Shiga Toxin-Producing E. coli O121 in flour
Shiga toxin-producing E. coli O121 and E. coli O26 strains have been identified in the USA and Canada in a flour-mediated outbreak. Consumers have been sickened mainly through eating, licking, or tasting raw, homemade dough or batter, and when young children were involved in pasta making, but cross-contamination has also occurred.

O26 and O121 strains belong to the STEC/VTEC (Shiga toxin-producing Escherichia coli (STEC), also known as verotoxin-producing E. coli (VTEC)) group.

Consumers have been sickened mainly by eating, licking, or tasting raw, homemade dough or batter and by involving young children in pasta making. There have also been cross-contamination cases where someone else in the household has made the pasta and caused the illness. Less attention is paid to food safety during baking, with flour as a raw material being perceived as basically safe by consumers.

The E. coli contamination typically indicates fresh faecal contamination, with a short survival time in a less than ideal environment. A possible explanation for the presence of E. coli in flour is that it is introduced by contamination of the wheat grain. During fertilisation and irrigation of wheat fields, the plant can take up the bacterium and it can be transferred to the grain. A favourable microclimate for E. coli can be created in the grain. Another possible cause of contamination in the milling plant is the use of unsafe water.

The German authority has started targeted monitoring for STEC/VTEC bacteria and decided to include flour in the monitoring plan, because there is no EU-wide information on the level of contamination across several Member States. A few Member States indicated that these serotypes are common, but they are not always able to identify the source. In a study in Switzerland, general E. coli contamination of flour was randomly sampled, and positive samples were found. The authority also found O26 in one flour sample.

In January 2020, the BfR's (German Federal Institute for Risk Assessment) report came out on the (re)assessment of the risk, sources and prevention options for STEC in flour. A full risk assessment could not be carried out, which the BfR plans to do in the future, but some recommendations were made.

For consumers, the general recommendations are: good general hygiene, avoid cross-contamination (flour vs. ready meals), avoid eating raw dough. It also discusses the facts known so far based on literature data e.g.: in low water-active food (like flour) E. coli cannot be inactivated at 70°C or at freezer temperature.

The detection method for E. coli is also proposed to be revised. Steps have already been taken in this direction, with reference to a publication (Mäde et al., 2017) that would complement the standard method with a "500 colony procedure".

The BfR study also sets out several research directions, as more information is lacking to understand the prevalence of E. coli in flour. Some examples are: mapping of pathways from grain to flour; prevalence of STEC as a function of flour origin (rice, wheat, etc.); technological research; alternative methods to reduce STEC (ionising radiation, ozone, etc.); VBNC status; kinetic parameters of STEC multiplication-destruction in flour, dough and other relevant matrices; prevalence in mills. Sampling of mills is also highly recommended by the BfR, thus reducing the data gap on STEC incidence.


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