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Mucosa-associated bacteria are increased in inflammatory bowel disease (IBD), which suggests the possibility of an increased source of digestible endogenous mucus substrate. We hypothesized that mucolytic bacteria are increased in IBD, providing increased substrate to sustain nonmucolytic mucosa-associated bacteria.Mucolytic bacteria were characterized by the ability to degrade human secretory mucin (MUC2) in pure and mixed anaerobic cultures. Real-time PCR was used to enumerate mucosa-associated mucolytic bacteria in 46 IBD and 20 control patients. Bacterial mucolytic activity was testedin vitrousing purified human MUC2.We confirm increased total mucosa-associated bacteria 16S rRNA gene in macroscopically and histologically normal intestinal epithelium of both Crohn's disease (CD) (mean 1.9-fold) and ulcerative colitis (UC) (mean 1.3-fold). We found a disproportionate increase in some mucolytic bacteria. MeanRuminococcus gnavuswere increased >4-fold andRuminococcus torques∼100-fold in macroscopically and histologically normal intestinal epithelium of both CD and UC. The most abundantly detected mucolytic bacterium in controls,Akkermansia muciniphila, was reduced many fold in CD and in UC. Coculture ofA. muciniphilawith MUC2 as the sole carbon source led to reduction in its abundance while it augmented growth of other bacteria.Mucolytic bacteria are present in healthy humans, where they are an integral part of the mucosa-associated bacterial consortium. The disproportionate increase inR. gnavusandR. torquescould explain increased total mucosa-associated bacteria in IBD.