DNA probe for detection of serogroup O157 enterohemorrhagic Escherichia coli

To develop a probe for the detection of serogroup O157 enterohemorrhagic Escherichia coli (EHEC), plasmid DNA extracts from 16 E. coli strains that hybridized with the CVD419 probe were screened for restriction fragments present in plasmids of serogroup O157 E. coli strains, but not in plasmids of non-O157 E. coli strains. Using a single O157:H7 E. coli strain (639I), 10 serogroup O157 E. coli specific fragments were then removed, radiolabeled and hybridized (42 degrees C) with colony blots of both groups of strains. A 2.0 kb SmaI fragment probe (VPM1) was the most specific for serogroup O157 EHEC. Using a larger set of 41 non-E. coli and 107 E. coli strains from human, animal and meat sources, VPM1 hybridized with all 49 serogroup O157 EHEC strains. None of 8 enterotoxigenic E. coli (ETEC), including serogroup O157 strains, nor any of the 41 non-E. coli with the VPM1 probe. However, this probe hybridized with 5 of 50 non-O157 E. coli which were verotoxin (VT) or CVD419 probe-positive. Increased hybridization stringency (45 degrees C) reduced the 5 false-positives to 2 negatives and 3 trace responses, which were easily distinguishable from positive responses.