TISSUE-CULTURE ASSAYS USING CACO-2 CELL-LINE DIFFERENTIATE VIRULENT FROM NONVIRULENT LISTERIA-MONOCYTOGENES STRAINS

Within the group of Listeria sp., only L. monacytogenes is pathogenic for humans and numerous studies oft. monocytogenes strains have descri bed non-virulent isolates. In this study, the potential value of two t issue culture assays (TCA) was analysed to ascertain the virulence pro perties oft. monacytogenes strains, initially typed for virulence usin g the immunocompromised mouse model (ICMM). The first assay assessed b oth the penetration into, and multiplication within, Caco-2 cells (PM assay); the second was a plaque-forming assay (PF assay). All the clin ical isolates (nine strains) were virulent in both TCA. Conversely, al l the non-pathogenic species (seven strains) were non-virulent in PM a nd PF assays. Compared with the virulence obtained in the ICMM with 29 Listeria strains, including 12 non-virulent L. monocytogenes strains, the sensitivity of both TCA was equal to 1. Specificity was 0.89 and 0.84 for the PF and PM assays, respectively. However, a study of strai ns exhibiting virulence differences in three other in vivo virulence m odels showed that ICMM only detected highly virulent strains. The spec ificity of the PF test could, therefore, be higher, and close to that obtained by the enumeration of viable bacteria in the spleen of mice i nfected by subcutaneous injection in the footpad and by intravenous in jection. Taken together, this study confirms the existence of low-viru lence L. monocytogenes strains and shows that the virulence status of some non-clinical L. monocytogenes isolates depends on the virulence m odels used. The data suggest that the PF assay could be used as a prim ary test to evaluate the virulence of Listeria strains in order to red uce the cost of testing all strains in vivo.