Isolation of E. coli from patient samples is performed by hospital and community laboratories close to where the patient resides or is admitted to hospital. For certain regions without adequate laboratory access, the Ontario Public Health Laboratory (OPHL) will also process primary clinical specimens.

The ministry’s Central Public Health Laboratory (CPHL) in Toronto receives all E. coli isolates from hospitals and community laboratories in Ontario for confirmation of subtype H7, which is the one that causes bloody diarrhea. All E. coli O157:H7 isolates then undergo a DNA fingerprinting method at CPHL to see if they are related. This method is called pulse field gel electrophoresis (PFGE).

The PFGE pattern is then forwarded to the National Medical Laboratory in Winnipeg. The national lab assigns a name to the PFGE pattern and compares it with other patterns in Canada and North America as part of a continually updated network called Pulsenet. If a cluster of similar patterns is identified, the local public health unit is immediately notified.

When public health units identify a cluster of reported and suspected E. coli O157-related cases, they forward samples of food and water consumed by those who became ill to the CPHL for culture of E. coli O157. Food and water testing is initiated at the same as the testing of patient isolates to avoid delays in identifying potentially implicated foods. If identified, the E. coli O157 isolates from food and water are tested along with patient samples using PFGE with the aim of identifying a potential match.

On rare occasions, the PFGE pattern can be the same but the isolates not related and not part of the same outbreak. To resolve this issue a more sophisticated fingerprinting method called Multiple Loci VNTR Analysis (MLVA) is performed. Currently this is done at the Centers for Disease Control (CDC) in Atlanta.

November 2008