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The prevalence of Listeria monocytogenesin different types of produce and on processing plant environments was investigated over a 4-year period in a large produce processing plant in Poland. Prevalence of L. monocytogeneswas 46% in frozen vegetables and 41.3% in swab samples taken from the plant environment. Survival studies using artificial inocula demonstrated that the number of Listeriain frozen produce stored for 100 days did not significantly decrease in relation to the initial contamination level. A subset of 129 L. monocytogenesisolates originating from produce and the plant environment were typed by pulsed-field gel electrophoresis. Seventy-six of these isolates were retyped by ribo- and serotyping. Thirteen pulsotypes and 18 ribotypes were distinguished. Persistent Listeriaisolates were found even when cleansing and sanitization was applied on a daily basis. Nine (69.2%) of 13 pulsotypes were recovered during a period of more than 2 years. L. monocytogenesof the same pulsotype was isolated from broccoli sampled directly before and after blanching, thus suggesting that blanching at 92 to 95°C for 4 to 8 min did not result in a Listeria-free product, most likely due to massive recontamination. This finding is of importance since blanching is the only critical control point in produce processing. Cross-contamination between the two lines was demonstrated through isolating L. monocytogenesstrains indistinguishable by pulsed-field gel electrophoresis from contaminated gloves and floor surfaces.

In this study, two typing methods, automated ribotyping and pulsed-field gel electrophoresis (PFGE), were evaluated for the subtyping of Listeria monocytogenes serotype 4b. The strains originated from patients and food samples collected in Austria during 2001–2005 and from Europe and North America in the World Health Organization collaborative study on the subtyping of this species. The largest group of Austrian clinical isolates was of the same PFGE subtype as those isolated from foodborne outbreaks in Switzerland and in the United States. Another subtype of clinical isolates from Austria was indistinguishable to that obtained from isolates responsible for a foodborne outbreak in the United States in 1985. Although the discriminatory power of PFGE was higher than that of automated ribotyping, some PFGE types were differentiated by ribotyping. Thus, combining data obtained by both automated ribotyping and PFGE increases the strain discrimination. Still, many of the Austrian strains remain indistinguishable from strains of foodborne outbreaks in other countries although there is no known epidemiological relation. This complies to previous studies which show the highly clonal nature of L. monocytogenes 4b strains which are responsible for both large outbreaks and sporadic cases.