Evaluation of Two Isolation and Two Nonisolation Methods for Detecting Naturally Occurring Salmonellae from Broiler Flock Environmental Drag-Swab Samples

Abstract

One hundred and fifty-five environmental drag-swab samples, collected from 15 broiler flocks, were examined for the presence of salmonellae using a conventional and modified isolation method and two nonisolation assays. The conventional isolation method consisted of primary overnight enrichment in Hajna's tetrathionate broth at 41 °C followed by inoculation to xylose-lysine desoxycholate and brilliant green agar plating medium. The modified isolation method employed the same overnight enrichment broths inoculated to xylose-lysine-tergitol 4 agar and brilliant green agar with novobiocin plating media. In the modified isolation method, any samples found Salmonella negative after primary enrichment were further evaluated using delayed secondary enrichment. The two commercial nonisolation assays examined were an enzyme-linked immunosorbent assay (ELISA) and a colorimetric DNA hybridization assay (DNAHA). Salmonellae were detected in 60 samples using the conventional isolation method and in 73 samples using the modified isolation method. Salmonellae were presumptively detected in 72 samples using the ELISA and in 67 samples using the DNAHA. The improved selectivity observed with the modified isolation method was primarily due to the incorporation of the xylose-lysine-tergitol 4 plating medium. The results from the modified isolation method revealed that both the ELISA and DNAHA were more specific, but less sensitive, than was indicated by the results from the conventional isolation method.