Understanding presumptive positives in meat testing
By Julie Archer - 07 August 2018
The majority of Escherichia coli strains are harmless and are generally seen as hygiene indicators in the food industry. However, a small number of strains of the Shigatoxin producing E coli (STEC) are highly virulent and have a low infective dose. These STEC E coli have caused a number of food borne outbreaks, associated with a variety of foods including undercooked meats, particularly burgers, some fresh produce, dairy products – particularly raw milk, and in Europe a large outbreak was linked with Egyptian fenugreek seeds.
Compulsory STEC testing
It’s now compulsory to test sprouted seeds in Europe and raw red meats in the USA for several serogroups of STEC. In Europe sprouted seeds must be tested for several STEC serogroups, collectively named the ‘Big 6’ E coli O26, O103, O111, O145, O157 and O104, and in the USA meat must be tested for the ‘Top 7’which includes the same serotypes except O45 and O121 instead of O104.
Highly specific testing
Specific DNA based PCR methods enable STEC E coli to be identified against a background of other closely related non-pathogenic E coli and other non-target microorganisms. Whilst PCR methods are complicated to run and interpret, they provide the specificity required. This approach is very different to the usual, agar plate based, methods for pathogens such as Salmonella and Listeria.
It is very important to remember that the any positive results are presumptive and not confirmed positives. PCR is done using a sample taken from an overnight enrichment broth and not from a colony on a plate, so a positive PCR result doesn’t necessarily mean the microorganism is present. However, a negative result will indicate that the organism is absent. Sometimes fragments of DNA from other strains or species present in the broth can combine and provide false positive PCR results. Therefore, it’s extremely important to treat any presumptive positive PCR results with caution, until they have been confirmed. To confirm a PCR result, you must also isolate the STEC E coli by growing it on a plate and repeating the PCR using an isolated colony from that plate.
Campden BRI have a UKAS accredited next day PCR based method for the detection of European Big 6 in sprouted seeds and the USA Top 7 STEC detection in red meats to provide the industry with quick and reliable results. We use our containment Class Level 3 laboratory (required to handle STEC E. coli) to confirm presumptive results rapidly so you can make decisions based on confirmed results.
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