Enhancing detection of STEC in the meat industry: insights into virulence of priority STEC

Detection of Shiga toxin-producing <i>Escherichia coli</i> (STEC) presenting high risk of human infections is challenging. In France, the latest Anses opinion categorized STEC in four groups based on their association with severe forms of clinical infection. STEC strains carrying the <i>eae</i> gene, particularly those of serogroups O157, O26, O111, O103, O145, O121, O45 and more recently O80 (top 8 serogroups), are usually monitored worldwide, whereas <i>eae</i>-negative STEC strains that are less clinically significant are not surveyed. Screening food enrichment broths with classical genetic markers (<i>stx</i>, <i>eae</i>) can overestimate the presence of highly virulent STEC, causing needless disruption and costs for food producers. Recently the updated MLG5C reference method introduced additional genetic markers (<i>espK</i>, <i>espV</i>) in the detection scheme to improve specificity and effectiveness of priority STEC detection in foodstuffs. This study, conducted on beef samples with a new method supporting the regulatory USDA-FSIS MLG5C.04 method, showed that 92% of the <i>stx</i>-positive samples carry <i>stx2</i> alone or in association with <i>stx1</i>. Among the <i>stx2</i>-positive samples, <i>stx2a</i> and/or <i>stx2d</i> subtypes dominate. Introduction of <i>espK</i>, <i>espV</i> markers on 868 <i>stx</i> ⁺/<i>eae</i> ⁺ beef enrichment broths reduced the number of presumptive positive results by 31%, compared to the ISO/TS 13136:2012 reference method. Subsequent analysis of the presumptive positives combining the O-group and the <i>eae</i>-subtype provided also a significant reduction of the number of the presumptive positive for the top 8 <i>eae</i>-positive STEC serogroups; and showed that O26, O103 and O157 were the most prevalent ones. Regarding the <i>stx</i> ⁺/<i>eae^-</i> samples, which are proportionally extremely predominant in beef as compared with the <i>stx</i> ⁺/<i>eae</i> ⁺ samples, 65% of them were positive for the serogroups monitored in this study (O91, O171, O174, O148, O146, O128 O113 and O104). The high occurrence of serogroup O113 observed in beef samples is not corroborated by the clinical data reported in France. Routine testing of beef samples should be revised to prioritize a hierarchical surveillance system based only on high risk STEC (STEC carrying the <i>eae</i> gene) and not on all STEC. This approach would provide Food Business Operators a significant improvement, saving time and costs while maintaining a high level of product safety.