In this study, a number of Listeria monocytogenes strains of different origins were evaluated for in vitro invasion capacity for various human cell types (monocytic THP-1, enterocytic Caco-2, and hepatocytic HepG2 cells) and for expression levels of specific virulence genes. For THP-1 cells, no differences between clinical and nonclinical L. monocytogenes strains in invasion capacity or in production of the proinflammatory cytokine interleukin-8 (IL-8) were observed, whereas for the Caco-2 and HepG2 cells, significant differences in invasion capacity were noticed. On average, the clinical strains showed a significantly lower invasion capacity than the nonclinical L. monocytogenes strains.

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Listeria monocytogenes is a major concern to producers of ready-to-eat foods because of the high mortality rate associated with listeriosis and the widespread nature of the organism. To investigate the prevalence of this pathogen in different ready-to-eat food products on the Belgian market, a variety of 252 ready-to-eat food products, mainly fish and meat products, were analyzed. Overall, L. monocytogenes was detected in 23.4% of the samples. The highest prevalence of L. monocytogenes was found in prepared minced meat (42.1%) and smoked halibut (33.3%). Contamination levels were in most cases low (<10 CFU/g); however, levels higher than 100 CFU/g were detected in some samples of smoked salmon, smoked halibut, and prepared minced meat. A high prevalence of Listeria innocua (15.8%) and Listeria welshimeri (36.8%) was detected in prepared minced meat. L. monocytogenes strains isolated from different contaminated products were subjected to repetitive element sequence-based PCR (REP-PCR) typing to determine possible associations with product type, producer, or market. REP-PCR patterns were analyzed using BioNumerics software, and seven different groups with at least 90% similarity were identified. The cluster analysis indicates that cross-contamination occurred at the producer and retail level. Serotype identification of the strains by PCR revealed that most belonged to the 1/2a(3a) serotype group.

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