Recovery of Salmonella and Listeria monocytogenes from Nonporous Surfaces Based on Surface Sampler Type

Abstract

Surface sampling devices ranging in material composition and size can be used in environmental monitoring programs. This study aimed to compare the bacterial recovery efficiency of surface samplers on stainless steel (SS) and polypropylene (PP) surfaces. Separate cocktails of Listeria monocytogenes and Salmonella enterica strains were spot inoculated (1 mL; 40 spots x 25 mu L) on SS and PP surfaces at high (7 log) and low (4 log) concentrations. A cellulose sponge sampler, polyurethane foam sponge sampler, and polyolefin nonwoven fabric sampler were utilized for bacteria recovery from SS and PP surfaces at two surface areas: 1x [929 cm2 (144 in2 )] and 2x [1858 cm2 (288 in2 )]. The effect of prewet volume (5 mL, 10 mL) on bacteria recovery from PP and SS was also investigated with the nonwoven fabric sampler at high inoculum level and 1x surface area. Three experimental trials were conducted totaling 336 samples, and recovery percentages were based on the CFU recovered divided by the initial CFU added to each surface. Statistical analysis was performed to determine whether sampler type, pathogen type, inoculum concentration, surface type, and surface area were significant predictors of recovery percentage. A significant five-way interaction (P = 0.0015) was observed between the predictor variables; therefore, no conclusions can be made regarding the main effects. The recovery percentage of L. monocytogenes was significantly higher than Salmonella from PP surfaces across all three sampler types. For the nonwoven fabric sampler, the 5 mL prewet volume yielded significantly higher recovery (P <= 0.05) for both bacteria combined at 10.66% (95% CI: 9.93, 11.44) compared to 3.09% (95% CI: 2.71, 3.52) recovery with the 10 mL prewet volume. However, the effect of volume on recovery depended on the interaction between surface type and inoculum level.