Antimicrobial resistance and genomic characterization of Listeria monocytogenes isolated from frozen prepared dishes from Huzhou City

ObjectiveTo investigate the contamination status of Listeria monocytogenes in frozen prepared dishes collected from Huzhou City during 2023—2024， and to elucidate the antimicrobial resistance and genetic characteristics of the isolates.MethodsA total of 127 retail frozen prepared dish samples were randomly collected in Huzhou City from 2023 to 2024. L. monocytogenes was isolated and identified according to the China national food safety standard GB 4789.30—2016. Minimum inhibitory concentrations （MICs） for 8 antibiotics were determined for 21 L. monocytogenes isolates using the broth microdilution method. Whole genome sequencing and bioinformatics analysis were conducted to analyze isolates lineages， serotypes， sequence types （STs）， clonal complexes （CCs）， core genome multi locus sequence types （cgMLSTs）， resistance genes， virulence genes， and stress response genes.ResultsL. monocytogenes was detected in 21 of 127 samples， with an overall detection rate of 16.54% （21/127）. Serotype 1/2a was dominant， accounting for 57.14% （12/21） of all isolates. The 21 isolates belonged to two lineages （Lineage Ⅰ and Ⅱ）， and were classified into 7 CCs and 7 STs. The predominant CC-ST types were CC121-ST121 （28.57%）， CC8-ST8 （23.81%）， and CC87-ST87 （19.05%）. All isolates carried the LIPI-1 pathogenicity island and key internalin family virulence genes such as inlA， inlB， and inlC. Premature stop codon （PMSC） mutations in inlA were observed exclusively in Lineage Ⅱ strains. One CC288-ST330 strain carried LIPI-3 associated virulence genes， and all CC87-ST87 strains carried LIPI-4， indicating more pathogenic potentiality. Strains of ST5， ST8， ST9， and ST155 carried stress survival island 1 （SSI-1） genes. In total， 95.24% （20/21） of isolates were sensitive to the tested antibiotics， while one isolate showed resistance to both the first-line antibiotic penicillin （PEN） and the broad-spectrum antibiotic meropenem （MEM）. cgMLST phylogenetic analysis revealed close evolutionary relationships among strains isolated from samples with the same manufacturer， and a possible persistence of ST8 strains was observed across two consecutive years in one production facility， suggesting ongoing contamination existed.ConclusionA certain level of L. monocytogenes contamination existed in frozen prepared dishes in Huzhou City， with particular concern for high-virulence ST87 strains and occasional antibiotic resistance. These findings highlight the need for food production enterprises to strengthen raw material control and process management， while also promoting consumer awareness of food safety during handling and cooking of prepared dishes to reduce potential health risks.