The sub-species characterization and antimicrobial resistance of Listeria monocytogenes isolated from domestic and imported food products from 2004 to 2011

Thirty-five Listeria monocytogenes strains isolated from domestic and imported food products including seafood, vegetables, and dairy foods were characterized by serotyping, molecular sub-typing, and antimicrobial susceptibility. L. monocytogenes serovars 1/2a and 1/2b strains were dominant as compared to other two serovars 4b and 1/2c strains. The dendrogram of AscI or ApaI-digested PFGE profiles of L. monocytogenes strains was classified into 23 (with 8 groups) or 3 (with 2 groups) different PFGE types, respectively. The AscI-digested groups consisted of the same serovar or food-source. Antimicrobials such as ampicillin, gentamicin, and trimethoprim-sulfamethoxazole are widely used in the treatment of listeriosis. Of the isolates used in this study, NCTR_LM14 and NCTR_LM57 were resistant to several classes of antimicrobials including aminoglycosides, penicillin, tetracycline, glycopeptide and fluoroquinolones. The multi-antimicrobial resistant isolates also showed higher efflux pump activity as compared to the antimicrobial sensitive strains NCTR_LM06 and L. monocytogenes EGD-e. This study demonstrates that L. monocytogenes isolates from various food products are genetically diverse with some isolates being resistant to more than 3 different antibiotic classes. This study also indicates that the efflux pump activity of the antibiotic resistant strains was higher than antimicrobial susceptible strains. Therefore, we propose that the antibiotic resistance observed in these strains may be conferred by the results of a highly active efflux pump system.