E. coli Facilitates Murine Campylobacter Infection Figure 5. Pro-inflammatory cytokine responses in the colon of C. jejuni infected infant mice. Conventionally colonized 3-weeks-old infant mice were orally infected with C. jejuni strain ATCC 43431 (43431, crossed circles) or B2 (solid circles) as described in Methods. Concentrations (pg per mg colon) of (A) nitric oxide, (B) TNF-a, (C) IL-6, (D) MCP-1, and (E) IFN-c were determined in supernatants of ex vivo colon cultures at day 6 p.i. by Griess reaction or CBA-ELISA, respectively. Uninfected mice (Na¨ıve, open circles) served as negative controls. Numbers of analyzed animals are given in parentheses. Means (black bars) and levels of significance (P-values) as compared to the respective group (determined by Student’s t-test) are indicated. Data shown are representative for three independent experiments. doi:10.1371/journal.pone.0035988.g005 infection in hosts with a compromized epithelial barrier function (e.g. Crohn’s disease patients with acute episodes). A high prevalence of C. concisus was recently detected in paediatric CD as well as adult UC patients [43,44]. Although distinct from IBD, irritable bowel syndrome was described to occur as a potential post-infectious event following C. jejuni infection [45]. Taken together, these findings once more point towards a correlation between inflammation, intestinal microbiota and susceptibility to C. jejuni infection. Given that colonization of the intestinal tract starts right after birth and complexity increases thereafter, the question arises whether infant mice already display a fully established colonization resistance. Neonatal mice had been used in Campylobacter research already three decades ago, investigating transmissibility from infant to dam, virulence factors, vaccines and the efficacy of bismuth subsalicylate in preventing the growth and colonization of the intestinal tract [35,36,37,39,46]. Remarkably, infection of 3-weeks-old mice right after weaning in our study revealed that C. jejuni B2, and to a less distinct extent strain ATCC 43431, could stably colonize the GI tract of mice. Moreover, infant mice displayed approximately 2–3 orders of magnitude higher E. coli counts in their feces as compared to 3-months-old adult animals. Thus, there is evidence that an altered microbiota towards higher enterobac- teria numbers (such as E. coli) might facilitate C. jejuni infection. When assessing C. jejuni induced clinical pathology due to blood- PLoS ONE | www.plosone.org 8 May 2012 | Volume 7 | Issue 5 | e35988