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Commensal bacteria play a role in the aetiology of inflammatory bowel diseases (IBD). High intestinal numbers ofEscherichia coliin IBD patients suggest a role of this organism in the initiation or progression of chronic gut inflammation. In addition, someE. coligenotypes are more frequently detected in IBD patients than others. We aimed to find out whether gut inflammation in an IBD mouse model is associated with a particularE. colistrain. Intestinal contents and tissue material were taken from 1-, 8-, 16- and 24-week-old interleukin 10-deficient (IL-10-/-) mice and the respective wild-type animals. Caecal and colonic inflammation was observed in IL-10-/- animals from the 8 weeks of life on accompanied by a lower intestinal microbial diversity than in the respective wild-type animals. Culture- based and molecular approaches revealed that animals with gut inflammation harboured significantly higher numbers ofE. colithan healthy controls. Phylogenetic grouping according to theE. coliReference Collection (ECOR) system and strain typing by random-amplified polymorphic DNA and pulsed-field gel electrophoresis revealed that all mice were colonized by one singleE. colistrain. The strain was shown to have the O7:H7:K1 serotype and to belong to the virulence-associated phylogenetic group B2. In a co-association experiment with gnotobiotic mice, the strain outnumberedE. coliECOR strains belonging to the phylogenetic group A and B2 respectively. A high number of virulence- and fitness-associated genes were detected in the strain's genome possibly involved in the bacterial adaptation to the murine intestine.