Characterization and pathogenic potential of Listeria monocytogenes isolates from the smoked fish industry

This study was designed to evaluate the hypothesis that some of the Listeri a monocytogenes subtypes associated with foods, specifically smoked fish, m ay have an attenuated ability to cause human disease. We tested this hypoth esis by using two different approaches: (i) comparison of molecular subtype s found among 117 isolates from smoked fish, raw materials, fish in process , and processing environments with subtypes found among a collection of 275 human clinical isolates and (ii) the evaluation of the cytopathogenicity o f industrial isolates. Ribotyping and PCR-restriction fragment length polym orphism typing of the hlyA and actA genes differentiated 23 subtypes among the industrial isolates and allowed classification of the isolates into thr ee genetic lineages. A significantly higher proportion of human isolates (6 9.1%) than industrial isolates (36.8%) were classified as lineage I, which contains human sporadic isolates and all epidemic isolates. All other indus trial isolates (63.2%) were classified as lineage II, which contains only h uman sporadic isolates. Lineage I ribotypes DUP-1038B and DUP-1042B represe nted a significantly higher proportion of the human isolates than industria l isolates (5.1%). Lineage II ribotypes DUP-1039C, DUP-1042C, and DUP-1045, shown previously to persist in the smoked fish processing environment, rep resented nearly 50% of the industrial isolates, compared to 7.6% of the hum an isolates. Representatives of each subtype were evaluated with a tissue c ulture plaque assay. Lineage I isolates formed plaques that were significan tly larger than those formed by lineage II isolates. Isolates from the smok ed fish industry representing three ribotypes formed no plaques or small pl aques, indicating that they had an impaired ability to infect mammalian cel ls. While L. monocytogenes clonal groups linked to human listeriosis cases and outbreaks were isolated, our data also suggest that at least some L. mo nocytogenes subtypes present in ready-to-eat foods may have limited human-p athogenic potential.