Characterization and Antimicrobial Resistance of Listeria monocytogenes Isolated from Food-related Environments

The purpose of this study was to determine the diversity and antimicrobial resistance of Listeria monocytogenes strains isolated from food-related environments in the United States. Nineteen unrelated strains of L. monocytogenes were recovered from approximately 1300 food and food processing environmental samples collected from 2007 to 2011 as part of the U.S. Food and Drug Administration pathogen surveillance program. The L. monocytogenes environmental isolates were characterized by serotyping, subtyping, and identification of antimicrobial resistance determinants. The serovars of L. monocytogenes were 1/2a, 4b, and 1/2b. PFGE using AscI digested total DNA showed genetic diversity; there were 10 PFGE pulse-types and 5 PFGE groups. All strains except one strain were susceptible to ampicillin, erythromycin, vancomycin, ciprofloxacin, and chloramphenicol but resistant to extended-spectrum cephalosporins (ESC). The environmental strains were predominantly resistant to streptomycin and tetracycline. No strain was resistant to 3 or more antimicrobial classes. All tetracycline- resistant strains were serotype 1/2a, and only tetM was amplified from the chromosomal DNA. This study, which reports the genetic diversity and antimicrobial resistance of foodborne L. monocytogenes, may be useful in food safety control programs to reduce the risk of transmission of L. monocytogenes to food products.