E. coli Facilitates Murine Campylobacter Infection Figure 2. C. jejuni colonization along the gastrointestinal tract of IL-102/2 mice with chronic colitis. Conventionally colonized 6-months- old IL-102/2 mice with chronic colitis were orally infected with C. jejuni strain ATCC 43431 as described in Methods. (A) At day 14 p.i. C. jejuni densities in distinct compartments of the gastrointestinal tract were determined by quantification of live C. jejuni in luminal samples (CFU, colony forming units; solid circles) taken from stomach, duodenum, ileum, and colon by cultural analysis and compared to wildtype controls (WT, open circles). (B) Furthermore, E. coli loads in luminal colonic samples of C. jejuni infected IL-102/2 mice (IL-10, open circles) were compared to healthy wildtype control animals (WT, open diamonds). (C) A complex molecular analysis of the colonic microbiota composition was assessed in IL-102/2 mice with chronic colitis (IL-10; solid circles) and compared to wildtype control animals (WT; open circles). Main gut bacterial groups were quantified by Real-Time-PCR analyses amplifying bacterial 16S rRNA variable regions and 16S rRNA gene numbers/ng DNA derived from luminal colonic samples: Enterobacteriaceae (EB), Enterococci (EC), Lactic acid bacteria (LB), Bifidobacteria (BIF), Bacteroides/Prevotella spp. (BP), Clostridium leptum group (CLEP), Clostridium coccoides group (CLOCC), Mouse intestinal bacteroidetes (MIB), and total eubacterial load (TL). Numbers of animals harboring C. jejuni or E. coli out of the total number of analyzed animals are given in parentheses. Medians (black bars) and significance levels (P- values) determined by Mann-Whitney-U test are indicated. Data shown were pooled from at least three independent experiments. doi:10.1371/journal.pone.0035988.g002 strikingly, the physiological CR against C. jejuni can be abrogated by elevating a single bacterial commensal gram-negative species such as E. coli within the complex commensal intestinal microbiota towards supra-physiological levels. These results point towards a pivotal role of E. coli in facilitating C. jejuni infection. Discussion Investigations of the molecular mechanisms underlying C. jejuni induced immunpathology in vivo are hampered by colonization resistance of mice due to their host-specific intestinal microbiota PLoS ONE | www.plosone.org 5 May 2012 | Volume 7 | Issue 5 | e35988