Purpose: When samples are pooled, the detection method used must be sensitive enough to detect one presumptive positive sample that has potentially been diluted by four negative samples. The objective of this study was to test the effectiveness of iQ-Check Listeria spp. at detecting various strains of Listeria in five-sample post-enriched pooled environmental sponge samples and determine the fractional positive detection limit of the kit with diluted pooled samples.
Methods: Listeria monocytogenes, L. innocua, L. welshimeri and L. ivanovii were cultured and inoculated each into one individual environmental sponge at a target level of <10 cells. Three competitor organisms (Staphylococcus aureus, Enterococcus gallinerum and Leuconostoc pseudomesenteroides) were cultured and inoculated into all five environmental sponges at a target level of >50 cells. After an overnight enrichment, one Listeria inoculated sample was pooled with four competitor inoculated samples. In addition, the L. monocytogenes sponge pool was diluted to verify the detection limit of the method.
Results: All inoculated samples were positive as were all wet pooled samples. While there was a 2-3 Cq difference between the individual sample run on its own and the pooled sample, all samples were still positive above the threshold. The L. monocytogenes dilution series showed the fractional positive level of the assay to be between 102–103 CFU/ml.
Significance: This study shows that the sensitivity of the iQ-Check Listeria spp. kit is not compromised when wet pooling was performed. Pooling samples using a method with a sensitive enough detection limit to withstand the dilution provides an economical way to test for Listeria.