Detection of Listeria spp. in food samples by immunomagnetic capture: ListerScreen method

A new assay, ListerScreen, has been developed to detect Listeria spp. in food. This method separates Listeria cells from enriched food samples by means of immunomagnetic beads. After magnetic capture, the beads are spread on PALCAM agar. The analysis time, including an 18-h enrichment and plate incubation, is 48 h for positive samples and 72 h for negative samples. Specificity studies of ListerScreen showed that all 52 strains of Listeria which were tested scored positive. When non-listeria strains were tested, 33 of 34 were negative. The one weak positive occurred only after prolonged incubation of the plates. The intrinsic sensitivity of magnetic separation is about 0.5 Listeria cells per ml; that is, one Listeria cell per assay (2 ml of enriched food). The detection limit of ListerScreen was tested by spiking four types of food products successively with four strains of Listeria. Whatever the strain and the food product tested, positives were detected at the lowest spike level (4 CFU/25 g). To evaluate accuracy, comparative studies were performed with 88 food samples naturally contaminated with Listeria spp. and 123 food samples without Listeria analyzed in duplicate with both ListerScreen and a standard cultural method. A total of 99% of the results were in agreement. ListerScreen provided more rapid results with all negative samples and 16% of positive samples. To evaluate reproducibility, a collaborative study between 10 laboratories was performed with milk spiked at levels from 12 to 117 CFU/25 ml. Except for one laboratory, identical results were obtained with 143 milk samples of 144. On the basis of these findings, the French Association of Normalization approved ListerScreen in April 1995.