Prevalence of Listeria Monocytogenes and Salmonella Enterica in Retail Produce Environments and Strategies for Their Control

John Lawrence Burnett, Purdue University


There has been a notable increase in produce-related foodborne illness attributed to L. monocytogenes and S. enterica. Recent studies show retail environments may be a source for L. monocytogenes contamination of deli meat; however, environmental prevalence of both L. monocytogenes and S. enterica in retail produce remains largely uncharacterized. Studies aiming to decrease the prevalence of L. monocytogenes in retail delicatessens have not shown significant successes. The first goal of this study was to determine the prevalence of Listeria monocytogenes and Salmonella enterica in retail produce environments and to elucidate their possible ecological niches. Thirty environmental samples per store were collected during daily operations monthly for six months in 30 retail produce departments across seven states. Selected samples were serially diluted and plated to determine aerobic plate count. Each sample was tested for L. monocytogenes and S. enterica using ROKA Atlas LmG2 and SEN assays, respectively. A total of 5,112 samples were tested for each pathogen. S. enterica was found during one sampling event in a single store; less than 0.1% of samples were positive overall. A total of 4.4% environmental samples tested positive for L. monocytogenes. L. monocytogenes was present on 8.1% of non-food contact surfaces and 1.6% of food contact surfaces tested; L. monocytogenes prevalence was highly variable among stores. Most of the positive L. monocytogenes samples were found in drains, floors, squeegees, or standing water. The odds of detecting L. monocytogenes increased 1.8-fold for every 1-log increase in APC (p < 0.0001). The data suggest that retail produce environments may be a significant source of L. monocytogenes, which may contaminate produce. However, S. enterica prevalence is very low and likely due to transient contamination. Further, APC may be a cost effective environmental monitoring tool that could indicate an environment capable of harboring L. monocytogenes. We subsequently evaluated the efficacy of SSOP changes, and employee-led department deep cleans to reduce L. monocytogenes prevalence in retail produce environments with the goal of preventing cross-contamination to ready-to-eat foods. Eight stores from the longitudinal study with >5.6% L. monocytogenes prevalence were selected for deep cleans, employee re-training and SSOP changes focused on eliminating standing water and enhanced harborage site sanitation. L. monocytogenes prevalence decreased by 53.1% in store 27 (χ2; p<0.0001), 49.9% in store 14 (χ2; p<0.0001), and 41.5% in store 5 (χ2; p=0.0008). There was no significant change in L. monocytogenes on NFCS in four stores (8, 17, 21, 28). As a group, the stores decreased prevalence on food contact surfaces (FCS) by 4.78% (17/198 to 4/105 (χ2; p=0.1199)). Standing water and floor drains remained those with the highest prevalence of L. monocytogenes. L. monocytogenes prevalence in retail environments can be reduced by deep cleans and changes in SSOPs. The overall decrease on FCS may suggest that the interventions decreased cross-contamination from NFCS, even in stores which failed to decrease prevalence on NFCS.




Oliver, Purdue University.

Subject Area

Food Science

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