A recent study published in npj Science of Food offers new insight into how Listeria monocytogenes biofilms lead to cross-contamination in ready-to-eat (RTE) food processing environments, specifically looking at cold-smoked salmon.

The research team, comprising scientists from the Spanish Higher Council for Scientific Research (CSIC) and the University of Córdoba, formed multispecies and single-species biofilms using bacteria taken from surfaces in food processing environments. They grew the biofilms under low-nutrient, low-Listeria load conditions, mirroring the real world. Using a modeling approach, the team then tracked Listeria transfer rates across 25 successive contacts between biofilms and salmon slices, revealing nuanced contamination profiles for different biofilm types.

The findings showed that L. monocytogenes from multispecies biofilms exhibited significantly higher growth in salmon than those from single-species biofilms. This suggests that coexisting microbes may enhance the pathogen’s survival and proliferation post-transfer.

Interestingly, while transfer rates declined with each contact, the structure of the biofilm influenced how tightly Listeria cells adhered. The researchers identified three distinct stages of detachment—initial, unstructured, and inner-layer transfer—each with unique contamination dynamics.

These findings complement those of another recent study that demonstrated L. monocytogenes’ ability to colonize preexisting multispecies biofilms within hours and persist over time, without disrupting the biofilm’s structure or matrix composition.

Additionally, the growth differences observed between multi-species and single-species biofilm cells are relevant within the context of European legislation. In 2024, the European Commission amended EU regulations on the microbiological criteria for foods to expand the requirements for food business operators regarding L. monocytogenes. Per the amended regulation, the responsibility for ensuring that L. monocytogenes is not present in 25 grams (g) of an RTE food now applies to all food business operators when the producer is unable to demonstrate that L. monocytogenes will not exceed the limit of 100 colony forming units per gram (CFU/g) of a product throughout its shelf life.

In light of this regulation, the researchers conducted a case study simulating a scenario where a single smoked salmon slice from an 80 g package was contaminated with L. monocytogenes from single- and multi-species biofilms.  After 15 days of refrigerated storage, the L. monocytogenes load in samples contaminated from the multi-species biofilm exceeded the regulatory limit of 100 CFU/g, underscoring the importance of considering multi-species biofilm contamination when designing challenge studies, facilitating more effective L. monocytogenes control.