Use of Immunomagnetic Capture on Beads to Recover Listeria from Environmental Samples

Abstract

Current methods of Listeria detection require days to complete and may fail to detect Listeria in some samples. In contrast, capture of Listeria 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 their bound Listeria, are isolated from other sample material and microorganisms in a magnetic field. The beads are plated on medium and incubated overnight. The next day, a replica of the plate is made onto a thin plastic membrane. The membrane is treated with an anti-Listeria antibody, an alkaline-phosphatase conjugated antibody that binds to the anti-Listeria antibody, and a substrate for alkaline phosphatase. This treatment produces colored spots that correspond to Listeria colonies on the master plate.Immunomagnetic capture was compared to a standard cultural method for analysis of environmental samples. Immunomagnetic capture detected Listeria in 100% of the samples, at contamination levels where a standard cultural method gave detection in only 36% of the samples. At lower levels of contamination, immunomagnetic isolation allowed detection of Listeria in 58% of the samples, while the cultural method failed to detect Listeria in any samples. Immunomagnetic capture resulted in recovery of injured Listeria. Because immunomagnetic capture did not rely on enrichment, the number of Listeria colonies obtained was related to the original level of contamination. Quantitative results were produced within 24 h for both positive and negative samples, while the cultural method produced qualitative results in 6 days (for positives) or 3 days (for negatives). Immunomagnetic capture is thus both more rapid and more sensitive than a standard cultural method for detection of Listeria in environmental samples.