DETERMINATION OF LISTERIA IN DAIRY AND ENVIRONMENTAL-SAMPLES - COMPARISON OF A CULTURAL METHOD AND A COLORIMETRIC NUCLEIC-ACID HYBRIDIZATION ASSAY

The efficiency of the International Dairy Federation (IDF) Listeria de tection method (IDF Standard 143:1990) was compared to that of a color imetric nucleic acid hybridization assay. Out of 250 naturally contami nated cheese and environmental samples tested, the IDF method revealed 153 and the Gene-Trak assay 144 positive samples, respectively. Hence , the Gene-Trak assay gave a false-negative rate of 5.9%. False-positi ve samples could not be detected. When Oxford and LPM agar were compar ed for suitability as the streaking medium for the Gene-Trak assay, mo re satisfactory results were obtained with the Oxford agar. Both the o verall number of positive samples and the mean optical density values of the Gene-Trak-positive samples were higher with Oxford than with LP M agar. The extension of sample enrichment time from 1 d (Gene-Trak-as say), and 2 d (IDF method), respectively, to 7 d resulted in a small i ncrease in the number IDF method-positive samples but led to a drastic decrease in the number of Gene-Trak-positive samples.