An in vivo assay for the detection of cytotoxic strains of Escherichia coli.

We have examined the local and systemic responses produced in mice by the inoculation of the footpad with cell-free sonicates from cytotoxic and non-cytotoxic strains of Escherichia coli. The cytotoxic strains, previously characterized in HeLa cell culture assay, produced one of the following cytotoxins: (1) cytotoxic necrotizing factor (CNF; 5 strains); (2) a cytotoxin that induced polynucleation of cells but was serologically distinct from CNF (4 strains); (3) Shiga-like toxin (3 strains). Whereas extracts from non-cytotoxic strains induced only a slight and transitory local edema, those from cytotoxic strains produced a marked local inflammatory response. This response, specific for the type of cytotoxin, was measured by increase in footpad thickness, persistence of inflammation and occurrence of necrosis of the leg extremity. Type 1 and 2 extracts produced persistent inflammatory lesions with, in the case of type 2 extracts only, necrosis of the leg extremity. Conversely, type 3 activity was characterized by a transient local infiltration with no apparent residual lesions. The mouse footpad test therefore provides an in vivo assay for the differential diagnosis of cytotoxic strains of E. coli.