This study was designed to evaluate the hypothesis that some of the Listeria monocytogenes subtypes associated with foods, specifically smoked fish, may have an attenuated ability to cause human disease. We tested this hypothesis by using two different approaches: (i) comparison of molecular subtypes 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 of industrial isolates. Ribotyping and PCR-restriction fragment length polymorphism typing of the hlyA and actA genes differentiated 23 subtypes among the industrial isolates and allowed classification of the isolates into three genetic lineages. A significantly higher proportion of human isolates (69.1%) than industrial isolates (36.8%) were classified as lineage I, which contains human sporadic isolates and all epidemic isolates. All other industrial isolates (63.2%) were classified as lineage II, which contains only human sporadic isolates. Lineage I ribotypes DUP-1038B and DUP-1042B represented a significantly higher proportion of the human isolates than industrial isolates (5.1%). Lineage II ribotypes DUP-1039C, DUP-1042C, and DUP-1045, shown previously to persist in the smoked fish processing environment, represented nearly 50% of the industrial isolates, compared to 7.6% of the human isolates. Representatives of each subtype were evaluated with a tissue culture plaque assay. Lineage I isolates formed plaques that were significantly larger than those formed by lineage II isolates. Isolates from the smoked fish industry representing three ribotypes formed no plaques or small plaques, indicating that they had an impaired ability to infect mammalian cells. While L. monocytogenes clonal groups linked to human listeriosis cases and outbreaks were isolated, our data also suggest that at least some L. monocytogenes subtypes present in ready-to-eat foods may have limited human-pathogenic potential.Listeria monocytogenes is responsible for nearly one-fourth of all estimated food-borne-disease-related deaths caused by known pathogens in the United States each year, which highlights its significance as a public health concern (25). The majority of human listeriosis cases occur in pregnant women, neonates, immunosupressed individuals, and the elderly (12). As a growing segment of our population falls into high-risk groups, improved methods for reducing the levels of L. monocytogenes in foods are essential. A better understanding of the ecology, transmission, and pathogenicity of this organism should facilitate development of effective strategies.While L. monocytogenes causes relatively few human disease cases, particularly compared to many other food-borne pathogens (25), it appears to be commonly present in raw and readyto-eat foods. U.S. Department of Agriculture data, for example, indicated a 2.5% prevalence of L. monocytogenes in 3,547 samples of ready-to-eat products surveyed in 1998 and a 4.6 and 2...

The development of Sarcocystis falcatula merozoites in bovine turbinate (BT) cell cultures is described and compared with development of Sarcocystis neurona merozoites. Merozoites of S. falcatula entered BT cell cultures and increased in size until 3 days post-inoculation when the nucleus of some merozoites developed lobes. Developing schizonts present at 4 days contained a lobed nucleus or appeared multinucleate. A single mature schizont was observed 4 days p.i. Schizonts were numerous 5 and 6 days p.i. Merozoites were produced from blastophores on the schizont. S. neurona merozoites developed to mature schizonts by 3 days p.i. in BT cells and a residual body was often present. Transmission electron microscopy revealed that S. falcatula merozoites possessed more micronemes than did S. neurona merozoites. Our study demonstrates that S. falcatula and S. neurona are not the same parasite.

Sarcocystis speeri Dubey and Lindsay, 1999 from the South American opossum Didelphis albiventris was successfully transmitted to the North American opossum Didelphis virginiana. Sporocysts from a naturally infected D. albiventris from Argentina were fed to 2 gamma-interferon knockout (KO) mice. The mice were killed 64 and 71 days after sporocyst feeding (DAF). Muscles containing sarcocysts from the KO mouse killed 71 DAF were fed to a captive D. virginiana; this opossum shed sporocysts 11 days after ingesting sarcocysts. Sporocysts from D. virginiana were fed to 9 KO mice and 4 budgerigars (Melopsittacus undulatus). Schizonts, sarcocysts, or both of S. speeri were found in tissues of all 7 KO mice killed 29-85 DAF; 2 mice died 39 and 48 DAF were not necropsied. Sarcocystis stages were not found in tissues of the 4 budgerigars fed S. speeri sporocysts and killed 35 DAE These results indicate that S. speeri is distinct from Sarcocystis falcatula and Sarcocystis neurona, and that S. speeri is present in both D. albiventris and D. virginiana.