Examination of Listeria monocytogenes prevalence among ready-to-eat foods in Japan revealed frequent (5.7 to 12.1%) contamination of minced tuna and fish roe products, and the isolates had the same virulence levels as clinical isolates in terms of invasion efficiency and infectivity in cell cultures and a murine infection model, respectively. Premature stop codons in inlA were infrequent (1 out of 39 isolates). Cell numbers of L. monocytogenes in minced tuna and salmon roe increased rapidly under inappropriate storage temperatures (from a most probable number [MPN] of 10 0 to 10 1 /g to an MPN of 10 3 to 10 4 /g over the course of 2 days at 10°C). Thus, regulatory guidelines are needed for acceptable levels of L. monocytogenes in these foods.Listeria monocytogenes causes listeriosis in humans mainly through consumption of ready-to-eat (RTE) foods. In Japan, the first reported food-borne listeriosis outbreak occurred in 2001, caused by contaminated cheese (16). Interestingly, this outbreak was detected from routine monitoring in the cheese manufacturing plant. Since this cheese was contaminated with L. monocytogenes at a most probable number (MPN) of 10 7 /g (16), individuals who had consumed cheese made in the plant were retrospectively examined and were found to have been infected. This was the first and only reported food-borne outbreak in Japan; however, we are unsure if previous or subsequent listeriosis outbreaks have occurred, as there are no official statistics on the incidence of listeriosis, due to the lack of a mandatory notification system (20). On the other hand, a questionnaire-based nationwide surveillance of hospitals estimated that an average of 83 listeriosis cases occur every year, which is equivalent to 0.65 per million inhabitants in Japan (20). Moreover, the pathogen has been detected in surveys of RTE foods at rates similar to those of other industrialized countries (21).Japan has a unique diet, comprising large quantities of raw RTE seafood, such as sashimi and sushi. Our previous study on L. monocytogenes contamination in such foods (11) revealed that minced tuna and fish roe products had high contamination rates (14.3% for minced tuna and 10.0 to 11.4% for fish roe products). In this study, we investigated L. monocytogenes prevalence in such RTE foods further, using a larger number of raw RTE seafood and other RTE food products. We also investigated the virulence potential of isolates in invasion efficiency and in a mouse model and determined whether each product type could support the growth of the pathogen. These results can provide baseline data for regulatory guidelines necessary for the safety of such products.Seafood products and other RTE foods were purchased from 229 different grocery stores and delicatessens located around Tokyo, Japan, between October 2004 and July 2008. By following a two-step enrichment procedure (11), five colonies on Palcam agars (Merck, Darmstadt, Germany) from each enrichment were randomly picked. Serotype was determined by the agglutination method using ...

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Listeria monocytogenes is of great concern as a foodborne pathogen. Many ready-to-eat foods are widely contaminated with this organism and have caused listeriosis outbreaks and sporadic cases in many countries. In Japan, there is a high incidence of L. monocytogenes contamination, specifically in raw ready-to-eat seafood. Identical L. monocytogenes subtypes have been isolated repeatedly from samples of food manufactured at a given store or processing plant, and researchers suspected that certain L. monocytogenes isolates have formed biofilms at these sites. A microtiter plate biofilm formation assay was conducted, and all raw ready-to-eat seafood isolates tested were able to form biofilms to various degrees. Biofilm formation by L. monocytogenes isolates of lineage I was significantly greater (P = 0.000) than that by isolates of lineage II. However, isolates of clonal lineages formed different levels of biofilms, indicating that the ability to form a biofilm is affected positively or negatively by environmental factors.

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