Food safety and quality

A tale of two pathogens


It was the best of times, it was the worst of times. It is the age of wisdom, it is the season of foodborne disease. Advances in microbiological risk assessment provide the spring of hope to overcome the despair caused by harmful foodborne bacteria.

Two new publications were released this week that will help risk managers address microbiological-related food safety issues, namely: Listeria monocytogenes and Shiga toxin-producing Escherichia coli (STEC). These two reports are part of the series of microbiological risk assessments (MRA) published by the Joint FAO/WHO Expert Meetings on Microbiological Risk Assessment (JEMRA).

We spoke with Kang Zhou, FAO Food Safety Officer and member of the JEMRA Secretariat, to better understand what was happening behind the scenes in the production of the reports.

  • Are the effects of Listeria and STEC on public health similar?

Both of these bacteria are foodborne pathogens and can infect anyone. Listeria monocytogenes disproportionally affects certain highly susceptible populations - pregnant women and immunocompromised individuals are two such groups. Children are particularly susceptible to acquiring severe forms of STEC infection. Although these pathogens can cause stomach upset and diarrhoea, they can both also cause severe life-threatening diseases.

Listeriosis outbreaks are often associated with ready-to-eat meat, unpasteurized dairy products and pre-cut fruits and vegetables. Unlike many other foodborne pathogens, Listeria can multiply at low temperatures, for example, in the fridge. It is ubiquitous in nature – found in both soil and water. Globally, Listeria monocytogenes was estimated to cause 14 000 illnesses in 2010. Listeriosis is infrequent, but is among the most deadly foodborne diseases known. It is estimated that 20 percent, or one in every five, Listeria infections results in death. Most cases of listeriosis are acquired when the patient, or the pregnant mother, eats contaminated foods.

People can get STEC from a variety of sources. However, infections are most frequently associated with eating beef, raw milk and raw milk cheese, leafy green and sprouts. Other sources include contaminated water and person-to-person transmission, or contact with infected animals or their environment. Worldwide, these infections result in approximately 1 million cases each year. Importantly, toxins produced by STEC spread outside gastrointestinal tract. These toxins are the leading cause of acute kidney failure in children.

  • Why were these two pathogens looked at by expert bodies at nearly the same time?

It was actually a coincidence that the JEMRA experts studied and discussed these two bacteria in the same year.

Listeriosis was first described as a foodborne pathogen about 40 years ago. The disease has long been attributed to consuming products such as raw milk, unpasteurized cheeses and processed deli meats. Outbreaks of listeriosis continue to occur across the globe today. However many of the recent outbreaks have been attributed to previously unreported food sources, including fresh and minimally processed fruits and vegetables, sprouts, and frozen vegetables and ice cream.

In addition to the changing patterns of listeriosis outbreaks, changes and advances in technology have provided new methods and tools to better characterize and understand Listeria and other pathogens. Because of these new developments in diagnostics and changes in the epidemiology of listeriosis, the JEMRA decided to undertake new work on Listeria in 2020.

Around this same time, the Codex Committee on Food Hygiene (CCFH) had growing concern about how to best control STEC. CCFH requested JEMRA to provide scientific advice on strategies and interventions to control STEC in beef, raw milk and cheese produced from raw milk, leafy greens and sprouts. The most recent JEMRA work on STEC builds on other JEMRA reports addressing the attribution, characterization and monitoring of STEC.

  • What has the JEMRA Secretariat learned from assessing the risks of these two pathogens – what conclusions can be drawn from these meetings?

The way we produce food is changing, as are the types of food people prefer to eat.  New ways to process and prepare foods are constantly being introduced. At the same time, pathogens have the potential to evolve or adapt and science is moving forward. We must leverage advances in science and technology, such as new molecular tools, to keep pace with these changes and provide risk-mangers up-to-date science on which they can make their decisions.

With new tools and more data in hand, JEMRA went back to the risk assessment basics- to better characterize the pathogens,  re-evaluate the dose-response models and determine the most important food sources responsible for infection with these pathogens. For STEC, we confirmed many of the same sources we have known for many years. In contrast, a number of foods that had not previously been linked to outbreaks of listeriosis were documented. These include rock melons, leafy greens and frozen vegetables.

With the pathogens and their sources described, we focused our attention on how to control and mitigate the risk of illness associated with these bacteria, starting with STEC in raw beef and dairy products. These results are reported in MRA 39, which was based on the attribution reports MRA 31 and 32. The new MRA 38 on Listeria lays the groundwork to develop an updated risk assessment model for that important pathogen. JEMRA’s next meetings later this year will focus on the burden of disease caused by Listeria in foods, and where risk can be mitigated across the entire food chain, starting on the farm. 

All in all, this scientific work adds up. It builds a more complete picture of these two pathogens and provides Codex with the advice they need to manage risk.


Related links

The new report on Listeria, MRA 38, and information about the upcoming JEMRA meeting on risk assessment of Listeria can be found here:

The new report on STEC, MRA 39, and the previous JEMRA work on STEC:


Photo: © FAO/Oliver Bunic

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