Genetic Diversity, Antimicrobial Resistance, and Virulence Profiles of Listeria monocytogenes Isolates from Nantong, China (2020-2023).

Listeria monocytogenes poses significant public health and food safety risks due to its environmental resilience and pathogenicity. In this study, we utilized whole-genome sequencing to characterize 15 L. monocytogenes strains isolated from Nantong, China (2020-2023), recovered from food and clinical samples. Antimicrobial susceptibility testing revealed that the Nantong isolates exhibited high resistance rates to ciprofloxacin (93.3%) and oxacillin (66.6%). Of particular concern, isolate NTLM03 exhibited the most extensive resistance profile, demonstrating resistance to six antimicrobials, including clindamycin, trimethoprim/sulfamethoxazole, erythromycin, chloramphenicol, ciprofloxacin, and tetracycline. Seven distinct multilocus sequence types were identified, with ST9 being the most prevalent. Virulence analysis revealed premature stop codons in the inlA gene of all ST9 isolates, a marker indicative of reduced virulence. In contrast, ST87 isolates carried the full-length inlA gene and the complete Listeria Pathogenicity Island-4 (LM9005581_70009 to LM9005581_70014), associated with high virulence. Core genome single nucleotide polymorphism (cgSNP) analysis demonstrated close relatedness (SNPs <20) among isolates from the same batch of meat products (NTLM03, NTLM04, NTLM05), suggesting a potential common contamination source. Furthermore, we examined the genetic relatedness of Nantong isolates with domestic and international L. monocytogenes strains. Two characteristic L. monocytogenes plasmids were assembled: the multidrug-resistant (MDR) plasmid pNTLM03 and the cadmium-resistant plasmid pNTLM08. Surprisingly, pNTLM03 shared a highly similar MDR region with Listeria innocua plasmids pLI42 and pLI203 from Chinese food sources. The transfer of antimicrobial resistance genes between Listeria species and other genera through various genetic mechanisms may increase the potential for the evolution of resistant L. monocytogenes strains, thereby increasing the difficulty of clinical treatment.