Escherichia coli is a Gram-negative bacterium common in the intestinal tract of humans and animals, where it performs important digestive functions. It is, therefore, a completely harmless commensal bacterium, except for the strains which have acquired the ability to cause injuries, sometimes very serious ones. The pathogenic strains of E. coli are generally responsible for diseases of the urinary tract (uropathogenic E. coli) or gastrointestinal tract (diarrhoeal E. coli), but also for septicemic forms and meningitis.
Characteristics of STEC
Among E. coli causes of gastrointestinal infections, the most fearful ones synthesize a very powerful toxin, capable of exceeding the intestinal site (where the bacterium resides) and reaching the vascular districts of the kidney, colon and brain, causing often irreversible lesions.
These are Shiga toxin-producing E. coli (STEC), sadly known for having caused even very large outbreaks of food poisoning.
The produced toxins are two (Stx1 and Stx2), of which there are numerous variants: four for Stx1 (a, c, d and e) and 15 for Stx2 (a-o). In addition to toxins, a characteristic of STEC is that of adhering firmly to the cells of the intestinal mucosa thanks to an adhesion protein, called “intimin”, synthesized by the eae gene, or to other adhesive factors.
The differentiation of STEC is mainly based on surface O antigens, in addition to flagellar H antigens, with their subdivision into serogroups (e.g. STEC O157) or serotypes (e.g. STEC O157:H7).
This classification allows us to assign different pathogenic powers to the various groups, depending on their involvement in cases of human infection. To date, in Europe, mainly the “Top-5” serogroups are being analyzed: O26, O103, O111, O145, O157.
Tank animals
The STECs, however, are not pathogenic for all living things. Many animal species can harbor them in the intestine in the absence of symptoms, as happens in domestic ruminants (cattle, sheep, goats, buffaloes) and wild ruminants (deer).
The elimination of STEC with faeces can cause contamination of the milk at milking or that of the meat during slaughter.
Food at risk
Among the foods at risk are, in addition to raw milk, raw milk cheeses, beef and other ruminant meats, which also include vegetables and water contaminated by animal waste.
As for beef, those consumed raw or undercooked, such as carpaccio, tartare and hamburgers, are at risk. In this regard, hamburgers were responsible for the first outbreaks of food poisoning, reported in the USA in the 80s and culminating in a multistate outbreak in 1993.
In these first cases, STEC O157 was identified as responsible for the infections in consumers. In subsequent decades, however, many other STECs were realized to be pathogenic to humans, such as the aforementioned Top-5.
An outbreak developed in Germany in 2011, in which more than 4.000 cases and around fifty deaths were reported, it was caused by STEC O104:H4, further expanding the number of E. coli serotypes on which to focus attention.
STEC disease in humans
Humans can get infected by direct contact with carrier animals (for example, with cattle and other ruminants), if there is close contact which facilitates the ingestion of microorganisms eliminated via faeces. In this regard, the behavior of children visiting farms or breeding farms, who caress the animals’ coats and then put their hands in their mouths, is very dangerous.
Another route of transmission is the interhuman one, within communities or family groups. Finally, there is the transmission route linked to the consumption of contaminated food. The infection is facilitated by the low infectious dose, as even fewer than 100 microorganisms may be sufficient.
STEC infections can have an asymptomatic course, especially in adults, or give rise to diarrheal forms, but also evolve towards haemorrhagic enteritis or Haemolytic Uremic Syndrome (HUS). The latter form is particularly severe since, in addition to the impairment of renal function, which requires the patient to undergo dialysis, neurological symptoms may appear, such as confusion, drowsiness, sensorial dulling, convulsions and coma.
The most serious forms of illness, which can even lead to death, occur in preschool children (0 to 4 years) and in the elderly (85 years and over).
Italian and European legislation
In Italy, food sector operators (FBOs) involved in the production and sale of meat must follow the rules of the Hygiene Package, a set of European Regulations issued since 2002.
These rules require compliance with rigorous hygiene standards during the slaughter of animals and the production of meat, as well as the application of HACCP plans.
However, a criterion for the search for STEC in beef, present only for sprouted seeds (responsible for the German outbreak in 2073), escapes the regulation on microbiological controls in food products (EC Regulation 2005/2011).
Prevention
Especially the people belonging to risk categories, such as very young children and the elderly, must be protected by preventing them from direct contact with carrier animals and by not giving them risky food products, such as raw milk, raw milk cheeses, especially if they are short-matured , vegetables not carefully washed and undrinkable water.
In the case of beef, great attention must be paid to meats consumed raw (tartare, carpaccio) which must be obtained in the strictest compliance with hygiene standards.
The burgers must be cooked appropriately, making sure that the heat also reaches the ‘heart’ of the product and not just its surface. In fact, the problem with products obtained with minced meat is that, unlike whole cuts in which the bacterial contamination is superficial, in them it becomes deep due to the reduction into fragments and their mixing. The burgers that caused the outbreaks in the USA were, in fact, partially raw in the center (with pink meat and not the traditional brown color that is observed after adequate cooking).
Silvia Bonardi
REFERENCES
Bell, Beth P., Marcia Goldoft, Patricia M. Griffin, Margaret A. Davis, Diane C. Gordon, Phillip I. Tarr, Charles A. Bartleson, et al. “A Multistate Outbreak of Escherichia coli O157:H7—Associated Bloody Diarrhea and Hemolytic Uremic Syndrome From Hamburgers: The Washington Experience.” Journal of the American Medical Association 272 (17): 1349–1353. doi:10.1001/jama.1994.03520170059036.
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Graduated in Veterinary Medicine and Specialist in Inspection of Food of Animal Origin and in Veterinary Public Health, she is Professor of Inspection and Control of Food of Animal Origin at the University of Parma.
