Listeria Detection after 24 Hours Using a Combined Sampler/Enrichment System and Romer Labs’ RapidChek® Listeria NextDay™

Introduction: Environmental sampling for Listeria presents challenges in large and complex facilities, such as aseptic handling of sponges and sample enrichment bags on the manufacturing floor, and assembling sample sets for laboratory processing.  Sample integrity and sampling workflow may be improved by using a sampler consisting of a pre-wetted, polyurethane foam swab incorporated into the screw cap of a 10 ml enrichment tube.  After swabbing, the sample and 1 ml neutralizing broth are delivered back to the tube, to which 9 ml enrichment broth is added.

Purpose: This study evaluated detection of low levels of Listeria monocytogenes by RapidChek® Listeria NextDay™ after 24 h incubation in a combined sampler/enrichment tube system with 9ml broth, and the traditional sampling sponge and enrichment bag with 60 ml broth.

Methods: For each method, 3 CFU/sample (n = 20) and 10 CFU/sample (n = 5) L. monocytogenes were spiked along with 1 x 10⁵ CFU/sample competitor background.  Samples were cold-stressed 24 h, and then incubated at 30 ± 2°C in the aforementioned volumes of enrichment broth for 24 h before boiling 0.4 ml from each enriched sample and testing by lateral flow device.  All samples were confirmed per USDA MLG 8.09 guidelines by streaking to MOX agar, followed by subsequent examination for colonies with Listeria spp. morphology and esculin hydrolysis.

Results: At 3 CFU/ml, both sampler/enrichment methods had 19 confirmed positive results.  At 10 CFU/ml, both sampler/enrichment methods had five confirmed positive results.  With a Mantel-Haenzel χ²of 0, both sampler/enrichment methods had no false positive or negative results, and indicated 100% sensitivity, specificity and accuracy.  The results indicate that both environmental sampling methods evaluated are equivalent.

Significance: The combined swab sampler/enrichment tube method can facilitate improved aseptic sampling, sample management, and laboratory workflow, while reliably detecting low levels of Listeria in environmental samples after 24 h incubation using this lateral flow test system.