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

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).