Evaluation of Control Strategies for Listeria monocytogenes in Retail Deli Environments with Evidence of High Prevalence

Haley Oliver
Purdue University

The project will evaluate control strategies for Listeria monocytogenes in retail delis identified as having a higher risk of L. monocytogenes prevalence and persistence.

Project is co-sponsored by the Food Marketing Institute Foundation.



The project will evaluate control strategies for Listeria monocytogenes in retail delis identified as having a higher risk of L. monocytogenes prevalence and persistence.  The proposed experiments will build on results from an ongoing AMIF, Food Marketing Institute Foundation (FMIF) and USDA funded study, which has identified niches in retail deli environments and utilized existing collaborations with retail chains.

Investigators hypothesize that implementation of control strategies for L. monocytogenes in retail delis identified as having a higher risk of prevalence and persistence will reduce the occurrence of L. monocytogenes in these environments and therefore the decrease the likelihood of cross-contamination of RTE foods.


Objective 1.  Conduct field studies in retail deli establishments to identify stores with potentially increased risk of high prevalence and persistence of L. monocytogenes in retail deli environments.  Survey development and distribution.  A survey was developed through revisions of the questionnaire previously used in the longitudinal study funded by AMIF in 2011. The questions capture physical characteristics of delis—information such as the presence or absence of fried chicken production in the deli, proximity of a seafood or raw meat department(s) to the deli, age of the building, type of flooring material, etc. —and differences in management practices among retail chains and among individual stores that may contribute to increased risk of L. monocytogenes prevalence and persistence.  Investigators distributed surveys electronically to all 50 deli managers in August and expected completion by early September. Responses will be compared with L. monocytogenes sampling data to identify store characteristics or management practices which potentially increase likelihood of L. monocytogenes contamination in retail delis.

Model development for sample site selection. Using data collected in the longitudinal study previously funded by AMIF, FSIS and the FMIF, a model was developed to predict the probability that a store has high L. monocytogenes prevalence. This predictive model is a logistic regression which uses the presence/absence of L. monocytogenes at a select number of sites to predict the probability that the store would have >10% L. monocytogenes positives across a large sample set (28 sites) over time. Model validation predicts a 97% success rate in identifying persistently, highly prevalent retail delis. The sample sites used to screen the delis (Table 2) are primary predictive sites used in the model and highly correlated secondary sites collected to further our understanding of the deli environment.

Identification of delis with high prevalence and evidence for persistent L. monocytogenes.Investigators tested 10 target sampling sites, based on the predictive model described above, in 50 stores across 6 states. This screening identified 13 stores with evidence of high prevalence. These stores are now being tested for L. monocytogenes on up to 20 food and non-food contact surfaces, including the initial 10 target sites. Results of monthly sampling for 3 months will be used to confirm identification of stores as highly prevalent before investment in interventions or changes to SSOPs (Obj. 2).  A modified version of the U.S. Food and Drug Association Bacterial Analytical Manual method was used to detect and isolate L. monocytogenes in environmental sponge samples (www.cfsan.fda.gov/~ebam/bam-10.html).  Samples scored as presumptively positive for Listeria spp. if typical Listeria-like colonies were observed on MOX plates after 48 h. Up to four presumptive L. monocytogenes colonies from LMPM will be molecularly confirmed using a L. monocytogenes PCR assay. Confirmed L. monocytogenesisolates a stored in 15% glycerol at –80°C. Pulsed Field Gel Electrophoresis (PFGE) subtyping will be performed on one representative isolate per sample using the standardized CDC PulseNet protocol.

Objective 2.  Implement practical and feasible control strategies to (i) reduce L. monocytogenes in retail deli environments and (ii) reduce cross-contamination to Ready-to- Eat (RTE) deli meats handled at retail. In order to achieve Obj. 2, investigators are working with corporate sanitarians and food safety managers from each retail chain enrolled in the study to select control strategies or optimized new potential strategies (e.g. deep cleans) for up to 10 of the target sites in the delis.  Control strategies selected for testing in Obj. 3 will be selected based on feasibility, cost, and likely impact. Control strategies finalized by corporate sanitarians and food safety managers will be implemented into in up to 10 stores with increased risk for high L. monocytogenes prevalence and persistence. The efficacy of control strategies will be tested as described in the experimental plan for Obj. 3.

Objective 3.  Test the efficacy of control strategies implemented in Obj. 2 through follow-up testing.  Retail delis with evidence for prevalent and persistent L. monocytogenes (Obj. 1) will be given a 3-month “wash-in” period for implementation and execution of control strategies identified in Obj. 2. At the end of the 3 month period, monthly environmental sampling of food and non-food contact surfaces will resume for 6 months in up to 10 stores with increased risk for L. monocytogenes prevalence and persistence. Specifically, monthly sampling as conducted in Obj. 1 will be repeated in up to 20 sites throughout the deli (food and non-food contact surfaces); isolates will be subtyped by PFGE.  Prevalence and persistence data collected after the implementation of control strategies will be compared to prevalence and persistence data collected in Obj. 1, which will serve as a baseline to determine success of intervention strategies.

Plans for Completion:  In order to complete Obj. 1, 2, and 3, investigators are committed to the following timeline:

Store information & floor plans returned (Obj. 1.; August 2012)

  • Deli Manager online survey responses completed (Obj. 1.; September 2012)
  • Sample stores with evidence for L. monocytogenes prevalence and persistence (once monthly) (Obj. 1; September-November 2012)
  • Plan interventions for identified stores (Obj. 2; October-December 2012)
  • Implement and execute interventions (e.g., deep cleans, enhanced SSOPs) (Obj. 2; January-March 2013)a
  • Monthly sampling to monitor intervention efficacy (Obj. 3; April-September 2013)

Finally, molecular subtyping (PFGE) will be performed throughout the study using the standardized CDC PulseNet protocol.  Molecular subtyping of L. monocytogenes isolates from retail environments will provide insight into remaining or additional harborage sites and routes of transmission associated with cross-contamination of deli meats at retail after our initial efforts to implement new control strategies. These data will be used to recommend best practices to the retail food industry and will be used to inform future iterations of the L. monocytogenes risk assessments and cross-contamination model.


Project status
Project code
Funded amount 
End date
15 months
July 2013

Research topic: