Survival rate of mice after transient colonization with Escherichia coli clones carrying variant Shiga-like toxin type II operons.

We have previously employed a streptomycin-treated mouse model to demonstrate that Escherichia coli clones expressing particular variant Shiga-like toxin type II operons differ in oral virulence, as judged by median survival time. Differences in virulence were not seen between all toxin variants, including two which differed significantly in cytotoxicity for Vero cells. In the present study, we have modified the animal model by withdrawing antibiotic selection and reintroducing normal mouse intestinal flora at various times after oral challenge with three different variant SLT-II-producing clones. This resulted in a transient colonization more akin to that seen in natural human infections. This has enabled detection of significant differences in survival rate between mice challenged with E. coli clones producing different SLT-II variants, which were not observed when colonization was maintained at high levels.