Food Safety Magazine


Testing for non-O157 STEC Escherichia coli in Beef

By Food Safety Net Services

In 2011, the United States Department of Agriculture (USDA) announced that Escherichia coli serogroups O26, O103, O45, O111, O121 and O145 will be declared adulterants in non-intact raw beef (raw ground beef, its precursors and tenderized steaks). This is in addition to E. coli serogroup O157:H7, which has been considered as an adulterant since 1994. Implementation was slated for March 5, 2012; however, the USDA-Food Safety and Inspection Service (FSIS) will begin testing for these six serogroups of STEC (known as Top 6 STEC) and enforcing the new policy on June 4, 2012.

The U.S. Centers for Disease Control and Prevention (CDC) tracks illnesses and outbreaks associated with both E. coli O157:H7 and Top 6 STEC. Over the last decade, they have been able to demonstrate that illness associated with O157:H7 is on the decline, while illness associated with STEC is on the increase. Although illness associated with STEC is generally though be to less severe than O157:H7, the European outbreak of O104 associated with sprouts shows that STECs can also cause widespread and serious human illness. It should be noted that this serotype is not classified as an adulterant under this new policy. The association of beef products with STEC illness is not well understood; however, there was an outbreak of O26 (three known cases) associated with beef products in mid-2010.

The announcement in the delay of the policy was received with relief by some members of the beef industry and with disappointment by some consumer groups. The stated purpose of delay was to allow the beef industry to evaluate and implement suitable rapid testing methods. A number of processors have taken the opportunity to collect data prior to June, while others are waiting for the policy to be implemented. While there may be hopes of a further delay, it is anticipated that the policy will indeed take effect on June 4 of this year.

Once the policy is implemented, the USDA-FSIS will commence with sampling and testing beef trim. FSIS sampling is expected to expand to include other ground beef components and ground beef. For trim testing, FSIS has stated that they will collect two 325-g samples, one to be tested for O157:H7 and the other to be tested for the Top 6 STEC. Currently the USDA-FSIS has two separate methods for the detection of O157:H7 and STEC, hence the need to collect and analyze two samples. However, recently the USDA has stated that the Top 6 method may also be used to screen samples for O157:H7, eliminating the necessity of testing two samples. At present, it is unclear how this will affect testing by the USDA.

Testing Options
There are a number of options currently available for testing the Top 6 STEC in beef. Currently most of these methods use genetic detection (PCR) for the detection of the organisms. The USDA-FSIS Microbiology Laboratory Guidebook (MLG) incorporates and enrichment followed by PCR analysis is for genes encoding the virulence factors shiga toxin and intimin (stx/eae). If the sample is positive, PCR is conducted for the serotype, also known as the O group (wzx gene). The MLG method is still relativity complex and labor intensive; however, a number of commercial options are available to obtain rapid test results, with a comparable time to result for the current E. coli O157:H7 offerings.

DuPont Qualicon offers the BAX Detection System Real-Time STEC suite, which provides a rapid analysis for the presence or absence of eae and stx. The BAX suite also offers rapid detection for the presence or absence of the O group. BioControl (Bellevue, WA) provides the Assurance GDS, and has several options available, including assays for the Top 6 alone, and E. coli O157:H7 in conjunction with the Top 6 in a multiplex assay. The GDS technology includes an immunocapture step followed by analysis for the presence or absence of eae and stx.

In addition, many new solutions are being developed. For example, bioMérieux is developing a comprehensive STEC solution that should be launched by the end of this year or early next year. The total solution includes an immunocapture step to concentrate the STEC, a detection assay that will detect both virulence markers and specific serotypes of concern and an improved selective plate to help differentiate STEC of concern.

When considering a STEC testing solution, beef processors need to consider the type of data they need to collect, the speed and reliability of the results and whether validation data is available.

Testing Strategy
Food Safety Net Services (FSNS) recommends that companies implement preliminary testing to pilot the methods and to determine false-positive rates. Careful consideration should be given to what type of product is tested, although not currently an adulterant a positive result does indicate the presence of pathogen in a product and should be considered when making product disposition decisions. FSIS has indicated that it is up to the manufacturer to determine product disposition and they will not act on positive results until June, unless there is foodborne illness associated with a product. After June 4, 2012, a positive-testing product would have to be diverted to cooking or another treatment that would render it suitable for human consumption, or the product can also be destroyed. A FSIS-detected positive will also result in a food safety assessment.

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