading cause of neonatal septicaemia, meningitis, and pneumonia in the industrialized world. Early onset (EOD) and late onset (LOD) diseases are defined in neonates by their occurring within or after the first week of life, respectively. Previous molecular epidemiological studies including ours have identified a capsular serotype III clone, referred to as CC-17 by Multi Locus Sequence Typing, as accounting for the vast majority of neonatal invasive diseases and for almost all cases of meningitis in LOD. Our working hypothesis is that host-environmental interactions may contribute to the colonization and persistence of the hypervirulent CC-17 GBS in the neonate. In this project, four teams from the APHP, Institut Pasteur Paris, Institut Pasteur Lille, and INRA will combine their complementary expertises to determine if reciprocal interactions between the intestinal microbiota and the innate and adaptive immune system may specifically facilitate the colonization of the neonate by the hypervirulent GBS CC-17 clone.