Prevalence, antimicrobial resistance and genetic diversity of Listeria monocytogenes isolated from chilled pork in Nanjing, China

•We investigate the prevalence of L.M isolated from chilled pork in Nanjing.•We characterize the isolates by serotyping, antibiotic resistance, and MLST subtyping analyses.•The contamination rate was 5.5%.•The majority of the isolates belonged to serotype which associated with pathogenicity.•Resistance of the L.M to commonly used antimicrobials presents a potential risk.

The objective of this study was to investigate the prevalence of Listeria monocytogenes isolated from chilled pork from open-air markets and supermarkets in Nanjing and to characterize the isolates by serotyping, antibiotic resistance, and MLST subtyping analyses. The results showed that 26 of 476 samples (5.5%) were positive for L. monocytogenes. All the isolates belonged to three serotypes, with serotype 1/2a being dominant (53.8%), followed by serotype 1/2b (23.1%) and serotype 1/2c (23.1%). The antimicrobial susceptibility tests revealed that 5 isolates (19.2%) were susceptible to all antimicrobials used; the remaining strains were resistant to 1 (19.2%), 2 (34.6%), 3 (11.5%), 6 (3.85%) or even 9 (3.85%) different antimicrobials. All the isolates were susceptible to penicillin and vancomycin. Most of the isolates were susceptible to rifampicin and gentamicin (96.2%), cefotaxime (92.3%), streptomycin (92.3%) and trimethoprim/sulfamethoxazole (92.3%). MLST subtyping analyses revealed that 26 isolates were grouped into 10 sequence types (STs). Therefore, this study indicates the high percentage of serotypes 1/2a and 4b, which are associated with human listeriosis; and the high level of multi-resistance could be a public health hazard.