USE OF IMMUNOMAGNETIC CAPTURE ON BEADS TO RECOVER LISTERIA FROM ENVIRONMENTAL-SAMPLES

Current methods of Listeria detection require days to complete and may fail to detect Listeria in some samples. In contrast, capture of List eria on microscopic immunomagnetic beads reduces test time to 24 h and improves sensitivity. In this approach, which eliminates enrichment, samples are mixed with immunomagnetic beads coated with anti-Listeria antibodies. Listeria in the sample bind to beads. The beads, with thei r bound Listeria, are isolated from other sample material and microorg anisms in a magnetic field. The beads are plated on medium and incubat ed overnight. The next day, a replica of the plate is made onto a thin plastic membrane. The membrane is treated with an anti-listeria antib ody, an alkaline-phosphatase conjugated antibody that binds to the ant i-Listeria antibody, and a substrate for alkaline phosphatase. This tr eatment produces colored spots that correspond to Listeria colonies on the master plate. Immunomagnetic capture was compared to a standard c ultural method for analysis of environmental samples. Immunomagnetic c apture detected Listeria in 100% of the samples, at contamination leve ls where a standard cultural method gave detection in only 36% of the samples. At lower levels of contamination, immunomagnetic isolation al lowed detection of Listeria in 58% of the samples, while the cultural method failed to detect Listeria in any samples. Immunomagnetic captur e resulted in recovery of injured Listeria. Because immunomagnetic cap ture did not rely on enrichment, the number of Listeria colonies obtai ned was related to the original level of contamination. Quantitative r esults were produced within 24 h for both positive and negative sample s, while the cultural method produced qualitative results in 6 days(fo r positives) or 3 days (for negatives). Immunomagnetic capture is thus both more rapid and more sensitive than a standard cultural method fo r detection of Listeria in environmental samples.