Susceptibility (re)-testing of a large collection of Listeria monocytogenes from foods in China from 2012 to 2015 and WGS characterization of resistant isolates

Objectives Our aim was to determine the antimicrobial susceptibilities of 2862 Listeria monocytogenes cultured from various foods in China and to use WGS to characterize the antimicrobial resistance and virulence genotypes of those expressing a resistance phenotype. Methods The susceptibilities of 2862L. monocytogenes were determined by broth microdilution. Twenty-eight L. monocytogenes were found to be resistant to one to four antibiotics. All 28 resistant isolates were subsequently sequenced using short-read high accuracy protocols. The corresponding genomes were assembled and further analysis was carried out using appropriate bioinformatics pipelines. Results All 28 resistant L. monocytogenes were classified into five STs (ST3, ST8, ST9, ST155 and ST515). Both ST9 and ST155 were dominant and their genotypes correlated with their resistance phenotypes. All ST9 isolates were MDR and could be phylogenetically classified into two clusters. One was relatively close to clinical origins and one to food. Downstream analysis of the genetic contexts in which these resistance genotypes were found suggested that these may have been acquired from other bacteria by horizontal transfer or insertion into the chromosome. All isolates harboured Listeria pathogenicity island (LIPI)-1 and LIPI-2, and only two harboured LIPI-3. Conclusions This study reported on the antimicrobial susceptibilities of 2862 foodborne L. monocytogenes along with the genomic characterization of 28 resistant isolates, 11 of which expressed an MDR phenotype. These data showed that this bacterium can acquire resistance by horizontal gene transfer in and between species. This study may necessitate a re-evaluation of risk to public health, associated with this bacterial species.