29G-11 |
Molecular characterization of resistant and sensitive Listeria monocytogenes strains and their interaction during pulsed electric field and high pressure processing |
B. H. LADO, E. M. Horton, and A. E. Yousef. Dept. of Food Science & Technology, Ohio State Univ., 2015 Fyffe Ct., 110 Parker Food Science Bldg., Columbus, OH 43210-1007 Strains of Listeria monocytogenes vary in resistance to pulsed electric fields (PEF) and high pressure (HPP) processing. A contaminated food is likely to harbor more than one strain of the same pathogen. Since extracellular protective factors can induce process resistance, interactions among strains of variable resistance are expected. This interaction may enhance the survival of the sensitive strain. This study aimed at characterizing processing-resistant and sensitive L. monocytogenes strains using molecular typing. Interactions among strains leading to enhanced survival to PEF and HPP was then assessed. Nine strains of L. monocytogenes were genotyped with pulsed field gel electrophoresis (PFGE) and arbitrarily primed-polymerase chain reaction (AP-PCR). Two L. monocytogenes strains were selected based on genetic relatedness and variation in resistance to PEF and HPP. To evaluate the survival of these strains individually, spontaneous antibiotic-resistant mutants were isolated by sequential plating on media that contain increasing levels of antibiotics. After PEF and HPP treatments, populations of processing-resistant and sensitive strains were determined in mixed cultures by plating on selective agar media. Hierarchical clustering divided the L. monocytogenes types into three similar groups with the PFGE and AP-PCR methods, but the PFGE was more discriminatory. Process-resistant and sensitive strains were found within each group. Strains OSY-8578 and Scott A were respectively the most and least resistant to PEF and HPP, and belonged to the same genotypic group. The mutants Rifr Neor OSY-8578 and Emr Ctcr Scott A were developed for survival assays. Processing the mixed culture resulted in a slight, but significant, protective effect for the process-sensitive strain (Scott A). Highly discriminatory genotyping methods are needed to track problematic strains of L. monocytogenes in food processing environment. The study also provides evidence that contamination of food with multiple strains may increase the risk of pathogen survival during processing.
Session 29G, Food Microbiology: General
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