Emerging threats: Listeria monocytogenes with acquired multidrug resistance from food in China, 2012-2022

Listeria monocytogenes is a foodborne pathogen that poses threat to food safety and public health. Generally, the rates of resistance to clinically important antibiotics in L. monocytogenes are low. This study aimed to investigate the prevalence and genetic characteristics of L. monocytogenes with acquired multidrug resistance (MDR) in food samples from China between 2012 and 2022. Of 8344 isolates collected, 34 (0.41 %) were identified as acquired MDR. The majority of acquired MDR isolates (n = 31, 92.3 %) belonged to hypovirulent clonal complex (CC) 9 (Lineage II, IIc), including 3 sequence types (ST) (ST9, n = 29; ST2458, n = 1; ST9-1LV, n = 1), which has remained dominant over the past decade. In 2022, three additional acquired MDR clones emerged: CC87/ST87 (Lineage I, IIb), CC8/ST8 (Lineage II, IIa), and CC155/ST705 (Lineage II, IIa), with CC87/ST87 and CC8/ST8 being notably associated with human listeriosis in Asia. The rep25_2_M640p00130 plasmid was the most common mobile genetic element among these acquired MDR isolates, consistently harboring seven types of antibiotic resistance genes, including aminoglycosides (ant(6)-Ia; aph(3')-III), trimethoprim (dfrG), macrolides, lincosamides and streptogramin B (MLS_b) (erm(B)), lincosamides (lnu(B)), pleuromutilins, lincosamides and streptogramin A (PLS_A) (lsa(E)), tetracyclines (tet(S)), and phenicols (catA), and flanked on one side by IS1216E. However, the diversity of acquired MDR-carrying plasmids increased from 2017 to 2022, with an increased prevalence among replicons including rep26_2_repA, rep26_4_repA, and rep26_1_pli0070/rep32_1_pli0023. Importantly, compared to the dominant hypovirulent CC9, which contained premature stop codons in the internalin gene inlA associated with adhesion and invasion, the newly emerged acquired MDR L. monocytogenes CC8/ST8 and CC155/ST705 maintained intact inlA gene and exhibited stronger adhesion and invasion phenotype in Caco-2 cells. These findings emphasize the need for continuous surveillance of acquired MDR L. monocytogenes, particularly the virulent CC8/ST8 and CC155/ST705, to mitigate risks to food safety and human health.