Investigating the genetic diversity of Listeria monocytogenes strains at the pig slaughterhouse to understand the source of strains isolated in the pork meat processing sector

Listeria monocytogenes (Lm) is a pathogenic bacterium responsible for Listeriosis. In France, the genetic diversity of Lm is known in the pig farming and finished pork meat products compartment only. The aim of the study was (i) to investigate the genetic diversity at the slaughterhouse, based on MLST-clonal complexes (CCs) (ii) to compare it to the genetic diversity observed in the other compartments and better understand the strain contamination routes through the pig and pork chain. A total of 290 environmental swabbing in warm and cold areas, on food contact and non-food contact surfaces were performed in two slaughterhouses of the West of France. Overall, 115 strains were isolated and belonged to twelve different CCs. According to the virulence classification, four hypervirulent strains (CC1, CC4, CC54, CC207), five medium virulent strains (CC8, CC20, CC21, CC59, C155) and three hypovirulent strains (CC9, CC121, CC31) were identified. No hypervirulent CC strains were detected on non-food contact surfaces. Strain CC diversity was higher in warm areas (H’-index=1.83) compared to cold areas (H’-index=1.53). The strains of CC9 (38%) and CC121 (17%) were the most detected in both slaughterhouses. CC9 and CC121 strains, either rarely or never detected at pig farming, were shown to appear at the slaughterhouse, and strongly set up in cold areas (cutting plants and finished pork products processing). CC21 and CC59 strains were associated to warm areas while CC31 and CC155 to cold areas. Additional sampling campaigns are ongoing to consolidate these results.