AutoMicrobic System for biochemical identification of Listeria species isolated from foods: collaborative study

A collaborative study was conducted to evaluate the performance of the AutoMicrobic System Gram-Positive Identification (GPI) and Gram-Negative Identification (GNI) test kits to biochemically characterize Listeria spp. Thirteen laboratories each tested 97 food and environmental isolates, representing the 7 species of Listeria, as well as 11 additional genera of Gram-positive rods. Each collaborator inoculated both a GPI and a GNI card with a pure culture of each organism. The AutoMicrobic System identified the isolates and printed out the biochemical results. The GPI card is used to obtain a species identification and a mannitol reaction result, and the GNI card is used to obtain rhamnose and xylose reaction results. Organisms are classified into species groups and can be further distinguished on the basis of hemolysis or nitrate reduction tests. The AutoMicrobic System method correctly classified 90.8% of the Listeria spp. isolates and 100% of the non-Listeria isolates. The AutoMicrobic System method was adopted first action by AOAC International for the biochemical characterization of Listeria spp. isolated from food and environmental sources.