Nonhemolytic strains of Listeria monocytogenes detected in milk products using VIDAS immunoassay kit

Abstract

In December 1995 detection of Listeria monocytogenes Sv 1/2a in milk products that were routinely sampled for investigation at the Austrian Federal Food Inspection Laboratory (Vienna) led the food manufacturer in question to withdraw his product from the market. While one of seven Listeria strains isolated from this food product using "VIDAS L. monocytogenes kit" was undoubtedly L. monocyotogenes, six strains were nonhemolytic. In classical bacteriology haemolysin is used as an important phenotypic property to differentiate L. monocytogenes from the apathogenic L. innocua species. Species identification by PCR and by Western blotting confirmed that all nonhemolytic strains were indeed L. monocytogenes. Our confirmation of nonhemolytic isolates as true L. monocytogenes strains underlines the considerable potential of the VIDAS system as a specific immunological easy-to-use and automated test kit for detection of L. monocytogenes in food products. Whether or not the presence of non-hemolytic Listeria monocytogenes in food products justifies legal actions should be addressed by the proper authorities.