Testing for Non-0157 STECs Leads to Development of New Lab Technology

Between the years 2009 and 2012, the food industry—and meat processing in particular—witnessed a wave of activity surrounding certain non-O157 Shiga toxin-producing Escherichia coli. The USDA Food Safety Inspection Service was petitioned in 2009 to declare E. coli strains O26, O45, O103, O111, O121, and O145, collectively known as the “Big Six,” adulterants, banning them from the nation’s food supply. In September 2011, these six strains achieved that status in recognition of the danger they pose to consumer health, given the fact that they produce the same potentially deadly toxins as O157 STEC.

According to the Centers for Disease Control and Prevention, an estimated 175,000 infections are caused by STEC each year. Approximately 36% of these incidences arise from the STEC O157 strain, despite rigorous testing by the meat and produce industries. The remaining 64% of illnesses can be traced to non-O157 strains. Although non-O157 STECs often cause milder cases of illness with less severe diarrhea, fewer hospitalizations, and an estimated single death per year according to CDC statistics, the number of infections significantly overshadows the more familiar O157 serogroup.

Awareness of the prevalence of non-O157 STECs has grown over several years. In April 2006, an outbreak of foodborne illnesses occurred in New York State, and a non-O157 STEC was suspected to be the culprit. This incident spurred the USDA into making inquiries into what kinds of surveillance states were doing and how they were handling the presence of these toxins. The agency discovered that in many cases, non-O157 strains were appearing in higher numbers than the O157 strain. In fact, in 2010, FoodNet’s foodborne illness burden data showed that the prevalence of non-O157 STECs exceeded that of O157.

On June 4, 2012, the FSIS implemented a sampling and testing program for beef trimmings only. To help the food industry and the public health arena comply with the new regulations and ensure that beef trimmings are free of these pathogens, new technologies in user-friendly kits are now commercially available to isolate these toxins in food samples.

One technology is the RapidFinder STEC Detection System developed by LifeTechnologies of Grand Island, N.Y., in cooperation with the USDA. Eric Liu, a product manager with Life Technologies’ food safety business, explained that the company has an ongoing relationship with the USDA that predates its STEC product development.

“That relationship gave us access to key opinion leaders– the scientists involved with designing the regulations–and it was able to give us insight into where the technology is likely to lead. Also, the development of the actual assay requires real world samples of the Big Six STEC, and our interaction with the USDA provided access to a diverse panel of real-world strains to use for testing and development purposes,” he said.

The system has been awarded a “Letter of No Objection” from the USDA, indicating that the company can pursue commercial customers for this product.

The RapidFinder workflow is a real-time polymerase chain reaction system composed of three parts: the STEC Sample Preparation Kit, which uses triptic soy broth to enrich the regulation-specified 375-gram sample of ground beef or beef trimming in as little as eight hours. A small sample of the enriched broth is then transferred to a magnetic particle processing instrument, where chemical and enzymatic lysis (bacterial cell destruction caused by a specific agent) releases DNA from the E. coli. Inside the instrument, the DNA binds to magnetic beads, which are then exposed to a magnet, separating the E. coli DNA out of the sample.

The second part is the screening assay, which tests the prepared sample for STX/EAE, the telltale genes that indicate that the target E. coli bacteria are present. If the sample yields negative results for these virulence factors, the meat is considered clean. If it tests positive for either gene, the next step is to confirm their presence and identify which strains have contaminated the meat. A confirmation assay is run only when the screening assay has detected STX and/or EAE. This second assay confirms the presence of target E. coli DNA and determines if it is O157 or one or more of the Big Six strains.