sion, intestinal disease manifestations such as enteritis and diarrhea are absent [12,13]. Murine models of C. jejuni infection have the disadvantage of sporadic colonization, absence of clinical signs of disease and/or lack of intestinal immunopathology [6,12]. This is in part due to the colonization resistance against C. jejuni exhibited by adult mice older than two months (commonly used in experimental settings) harboring a conventional intestinal micro- biota [12]. Results from the 1980s demonstrated that colonization resistance was abrogated in isolator raised germfree (GF) mice. In these animals C. jejuni colonized the entire gastrointestinal (GI) tract and induced clinical signs of enterocolitis including granulocyte infiltrates, bloody diarrhea, and humoral immune responses [14,15,16]. More recent investigations showed that colonization resistance is mainly caused by the host-specific intestinal microbiota composition [17,18]. Previous work in our group revealed that colonization resistance of mice against C. jejuni is abrogated in gnotobiotic (GB) mice generated by quintuple antibiotic treatment as well as in GB mice replenished with human flora (hfa) [17]. In contrast to isolator raised germfree mice, however, GB and hfa animals display a fully established immune system. The finding that GB mice replenished with murine flora were completely resistant to C. jejuni proved that colonization resistance is caused by the host-specific microflora composition [17]. Most strikingly, C. jejuni infected GB and hfa mice displayed characteristic inflammatory responses seen in human campylo- bacteriosis [17]. The major role of the microflora composition in colonization resistance against C. jejuni was further supported by the recent finding that obesity caused by both feeding human ‘‘Western style’’ cafeteria diet (CAF) as well as leptin deficiency in ob/ob2/2 mice rendered mice susceptible to C. jejuni infection [18]. Interestingly, quantitative analysis of main gut bacterial groups revealed 2–3 orders of magnitude higher loads of commensal E. coli in C. jejuni susceptible animals in all these models as a common feature as compared to respective resistant controls [17,18]. Recent studies on Salmonella enterica also point towards a pivotal role of microbiota composition in host susceptibility to enteropathogenic bacteria [19,20]. Mice displaying high com- mensal E. coli densities were more susceptible to S. enterica induced gut inflammation [19,20,21]. We have previously shown that murine models of acute small as well as large intestinal inflammation were associated with tremendous changes of the intestinal microbiota with decreasing species diversity and lactobacilli loads and an intestinal overgrowth with commensal E. coli and Bacteroides/Prevotella spp. [22,23,24,25]. We could further show that TLR-4-signalling of E. coli LPS initiated and perpetuated acute ileitis and colitis in mice [23,24]. Thus, these experimental models were mimicking immunopathological key features of human IBD Crohn’s disease (CD) and ulcerative colitis (UC). The exact etiology of IBD remains unknown and the pathogenesis seems to be multifactorial, involving the complex interaction between the intestinal microflora, host immune and genetic factors as well as environmental stimuli [26,27], Gram- negative bacteria such as E. coli and Bacteroides spp. have been reported to accumulate at inflamed tissue sites in IBD patients with active intestinal inflammation, and further potentiate immunopa- thology by translocation via microlesions and ulcerations [28,29]. It is under current debate whether IBD patients might be more susceptible to intestinal pathogens such as Campylobacter or Salmonella spp. and whether acute bacterial enterocolitis is one of the factors inducing or exacerbating IBD in susceptible individuals [26,27,30]. Results from population-based studies point towards a potential role of enteric Campylobacter or Salmonella infections in IBD development [31,32,33]. E. coli Facilitates Murine Campylobacter Infection In the present study, we investigated colonization resistance in mice displaying an altered gut microbiota composition under conditions of acute or chronic intestinal inflammation as well as in infant mice after weaning (lacking signs of intestinal inflammation) having all elevated intestinal E. coli loads in common. Finally we asked the question whether feeding healthy adult mice with live E. coli and thus artificially elevating the enterobacteria population within the commensal gut microbiota towards supra-physiological loads might overcome colonization resistance and renders the animals susceptible to C. jejuni. The results clearly demonstrate that colonization resistance is completely abrogated under conditions of elevated E. coli loads within the commensal intestinal microbiota. Following infection, C. jejuni induced pro-inflammatory immune responses mimicking key features of human campylobacteriosis. Taken together, the murine C. jejuni infection models presented here proved to be suitable for reproducible investigations of factors that predispose C. jejuni colonization and trigger the onset and progress of infection and immunopathology. Results T. gondii-induced Acute Ileitis Abrogates Colonization Resistance of Mice Against C. jejuni Infection Colonization resistance against C. jejuni exerted by mice harbouring a conventional intestinal microbiota can be overcome by modification of the gut microbiota composition. We therefore investigated C. jejuni infection capacities in mice with acute or chronic intestinal inflammation displaying distinct microbiota shifts towards higher intestinal enterobacteria loads. Three-months-old adult C57BL/6 mice harboring a conven- tional gut microbiota were infected perorally with T. gondii in order to induce acute ileitis. Four days thereafter, the time point when initial histopathological changes in the ileum mucosa can be observed, mice were orally subjected to 109 CFU C. jejuni ATCC 43431 by gavage. At day 7 following T. gondii infection, mice suffered from acute ileitis and were prone to death. Interestingly, by that time (i.e. 3 days following C. jejuni ATCC 43431 infection) C. jejuni ATCC 43431 could be cultured from the entire GI tract of diseased, but not healthy mice with high loads of 109 CFU/g in duodenum, ileum and colon (Fig. 1A). Notably, the E. coli concentrations in the ileum lumen had increased by more than 6 orders of magnitude up to more than 1010 CFU/g during ileitis development (Fig. 1B). Live C. jejuni ATCC 43431 and commensal E. coli translocated into MLNs draining the small intestine (10060% and 78.667.1%, respec- tively), spleen (57.161.49% and 50.060.68%, respectively) and liver (57.160% and 42.960%, respectively) (Fig. 1C). Strik- ingly, in 92.960.68%, but only in 7.160.7% of mice with acute ileitis following T. gondii infection, live C. jejuni ATCC 43431 and E. coli, respectively, could be cultured from cardiac blood indicating C. jejuni bacteremia (Fig. 1C). A molecular survey of the ileum microbiota composition revealed that concentrations of total eubacteria, enterobacteria, enterococci and Bacteroides/ Prevotella spp. were increased in T. gondii-induced acute ileitis, whereas the lactobacilli population was lower as compared to healthy mice (Fig. 1D). Thus, during acute small intestinal inflammation accompanied by distinct intestinal microbiota shifts towards E. coli overgrowth in the GI tract colonization resistance against C. jejuni is abrogated with hazardous consequences for the host, as indicated by translocation and systemic spread of the pathogen. PLoS ONE | www.plosone.org 2 May 2012 | Volume 7 | Issue 5 | e35988