It is clear evidence that the microbiota, or intestinal flora, play a critical part in Inflammatory Bowel Diseases. Based on samples from families with several cases of Crohn's disease, this work aims at emphasizing and characterizing bacteria specifically associated with this condition, and identifying at-risk subjects (non-diseased) who carry this microbial species to consider providing them with a preventive treatment targeting the species. This study will also follow for the first time the mode of transmission of microbial species over time in a family.