Virulence of C. rodentium strains. (A) Data depict the number of C. rodentium CFU recovered from colonic tissues of individual mice, on day 13 postinfection. Mice infected with the wild-type strain had a high pathogen burden. In contrast, mice infected with the cesD2-mutant strain (ICC173) had lower bacterial counts, although the difference was not statistically significant (P > 0.05). In mutant strains complemented with either pICC245 or pICC246, the CFU level was restored to wild-type level. (B) The distal 8 cm of the colons were weighed 13 days postchallenge. The weights of colons from mice infected with the wild-type (WT) strain or complemented mutant strains, carrying either pICC245 or pICC246, were equivalent and significantly greater than those of colons from mice infected with the mutant strain and uninfected controls (P < 0.1). There was no significant difference between the colon weights of uninfected mice and mice infected with ICC173 (P > 0.05).