COMPARISON OF NUCLEIC-ACID HYBRIDIZATION ASSAYS AND BIOCHEMICAL-CHARACTERIZATION TESTS FOR THE CONFIRMATION OF LISTERIA-MONOCYTOGENES

A comparison was made of various biochemical and nucleic acid hybridization assays used for the confirmation of Listeria monocytogenes. Identification and confirmation tests included traditional carbohydrate utilization tests, several types of hemolysis assays, an immunocompromised mouse-pathogenicity test, the API(R) LISTERIA identification system, the Biolog GP MicroPlate(TM), a prototype Biolog Listeria MicroPlate(TM), and two nucleic acid hybridization assays (a slot-blot assay and the commercially available AccuProbe(TM) L. monocytogenes culture confirmation test). A panel of 44 Listeria isolates, including representatives from all six existing Listeria species, was tested. A considerable number of the L. monocytogenes isolates tested gave ambiguous reactions on the CAMP-hemolysis assay. In many instances, the Biolog GP MicroPlate(TM) was unable to differentiate L. monocytogenes from other Listeria species. Reactions on the prototype Listeria MicroPlate(TM) mirrored those of traditional biochemical tests, but additional analyses were necessary to differentiate between L. monocytogenes and Listeria innocua. The API(R) LISTERIA identification system could readily differentiate between the latter two Listeria species without the need for additional tests. Atypical L. monocytogenes isolates (nonhemolytic and nonpathogenic or hemolytic and nonpathogenic) were correctly identified by the API(R) LISTERIA system. A perfect correlation was observed between results from the slot-blot format hybridization assay and the AccuProbe(TM) test. The probes used in the two assays hybridized with all L. monocytogenes isolates, regardless of hemolytic reaction or mouse pathogenicity. The commercially available assays examined in this study were all easy to use and capable of providing results within 24 hours.