Whole genome sequencing reveals transmission dynamics and persistence of Listeria monocytogenes along the poultry slaughtering and processing chain.

Listeria monocytogenes (Lm) is a gram-positive foodborne pathogen responsible for severe invasive human and livestock listeriosis. The dissemination of Lm throughout the poultry production chain significantly increases the risk of human listeriosis. In this study, we investigated the epidemiology and genetic characteristics of Lm in a poultry slaughterhouse using whole-genome sequencing (WGS). Lm contamination was detected in 8.32 % (77/925) of slaughterhouse carcasses, 9.86 % (102/1035) of environmental samples, and 13.28 % (32/241) of retail chicken, showing a gradual increase along the production chain. Moreover, contamination levels rose progressively throughout processing, reaching their highest at the quick-freezing and refrigeration stages after evisceration. Cross-contamination between poultry carcasses and processing equipment surfaces (8 ≤ single-nucleotide polymorphism distance (SNP) ≤ 21) was major driver of Lm transmission in slaughterhouse. Core-genome multilocus sequence typing (cgMLST) and SNP analyses identified four persistent Lm clusters (clonal complex 8, 9, 19 and 87), indicating long-term contamination on slaughterhouse processing equipment surfaces. Notably, all serotype 1/2b isolates belonged to cluster CC87 and harbored multiple stress resistance genes, demonstrating enhanced adaptability to slaughterhouse environments. In addition, CC87 strains showed reduced susceptibility to ampicillin (100 %) and gentamicin (66.7 %), highlighting significant therapeutic challenges and public health risks. These findings underscore the importance of implementing robust epidemiological surveillance throughout the poultry production chain to prevent Lm contamination and reduce the risk of human listeriosis.