Specific DNA probes to detect Escherichia coli strains producing cytotoxic necrotising factor type 1 or type 2

Cytotoxic necrotising factors type 1 (CNF1) and type 2 (CNF2) are produced by many Escherichia coli strains isolated from man and animals with intestinal or extra-intestinal colibacillosis. In most laboratories, CNF-producing strains are detected by a cell cytotoxicity assay and confirmed with a neutralisation assay or a mouse footpad assay. In this study, we sought to determine whether DNA probes could detect clinical isolates of E. coli producing CNF2 or CNF1, or both, without the need for cell cultures or animal assays. Two internal fragments of the gene encoding CNF2 were used as DNA probes: a 875-bp XhoI-PstI DNA fragment and an adjacent 335-bp PstI-ClaI fragment. A positive response with both DNA probes was associated with CNF2-producing strains, whereas a positive response with only the 335-bp probe was associated with CNF1-producing strains. Results of colony hybridisation experiments with 185 clinical isolates of E. coli demonstrated that these DNA probes detected CNF2-producing strains with a sensitivity and specificity of 100% and CNF1-producing strains with a sensitivity and specificity of 99%. These two DNA probes should greatly facilitate epidemiological studies to assess the importance of CNF-producing strains as agents of diarrhoea and septicaemia.