title: The intestinal flora in inflammatory bowel disease: normal or abnormal?
abstract: OBJECTIVE
  The human gut is the natural habitat for a large and dynamic bacterial community. There is a substantial body of evidence implicating the resident flora in the pathogenesis of chronic intestinal inflammation. This review discusses the possible role of a microbial imbalance in the pathophysiology of inflammatory bowel disease.
  
  
  RESULTS
  Recently developed molecular biologic tools suggest that a sizeable part of the microbial populations of the human gut remains to be defined. Conversely, the relevance of gut bacteria in the host's physiology is well documented. The specialised lymphoid follicles of the gut mucosa are the major sites for induction of effector and regulatory mechanisms of the intestinal immune system, and it is now becoming clear that resident and in-transit microorganisms play an essential role in the homeostasis of local and systemic immunity. An infectious origin of inflammatory bowel disease is not supported by our current knowledge. Several studies, however, have disclosed substantial differences in the intestinal flora between patients with inflammatory bowel disease and healthy subjects, in regard to both composition and mucosal colonisation. Aggressive species are abundant on the inflammatory bowel disease mucosal surface whereas protective genera are underrepresented, but the biologic relevance of these changes needs further investigation.
  
  
  CONCLUSIONS
  A balanced microbial environment would likely help in both prevention and control of inflammatory bowel disease. Research is needed to identify microorganisms able to mediate immunoregulation in the gut mucosa.
authors.affiliation: Digestive System Research Unit, University Hospital Vall d'Hebron, Barcelona, Spain. fguarnera@medynet.com
authors.forename: Francisco
authors.initials: F
authors.lastname: Guarner
mesh_terms: D001420:Bacteria, Aerobic / Q000502:physiology*; D001421:Bacteria, Anaerobic / Q000502:physiology*; D018988:Bacterial Translocation; D006801:Humans; D015212:Inflammatory Bowel Diseases / Q000382:microbiology*; D007413:Intestinal Mucosa / Q000382:microbiology*; D013559:Symbiosis