We reported on Labor day weekend that health authorities in Michigan were investigating a cluster of E. coli O157:H7 illnesses that appeared to match each other, and were thus thought to have come from the same source. In fact, Michigan state and local health officials recently announced their conclusion that the three confirmed cases are actually from quite different strains of E. coli O157:H7. Thus, it appears that there was no single source of infection.
This is a good example of the utility of microbiology in investigating foodborne disease, and how it relates to the science of epidemiology in tracking outbreaks. The testing done to distinguish E. coli O157:H7 from its other E. coli counterparts is called serotyping. Pulsed-field gel electrophoresis (“PFGE”), sometimes also referred to as genetic fingerprinting, is used to compare E. coli O157:H7 isolates to determine if the strains are distinguishable.
Through PFGE testing, isolates obtained from the stool cultures of probable outbreak cases can be compared to the genetic fingerprint of the outbreak strain, confirming that the person was in fact part of the outbreak. Because PFGE has proved to be such a powerful outbreak investigation tool, PulseNet, a national database of PFGE test results, was created to enable health investigators to track more-than-baseline occurrences of specific strains of bacteria to determine whether an outbreak is occuring.
PFGE testing, in combination with other methods of identifying similarities between the genetic profiles of bacterial isolates, must be employed alongside more traditional epidemiological methods. These would include consideration of food exposures for any matching illnesses, geographical proximity between cases, the time of exposure as it relates to the existence of an implicated food in the marketplace, and other things.
Good work by state and local Michigan health authorities. It is just as important to quickly and efficiently determine what illnesses are not related to each other as those that are.