Performance Assessment of the GENE-TRAK® Colorimetric Probe Assay for the Detection of Foodborne Salmonella spp.

Abstract

A performance assessment of the GENE-TRAK® colorimetric probe assay using pure cultures and naturally contaminated foods and animal feeds underlined the high sensitivity and specificity of this DNA-rRNA diagnostic system. The probe effectively identified 110 (100%) strains of Salmonella spp. and yielded no false-positive reactions in the examination of 61 pure cultures of nonsalmonellae. Of the 53 contaminated raw meats and other high-moisture samples examined in this study, 47 (88.7%) were detected using the GENE-TRAK®analytical protocol. The system also identified 21 (95.5%) of 22 contaminated low-moisture foods and animal feeds. The overall sensitivity and specificity of the GENE-TRAK assay procedure with the naturally contaminated foods examined in this study was 90.7 and 100%, respectively. Ancillary work showed that the choice of selective enrichment conditions played a determinant role in the performance of the probe system. Although the GENE-TRAK protocol relies on varying temporal regimens of selective enrichment in tetrathionate brillant green (TBG35) and selenite cystine (SC35), and postenrichment in gram-negative (GN) broth for high and low-moisture food products, our results suggest that, irrespective of product type, probing of GN (6-h) postenrichment cultures inoculated from homologous TBG43 and SC35 (24-h) cultures and subsequent combination of these postenrichment cultures into a single probe assay would enhance the performance of the GENE-TRAK system to a level of unfailing sensitivity and specificity. Attempts at method brevity through direct probing of TBG43, TBG35, SC35, and Rappaport-Vassiliadis (RV43) enrichment cultures proved unsuccessful where probing of short (6-h) enrichment cultures identified ≤15.1 % and ≤72.7% of contaminated high- and low-moisture foods, respectively. Direct probing of 24 h enrichment cultures yielded homologous values of ≤17.0% and ≤77.3%. These findings suggest that components in the selective enrichment broths inhibit the probe assay. This negative effect was most prominent with RV43 enrichment cultures where most samples known to contain Salmonella spp. yielded false-negative reactions. Treatment of RV43 cultures, with 0.1 M sodium citrate prior to probe assay was partially effective in neutralizing the inhibitory agent(s).