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Laboratory Diagnosis of Shiga Toxin-Producing Escherichia coli


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December 2010

Beyond Hot Topic is an opportunity for viewers to submit questions to the Hot Topic presenter. The opportunity to submit questions for this topic is now closed.

The following questions were submitted by viewers and answered by the presenter, Dr. Thomas Grys, Assistant Laboratory Director, Mayo Medical Laboratories New England, Department of Laboratory Medicine and Pathology at Mayo Clinic. Questions are presented as submitted (unedited).

  1. The RT-PCR used to detect Shiga toxin-producing EC, was it developed at the Mayo lab?

    Yes, the real-time PCR assay used to detect Shiga toxin was developed in the Division of Clinical Microbiology.  

  2. Can a 0157 strain be Shiga toxin negative?

    Yes, on rare occasions, strains of enterohemorrhagic E coli (EHEC) may be identified that lack the genes encoding Shiga toxin 1 and/or 2 (note that Shiga toxin-negative E coli are, by definition, no longer STEC). There is some debate about how common this is. Many investigators believe that it is a phenomenon due to identification of an isolate that has lost the phage-encoded gene during an infection, or when an isolated strain is passaged in the laboratory and subsequently loses the gene. In general, the issue of Shiga toxin-negative strains is not one that diagnostic laboratories need to be concerned with at this time.


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