Rapid Detection of Salmonella enteritidis in Pooled Liquid Egg Samples Using a Magnetic Bead-ELISA System

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Abstract
An assay was developed to shorten the time necessary to detect Salmonella enteritidis (SE) in contaminated egg pools. The immunomagnetic separation (IMS)-based assay used the DynabeadsTM Anti-Salmonella, a magnetic bead with mouse anti-Salmonella antibodies affixed to the surface, to bind the SE in the egg pools. The bound SE were concentrated by a magnet and were detected via an enzyme-linked immunosorbent assay (ELISA) (IMS-ELISA) employing a monoclonal anti-SE flagellar proteins (flagellins) antibody. Following the ELISA, the beads were plated onto differential media (IMS-direct).The efficacy of the assay for detecting SE was compared with that of the standard assay, direct plating, in pooled egg samples spiked with low concentrations of SE and incubated at 37°C for 24 to 96 h. Conventional direct plating of egg samples required a total of 48 h before SE could be identified in egg pools, compared with 24 h for the IMS-ELISA. Plating of the beads (IMS-direct) to confirm the presence of SE required a further 24 h. The IMS-ELISA could detect SE at concentrations of 105 to 106 SE cells per ml, comparable to that shown previously for direct plating. The IMS-direct could detect SE at 104 SE cells per ml of egg pool. In egg pools initially contaminated with 10 SE cells per ml, the organism grew to levels by 24 h at 37°C where 100% of the pools were positive for SE by all three detection methods. In egg pools initially contaminated with 1 SE cell per ml, 61% of pools were detected by direct plating and IMS-ELISA and 72% were detected by IMS-direct. Similar detection frequencies were observed for a second SE isolate. The IMS-ELISA provides an SE detection rate comparable to direct plating but achieves the result 24 h sooner. The IMS-direct was the most sensitive means of detecting the SE.

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