Dr Declan J Bolton is a Principal Research Officer, Food Safety Department, Teagasc Food Research Centre, Ashtown, Dublin 15.
Verocytotoxigenic E. coli (VTEC), also called Shiga toxin-producing E. coli (STEC), are now one of the most common causes of foodborne bacterial infection worldwide. The symptoms in humans range from mild diarrhoea to hemorrhagic colitis, hemolytic uremic syndrome (HUS), and thrombocytopenic purpura.
The VTEC group includes many different serogroups, some of which are more prevalent in human infections. Moreover, different strains may carry different combinations of virulence genes in addition to the vtx gene(s). While the vast majority of pathogenic strains are Enteropathogenic E. coli (EPEC) that carry vtx genes, a major outbreak (4,321 confirmed cases, 852 HUS and 54 deaths) was associated with an Enteroaggregative E. coli (EAEC) in 2011 in Europe.
The serogroups most frequently associated with severe human disease are O157, O26, O111, O103, and O145 and are regarded as the “top five”. After the 2011 outbreak, O104 was added to this list. Other serogroups including O91, O113, O117, O146 and O128 are also associated with human illness1.
SAFE FOR HUMANS?
The European Food Safety Authority (EFSA) recently concluded that it is not possible to define human pathogenic VTEC absolutely, either in terms of serogroup or virulence gene combination. Although additional genes such as eae (for VTEC) and aaiC plus aggR (for EAEC that have acquired the vtx gene) may be associated with a higher risk in terms of occurrence and severity of illness, their absence does not mean the strain is non-pathogenic. The question therefore arises, if vtx genes are detected in a ready-to-eat (RTE) food, should this food be declared unfit for human consumption?
Policy-makers often use the precautionary principle to justify discretionary decisions when the possibility exists of harm and in the absence of clear scientific knowledge. This principle was recently applied in the revision of Article 14 of Regulation (EC) No 178/2002, which concluded that all RTE foods testing vtx-positive by PCR are ‘unsafe’ and should be ‘withdrawn or recalled from retail’ and subject to ‘corrective measures to eliminate the VTEC hazard’. As there is no corrective action for some RTE foods, the only option is disposal, which is expensive and arguably contrary to efforts to supply food to the ever-expanding global population.
GENES LINKED TO ILLNESS
The risk of serious human illness is related to the presence of vtx genes, but only when present with other key virulence factors in the same organism. To the best of our current knowledge, these other virulence factors are eae (intimin production) or aaiC (secreted protein of EAEC) plus aggR (plasmid encoded regulator)1.
Thus, as concluded by EFSA, an RTE product contaminated with O157, O26, O103, O145, O111 or O104 in combination with vtx and eae or vtx and aaiC plus aggR genes, presents a high risk for diarrhoea and HUS. Other serogroups, in combination with the same genes, present a high risk for diarrhoea and an unknown risk for more serious illness. Outside of this serogroup-virulence gene combination framework, the risk and severity of human illness is unknown.
PCR TESTING NEEDED
All RTE products should therefore be tested initially using PCR for the presence of vtx genes. If these are absent, the RTE food does not present a risk of VTEC infection. If PCR positive, E. coli should be isolated from the food and tested for the presence of vtx plus eae (VTEC) and vtx, aaiC plus aggR (EAEC-VTEC) using PCR methods.
If these gene combinations are present, the food should be considered unsafe for human consumption and disposed of or used in a product that will be subject to cooking at temperatures sufficiently high to kill any E. coli present. The main issue, however, is what to do with RTE foods that are contaminated with E. coli that are vtx positive but negative for eae or aaiC and aggR.
Such E. coli may have alternative attachment factors such as saa (STEC agglutinating adhesion), common in O113 strains. Alternatively they may not be associated with illness in humans. Put simply, the risk is unknown.
IRISH RESEARCH
Research undertaken at Teagasc Food Research Centre, Ashtown2,3 found that, on average, 15 percent of VTEC isolates that are eae negative carry the Saa mechanism.
Some of these strains; O33:H11 and ONT: H11 each carried vtx gene variants (Vtx2c and Vtx2dact) associated with an increased risk of HUS4. Further evidence for the importance of eae negative VTEC in human infections comes from the human infection data reported to the European Centre for Disease Prevention and Control (ECDC). Between 2007 and 2010, 770 (10.6%) of confirmed cases, where the causative VTEC was isolated and characterised, were attributed to eae negative VTEC1.
Based on these data, it can be concluded that consuming RTE foods contaminated with eae negative VTEC would inevitably result in human infections and such food therefore represents a risk to public health. There seems little option but to remove such products from the food chain unless corrective action, such as heat treatment, can be applied to kill any VTEC present.
References
1. EFSA (2013). EFSA Panel on Biological Hazards (BIOHAZ); Scientific Opinion on VTEC-seropathotype and scientific criteria regarding pathogenicity assessment. EFSA Journal 2013;11(4):3138.
2. Monaghan, A., Byrne, B., Fanning, S., Sweeney, T., McDowell, D. and Bolton D. J. (2011) Serotypes and Virulence Profiles of non-O157 Shiga-Toxin producing Escherichia coli (STEC) from Bovine Farms. Applied and Environmental Microbiology, 77 (24), 8662-8668.
3. Monaghan, A., Byrne, B., Fanning, S., Sweeney, T., McDowell, D. and Bolton D. J. (2012). Serotypes and virulotypes of non-O157 shiga-toxin producing Escherichia coli (STEC) on bovine hides and carcasses. Food Microbiology, 32 (2), 223-229.
4. Bolton, D. J. (2010) Verocytotoxigenic (Shiga Toxin Producing) Escherichia coli: Virulence Factors and Pathogenicity in the Farm to Fork Paradigm. Foodborne Pathogens and Disease 8(3): 357-365.
Comments