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Marler Blog Providing Commentary on Food Poisoning Outbreaks & Litigation

Genetic Fingerprinting – PFGE and MLVA – Scientists Expand Testing Methodology to Distinguish Cookie Dough E. coli Illness Cases

The CDC has confirmed that 69 people, ages 2 to 67, are linked to an E. coli O157:H7 outbreak linked to the consumption of refrigerated Nestle Toll House Cookie Dough. The links are both the food histories as well as the discovery of the outbreak strain of the E. coli O157:H7 bacteria. Nestle has recalled the product and the Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) continue to investigate both illnesses and the cause of the contamination. It is presumed that environmental samples are being tested from the Nestle manufacturing facility as well as recalled cookie dough. To date no non-human samples have tested positive for E. coli O157:H7. However, in most outbreaks (excluding some recent successes in the Dole 2006 E. coli O157:H7 outbreak and the Peanut Corporation of America Salmonella outbreak) the likely offending product (even left-over’s) and the plant that made it, do not produce positive samples.

In addition to interviewing sickened individuals about potential exposures to E. coli O157:H7, the CDC has conducted further scientific tests to determine the link between all sick individuals and the consumption of Nestle Toll House Cookie Dough.

Pulsed-field gel electrophoresis (PFGE), also referred to as “genetic fingerprinting,” is a process used in molecular microbiology to compare E. coli O157:H7 isolates to determine if the strains are distinguishable. In 1993 public health investigators successfully used PFGE analysis to distinguish patients who acquired E. coli O157:H7 after eating contaminated hamburgers from persons infected with E. coli O157:H7 from other sources. The CDC has established PFGE as the molecular test of choice used by public health laboratories for genetic typing of E. coli O157:H7 and other pathogenic bacteria. To date, the 69 sickened individuals share the same “genetic fingerprint” of the E. coli O157:H7 bacteria found in their stool cultures.

Perhaps because cookie dough is a product that has not been linked to E. coli O157:H7 in the past, and because the “genetic fingerprint” found in this outbreak has been seen since 2005, the CDC employed additional scientific testing to assure consumers and Nestle of the actual link between the product and the illnesses.

Other genetic testing methods have been developed, including Multiple-Locus Variable-Number Tandem Repeat Analysis (MLVA). This test has proven to be highly reproducible and portable, features especially important in outbreak investigations. MLVA is a polymerase chain reaction (PCR) based technique used to target tandem repeats, which are areas of the bacterial genome that evolve rapidly. Thus far the CDC has linked 46 of the 69 people by MLVA as well as PFGE. These results, along with the common ingestion of Nestle Toll House Cookie Dough accounted for the recall of the product. Studies have shown that PFGE and MLVA results are well correlated. Evaluating MLVA results in tandem with PFGE results in an outbreak situation allows investigators to further delineate outbreak related illnesses to non-outbreak cases. This has proven to be valuable when the outbreak strain is one that is detected frequently and assumed to have many sources.

So, where does all this science leave us? We know that we have at least 69 individuals who share both the consumption of Nestle Toll House Cookie Dough and the E. coli O157:H7 recovered from their stool match by PFGE. We also know that at least 46 tested by the more sensitive test, MLVA, also match. This is very significant and gives the CDC, FDA, Nestle and its consumers, proof that a link has been found between the product and the illnesses. What we do not yet have are environmental (product and/or plant) sample data, which may or may not have results. However, even without those results the work done by the CDC and FDA (along with state and local health departments) is compelling proof of the link between the illnesses and the Nestle product.

  • John Munsell

    This story should prove that testing has little value in food safety. Huh? Well, for the last five years, a popular mantra within the meat industry is the phrase “We can’t test our way to safe food”. Kinda true. Let me explain. Test results, BY THEMSELVES, will not result in safer food. If we merely accumulate adverse test results, and file them away inacessible to government oversight (this is common), then no improvements will be mandated, or unilaterally implemented by food companies. However, if FDA and USDA would be granted unrestricted access to ALL test results, and had the authority to require corrective actions to prevent recurrences, then we would experience safer food (not guaranteed safe, but safer). So, when government regulators and food manufacturers make the bogus statement that we can’t test our way to safe food, you now know their reasoning: diminish testing, and restrict government access to test results, because we’ve got something to hide. These PFGE test results have conclusively revealed a lethal problem, which cannot now be hidden. John Munsell