Method Modification for the Atlas Listeria Environmental LE Detection Assay Using FoodChek Actero Listeria Enrichment Media and Half-Fraser Media for the Detection of Listeria spp. from Environmental Surfaces.

Two candidate method modifications for the Atlas Listeria Environmental LE Detection Assay were compared with the U.S. Department of Agriculture (USDA)-Food Safety and Inspection Service Microbiology Laboratory Guidebook 8.09 (MLG 8.09) method for detection of Listeria spp. on stainless steel, polyvinyl chloride (PVC), and sealed concrete surfaces. For LE candidate method 1, samples were enriched in FoodChek Actero Listeria Enrichment Media [ALEM; Performance Tested MethodSM (PTM) 111201] at 35 ± 2°C for 18 to 24 h and evaluated for a range of analytical sample volumes. For LE candidate method 2, the current Roka PTM using 90 mL of Half-Fraser broth for enrichment at 35 ± 2°C was evaluated at 24 h with a reduced sample volume. These comparisons were made in multiple studies across the three environmental surfaces. Within each method and study, a total of 5 samples were uninoculated, 20 samples were inoculated with Listeria spp. at a low level to target fractional positivity, and 5 samples were inoculated with Listeria spp. at a high level to approach a probability of detection of 1. Inclusivity and exclusivity studies were also conducted for the LE method in combination with Half-Fraser and ALEM. The Atlas Listeria Environmental LE Detection Assay detected all 50 inclusive organisms, including 25 strains of L. monocytogenes and 5 strains of each of the other five common species of Listeria (L. innocua, L. welshimeri, L. ivanovii, L. seeligeri, and L. grayi) and none of the 30 exclusive organisms across all media and with both 200 and 2000 µL sample volumes. For the LE candidate method 1 studies, no significant differences were observed within the Roka ALEM method at 18, 20, or 24 h and for both the 200 and 2000 µL sample volumes as compared with the paired culture outcome. However, the ALEM method performed significantly better as compared with the unpaired reference method for sealed concrete and stainless steel. For the LE candidate method 2 studies, no significant differences were observed within the Roka HF method at 24 h for the 200 and 2000 µL samples as compared with the paired culture outcomes and unpaired reference method outcomes across the surfaces. The independent laboratory studies observed no significant differences in performance between the USDA/MLG 8.09 reference method and candidate methods 1 or 2, respectively, across the evaluated parameters. Overall, the candidate method 1 modification parameters and candidate method 2 sample parameters for the Atlas Listeria Environmental LE Detection Assay were statistically equivalent to or better than the reference method for detection of Listeria spp. on stainless steel, PVC, and sealed concrete surfaces, providing greater flexibility in method application for end users.