The characterization of a novel hybrid multidrug resistance plasmid in foodborne Listeria monocytogenes discovered in China.

Listeria monocytogenes is a Gram-positive foodborne pathogen that causes listeriosis with a high mortality rate among high-risk populations. Although combination therapy with ampicillin and gentamicin is the standard treatment, the emergence of multidrug-resistant strains poses a growing public health threat. To explore the mechanism of multidrug resistance, the strain 466-3LM was analyzed using antimicrobial susceptibility testing, plasmid stability assessment, whole genome sequencing, and conjugation experiments in this study. Results showed that 466-3LM was resistant to trimethoprim/sulfamethoxazole, erythromycin, tetracycline, kanamycin, tiamulin, streptomycin, clindamycin, and chloramphenicol. Its resistance primarily originated from the 95.7 kb plasmid (designated as pLM466-3), which contained multiple resistance genes and was likely a novel hybrid formed by recombination of pNH1 and pLMR479a. Without antibiotic selection pressure, the chloramphenicol resistance gene catA8 and IS1216 element excised from pLM466-3 plasmid, reducing chloramphenicol resistance in the derived 466-3LMcured strain. Further investigation indicated that the resistance gene cluster of 466-3LM could form translocatable units (TUs) and spread via circular intermediates. Conjugation experiments demonstrated that 466-3LM failed to transfer its resistance to Escherichia coli C600, L. monocytogenes 10403S, and Streptococcus mutans UA159. In conclusion, a novel hybrid plasmid pLM466-3 emphasizing the urgent need for "One Health" surveillance to control the spread of such high-risk hybrid plasmid.