Determination of Listeria in Dairy and Environmental Samples: Comparison of a Cultural Method and a Colorimetric...

The efficiency of the International Dairy Federation (IDF) Listeria detection method (IDF Standard 143:1990) was compared to that of a colorimetric nucleic acid hybridization assay. Out of 250 naturally contaminated 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-positive samples could not be detected. When Oxford and LPM agar were compared for suitability as the streaking medium for the Gene-Trak assay, more satisfactory results were obtained with the Oxford agar. Both the overall number of positive samples and the mean optical density values of the Gene-Trak-positive samples were higher with Oxford than with LPM agar. The extension of sample enrichment time from 1 d (Gene-Trak-assay), and 2 d (IDF method), respectively, to 7 d resulted in a small increase in the number IDF method-positive samples but led to a drastic decrease in the number of Gene-Trak-positive samples.