CSI Listeria: Investigating foodborne pathogens

A multi-institute consortium, led by Teagasc, has sought to understand the behaviour of Listeria monocytogenes in foods by studying a collection of isolates from foods and food processing environments in Ireland. The research team found that not all isolates are created equal, raising the question: should they be treated differently in terms of food safety management?

Listeria monocytogenes is an organism found widely in the environment but it can cause food poisoning if it enters the food chain. Although infection with Listeria from consuming contaminated foods is extremely rare, with typically fewer than 1,700 cases per year in the EU, it can be a serious disease for those with underdeveloped or suppressed immune systems – the young, the old and the immunocompromised – and for pregnant women.

While cooking will kill the organism, it can grow at refrigeration temperatures. At-risk products are generally ready-to-eat (RTE) foods, foods that are consumed directly from the fridge without cooking. Examples include deli meats, fresh cheeses, and prepared salads such as coleslaw and salad leaves.

The Listeria Challenge Studies project sought to investigate a large collection of isolates gathered from the processing environment of RTE food products – dairy, meat, seafood and vegetables – to understand:

• how their genetics are linked to the physical characteristics they display
• how they respond to the stresses encountered in the food processing environment
• how their response to stress influences how they behave in a food product
• how they can be controlled by the food business operator.

Dealing with stress

“The ability of Listeria monocytogenes to endure external stresses determines its ability to grow in food products,” explains Principal Research Officer Olivia McAuliffe. “Analysis of our Listeria collection revealed a significant level of heterogeneity amongst isolates in response to such stresses. Some isolates exhibited extensive tolerance to low temperature and commonly used food preservatives, such as organic acids and salt, while others appeared to have developed resistance to sanitisers commonly used in food processing plants to prevent microbial growth. Indeed, some isolates displayed multiple resistances, making these particularly hard to control for food businesses.”

Examining the genome sequences of these isolates revealed some interesting information. Although the isolates were collected from a diverse range of food products and food processing plants, several groups of isolates with highly similar genome content could be traced to single or multiple food businesses, providing evidence that these strains can persist in food processing environments. Some were associated with only one particular processing environment, suggesting that these isolates had adapted to that particular niche, while others were found across different processing environments, suggesting a more generalised persistence ability. Interestingly, some of the isolates were related to human outbreak isolates, providing evidence for a potential link between Listeria monocytogenes that are prevalent in the food chain and cases of foodborne infection.

Pictured at the 2023 Symposium on Listeria monocytogenes in Foods in Ashtown, Dublin are project partners and invited speakers (l-r):  Fabian Commichau (University of Hohenheim), Kaye Burgess (Teagasc), Achim Schmalenberger (University of Limerick), Birgitte Kalpollitis (University of Southern Denmark), Olivia McAuliffe (Teagasc), Ana Allende (CEBAS-CSIC) and Conor O’Byrne (University of Galway)

A side of Listeria with your spinach?

One of the main objectives of the project was to establish how these isolates behave in various RTE food products. Consumption of salad leaves has increased dramatically in recent years as consumers look to increase their intake of these healthy products. Pre-prepared salad leaves tend to be consumed raw in salads and smoothies, and there has been an increasing number of product recalls and foodborne outbreaks associated with these types of products in recent times – a significant challenge for food producers.

Identifying particular isolates from the collection that were derived from vegetable products, the project team examined their growth on lettuce, spinach and rocket and confirmed that these products can support the growth of Listeria, reaching numbers that could cause infection over the nine-day testing period. What was especially concerning in these findings was that spinach leaves in particular showed very little visual deterioration over this time. The leaves still looked fit for consumption after nine days, despite the high numbers of Listeria present. Such contaminated spinach could pose a serious health risk to consumers. There were also many environmental factors influencing the growth of the organisms on these leaves, including the leaf variety, the season, and whether the leaves were cultivated in polytunnel or open fields.

More work to do

At present, European Union regulations treat all Listeria isolates as equal, in terms of their resistance behaviour and their virulence, or ability to cause infection. Differences in stress responses and growth behaviour have been documented in the Teagasc collection, while research from the team is also demonstrating differences in the potential virulence of these isolates.

“However, there is more work to do to discriminate between truly virulent strains and non-virulent strains,” says Olivia. “In time, the work gathered here will contribute to the identification of the most resistant and virulent stains. This will allow control measures to be targeted directly towards the most problematic strains for the industry and for predictive models to be built to enable food producers to assess the risk that these strains present to their business.”

The project wrapped up with a gathering of the project partners together with national and international experts at the 2023 Symposium on Listeria monocytogenes in Foods, May 24-25, Ashtown, Dublin. 

“Some isolates showed extensive tolerance to common food preservatives, while others have resistance sanitizers used in food processing plants.”

Acknowledgements

The contributors wish to thank project partners Francis Butler and Seamus Fanning (University College Dublin).

Contributors

Olivia McAuliffe, Principal Research Officer, Teagasc Food Research Centre, Moorepark, Cork. Emaile: olivia.mcauliffe@teagasc.ie

Conor O’Byrne, Professor of Microbiology, University of Galway.

Achim Schmalenberger, Senior Lecturer, University of Limerick.

Cormac Gahan, Professor of Microbiology & Pharmacy, University College Cork.

Funding

The Listeria Challenge Studies project was funded by the Department of Agriculture, Food and the Marine (ref. 17F244).

[pic credit] Liudmila Chernetska/istockphoto.com