Validation of a 24-hour Immunochromatographic Test Strip-based Method for the Detection of Listeria spp. on Environmental Surfaces

Introduction: The USDA-FSIS has recently proposed a requirement for food processing establishments to withhold the release of product into commerce pending pathogen test results.  For ready-to-eat (RTE) food processors, environmental monitoring of food contact surfaces for Listeria spp. is commonly used as an indicator of the potential for L. monocytogenes contamination.   In order to reduce the time-to-result and therefore the cost of holding product pending test results, we have developed a simple, 24-hr, immunochromatographic test strip-based method for the detection of Listeria spp. on relevant environmental surfaces. 

Purpose: The purpose of this study was to validate the performance of a new 24-hr, immunochromatographic test strip-based method for the detection of Listeria spp. on relevant environmental surfaces. 

Methods: Environmental surfaces were spiked at levels ranging from 50 to 400 CFU per surface.  A total of 120 spiked samples were tested by the SDIX method at 24 and 40 h and the USDA-FSIS cultural reference method.  Inclusivity/exclusivity studies evaluated 50 Listeria strains and 35 non-Listeria bacterial strains. 

Results: Total confirmed positives were 49, 54, and 48 for the SDIX 24 h method, the SDIX 40 h method, and the USDA-FSIS cultural reference method, respectively.  Non-spiked samples from all environmental surfaces were reported as negative for Listeria spp. by all methods.  The overall Chi square was 0.017 (P = 0.104) and 0.611 (P = 0.566) after a 24 h and 40 h enrichment, respectively, indicating that the test method was equivalent in performance to the reference method at both enrichment times.  The method showed 100% sensitivity and 97% specificity for Listeria spp.

Significance: The new test method should provide the end user with a rapid and reliable tool for monitoring and controlling Listeria species in the food processing environment and by doing so, minimize the contamination of food products by Listeria monocytogenes.