Genomic Insights into Antibiotic Resistance and Virulence of <i>Listeria monocytogenes</i> Isolated from Chongqing, China

<i>Listeria monocytogenes</i> (<i>L. monocytogenes</i>) is a pathogen of significant concern in food due to its ability to survive and multiply under harsh environmental conditions, such as high osmotic pressure, low temperatures, and freezing. This bacterium can cause listeriosis, a severe infection particularly dangerous for high-risk groups including newborns, pregnant women, and immunocompromised patients, due to its high morbidity and mortality rates. This study aimed to investigate the molecular epidemiological characteristics of <i>L. monocytogenes</i> isolated in Chongqing, southwest China. A total of 72 <i>L. monocytogenes</i> isolates collected between 2015 and 2022 were analyzed using whole-genome sequencing (WGS). Multilocus sequence typing (MLST) revealed 15 sequence types (STs), with ST9 (20.83%), ST87 (19.44%), and ST8 (13.89%) being the most prevalent. The isolates were classified into two phylogenetic lineages and four serotypes, with serotypes 1/2b (lineage I) and 1/2a (lineage II) representing 36.11% and 41.67%, of the isolates, respectively. Antibiotic resistance gene analysis showed a high prevalence of the tetracycline resistance gene <i>tet</i>(M), β-lactam resistance gene <i>blaZ</i>, and erythromycin resistance genes <i>msr(A)</i>, <i>msr(D)</i>, and <i>mef(A)</i>. All isolates contained <i>Listeria</i> pathogenicity islands (LIPI-1) and LIPI-2; 12 isolates carried LIPI-3, and 17 isolates carried LIPI-4, with all ST87 isolates harboring LIPI-4. The ST87 isolates were primarily sourced from meat products. These findings indicate that <i>L. monocytogenes</i> isolates in Chongqing harbor multiple virulence and antibiotic resistance genes, underscoring the need for ongoing surveillance and risk assessment, particularly for ST87 in meat products.