E. coli Facilitates Murine Campylobacter Infection Figure 4. Inflammatory and immune cell responses following C. jejuni infection of infant mice. Conventionally colonized 3-weeks-old infant mice were orally infected with C. jejuni strain ATCC 43431 (crossed circles) or B2 (solid circles) as described in Methods. The average numbers of apoptotic cells (positive for caspase-3, panel A), neutrophilic granulocytes (neutrophils, positive for MPO-7, panel B), macrophages (positive for F4/80, panel C), T-lymphocytes (positive for CD3, panel D), regulatory T-cells (Tregs, positive for FOXP3, panel E) and B-lymphocytes (positive for B220, panel F) from at least six high power fields (HPF, 6400 magnification) per animal were determined microscopically in immunohistochemically stained colon sections at day 6 p.i. 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) determined by the Student’s t-test are indicated. Data shown are representative for three independent experiments. doi:10.1371/journal.pone.0035988.g004 the clinical signs of enterocolitis following C. jejuni infection remained rather subtle. The reason for this discrepancy in colitis severity is most likely attributed to differences in the genomic contents of the C. jejuni strains used. The intestinal inflammation models exerting a compromised colonization resistance against C. jejuni had all in common significantly higher E. coli loads at inflamed tissue site, for instance, when compared to healthy controls with an intact colonization resistance, irrespective whether the inflammatory condition was acute or chronic, located in the small or large intestine. These findings are well in line with results from previous studies pointing towards a weakness of the physiolog- ical barrier during intestinal inflammation [19,20,21,42]. Bacteremia and sepsis display hazardous sequelae of C. jejuni PLoS ONE | www.plosone.org 7 May 2012 | Volume 7 | Issue 5 | e35988