Detection of Shiga Toxin

E. coli vs. Shiga toxin-producing E. coli - what’s the difference and what’s the problem?

By Roy Betts - 22 October 2020

E. coli, STEC, VTEC, O157… confused about the terms used for different groups of Escherichia coli and their relative food safety risks? You’re not alone. We often hear about the presence of “E. coli” and their use as a “faecal indicator”, or of STEC and its pathogenicity and symptoms, but regardless of what you may already know about them, to understand E. coli we must start at the beginning.

Emergence of kidney failure-causing E. coli

Shiga toxin-producing E. coli (STEC) are relatively newly-recognised pathogens. The initial cases were noted in the early 1980s, with the first issues centred on one now infamous (among microbiologists at least) serogroup: E. coli O157:H7 (often shortened to E. coli O157). The source? Usually improperly handled or undercooked beef and beef products. It soon became clear that this organism had a low infective dose and could cause serious illness. Symptoms included severe bloody diarrhoea and haemolytic uremic syndrome which could result in kidney failure.

Over the next few years, E. coli O157 was found to be the cause of outbreaks originating from a range of different foods: dairy products, various cooked meats, fruits, vegetables and salads were all implicated in outbreaks. Concern only grew when officials realised that it wasn’t just E. coli O157 that was causing these issues, there were a range of E. coli serogroups capable of causing similar serious illnesses. This was when the umbrella term of ‘STEC’ - sometimes referred to as Verocytotoxin-producing E. coli (VTEC) - came into use and encompassed all the pathogenic E. coli capable of producing the Shiga toxin.

Where is E. coli found?

You may be surprised to hear that E. coli is a very common organism. It’s found in vast numbers in animal and human gut contents where they usually do us no harm at all. You’ll also find it widespread in the environment, whether that’s in waters or soils.

The presence of “generic” E. coli in foods should be viewed as a hygiene indicator. We would like there to be none, we may accept a very low level on some types of food, but elevated levels would suggest an immediate improvement in hygiene is needed. Then we have pathogenic E. coli, of which STEC are probably the most potent.

STEC: more than just a pain in the neck

STEC are a group of E. coli that contain genes coding for the “Shiga toxin” (stx) which is also known as Verocytotoxin. Although E. coli O157 is the most well-known of this group, many hundreds of STEC serogroups exist and all have the potential to cause human illness. Some strains can also attach to the gut wall, causing an even more severe illness.

If STEC are consumed within a food, they will begin to multiply when they reach the gut. Those that can attach to the gut wall will do so potentially causing the first primary symptom of severe STEC infection: haemorrhagic colitis or severe bloody diarrhoea. If that isn’t bad enough, the Shiga toxin produced by this organism can enter the bloodstream and potentially cause kidney damage, leading to haemolytic uremic syndrome. These severe symptoms often result in hospitalisation and, in the worst cases, death.

The infective dose for STEC is known to be very low. The ingestion of just ten cells is believed to cause illness. The low infective dose together with its symptoms make person-to-person spread between those that are ill and healthy individuals possible - unless great care is taken with personal hygiene.

What about non-O157 STEC organisms?

Whilst the O157 serogroup is still the major cause of STEC-related illnesses in many countries, it’s now clear that there are a wide range of other serogroups that have caused serious outbreaks. As a result, various countries have taken measures to test for, and prevent the sale of, contaminated products. For example, in the USA there is a legal requirement to test various beef cuts for STEC serogroups O26, O45, O103, O111, O121 and O157, as it’s now recognised that they have all caused, or have the potential to cause, serious illness. In the EU, manufacturers of sprouted seeds have a legal requirement to test seeds destined for sprouting or sprouted seed irrigation water for STEC serogroups O157, O26, O103, O111, O145 and O104:H4. Any samples found positive may not be placed on the market.

Our views on STEC are changing as the reliance we have on specific pathogenic serogroups is challenged. It comes as recent outputs from the European Food Safety Authority (EFSA) state that any STEC regardless of serogroup has the potential to cause severe illness. In the future we will not be looking for certain serogroups but for the presence of any STEC as a major safety risk.

Implicated Foods

When STEC infections were first widely recognised in the late 1980s, the sole serogroup of concern was E. coli O157 - primarily linked to comminuted beef products such as burgers which had been improperly cooked. Since then, O157 has been linked not only with raw/undercooked meats but with a variety of dairy products including milk and cheeses made from both cow’s and goat’s milk.

Fresh produce has also been widely reported to be a cause of E. coli O157 outbreaks, with spinach, lettuce, watercress, apples, nuts and sprouted seeds being linked with the organism. All these routes of infection originate from the primary animal source (as previously mentioned, E. coli is generally an organism that colonises the animal gut). Raw meats can become contaminated through poor abattoir hygiene, with faecal material coming into contact with prepared carcasses; dairy products through poor dairy hygiene during milking; and fresh produce through contamination via contaminated irrigation water, water run-off from animal-grazed fields, or simply wild animals traversing crop fields.

It has also been recognised that poor hygiene during food preparation (direct or indirect contact between the raw and final product) can result in a range of ready-to-eat foods, e.g. cooked sliced meats, causing outbreaks.

A report on food categories causing STEC illness published by WHO in 2019, indicated that in the USA 40% of traceable cases were caused by beef - in Europe it was 30%. In both areas, beef caused more cases than any other single food type. It is quite a concern that the 2020 EFSA surveillance report on STEC covering 2018 data noted a 41% increase in cases in Europe compared to the previous four years. EFSA stated that undercooked ground beef and other meats were a significant risk factor for acquiring infection.

How can STEC be controlled?

With the above in mind, you may be shocked to hear that STEC are not that difficult to control. So, when it comes to:

There are reports that STEC are acid resistant. It is difficult to determine if the group are any more resistant than non-STEC E. coli, however there have been numerous publications showing that various strains of STEC can survive at pH 4, and food with pHs as low as 3.6 have been implicated in outbreaks. Generally, a low pH cannot be used as a measure to inactivate STEC without validation. The best controls in fresh produce is via hygienic growth/harvesting and careful use of irrigation waters.

Cross-contamination can be controlled using simple hygiene measures. In outbreaks that have been traced to cross contamination, a breakdown in simple hygiene control was the cause. The issue has been recognised by the UK Food Standards Agency (FSA). They’ve since issued specific guidance to food businesses on control of O157 that detail simple hygienic measures that minimise the risks of cross-contamination between raw meats and ready-to-eat foods.

In Europe, STEC can only be handled within a Category 3 pathogen laboratory which makes method validation difficult for many laboratories. ISO Technical Specification 13136 details a method for the detection of STEC from foods and a range of proprietary kits are also available for laboratories to use. Indeed, we are accredited to ISO 17025 to perform this testing.

One of the key problems is not the test itself, but the interpretation of results. This is not a test that provides a simple yes/no answer, but many answers. It will show presence of stx genes, eae genes and determine presence of one of the top six serogroups - but how are these various results interpreted and food safety decisions made? What happens if one of the top serogroups is detected, but it contains no stx genes? The organism is not an STEC, but is it a risk to consumers? This is just one example of the interpretational difficulties faced by those doing STEC testing and demonstrates the need to seek expert food microbiological assistance to help in decision making. Get in touch with me to find out how we can help you with this.

Roy Betts
+44(0)1386 842075
roy.betts@campdenbri.co.uk

Roy Betts

About Roy Betts

Roy Betts is the Microbiology Ambassador at Campden BRI, an independent international food consultancy and research organisation based in the UK. Read more...


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