Focus on EHEC - Detection of STEC in Foods of Plant Origin and of Animal Origin
STEC, VTEC, EHEC? A classification
Feb 2023 (Update). Most E. coli (Escherichia coli) strains are apathogenic intestinal commensals (intestinal co-inhabitants) or symbionts (e.g. displacement of pathogenic germs) that do not cause diseases. The exceptions are STEC (Shigatoxin-producing E.coli) and VTEC (Verotoxin-producing E.coli). STEC and VTEC can be used as synonyms here, although for the sake of simplicity only the term STEC will be used in the following.
A subgroup of STEC are EHEC (Enterohaemorrhagic Escherichia coli). They are the most dangerous for the European region and the pathogenic E. coli most frequently found in food and raw materials. EHEC is the clinical medical term used when a STEC has triggered an enterohaemorrhagic syndrome (HUS) in humans and the case of illness has been confirmed. EHEC cannot be tested for in food; at best, STEC is referred to as presumptive (potential) EHEC.
In addition to the above, infections with so-called EPEC (Enteropathogenic E. coli) occur in infants. The group of ETEC (Enterotoxic E. coli (syn. travel diarrhoea E.coli) as well as EIEC (Enteroinvasive E.coli) and EAgg (Enteroaggregative E.coli) can also be distinguished by means of serotyping and molecular biological examinations. These subgroups are difficult to detect in the routine microbiological examination of food. This is only possible in a special laboratory by means of PCR or serotyping of a larger number of cultivated E. coli germs.
Occurrence of STEC in food
According to evaluations of the Federal Institute for Risk Assessment (BfR), STEC are mainly detected in meat, meat products, raw milk and raw milk products of ruminants such as cattle, sheep and goats. In addition, products of wild ruminants and wild boars but also plant foods, such as sprouts, are affected. Recently, flours and other natural products (e.g. dried grasses) have been tested positive. It should be noted that STEC occur only sporadically and cannot be avoided by hygienic measures in food processing. The minimisation of the fecal contamination must therefore take place early during primary production.
STEC in cereal flours and raw doughs
An evaluation by the Federal Institute for Risk Assessment (BfR, 20 January 2020[1]) showed that "STEC are detectable in a considerable proportion (10 to 30%) of the [investigated] flours". This was based on the results of the German-wide monitoring plan (BüP) in 2018. 50 STEC were detected in 328 wheat, spelt and rye flour samples examined (15.2%), whereby in some cases several sub-samples were taken and examined (Federal Office of Consumer Protection and Food Safety (BVL) 2019[2]).
This is also consistent with the results from the investigations of commonly available wheat flours in the Eurofins laboratories for microbiology in Germany. In associated cultures, a multiplication of STEC clones could also be detected, which indicates a potential for infection.
Accordingly, the raw doughs offered primarily in the USA, known as "cookie dough", can be hygienically problematic and should only be made with heat-treated flour and, to avoid infection by Salmonella, with pasteurised egg. Commercial flours should carry a warning label for raw consumption. For the assessment of positive samples, the ALTS made recommendations in its statement 2021/88/02 of the 88th Workshop on the assessment of STEC in flour and products derived therefrom[3].
STEC as pathogens
Even if STEC are not generally human pathogenic, it should be taken into account that even very low infective doses can lead to the disease. Therefore, for ready-to-eat foods, as with other food pathogens, a zero tolerance is required (negative result in 25 g of food). According to EFSA's scientific advice, there was no agreement on a valid marker to assess the risk to the consumer, as there is no single or combination of markers that fully define the pathogenic STEC. The BfR is following and is already linking the assessment of the risk of STEC strains to the detection of the Shiga toxins or the genes coding for them and the isolation of a corresponding viable STEC strain.
Consulting and analysis
Eurofins offers 24 h release analyses using the PCR rapid method. If the result is positive, a test for the ability to multiply can be added. So far, there is no reliable marker for the infectivity of an isolate. An indication is the detection of the eae gene. The serotype can be determined, on which a recall decision may depend in individual EU states. In Germany, the assessment is made according to the ALTS recommendation/VO (EU) 178[4] and evaluated accordingly according to the type of use and consumer information.
We can also advise you on further analysis options, such as whole genome sequencing and the storage of isolates within the framework of the Zoonoses Regulation and infection chain tracing within the context of your supplier monitoring.
Identify your need for action through targeted consultation on your products: Please ask your personal customer service manager or contact our expert Dr Hans-Georg Leusch directly.
References
[1] BfR opinion No 004/2020 of 20 January 2020: Escherichia coli in flour – sources, risks and prevention
[2] Bundesweiter Überwachungsplan 2018 (in German)
[3] 88. Arbeitstagung des ALTS, p. 2-3: 2021/88/02 Beurteilung von STEC (syn. VTEC) in Mehl und daraus hergestellten Erzeugnissen wie Backmischungen und Frischeteige (in German)
[4] BVL - Startseite - Empfehlungen zur Einstufung bedenklicher Keime als wahrscheinlich gesundheitsschädlich i. S. des Art. 14 Abs. 4 oder als inakzeptable Kontamination i. S. des Art. 14 Abs. 5 der VO (EG) Nr. 178/2002 (bund.de) (in German)
