Purpose: The objective of this study was to determine the prevalence of antimicrobial resistance in Listeria monocytogenes strains isolated from food and food-related environments in the US.
Methods: Forty-two unrelated strains of L. monocytogenes were recovered from approximately 1300 food and food processing environmental samples between 2007 and 2011 as part of the FDAs pathogen surveillance program. Environmental isolates (n=19) and food isolates (n= 23) were characterized by serotyping and subtyping, antimicrobial resistance determinants, pulsed-field gel electrophoresis (PFGE), and plasmid profiles.
Results: The predominant serovars of L. monocytogenes from foods were 1/2a and 1/2b, but from the environment serovars 1/2a and 4b predominated. PFGE using AscI digested total DNA showed genetic diversity; there were 24 PFGE pulse-types and 8 PFGE groups. All strains were susceptible to erythromycin, vancomycin, ciprofloxacin, and chloramphenicol, but resistant to nalidixic acid. Eight (42.1%), six (31.6%), and four (21.1%) environmental strains were resistant to streptomycin, tetracycline, and ampicillin, respectively. Five (26.3%) strains were resistant to 3 or more antimicrobial classes. Six out of seven tetracycline-resistant strains were recovered from food-related environments, whereas only one was recovered from food. These six tetracycline-resistant strains were serotype 1/2a, and only tetM was amplified from the chromosomal DNA. Two of resistant environmental isolates contained plasmids, ranging from 25 to 90 kbp.
Significance: These results suggest that epidemiological and antimicrobial resistance monitoring data from food-processing environments may be used to reduce the risk for human listeriosis resulting from the transmission of the pathogen to food products.