Bacterial populations remain stable despite cleaning efforts in ready-to-eat food facilities, according to researchers.

Scientists from the Quadram Institute and UK Health Security Agency (UKHSA) are working on how Listeria persists in ready-to-eat (RTE) food production environments.

Researchers hope that understanding how Listeria survives in these environments could inform better laboratory testing of cleaning methods. Funding came from the Biotechnology and Biological Sciences Research Council.

In 2019, six people died in a Listeria outbreak caused by contaminated ready-to-eat sandwiches served in several hospitals across the UK. A recent inquest at Manchester Crown Court found Beverley Sowah, 57, and Enid Heap, 84, died after eating contaminated chicken mayo sandwiches.

Despite well-implemented strategies to disinfect facilities and control microbial risks, Listeria can breach food safety barriers and cross-contaminate products. This is particularly dangerous in RTE foods wher consumers will not destroy bacteria by heating the food before eating it.

Researchers wanted to understand the potential sources of cross contamination and the factors that contribute to the survival of Listeria monocytogenes in such environments.

The team sampled the floor of a RTE food factory that had detected Listeria monocytogenes in non-food contact areas of the plant. They sampled different sites: a preparation area, wher ingredients were kept at 4 degrees C (39.2 degrees F), and a production area wher food was assembled and packaged, kept at 10 degrees C (50 degrees F). They sampled the sites over 10 weeks, before and after cleaning.

Listeria and supporting microbes
Culturing and genetic analysis on samples was done to identify which bacteria were present and in what proportions. Results showed the populations of bacteria that coexist with Listeria monocytogenes were stable over time and have adapted to conditions on the factory floor, including food safety controls. 

“As Listeria monocytogenes is supported by a stable community of other bacteria, we may now need to develop new strategies to alter the whole bacterial population to effectively eliminate the pathogen,” said Maria Diaz from the Quadram Institute.

Overall bacterial populations, and proportions of bacteria were stable before and after cleaning.

“The populations are very stable, and cleaning is not shifting the composition – it’s not letting one bacterium grow over another. After cleaning, the bacteria reduce in numbers and the bacterial load is lower, making cross contamination less likely,” said Diaz.

There was a difference between areas of the site at different temperatures; suggesting that bacterial populations are adapted to different environments in the facility. It also indicates that bacteria in the factory are established populations and not introduced from outside sources – as despite movement of staff between them, the populations remained stable.

Diaz will present her data at the Microbiology Society Annual Conference, which is scheduled from April 8 to 11 at Edinburgh International Convention Centre.

Meanwhile, in September 2024, the Quadram Institute is hosting the 21st International Symposium on Problems of Listeria and Listeriosis.

The meeting will bring together the Listeria research community and other food and health stakeholders to discuss new discoveries and opportunities to understand and control the pathogen.