Prevalence, Characterization and Genetic Diversity of Listeria monocytogenes in Ready-to-Eat Raw Salmon (Salmo salar) and Trout (Oncorhynchus mykiss) Products

Listeria monocytogenes is a high-risk pathogenic bacterium associated with ready-to-eat foods and poses a potential threat to consumer health. This study aimed to investigate the prevalence, characterization and genetic diversity of L. monocytogenes in ready-to-eat raw salmon and trout products obtained from physical stores and online stores in China. Out of 150 samples analyzed, 23 (15.3%) were positive for L. monocytogenes. Among these positive samples, three (12%) were from Japanese restaurants, four (16%) from farmers markets, one (2.9%) from large supermarkets and fifteen (30%) from e-commerce platforms, and only one sample showed a contamination level exceeding 100 most probable number (MPN)/g. The isolates from positive samples demonstrated a concrete public health risk through several findings: twenty-three L. monocytogenes exhibited varying degrees of cytotoxicity, ranging from 7.6% to 71.8%. Compared with the reference strain ATCC 19115, five of these isolates were highly cytotoxic, a result that was validated by mouse survival rate experiment, which also confirmed their high virulence at tested dose. All isolates were resistant to cefuroxime sodium, ceftriaxone, cefepime and nalidixic acid, and 13% showed resistance to sulphamethoxazole-trimethoprim. Three serogroups were identified, with serogroup Ⅰ.1 (1/2a, 3a) being the most prevalent (65.2%). These isolates were grouped into eight sequence types, with ST8 (34.8%) and ST87 (30.4%) dominating. All isolates carried virulence genes associated with LIPI-1 andmultiple internalin genes (inlA, inlB, inlJ and inlK), confirming their potential pathogenicity. Additionally, the isolates harbored antimicrobial resistance genes lin and FosX. The five highly virulent isolates exhibited the highest genetic similarity to J2-031 (GCA_000438645.1) and C1-387 (GCA_000438605.1). The results provided valuable information for Chinese regulatory authorities to strengthen the risk monitoring of L. monocytogenes in ready-to-eat raw salmon and trout products.