Submitted to: Applied and Environmental Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/26/2006
Publication Date: 7/1/2006
Citation: Shen, Y., Liu, Y., Zhang, Y., Cripe, J., Conway, W.S., Meng, J., Hall, G., Bhagwat, A.A. 2006. Isolation and characterization of listeria monocytogenes strains from ready-to-eat foods in florida. Applied and Environmental Microbiology. 72:5073-5076. Interpretive Summary: Listeria monocytogenes is an important food-borne pathogen that can have a high mortality rate if consumed in high numbers. The severity of the illness also varies among various strains of this pathogen. Only limited information exists related to growth, biochemical and genetic characteristics of L. monocytogenes strains isolated from ready-to-eat foods. We used DNA-based analysis in combination with bacterial properties such as resistance to antibiotics and stomach acidity as well as to bacteriophages (viruses of bacteria) to classify 91 different L. monocytogenes isolates obtained from ready-to-eat foods. The characterization of L. monocytogenes isolates with the proposed scheme enabled us to determine the potential risk associated with individual strains. Determining the susceptibility of L. monocytogenes to bio-control agents such as bacteriophages will directly help in the development of novel technologies for the microbial safety of ready-to-eat foods. Both the food-preparation industry and consumers will benefit from the results of this research.
Technical Abstract: During the year 2003, food samples consisting of ready-to-eat foods were examined for the presence of Listeria monocytogenes. Out of 3,063 samples tested, 91 (2.97%) samples were positive for L. monocytogenes based on conventional biochemical characteristics and serology. PCR-based serogroup determination and pulse field gel electrophoresis (PFGE) following Apa I and Asc I digestions classified the 91 isolates into 3 putative serogroups (1/2b/3b, 1/2a/3a and 4) and 31 PFGE types, respectively. Lineage 1 strains (serogroups 1/2a and 3a) outnumbered lineage 2 (serogroups 1/2b, 3b and 4) strains 56 to 34. The glutamate-dependent acid-resistance (GDAR) system was functional in all the strains, except in two strains where the system required anaerobic growth for induction. Although 71 isolates (78%) carried multiple antibiotic resistance and exhibited resistance either to sulfomethoxazole, ciprofloxacin, or tetracycline in addition to nalidixic acid, all isolates were susceptible to ampicillin, penicillin, gentamicin, and trimethoprim. A bacteriophage cocktail, specific for L. monocytogenes strains that was previously shown to be an effective biocontrol agent, killed 65 of 91 (71%) isolates, 4 out of 6 serogroup 4b(d,e) isolates were resistant to this treatment. A combination of DNA-based classification procedures with phenotypic analysis resulted in improved characterization of the isolates. This information would be beneficial for epidemiological and public health research of L. monocytogenes.