Listeria monocytogenes: a surprisingly adaptive bacterium
Listeria monocytogenes is a ubiquitous bacterium found in food, livestock and wild animals, soil, water and vegetation. The European ListAdapt project focused on the ability of strains of this bacterium to adapt to these different environments. It found that this ability is independent of the strains' environments of origin or their belonging to a given sub-group.
Listeria monocytogenes is the bacterium responsible for listeriosis in humans. This disease remains rare in France but is the second leading cause of death from foodborne illness. It can lead to septicaemia or infection of the central nervous system, while infection in pregnant women can lead to miscarriage, preterm delivery or serious neonatal infection.
The aim of the European ListAdapt project was to understand the mechanisms by which Listeria strains adapt to their environment. This is because the bacterium has many strains that live in different environments: "We had previously observed that certain strains were found in food but not in animals or in the natural environment, and vice versa," explains Sophie Roussel, project coordinator and head of the research team in the Salmonella and Listeria Unit at ANSES's Laboratory for Food Safety in Maisons-Alfort.
Understanding their mechanisms of adaptation would make it possible to identify which strains could proliferate in contact with food and which may be resistant to a disinfectant or antibiotic, for example. ListAdapt was funded from 2018 to 2020 by the One Health EJP programme, coordinated by ANSES. This project brought together eight partners from seven different European countries. Within ANSES, in addition to the Laboratory for Food Safety, which coordinated the project, the Fougères Laboratory was involved in studying resistance to antibiotics and biocidal products, while the Ploufragan-Plouzané-Niort Laboratory helped obtain the genomes with its high-throughput sequencing platform.
A total of 1485 new strains of bacteria
Assisted by institutes and laboratories across Europe, the project members collected 1485 new strains of Listeria monocytogenes. Some were sampled especially for ListAdapt. Others came from existing collections but had not yet been studied. "These strains cover the full diversity of Listeria monocytogenes," stresses the scientist. "They come from a variety of sources and belong to 80 clonal families." Clonal families, also known as clonal complexes, are sub-groups of Listeria monocytogenes differentiated on the basis of certain genes. The first finding was that certain clonal families are preferentially found in specific environments. But contrary to the scientists' assumptions, this is not due to an increased ability to survive in these environments.
Different characteristics between strains of the same clonal family
The scientists selected a sub-panel of 100 environmental or animal strains and 100 food strains from among the collected strains, in order to study their characteristics in detail. "We discovered, for example, that certain strains taken from food can survive in soil while others cannot, even if they belong to the same clonal family", explains Sophie Roussel. The ability of bacteria to form biofilms, adhere to surfaces and resist disinfectants and antibiotics was also examined. Here too, there was high variability within the same clonal family.
Some of the results suggest that the differences in adaptability between strains may be due to the cumulative effect of small genetic variations. However, further studies are needed to confirm this idea. "There is a vast amount of data. Even though the project has officially ended, we will still need another 2 or 3 years to analyse everything," concludes Sophie Roussel.