Comparison of 3M[trade mark sign] Petrifilm[trade mark sign] Environmental Listeria Plates against Standard Enrichment Methods for the Detection of Listeria monocytogenes of Epidemiological Significance from Environmental Surfaces

Environmental monitoring using sensitive methods for detection and elimination of harborage sites of Listeria monocytogenes is key to the control of this organism. The 3M[trade mark sign] Petrifilm[trade mark sign] Environmental Listeria (EL) Plate--a no enrichment method--was compared with the USDA/FSIS, modified USDA/FSIS (mUSDA), and ISO methods for detection/recovery of L. monocytogenes on 4 environmental surfaces (brick, dairy board, stainless steel, and epoxy resin). The efficacy of 3 sampling devices including the Microbial-Vac system®, environmental sponge, and 3M Quick swab in recovering epidemiologically significant strains of uninjured and sublethally injured L. monocytogenes from environmental surfaces was evaluated. Environmental surfaces were inoculated with Listeria to obtain final cell densities of approximately 10 to 100 CFU/100 cm². The surfaces were then sampled and processed. For all methods, percent recovery (% samples where Listeria was detected) was significantly higher (P < 0.05) for uninjured cells (75% to 100%) compared to injured cells (58.9% to 81.1%). The Petrifilm EL Plate method efficiently recovered both low level and injured Listeria populations from environmental test surfaces when used in conjunction with environmental sponge and the 3M Quick swab sampling. The mUSDA method was superior to all other methods for recovering both uninjured (100% recovery) and injured L. monocytogenes (80.8% to 81.1% recovery). Sponges and swabs were equally effective in recovering uninjured and injured Listeria and were significantly different (P < 0.05) from the Microbial-Vac system. The findings indicate that both mUSDA and Petrifilm EL Plate methods can be used for the detection of potentially injured Listeria on food processing environmental surfaces.