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.