Novel Approaches for the Efficient Sampling and Detection of Listeria monocytogenes and Brochothrix thermosphacta on Food Contact Surfaces

The primary step in the microbiological assessment of highly dynamic and complex food processing conditions is environmental sampling. The objectives of this study were to: (1) compare the efficacy of four sampling devices including Microbial-Vac system (MV), cellulose sponge (SP), polyester swab (SW) and composite tissue (CT), for the recovery of Listeria monocytogenes and Brochothrix thermosphacta on five surfaces and (2) to determine if there was a significant difference between the recovery of low (10 CFU/900cm²) and high (100 CFU/900cm²) L. monocytogenes inoculum levels using the sampling devices in a simulated food processing environment. Surfaces used for this study were stainless steel (SS), polyethylene cutting board (CB), polyurethane conveyor belt (PB), open hinge flat top conveyor belt (FT) and mesh conveyor belt (MB). Food contact surfaces were inoculated with L. monocytogenes to obtain a final cell population of 10 (low) or 100 (high) CFU/900 cm². An average cell density of 10,000 CFU/25 cm² was used for inoculating B. thermosphacta on each of the surfaces. Inoculated surfaces were dried and held for two hours at 4˚C then sampled and processed for detection. Because L. monocytogenes is a "zero tolerance" pathogen in ready-to-eat foods, the qualitative analysis included an enrichment step to detect presence/absence in the sample. In comparison, B. thermosphacta was directly plated in order to quantify the recovery capability of each device. Results indicated for recovery of 100 CFU/900 cm² L. monocytogenes, there was no difference among devices on SS, CB or PB surfaces (p>0.05). However, a significant difference was detected at 10 CFU/900 cm² on SS between MV and CT, 62.97 and 17.34%, respectively (p=0.0086). Results for FT indicated MV was superior over SP and SW (p=0.0004) for detection of high and low L. monocytogenes. There was no difference for the quantitative recovery of B. thermosphacta on PB and SS; however, there was a difference (p=0.0371) among devices on CB indicating MV was superior over SP and CT. The swab recovered 3.25 log CFU/25cm² from flat top belts and was significantly lower (p=0.0259) than MV and SP devices, 4.29 and 4.12 log CFU/25cm², respectively.