The human gastrointestinal tract and oral microbiota in inflammatory bowel disease: a state of the science review

Inflammatory bowel disease (IBD) includes a spectrum of diseases from ulcerative colitis (UC) to Crohn's disease (CD). Many studies have addressed the changes in the microbiota of individuals affected by UC and CD. A decrease in biodiversity and depletion of the phyla Bacteroidetes and Firmicutes has been reported, among others. Changes in microbial composition also result in changes in the metabolites generated in the gut from microbial activity that may involve the amount of butyrate and other metabolites such as H2S being produced. Other factors such as diet, age, or medication need to be taken into consideration when studying dysbiosis associated with IBD. Diverse bacterial species have been associated specifically or non-specifically to IBD, but none of them have been demonstrated to be its ethiological agent. Recent studies also suggest that micro-eukaryotic populations may also be altered in IBD patients. Last, but not least, viruses, and specially bacteriophages, can play a role in controlling microbial populations in the gastrointestinal tract. This may affect both bacterial diversity and metabolism, but possible implications for IBD still remain to be solved. Dysbiosis in the oral microbiome associated with IBD remains an emerging field for future research.