Purpose: To compare the Atlas® Listeria monocytogenes LmG2 Detection Assay to the Dupont™ BAX® System PCR Assay for detection of Listeria monocytogenes in RTE meat matrices using a relative Limit of Detection (rLOD) approach.
Methods: Hot dog and deli turkey were bulk batch inoculated targeting 4 - 8 CFU/sample, held at 4°C for 48 - 72 h and then portioned into twenty replicate samples per matrix and method. Five un-inoculated samples were included for each method. For the Atlas®, 1:9 and 1:4 enrichment ratios were prepared in PALCAM broth + 0.02 g/l nalidixic acid and incubated at 35 ± 2°C for 24 h (25 g hot dogs) or 48 h (125 g deli turkey), respectively. For the BAX, a 1:9 dilution was prepared in UVM broth for both matrices, and incubated at 30 ± 2°C for 24 h (25 g hot dogs) or 26 h (125 g deli turkey), respectively, prior to secondary enrichment in MOPS-BLEB for 24 h prior to analysis. End-point enriched samples were 10-fold serially diluted and subjected to testing by each respective method. Culture based MPN was utilized to estimate starting concentrations.
Results: For hot dogs, the Atlas detected 20/20 positives for the 1:1000 dilution, whereas BAX detected 7/20. The rLOD value was 14.383, indicating significantly different method sensitivities (P < 0.001). Similarly, for deli turkey at the 1:10,000 dilution, Atlas detected 14/20 positives and BAX 0/20. The rLOD value was 112.441, indicating significantly different method sensitivities (P < 0.001). Un-inoculated replicates were negative for both methods. Between test Chi-square analyses at each dilution further supported rLOD analyses.
Significance: Rapid methods with enhanced sensitivity may provide more accurate test results for products with extremely low level contamination.