Detection of Non-O157 Shiga Toxin–Producing Escherichia coli in 375 Grams of Beef Trim Enrichments across Multiple Commercial PCR Detection Platforms

Abstract

Although serotype O157:H7 remains the pathogenic Shiga toxin-producing Escherichia coli (STEC) of primary concern worldwide, some focus in the United States has shifted to six particular non-O157 STEC serogroups (O26, O45, O103, O111, O121, and O145). Some of these serogroups have also emerged as concerns elsewhere around the world, including Europe. The objective of this work was to compare commercial detection methods with the U.S. Department of Agriculture (USDA) reference method for detection of non-O157 STEC in 375 g of beef trim using a limit of detection study design. Overall, the commercial platforms performed well, showing similar levels of sensitivity for detection of presumptive positives for O45, O26, O103, and O121 (PCR screen results only). For O111, one method that utilizes an integrated immunomagnetic separation and PCR approach was more sensitive than a PCR-only screen approach. Additionally, one commercial method showed more presumptive and confirmed positives overall. Use of an immunomagnetic separation tool, such as antibody-coated beads, aided considerably with the confirmation procedures and is an important step when confirming suspect samples. A secondary goal of this study was to evaluate isolation and International Organization for Standardization confirmation protocols used in Europe compared with strategies provided by the USDA Microbiology Laboratory Guidebook (MLG). Generally, results from the USDA confirmation plates (modified Rainbow agar) were better than the European Union confirmation plates (MacConkey agar with or without rhamnose). In summary, detection of non-O157 STEC in 375 g of beef trim can be performed by any of the three methods on the market evaluated in the study.

Comparison of Six Chromogenic Agar Media for the Isolation of a Broad Variety of Non-O157 Shigatoxin-Producing Escherichia coli (STEC) Serogroups

“…Supplementation of the medium with 0.8 mg/L potassium tellurite and 10 mg/L sodium novobiocin is recommended by the manufacturer for samples with high microbiological background, whereas 0.05 mg/L cefixime, 0.15 mg/L potassium tellurite, and 5 mg/L novobiocin is recommended by the USDA STEC [ 17 ]. However, multiple studies demonstrated that both supplementations were unable to support the growth of a substantial proportion of STEC strains tested [ 9 , 15 , 18 ]. Further, Kase et al [ 9 ] showed that the addition of washed sheep’s blood to RBA substantially reduced these inhibitions.…”