Although contamination is rare, listeriosis is the second leading cause of death from foodborne illness in France. This infection is caused by the bacterium Listeria monocytogenes, which can be found in the environment and in many foods, mainly cooked delicatessen meats, cheeses and smoked fish. The people most at risk are those who are immunocompromised or elderly, as well as pregnant women, who can suffer miscarriages. The presence of Listeria monocytogenes in a food production plant can cause cases of poisoning over several years if the contamination is not dealt with. It is therefore important to trace the origin of poisoning cases.
ANSES is the European reference laboratory for Listeria monocytogenes. Following several years of research, it has developed a PCR test that can identify the genetic group to which a strain of Listeria monocytogenes belongs, in less than a day and for a cost of less than €10. This is a significant step forward compared with the method normally used, multilocus sequencing typing (MLST), which requires three to five days of analysis and costs €150 per strain tested.
"The test we have developed can identify the 30 genetic groups of Listeria monocytogenes, known as clonal complexes, most commonly found in food in Europe" explains Benjamin Félix, project leader in the ANSES Laboratory for Food Safety. "Our method is very useful for countries that don't have the financial resources to carry out routine sequencing of whole genomes, or when a large number of samples need to be analysed, as it can be used to quickly make an initial selection."
The test is based on the detection of DNA sequences that are characteristic of each strain group. The new method was published in the journal Microbiology Spectrum in May. As well as the ANSES Laboratory for Food Safety, the Italian, Hungarian and Dutch national reference laboratories for Listeria monocytogenes and several French agri-food technical institutes contributed to this development. The ANSES Ploufragan-Plouzané-Niort Laboratory also helped with the bioinformatics analyses.
An example of its use: fish-based products responsible for listeriosis in the Netherlands
Reference laboratories in several European countries have already been trained in this new method, showing its value in real-life situations. For example, the Dutch food safety authority used the test to determine the source of listeriosis cases that have been reported in the last few years. Two manufacturers of fish-based products were suspected of being behind the contamination. Samples were taken from areas of the plants most at risk of contamination, such as cutting machines, trays and conveyor belts. Thanks to the speed and low cost of the PCR test, it was possible to run 200 analyses in order to select six strains for priority sequencing. This then identified the plant responsible for the contamination.