Results of Data Analysis for the Listeria 

monocytogenes RLm Risk-based Sampling 

Program, Calendar Year 2008
 

Stephen W. Mamber, USDA 

September 2010 
 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

ACKNOWLEDGEMENTS
 

This report is the result of efforts and contributions from across the Food Safety and Inspection Service. 

The Data Analysis and Integration Group (DAIG) of the Office of Data Integration and Food Protection 

developed this report in consultation with experts from other program areas, including Headquarters and 

field personnel. Key contributions were made by the following individuals: 

Kristina Barlow 

Nelson Clinch 

Erin Dreyling 

Emilio Esteban 

Peter Evans 

Gail Graves 

Susan Knower 

David LaBarre 

Loren Lange 

Priscilla Levine 

Harry Marks 

Evelyne Mbandi 

Paul Uhler 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

TABLE OF CONTENTS
 

Executive Summary............................................................................................................. 1
 

1. Introduction.......................................................................................................................... 2
 

1.1	 Background................................................................................................................ 3
 

2. Data Collection Design and Implementation..................................................................... 5
 

3. Data Analysis Procedures.................................................................................................... 6
 

4. Results and Discussion......................................................................................................... 7
 

4.2	 Classification of Lm-Positive Samples, within Establishments, Calendar Year 

2008........................................................................................................................... 10
 

4.3	 Types and Sources of Lm-Positive Samples, Calendar Year 2008...................... 10
 

4.4	 Isolate PFGE Pattern Typing Results ................................................................... 13
 

4.5	 Cross-contamination and Harborage .................................................................... 14
 

4.6	 Results Based on Alternatives Used to Control L. monocytogenes ..................... 15
 

4.7	 Results by Establishment HACCP Size................................................................. 16
 

4.8	 Results by Production Volume............................................................................... 16
 

4.9	 Results by Food Product Category ........................................................................ 17
 

4.10	 Results by Geographic Region ............................................................................... 17
 

4.11	 Results by Season and Month................................................................................. 18
 

4.12	 Trend Analysis of Combined RLm Data: April 2006 through December 2008 26 

5. Summary and Conclusions................................................................................................ 31
 

Appendix A: FSIS Scheduling Criteria for Routine Lm Risk-based (RLm) Sampling 

Program................................................................................................................. 34
 

LIST OF TABLES 

Table 4.1.1. Detection of L. monocytogenes in Product, Contact Surface, and 

Environmental Samples, Calendar Year 2008..................................................... 7
 

Table 4.1.2. Number and Percentage of Establishments with at Least One Lm-Positive 

Sample, Calendar Year 2008................................................................................. 9
 

Table 4.2.1. Classification of Lm-Positive Samples from Establishments with at 

Least One Positive Sample, Calendar Year 2008 .............................................. 10
 

Table 4.3.1. Types of Lm-Positive Product Samples, Calendar Year 2008.......................... 11
 

Table 4.3.2. Types of Lm-Positive Contact Surface Samples, Calendar Year 2008............ 11
 

Table 4.3.3. Lm-Positive Rate for the Main Types of Contact Surface Samples, 

Calendar Year 2008.............................................................................................. 12
 

Table 4.3.4. Types of Lm-Positive Environmental Samples, Calendar Year 2008.............. 12
 

Table 4.3.5. Lm-Positive Rate for the Main Types of Environmental Samples, 

Calendar Year 2008.............................................................................................. 13
 

Table 4.3.6. Lm-Positive Rate for RLMENVR and INTENV Floor Mat Samples, 

Calendar Years 2006-2007 and 2008 .................................................................. 13
 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Table 4.4.1. Patterns and Occurrence of Lm PFGE Subtypes Isolated in the RLm 

Program, Calendar Year 2008 ............................................................................ 14
 

Table 4.5.1. Incidence of Multiple Isolations of the Same PFGE Subtype within the Same
 

Establishment, Calendar Year 2008 ................................................................... 15
 

Table 4.6.1. Detection of L. monocytogenes by Control Alternative, Calendar Year 

2008 ........................................................................................................................ 19
 

Table 4.7.1. Detection of L. monocytogenes by Establishment Size, Calendar Year 

2008 ........................................................................................................................ 21
 

Table 4.8.1. Detection of L. monocytogenes as a Function of Establishment Annual 

Production Volumes, Calendar Year 2008......................................................... 21
 

Table 4.10.1. Detection of L. monocytogenes by Geographic Region, Calendar Year 2007 . 24
 

LIST OF FIGURES 

Figure 4.1.1. Samples Collected and Tested for L. monocytogenes, Calendar Year 2008...... 8
 

Figure 4.1.2. Percentage of Lm-Positive Samples, Calendar Year 2008.................................. 8
 

Figure 4.1.3. Percentage of Establishments with Lm-Positive Samples, Calendar Year 

2008 .......................................................................................................................... 9
 

Figure 4.6.1. Percentage of Lm-Positive Product and Contact Surface Samples by 

Figure 4.6.2. Percentage of Lm-Positive Environmental Samples by Control
 

Figure 4.7.1. Percentage of Lm-Positive Product and Contact Surface Samples by 

Figure 4.7.2. Percentage of Lm-Positive Environmental Samples by Establishment
 

Figure 4.8.1. Percentage of Lm-Positive Samples as a Function of Establishment 

Figure 4.9.1. Percentage of Lm-Positive Samples by Establishment Food Production 

Figure 4.10.1. Percentage of Lm-Positive Product, Contact Surface and Environmental
 

Figure 4.11.1. Percentage of Lm-Positive Product, Contact Surface and Environmental
 

Control Alternative, Calendar Year 2008.......................................................... 19
 

Alternative, Calendar Year 2008 ........................................................................ 20
 

Establishment HACCP Size, Calendar Year 2008 ............................................ 22
 

HACCP Size, Calendar Year 2008 ..................................................................... 22
 

Annual Production Volumes, Calendar Year 2008........................................... 23
 

Category, Calendar Year 2008............................................................................ 23
 

Samples by Geographic Region, Calendar Year 2008..................................... 25
 

Samples by Season, Calendar Year 2008.......................................................... 25
 

Figure 4.11.2. Percentage of Lm-Positive Samples by Month, Calendar Year 2008 ............ 26
 

Figure 4.12.1. Percentage of Total Samples Positive for L. monocytogenes,
 

April-December 2006 and Calendar Years 2007 and 2008............................. 28
 

Figure 4.12.2. Percentage of Product and Contact Surface Samples Positive for L.
 

monocytogenes, April–December 2006 and Calendar Years 2007 and 2008. 28
 

Figure 4.12.3. Percentage of Environmental Samples Positive for L. monocytogenes, 

April-December 2006 and Calendar Years 2007 and 2008 ............................. 29
 

Figure 4.12.4. Percentage of Establishments with at Least One Lm-Positive Sample, 

April-December 2006 and Calendar Years 2007 and 2008............................. 29
 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.12.5. Percentage of Establishments with Lm-Positive Product and Contact
 

Surface Samples, April-December 2006 and Calendar Years 2007 and 

2008....................................................................................................................... 30
 

Figure 4.12.6. Percentage of Establishments with Lm-Positive Environmental Samples, 

April–December 2006 and Calendar Years 2007 and 2008 ............................ 30
 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

ACRONYMS AND ABBREVIATIONS 

CDC Centers for Disease Control and Prevention 

DAIG Data Analysis and Integration Group 

EIAO Enforcement, Investigations, and Analysis Officer 

FSA Food Safety Assessment 

FSIS Food Safety and Inspection Service 

GMP Good manufacturing practice 

HACCP Hazard Analysis and Critical Control Point 

IVT Intensified Verification Testing 

Lm Listeria monocytogenes 

LSFS Laboratory Sample Flow System 

NOIE Notices of Intended Enforcement 

NR Noncompliance Record 

OFO Office of Field Operations 

OSEL Outbreaks Section of the Eastern Laboratory Microbiology Branch 

PBIS Performance-based Inspection System 

PFGE Pulsed field gel electrophoresis 

PHV Public Health Veterinarian 

RLm Routine Listeria monocytogenes Risk-based Sampling Program 

RTE Ready-to-eat 

USDA United States Department of Agriculture 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

EXECUTIVE SUMMARY
 

The Food Safety and Inspection Service’s (FSIS’s) Listeria monocytogenes Risk-based RLm sampling 

program is designed to detect L. monocytogenes (Lm) contamination from three types of samples: Postlethality environmentally exposed ready-to-eat (RTE) meat and poultry products, RTE food contact 

surfaces and noncontact environmental sources. The Agency analyzed results of Lm testing of meat and 

poultry product, food contact surface, and environmental (nonfood contact) samples collected under the 

RLm sampling program for calendar year 2008. These analyses, which included 6,006 samples from 204 

establishments, focused on the following: 

incidence and categorization of positive Lm samples from sampled establishments; 

types, sources, and pulsed field gel electrophoresis (PFGE) subtyping of Lm isolates from the 

positive samples; 

descriptive summaries with respect to 

– Lm control alternatives employed by the establishment, 

– establishment Hazard Analysis and Critical Control Point (HACCP) size, 

– establishment production volume, 

– FSIS District, 

– geographic location of the establishment, and 

– season or month of sample collection; and 

trends in percentage of positive results from April 2006 (program inception) through 2008. 

For calendar year 2008, 0.5% (5/959) of product samples, 0.6% (19/3,322) of contact surface samples, 

and 2% (35/1725) of environmental samples tested positive for L. monocytogenes. This included five 

product samples from four separate establishments. About 1 in 25 establishments had Lm-positive contact 

surface samples, and about 1 in 6 establishments had Lm-positive environmental samples. The five Lmpositive products were three chicken products, one beef product, and one pork product. Containers, 

blades, tables, and trays were the chief types of positive contact surface samples. Drains, wheels, floors, 

floor mats, and squeegees were the most common sources of positive environmental samples; 20% of the 

tested floor mat samples were positive for Lm. PFGE subtyping results yielded 31 distinct patterns among 

59 isolates tested. Matching PFGE subtypes were obtained among multiple isolates from each of six 

establishments. 

Results of analysis based on Lm control alternatives showed that the majority of positive samples were 

obtained from establishments employing Lm control Alternative 2b (antimicrobial treatment/high-risk) 

and Alternative 3 (sanitation only/highest risk). Most of the positive RLm samples were from 

establishments that produced deli meats, hot dogs, and cooked products. Positive product and contact 

surface samples were isolated mainly from establishments that produced between 10,000 and 10,000,000 

pounds of product per year. (This encompasses about 85% of all establishments, but less than 15% of 

total production volumes). Positive environmental samples were obtained at all times of the year, whereas 

most of the positive contact surface samples were obtained mainly between May and September. 

Analysis of multi-year (2006-2008) data for the RLm program revealed statistically significant increases 

for RLMPROD (RLm product) and RLMCONT (RLm food contact surface) samples over time. There 

also was a downward trend in the percentage of establishments with at least one Lm-positive sample. The 

net effect of these results is that between 2006 and 2008, more Lm-positive samples were being isolated 

from a stable or decreasing percentage of Lm-positive establishments. 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

1. INTRODUCTION 

FSIS conducts regulatory microbiological testing of ready-to-eat (RTE) meat and poultry products for 

three microorganisms: Listeria monocytogenes (Lm), Salmonella, and Escherichia coli O157:H7. FSIS 

began risk-based testing of RTE products for L. monocytogenes in 2004. One of the first of these riskbased sampling and testing programs was ALLRTE (random verification sampling of all RTE meat and 

poultry products). All establishments were considered at equal risk under the ALLRTE sampling 

program. 

FSIS regulations also mandated the reporting of various production factors by establishments producing 

RTE meat and poultry products that were exposed to the environment after a lethality treatment. This 

served as the basis for sampling programs based on the risk characteristics of the producing 

establishment. RTE001, a sampling and testing program for RTE meat and poultry products based on 

establishment risk factors, was initiated in January 2005. An Lm risk-based sampling project named the 

Routine Lm Risk-based (RLm) Sampling Program was then initiated in April of 2006. While RTE001 

involves sampling and testing of the RTE meat and poultry products themselves, the RLm program 

includes sampling and testing of products, product contact surfaces, and environmental surfaces. This 

makes the RLm program a proactive sampling project; that is, RLm provides a means of identifying 

establishments with a higher risk of Lm contamination in the food processing environment before product 

contamination can actually be demonstrated. In addition, a Food Safety Assessment (FSA) is conducted at 

the establishment in conjunction with RLm sampling and testing. Unlike the ALLRTE and RTE001 

programs in which samples are also tested for Salmonella and E. coli O157:H7, in the risk-based RLm 

program, samples are collected and tested for L. monocytogenes only. 

The RLm testing program consists of the following three concurrent sampling projects: 

1.	 RLMPROD—the routine risk-based testing of intact RTE food product samples throughout the 

selected production shift; 

2.	 RLMCONT—the routine risk-based testing of surfaces that have direct contact with RTE 

product(s) in the RTE production area (e.g., conveyor belts, storage racks, slicer blades, loaders, 

table tops); and 

3.	 RLMENVR—the routine risk-based testing of environmental (nonfood contact) surfaces in the 

RTE production areas (e.g., floors, drains, walls, floor mats). 

Samples collected under the RLm program are limited to establishments subject to 9 CFR Part 430 (i.e., 

establishments in which RTE products are exposed to the post-lethality environment (see 

http://www.access.gpo.gov/nara/cfr/waisidx_08/9cfr430_08.html)). 

In accordance with FSIS Directive 10,240.5, Verification Procedures for Enforcement, Investigations and 

Analysis Officers (EIAOs) for the Listeria monocytogenes (Lm) Regulation and Routine Risk-Based 

Listeria monocytogenes (RLm) Sampling Program, FSIS Enforcement, Investigations, and Analysis 

Officers (EIAOs) and Public Health Veterinarians (PHVs) trained in EIAO methodologies are responsible 

for conducting RLm sampling and assessing whether the establishment’s food safety system complies 

with 9 CFR Part 430 (see http://www.fsis.usda.gov/OPPDE/rdad/FSISDirectives/10240.5Rev2.pdf). 

The selection of establishments for food contact and environmental swab sampling and number of 

samples for testing was previously based on an FSIS risk-based sampling algorithm. The risk-ranking 

algorithm was maintained using data from various Agency resources, including information from FSIS 

Form 10,240-1 (in particular, food product category and sanitation alternative for controlling Listeria) and 

the establishment’s sample history. Once the risk ranking had been established, additional scheduling 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

criteria were employed to select establishments for testing. However, starting in July 2008, the top 95% 

volume establishments were sampled. Scheduling criteria for the RLm program for 2008 were posted on 

the FSIS Web site and are included here as Appendix A. The risk ranking is updated monthly. A 

scheduling memo is sent to districts to inform them of the establishments selected for RLm sample 

collection activity. 

FSIS previously tabulated, analyzed, evaluated, and reported on Lm data collected under the RLm 

program since its inception in April 2006 through calendar year 2007 (see 

http://www.fsis.usda.gov/PDF/Results_Data_Analysis_Lm.pdf). Accordingly, the objectives of this report 

were to (1) obtain, tabulate, and review sampling results for calendar year 2008; (2) evaluate the data with 

respect to various parameters; and (3) identify possible trends in the data based on annual results for 2006 

through 2008. 

1.1 Background 

The RLm program was conceived as a means of routinely collecting and testing three types of samples 

(RTE meat and poultry product, food contact surface, and environmental) in post-lethality production 

areas where L. monocytogenes may be present. The impetus for this sampling and testing effort was the 

need to determine if, or how well, establishments were controlling Lm contamination in post-lethality 

exposed RTE products based on regulation 9 CFR 430. FSAs are conducted in conjunction with sample 

testing to evaluate the food safety practices of establishments producing post-lethality exposed RTE 

products, particularly those establishments considered to be high risk. 

This examination of high-risk establishments on a routine basis was conceptually similar to that of 

Intensified Verification Testing (IVT). IVT involves the collection and testing of food contact surface and 

non-food contact environmental samples (in addition to product) in response to RTE meat and poultry 

product samples that initially test positive for L. monocytogenes in the ALLRTE and RTE001 sampling 

programs. What sets the RLm program apart from IVT (and indeed, from other FSIS sampling programs 

for foodborne pathogens) is that the routine testing of samples from food contact surfaces and processing 

environments is not done in response to positive product samples. Rather, the purpose of such testing is to 

proactively detect the presence of L. monocytogenes in establishments, even in the absence of actual 

product contamination, and to take corrective actions accordingly. (With respect to positive samples from 

the RLm program, one way of ensuring the effectiveness of any corrective actions is IVT itself, employed 

as a follow-up when a sample tests positive for L. monocytogenes.) 

In conducting the RLm program, FSIS anticipated it would be able to assess the compliance of 

establishments with regulation 9 CFR 430 regarding the control of L. monocytogenes in post-lethality 

exposed RTE products and help ensure that RTE products are safe for consumption at the end of the 

production process. With the RLm program in place, FSIS has the ability to verify and evaluate Lm 

control alternatives and sanitation practices at individual establishments that would not be possible with 

normal day-to-day inspection. The RLm program was also designed, in part, to increase confidence in the 

effectiveness of a given establishment’s control measures and interventions (alternatives). 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

RLm testing involves collecting multiple samples as a unit—3 product samples, 10 product contact 

samples, and 5 environmental (non-product contact) samples—during a production shift.1 (It should be 

noted that it is up to the EIAO to select products based on which ones are considered to be the highest 

risk, as well as selecting appropriate contact and environmental surfaces for collection and testing). This 

contrasts markedly with RTE001 and ALLRTE, in which a single RTE product sample is collected at a 

given point in time. Moreover, because RLm sampling at each establishment is done in conjunction with 

an FSA, an in-depth evaluation of food safety practices is possible. The product, contact surface, and 

environmental sample data collected from the establishments can help identify possible risk factors that 

could be associated with positive results. For example, testing of food contact and environmental samples 

may permit the identification of establishments where there is evidence of control issues, such as 

harborage (sites of Lm survival or persistence) or poor sanitation practices. 

Because the RLm program was intended to be a routine sampling program to complement the FSA 

process, FSIS has the expectation that establishments selected for sampling should be in compliance with 

all regulatory standards because those establishments are selected for sampling on the basis of risk rather 

than for any particular cause. Accordingly, FSIS evaluates establishments producing RTE products, first 

and foremost, to ensure the safety of these products and, thus, to protect the public from foodborne 

Listeria infections. If a given establishment has positive results from the RLm sampling, FSIS takes 

enforcement actions as necessary to address product contamination and adulteration. Furthermore, the 

positive results serve as the impetus for focusing inspection efforts and intensifying inspection resources 

in that establishment. Such results may indicate poor HACCP design, execution, or both. In addition to 

determining the vulnerabilities and the adequacy of the establishment’s food safety practices with respect 

to L. monocytogenes, FSIS develops and implements policies to improve the effectiveness of the 

establishment’s Listeria contamination control practices. 

Initially, the numbers of 18-sample units used at each establishment were based on the number of production lines. Currently, 

the number of sampling units is based on establishment Hazard Analysis and Critical Control Point (HACCP) size, with three, 

two, and one sample units used at establishments classified as HACCP sizes Large, Small, and Very Small, respectively. This 

system provides for consistency with respect to the logistics of sample collection and testing. It should also be noted that the 

ratio of 10:5 for contact and environmental samples is because positive contact samples have defined regulatory consequences, 

which is not the case for positive environmental samples. 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

2. DATA COLLECTION DESIGN AND IMPLEMENTATION 

Data routinely generated from the 2008 RLm program were used for all analyses. FSIS tabulated the 

routine risk-based sample information for L. monocytogenes in RTE meat products that were collected on 

FSIS sampling forms. The data consisted of product, contact surface, and environmental test results for 

samples that were collected and tested for L. monocytogenes. Information regarding establishment 

parameters such as control alternative and production volumes, was obtained from establishmentprovided data contained in FSIS Form 10,240-1. These data were extracted from the FSIS Data 

Warehouse (M2K database) via Laboratory Sample Flow System (LSFS). Supplementary data were 

obtained using the Performance-based Inspection System (PBIS) reader. 

The PFGE pattern data related to isolate subtyping of Lm-positive samples were provided by the 

Outbreaks Section of the Eastern Laboratory Microbiology Branch (OSEL) of FSIS. Actual PFGE pattern 

designations and pattern matching were done by the Centers for Disease Control and Prevention (CDC). 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

3. DATA ANALYSIS PROCEDURES 

FSIS calculated the numbers of positive samples and percentage of positive samples for product, contact 

surface, and environmental samples for calendar year (CY) 2008. The data analyzed were based on 

sample collection dates (January 1, 2008 through December 31, 2008)1

. The analyses which included 

6,006 samples from 204 establishments in 2008, focused on the following: 

the incidence and categorization of positive Lm samples from sampled establishments; 

types, sources, and PFGE subtyping of Lm isolates from the positive samples; 

descriptive summaries with respect to the following: 

– Lm control alternatives employed by the establishment, 

– establishment HACCP size, 

– establishment RTE production volumes, 

– FSIS District, 

– geographic location of the establishment, 

– season or month of sample collection; and 

trends in percentages of positive results from 2006-2008 (all based on sample collection date). 

All data analyses were performed through data handling and evaluation techniques using Microsoft Office 

Excel. 

FSIS routinely posts summarized annual data for their microbiological testing programs for ready-to-eat (RTE) meat and 

poultry products (http://www.fsis.usda.gov/Science/Micro_Testing_RTE/index.asp#results08). Prior to calendar year 2008, 

results that were posted were based on sample analysis completion date. Beginning in January 2008, results are being posted 

by sample collection date to include all samples collected within the calendar year. This aligns FSIS' activities in this area with 

those of other Federal partners. As it happens, the 2008 RLm results by collection date are directly comparable with the RLm 

results from 2006 and 2007 because those results are the same regardless of whether collection date or completion date is used 

(i.e., in 2006 and 2007, all samples collected had their analyses completed within the same calendar year). 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

4. RESULTS AND DISCUSSION 

Data collection under the RLm sampling program began in April 2006. The results for the periods April 

through December 2006 and calendar year 2007 are found in an online report at 

http://www.fsis.usda.gov/PDF/Results_Data_Analysis_Lm.pdf. Sampling data collected under RLm in 

calendar year 2008 included product, contact surface, and environmental sampling results. Some analyses, 

notably for types of samples and for data trends over time (2006 through 2008), required the evaluation of 

combined multi-year data. Beginning in 2008, summarized RLMPROD and other RLm data are being 

reported by FSIS based on the date of sample collection in order to better align FSIS data reporting 

activities with that of other Federal partners. Thus, all data analyzed for this report, including any data 

that involved multiple years (2006-2008) are based exclusively on sample collection dates in order to 

make them directly comparable on an annualized basis. However, no changes needed to be made in the 

above-cited 2006-2007 report, because, in this instance, the 2006 and 2007 results are identical regardless 

of whether collection date or analysis completion date is utilized for reporting (i.e., in 2006 and 2007, all 

samples collected had their analyses completed within the same calendar year). 

4.1 RLm Testing Results for Calendar Year 2008 

Table 4.1.1 and Figures 4.1.1 and 4.1.2 show the results of testing 6,006 samples from 204 establishments 

in the RLm sampling program for calendar year 2008 (based on sample collection date). This 

encompasses the following: 

959 RTE food product samples, 

3,322 contact surface samples, and 

1,725 environmental surface samples. 

Overall, 1% of the samples (59 of 6,006 samples) were positive for L. monocytogenes. Five (0.5%) of the 

product samples were positive for L. monocytogenes, while positive results were obtained for 19 (0.6%) 

contact surface and 35 (2%) environmental samples. 

Table 4.1.1. Detection of L. monocytogenes in Product, Contact Surface, and 

Environmental Samples, Calendar Year 2008
 

Sample Type 

Total 

Collected 

Positive Samples 

No. % 

Product 959 5 0.52 

Contact surface 3,322 19 0.57 

Environmental 1,725 35 2.03 

Total 6,006 59 0.98 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.1.1. Samples Collected and Tested for L. monocytogenes, 

Calendar Year 2008
 

Figure 4.1.2. Percentage of Lm-Positive Samples, Calendar Year 2008
 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Of the 204 establishments tested in 2008, 1 of every 6 establishments (33, or 16.3%) had at least one Lmpositive sample (product and/or contact and/or environmental). The results of sorting the establishment 

data by type of testing program are shown in Table 4.1.2 and Figure 4.1.3. Most of the establishments 

with Lm-positive samples had positive results for environmental sampling (29, or about 14%), while 9 

(4.4%) had positive results for contact surface, and 4 (2%) had positive results for product sampling. 

Table 4.1.2. Number and Percentage of Establishments with at Least One Lm-Positive 

Sample, Calendar Year 2008 

Sample Type 

Establishments 

No. % 

Product 4 2.0 

Contact surface 9 4.4 

Environmental 29 14.1 

Combined (204 establishments sampled) 33* 16.1 

*Total includes establishments with more than one positive sample type. See Section 4.2. 

Figure 4.1.3. Percentage of Establishments with Lm-Positive Samples, Calendar Year 2008 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

4.2 Classification of Lm-Positive Samples, within Establishments, Calendar Year 2008 

Lm-positive samples from the establishments were further characterized based on which establishments 

had samples that were positive in all three sample sites or in six other possible combinations of sample 

sites. The results of this categorization are shown in Table 4.2.1. A total of 33 establishments (or about 

16% of the 204 establishments for which samples were collected and tested) had Lm-positive samples: 

Most of the positive establishments (24 of 33, or about 72% of all establishments with at least 1 

positive sample) were positive only for environmental samples. 

Another three establishments (9.1%) were positive only for contact surface samples. 

Two establishments (6.1%) were positive for both contact surface and environmental samples. 

Of the four establishments with positive product samples, one also was positive for contact 

surface samples, while the other three yielded positive results in all three sample sites. 

These results suggest that in establishments with Lm-positive product samples, one may also encounter 

positive contact and/or environmental samples. Still, over 80% of these establishments had positive 

samples in only one category (largely environmental) within the RLm sampling program. 

Table 4.2.1. Classification of Lm-Positive Samples from Establishments with at Least One 

Positive Sample, Calendar Year 2008 

Time 

Period 

Environmental 

Only 

Contact 

Surface 

Only 

Environmental 

and Contact 

Surface 

Product 

and 

Contact 

Surface All Types 

Total 

Establishments 

CY 2008 24 (72.1%) 3 (9.1%) 2 (6.1%) 1 (3.0%) 3 (9.1%) 33 

Note: Values in parentheses are percent positive relative to all establishments with at least one positive sample. 

4.3 Types and Sources of Lm-Positive Samples, Calendar Year 2008 

A total of 59 Lm-positive samples were obtained under the RLm sampling program in 2008. Of these, 5 

were from product, 19 were from contact surfaces, and 35 were from environmental surfaces. Tables 4.3.1 

through 4.3.5 show a breakdown of the types and sources of Lm-positive samples by category. These 

results are as follows: 

There were four different product types represented among the five meat and poultry products 

that were positive in calendar year 2008 (Table 4.3.1). 

Of the 19 positive contact surface samples, there were 4 positive container (or bin, tub, or bag) 

samples, 3 positive blade samples, 3 positive table samples, and 2 positive tray samples. One 

positive was found in 7 other contact surface sample types (Table 4.3.2). 

–	 The rates of Lm-positive samples relative to the numbers of containers, blades, tables, and 

trays sampled ranged from 0.8 to 3% (Table 4.3.3). 

Of the 35 positive environmental surface samples (Table 4.3.4), the highest number of positives 

were found in drains (11, or about 31% of all environmental positives), followed by floors (4), 

wheels (4), and floor mats (3). 

–	 The rates of Lm-positive samples relative to the approximately 440 drain, wheel, and floor 

samples ranged from 3.6 to 4.6% (Table 4.3.5). 

–	 The rate of Lm-positive floor mats relative to the 15 floor mats sampled was 20%. 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Table 4.3.1. Types of Lm-Positive Product Samples, Calendar Year 2008 

Product Type Positives 

Beef, Large Mass, Whole Muscle (Brisket) 1 

Chicken, Small Mass, Whole Muscle (Chicken Teriyaki) 1 

Pork, Sliced, Diced/Shredded (Pork Product) 1 

Chicken, Sausage Type Product, Peeled (Chicken Franks) 2 

Total 5 

Table 4.3.2. Types of Lm-Positive Contact Surface Samples, Calendar Year 2008 

Contact Surface Type 

Positive Samples 

No. % 

Container/Bag/Tub/Bin 5 26.3 

Blade (2 slicer; 1 shredder) 3 15.8 

Table 3 15.8 

Tray 2 10.5 

Packaging Film 1 5.3 

Gloves 1 5.3 

Grinder 1 5.3 

Hands 1 5.3 

Loader 1 5.3 

Shredder Entry Chute 1 5.3 

Total 19 100.0 

11
 
 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Table 4.3.3. Lm-Positive Rate for the Main Types of Contact Surface Samples, 

Calendar Year 2008
 

Contact Surface Type 

Total 

Collecteda 

Positive Samples 

No. % 

Container/Bag/Tub/Bin 135 4 3.0 

Blade 175 3 1.7 

Table 390 3 0.8 

Tray 85 2 2.4 

a

Approximate numbers from sample description listings. 

Table 4.3.4. Types of Lm-Positive Environmental Samples, Calendar Year 2008 

Environmental Surface Type 

Positive Samples 

No. % of All Positives 

Drain 11 31.4 

Floor 4 11.4 

Wheel(s) 4 11.4 

Floor Mat 3 8.6 

Squeegee 2 5.7 

Dolly 1 2.9 

Foil Wrap 1 2.9 

Fork Lift 1 2.9 

Freezer Curb (under drain) 1 2.9 

Gloves 1 2.9 

Hose 1 2.9 

Other ("Sauger Green with Duct Tape") 1 2.9 

Pallet 1 2.9 

Slicer 1 2.9 

Trash Can Bottom 1 2.9 

Wall 1 2.9 

Total 35 100.0 

12
 
 

 

 

 

 

 

 

 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Table 4.3.5. Lm-Positive Rate for the Main Types of Environmental Samples, 

Calendar Year 2008
 

Environmental Surface Type 

Total 

Collecteda 

Positive Samples 

No. % 

Drain 240 11 4.6 

Floor 90 4 4.4 

Wheel(s) 110 4 3.6 

Floor Mat 15 3 20.0 

Squeegee 20 2 10.0 

a

Approximate numbers from sample description listings (actual number for mat). 

While absolute numbers of RLMENVR samples from floor mats are relatively low, the percent positive 

rate for floor mats was 20% in calendar year 2008. Retrospective data analysis of percent positive rates 

for floor mats in RLMENVR also exceeded 20% in 2006-2007. A similar situation exists for INTENV 

samples (environmental samples collected as part of Intensified Verification Testing). These results raise 

concerns that floor mats in RTE production establishments may be an important environmental source of 

L. monocytogenes contamination. 

Table 4.3.6. Lm-Positive Rate for RLMENVR and INTENV Floor Mat Samples, 

Calendar Years 2006-2007 and 2008
 

Sampling Program Total Collecteda 

Positive Samples 

No. % 

RLMENVR (2006-2007) 12 3 25 

RLMENVR (2008) 15 3 20 

INTENV (2006-2007) 11 4 36.4 

INTENV (2008) 8 2 25 a

Actual numbers from sample description listings. 

4.4 Isolate PFGE Pattern Typing Results 

PFGE analysis was performed on all 59 isolates derived from the positive RLm samples. The results from 

RLm sampling and PFGE analysis of isolates can be used to determine cross-contamination between 

products and/or contact surfaces and or the production environment. Furthermore, in conjunction with 

PFGE analysis of isolates from Intensified Verification Testing (IVT), RLm sampling and PFGE analysis 

can help to identify both possible cross-contamination and harborage (persistence of specific PFGE 

pattern types over time) within establishments. As shown in Table 4.4.1, a total of 31 different PFGE 

pattern types were observed among the 59 isolates.1 Ten of these patterns were isolated multiple times, 

with 1 pattern type isolated 13 times, 1 pattern type isolated 6 times, 1 pattern type isolated 5 times and 7 

pattern types isolated 2 times (Table 4.4.1). 

1 PFGE pattern types are designated by CDC and are part of the PulseNet database, which was accessed for purposes of this 

analysis. Actual pattern types are not shown. 

13
 
 

 

 

 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Table 4.4.1. Patterns and Occurrence of Lm PFGE Subtypes Isolated in the RLm 

Program, Calendar Year 2008
 

PFGE 

Pattern 

Type 

Occurrence of Pattern Type 

No. of 

Total 

Isolates 

% of 

Total 

Isolates 

Rank in FSIS 

List of Top 10 

PFGE 

Patternsa RLMPROD RLMCONT RLMENVR 

1 2 8 3 13 22.0 1 

2 1 4 1 6 10.2 —

3 2 2 1 5 8.5 —

4 2 2 3.4 10 

5 2 2 3.4 —

6 2 2 3.4 2 

7 2 2 3.4 —

8 1 1 2 3.4 —

9 2 2 3.4 5 

10 2 2 3.4 3 

11 1 1 1.7 —

12 1 1 1.7 —

13 1 1 1.7 —

14 1 1 1.7 —

15 1 1 1.7 4 

All other 

pattern 

types (single 

occurrences) 

16 16 27.1 —

Total 5 19 35 59 100 a The FSIS list of top 10 PFGE patterns encountered is maintained by the Microbiology Division of the Office of 

Public Health Science (OPHS). 

It should also be noted that FSIS (specifically, the Microbiology Division of OPHS) keeps a list of the 10 

top PFGE patterns encountered. Of the 31 PFGE patterns encountered, 6 patterns from 22 isolates 

(representing about 37% of all isolates) were on the current top 10 pattern list. The number one pattern 

from the FSIS list was also the number one pattern from the RLm sampling program (Table 4.4.1). The 

fact that the other patterns were not on the current FSIS pattern list does not exclude the possibility that 

any of these isolates could be involved in Listeria infections in humans. 

4.5 Cross-contamination and Harborage 

Several of the establishments had multiple isolations of the same PFGE pattern type from positive 

samples. In five of six instances, the same PFGE pattern was observed for multiple product and/or contact 

surface and/or environmental isolates obtained within the same establishment. This indicated possible 

cross-contamination between products, product contact surfaces, and environmental locations (Table 

4.5.1). This included the following: 

14
 
 

 

 

 

 

 

 

 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Two establishments with matching product, contact surface, and environmental isolates; 

two establishments with matching product and contact surface isolates; and 

one establishment with matching contact surface and environmental isolates. 

In the sixth establishment (#5 in Table 4.5.1), two different environmental isolates had the same PFGE 

pattern, which could also be considered to be evidence of cross-contamination. In all, these results 

demonstrate the utility of collecting multiple product, contact surface or environmental samples for the 

purpose of detecting isolates with a common PFGE pattern from different sources within a single 

establishment. 

Evidence of harborage over time could not be determined from the RLm results alone because 

establishments with positive results were not sampled multiple times under this program. FSIS 

systematically reviews PFGE data across all Lm sample sites to determine whether harborage or crosscontamination may have occurred within particular establishments and may use the information as a basis 

to take further actions. A comparison of PFGE pattern types from RLm and IVT isolates obtained at 

different times from the same establishments in 2006 and 2007 revealed harborage in 2 of 4 (50%) of 

those establishments. This information is being used for the purpose of developing regulations that would 

require IVT in establishments with positive RLMPROD and/or positive RLMCONT samples. (At this 

time, positive RLMENVR samples do not have regulatory impact). The combined RLm, IVT, and PFGE 

results should yield important information regarding both harborage and cross-contamination in 

establishments with contaminated products and/or contact surfaces. 

Table 4.5.1. Incidence of Multiple Isolations of the Same PFGE Subtype within the Same 

Establishment, Calendar Year 2008 

Establishment 

PFGE Pattern Type 

Within Establishment 

Occurrence of Pattern Type 

RLMPROD RLMCONT RLMENVR Total 

1 1 1 2 3 

2 1 1 6 1 8 

3 2 1 4 5 

4 3 2 2 1 5 

5 5 2 2 

6 8 1 1 2 

Note: Establishment is a numerical ranking, not an identifier. Pattern types are the same as in Table 4.4.1. 

4.6 Results Based on Alternatives Used to Control L. monocytogenes 

For the RLm sampling program, establishments use one or more of four possible contamination control 

procedures, or control alternatives, for eliminating or inhibiting the growth of L. monocytogenes in the 

particular RTE products produced by each establishment. The four alternative categories are the 

following: 

Alternative 1, the lowest-risk category, involves using both a post-lethality treatment (which 

could be a physical treatment or an antimicrobial agent) “that reduces or eliminates 

microorganisms on the product AND an antimicrobial agent or process that suppresses or limits 

the growth of L. monocytogenes” (FSIS Directive 10,240.4, Revision 1, 3/15/2006).

15
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Alternatives 2a (or Alternative 2 choice 1) and 2b (or Alternative 2 choice 2), the next higherrisk categories, provide the option of either a post-lethality treatment the kills or inhibit 

microorganisms (2a) or an antimicrobial agent or process that specifically inhibits L. 

monocytogenes (2b). 

Alternative 3, the highest-risk category, requires the “use of sanitation procedures only” (FSIS 

Directive 10,240.4). 

Accordingly, one would expect the potential of encountering L. monocytogenes in product, contact 

surface, and environmental samples to be the least in Alternative 1 establishments and the greatest in 

Alternative 3 establishments. 

The percentages of product, contact surface, and environmental samples positive for L. monocytogenes 

with respect to the four major RTE Lm control Alternatives (1, 2a, 2b, and 3) for calendar year 2008 are 

shown in Table 4.6.1 and Figures 4.6.1 and 4.6.2. All five Lm-positive product samples encountered in 

2007 were from Alternative 2 or Alternative 3 establishments. The same was true for all Lm-positive 

contact surface samples. About 11% of environmental samples that were positive for Lm were from 

individual establishments that were using both Alternatives 1 and 3, while about 5% of the sample 

positives were from individual establishments that employed both Alternatives 1 and 2. Although the risk 

associated with employing Alternative 1 as a contamination control measure is low, it is uncertain 

whether these establishments were employing Alternative 1 or Alternatives 2 or 3 at the time of (or in the 

location of) sample collection. The results may represent a demonstration of the effectiveness of postlethality treatments, as none of the Lm-positive product or contact surface samples were from 

establishments that employed Alternative 1 (or Alternative 2a alone) for pathogen control. 

4.7 Results by Establishment HACCP Size 

The percentages of product, contact surface, and environmental samples positive for L. monocytogenes, 

with respect to each establishment’s HACCP size of large, small, or very small for calendar year 2008, 

are shown in Table 4.7.1 and Figures 4.7.1 and 4.7.2. In 2008, all five positive product samples were 

from small establishments, while all contact samples were from either small or very small establishments. 

Positive Lm environmental samples occurred in establishments across all three HACCP sizes, with the 

percentages of 2.5%, 2.1%, and 0.9% for large, small, and very small establishments, respectively. 

4.8 Results by Production Volume 

Results were analyzed as a function of the annual production volumes of RTE food products. This 

information, when available, is provided by the producing establishments on FSIS Form 10,240-1. The 

observations made are, thus, dependent on the accuracy of the production volumes supplied. The 

percentages of product, contact surface, and environmental samples positive for L. monocytogenes with 

respect to the production volumes for calendar year 2008 are shown in Table 4.8.1 and Figure 4.8.1. The 

results indicated that Lm-positive product and contact surface samples were most commonly found in 

establishments with production volumes in the range of 10,000-10,000,000 pounds per year. Positive 

environmental samples were present in establishments at all but the lowest production volumes (1,000 

pounds per year or less). The data on a percentage basis suggest that Lm-positive product and contact 

surface samples may occur in establishments that produce in a specific range of volumes. Positive product 

and contact surface samples were absent at both the lower and upper ends of the production volume 

ranges. The decreased or absent levels of positive environmental samples in those establishments at the 

lower end of the production volume range (<=10,000 pounds per year) is also of interest. Further 

Large establishments have 500 or more employees, Small establishments have 10 or more employees, but fewer than 500, and 

Very Small establishments have fewer than 10 employees or less than $2.5 million in annual sales. 

16
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

comparisons of results as a function of production volumes are warranted, provided that such volumes can 

be estimated as accurately as possible. 

4.9 Results by Food Product Category 

Positive samples were analyzed as a function of the nine food product categories found on FSIS Form 

10,240-1.2 The percentages of product, contact surface, and environmental samples positive for L. 

monocytogenes with respect to the nine above-named food product categories for calendar year 2008 are 

shown in Figure 4.9.1. The results indicated that Lm-positive samples were often found in establishments 

that produced multiple products, mainly deli meat and/or cooked products and/or hot dog products. 

However, the highest percentages of Lm-positive samples were found a) in establishments that produced 

deli/hotdog/cooked/dried products and b) in establishments that produced deli/cooked/salad products (see 

Figure 4.9.1). 

4.10 Results by Geographic Region 

To explore possible geographic influences on the detection of L. monocytogenes, individual states were 

classified into the following geographic regions3

: 

Northeast: Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New 

Jersey, New York, Pennsylvania, Rhode Island, Vermont, and Washington, DC 

North Central: Illinois, Indiana, Iowa, Michigan, Minnesota, Ohio, and Wisconsin 

Southeast: Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, 

Tennessee, Virginia, West Virginia, Puerto Rico, and the U.S. Virgin Islands 

Southwest: Arkansas, Kansas, Louisiana, Missouri, Nebraska, New Mexico, Oklahoma, and 

Texas 

West: Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, North Dakota, 

Oregon, South Dakota, Utah, Washington, Wyoming, American Samoa, Guam, and the Northern 

Marianas Islands 

The percentages of product, contact surface, and environmental samples positive for L. monocytogenes 

within these five broad geographic regions for calendar year 2008 are shown in Table 4.10.1 and Figure 

4.10.1. In 2008, four of the five Lm-positive product samples were from establishments located in the 

Western region of the country (the fifth was from the North Central region). Lm-positive contact surface 

samples were fairly evenly distributed across the five regions (range, 0.8-1.4%). The Lm percent positive 

rates for environmental samples ranged from 1.6-4.4% across four of the five geographic regions; there 

were no positive environmental samples from the Southwest region in 2008. 

2	 The nine 10,240-1 categories are deli products sliced at the producing establishment, deli products to be sliced after 

distribution, hot dog products, fully cooked products, fermented products, dried products, salt-cured products, frozen products, 

and salad/spread/pâté products. 3	 This classification is taken from the following FSIS manuscript: Naugle et al., Journal of Food Protection 69:2607-2614, 2006. 

17
 
 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

4.11 Results by Season and Month 

To explore possible seasonal influences on the detection of L. monocytogenes, positive product, contact 

surface, and environmental results were categorized based either on season (quarter) or month of the year. 

The percentages of product, contact surface, and environmental samples positive for L. monocytogenes by 

season and by month for calendar year 2008 are shown in Figures 4.11.1 and 4.11.2, respectively. All of 

the Lm-positive product samples and most of the positive contact surface samples were obtained in the 

spring and summer (notably, between May and September). In contrast, Lm-positive environmental 

samples were isolated across all seasons or months of the year. 

18
 
Table 4.6.1. Detection of L. monocytogenes by Control Alternative, Calendar Year 2008 

Alternative 

No. of 

Establishments 

Product Samples Contact Surface 

Samples Environmental Samples Total Samples 

Amount 

Positive 

Samples 

Amount 

Positive 

Samples 

Amount 

Positive 

Samples 

Amount 

Positive 

Samples 

No. % No. % No. % No. % 

1 5 21 0 0.0 70 0 0.0 35 0 0.0 126 0 0.0 

3 87 328 1 0.3 1167 7 0.6 625 12 1.9 2120 20 0.9 

2A and 2B 18 90 0 0.0 339 0 0.0 169 4 2.4 598 4 0.7 

2B 41 238 2 0.8 801 2 0.2 406 10 2.5 1445 14 1.0 

Mixed 1/2/3 5 27 0 0.0 90 0 0.0 45 0 0.0 162 0 0.0 

Mixed 1/2 7 38 0 0.0 128 0 0.0 65 3 4.6 231 3 1.3 

Mixed 1/3 1 6 0 0.0 16 0 0.0 9 1 11.1 31 1 3.2 

Mixed 2/3 40 211 2 0.9 711 10 1.4 371 5 1.3 1293 17 1.3 

Total 204 959 5 0.5 3322 19 0.6 1725 35 2.0 6006 59 1.0 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

19
Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.6.1. Percentage of Lm-Positive Product and Contact Surface Samples by Control 

Alternative, Calendar Year 2008 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1 3 2A and 

2B

2B Mixed 

1/2/3

Mixed 

1/2

Mixed 

1/3

Mixed 

2/3

% Positive

Lm Alternative

Product

Contact Surface

 

0.0

2.0

4.0

6.0

8.0

10.0

12.0

1 3 2A 

and 

2B

2B Mixed 

1/2/3

Mixed 

1/2

Mixed 

1/3

Mixed 

2/3

% Positive

Lm Alternative

Figure 4.6.2. Percentage of Lm-Positive Environmental Samples by Control Alternative, 

Calendar Year 2008
 

20
 
 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Table 4.7.1. Detection of L. monocytogenes by Establishment Size, Calendar Year 2008 

Establishment 

HACCP Size 

Product 

Samples 

Positive 

Samples Contact 

Surface 

Samples 

Positive 

Samples Environmental 

Samples 

Positive 

Samples Total 

Samples 

Positive 

Samples 

No. % No. % No. % No. % 

Large 214 0 0.0 737 0 0.0 367 9 2.5 1,318 9 0.7 

Small 621 5 0.8 2,157 17 0.8 1125 24 2.1 3,903 46 1.2 

Very Small 124 0 0.0 428 2 0.5 233 2 0.9 785 4 0.5 

Total 959 5 0.5 3,322 19 0.6 1725 35 2.0 6,006 59 1.0 

Table 4.8.1. Detection of L. monocytogenes as a Function of Establishment Annual Production Volumes, Calendar Year 2008 

Establishment 

Annual Production 

Volume Range, 

Pounds* 

Product 

Samples 

Positive 

Samples Contact 

Surface 

Samples 

Positive 

Samples 

Environmental 

Samples 

Positive 

Samples 

Total 

Samples 

Positive 

Samples 

No. % No. % No. % No. % 

~1000 6 0 0.0 20 0 0.0 10 0 0.0 36 0 0.0 

~10000 56 0 0.0 186 1 0.5 92 1 1.1 334 2 0.6 

~100000 156 1 0.6 556 5 0.9 290 5 1.7 1,002 11 1.1 

~1000000 265 2 0.8 923 11 1.2 498 14 2.8 1,686 27 1.6 

~10000000 401 2 0.5 1,378 2 0.1 706 11 1.6 2,485 15 0.6 

~100000000 75 0 0.0 259 0 0.0 129 4 3.1 463 4 0.9 

*Approximate, based on information collected on FSIS 10,240-1 forms. 

21
 
 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.7.1. Percentage of Lm-Positive Product and Contact Surface Samples by 

Establishment HACCP Size, Calendar Year 2008
 

Figure 4.7.2. Percentage of Lm-Positive Environmental Samples by Establishment
 

HACCP Size, Calendar Year 2008
 

22
 
 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.8.1. Percentage of Lm-Positive Samples as a Function of Establishment Annual 

Production Volumes, Calendar Year 2008
 

Figure 4.9.1. Percentage of Lm-Positive Samples by Establishment Food Production
 

Category, Calendar Year 2008
 

23
 
Table 4.10.1. Detection of L. monocytogenes by Geographic Region, Calendar Year 2007 

Region 

Product 

Samples 

Positive 

Samples Contact 

Samples 

Positive 

Samples Environmental 

Samples 

Positive 

Samples Total 

Samples 

Positive 

Samples 

No. % No. % No. % No. % 

North Central 229 1 0.4 778 6 0.8 392 10 2.6 1399 17 1.2 

Northeast 153 0 0.0 534 5 0.9 295 13 4.4 982 18 1.8 

Southeast 156 0 0.0 598 5 0.8 311 5 1.6 1065 10 0.9 

Southwest 247 0 0.0 829 11 1.3 411 0 0.0 1487 11 0.7 

West 174 4 2.3 583 8 1.4 316 7 2.2 1073 19 1.8 

Total 959 5 0.5 3322 35 1.1 1725 35 2.0 6006 75 1.2 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

24
Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.10.1. Percentage of Lm-Positive Product, Contact Surface and Environmental 

Samples by Geographic Region, Calendar Year 2008 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

North Central Northeast Southeast Southwest West

% Positive Samples

Product

Contact Surface

Environmental

 

 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Winter Spring Summer Fall

% Positive Samples

Product

Contact Surface

Environmental

Figure 4.11.1. Percentage of Lm-Positive Product, Contact Surface and Environmental 

Samples by Season, Calendar Year 2008
 

25
 
 

 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Jan Feb Mar Apr MayJun Jul AugSep Oct NovDec

% Positive Samples

Product

Contact Surface

Environmental

 

 

 

 

 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.11.2. Percentage of Lm-Positive Samples by Month, Calendar Year 2008 

4.12 Trend Analysis of Combined RLm Data: April 2006 through December 2008 

Because the 2006 data begin in April, numbers of samples and Lm-positive results for 2006, 2007, and 

2008 cannot be compared directly on a year-to-year basis. However, all 3 years’ worth of data can be 

compared based on percentages of Lm-positive samples in each category analyzed. Accordingly, Figures 

4.12.1 through 4.12.3 show comparative results for the rates of Lm-positive total, product, contact surface, 

and environmental samples for each year. Based on these results, the Lm-positive rates did the following: 

increased for product samples (0.2% and 0.5% in 2007 and 2008, respectively, versus 0.0% in 

2006),
 

increased for contact surface samples (0.4% and 0.6% in 2007 and 2008, respectively, versus 

0.2% in 2006), and 

were virtually unchanged for environmental samples (1.8-2% over the 3-year sampling period). 

Logistic regression analysis was used to evaluate the upward trends in the percentages of positive results 

observed for the product and contact surface samples (SAS PROC LOGISTIC procedures). This revealed 

statistically significant increases for product and contact surface samples with time (P = 0.08 and P = 

0.05, respectively). If the product and contact surface data are pooled, the increase over time yields a P 

value of 0.01. The model assumes that samples were collected with equal probability. In actuality, the 

sampling was such that this approximation might not be valid. Nevertheless, the increased incidence for 

the food contact surface areas and product over the last 3 years points to the existence of potential 

problems that are not being addressed. No statistically significant changes were observed for the 

environmental samples over time (P = 0.6). 

26
 
 

 

 

 

 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Trends in the percentages of establishments with at least one positive sample in any of the three sample 

sites for 2006 through 2008 are shown in Figure 4.12.4. In 2006 and 2007, about one of every five 

establishments had at least one positive sample; while in 2008, the same was true for about one of every 

six establishments. Figures 4.12.4 and 4.12.6 show comparative results for the rates of establishments 

with at least one positive product, contact surface, and environmental sample. Based on these results, the 

percentage of establishments with at least one positive sample 

increased for product samples (1.6% and 2% in 2007 and 2008, respectively, versus 0% in 2006), 

was similar for contact surface samples (about 4-5% over the 3-year sampling period), and 

was similar for environmental samples (about 14-17% over the 3-year sampling period). 

There appeared to be a downward trend in the percentage of establishments with Lm-positive samples. 

However, logistic regression analysis showed that none of the establishment trends were statistically 

significant (P>0.15 for environmental, P>0.3 for product, and P>0.5 for contact samples). 

If one examines the data for both samples and establishments, there is an interesting dichotomy: While in 

general the percentages of positive samples increased from 2006 through 2008, the percentages of 

establishments with positive samples trended down slightly during that same period. This means that in 

2008, there were more positive samples within a smaller percentage of establishments than there were in 

either 2006 or 2007. However, it must be reiterated that such changes over time may not be comparable 

because of differences in establishment prioritization for the RLm sampling program. 

27
 
 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.12.1. Percentage of Total Samples Positive for L. monocytogenes, 

April-December 2006 and Calendar Years 2007 and 2008
 

Figure 4.12.2. Percentage of Product and Contact Surface Samples Positive for 

L. monocytogenes, April-December 2006 and Calendar Years 2007 and 2008
 

28
 
 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.12.3. Percentage of Environmental Samples Positive for L. monocytogenes, 

April-December 2006 and Calendar Years 2007 and 2008
 

Figure 4.12.4. Percentage of Establishments with at Least One Lm-Positive Sample,
 

April-December 2006 and Calendar Years 2007 and 2008
 

29
 
 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Figure 4.12.5. Percentage of Establishments with Lm-Positive Product and Contact Surface 

Samples, April-December 2006 and Calendar Years 2007 and 2008
 

Figure 4.12.6. Percentage of Establishments with Lm-Positive Environmental Samples,
 

April-December 2006 and Calendar Years 2007 and 2008
 

30
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

5. SUMMARY AND CONCLUSIONS 

FSIS analyzed data with respect to the detection of L. monocytogenes in product, contact surface, and 

environmental samples collected under the RLm sampling program for calendar year 2008. Overall, 6,006 

samples from 204 establishments were tested in 2008. Percentages of Lm-positive product, contact 

surface, and environmental samples were 0.5%, 0.6%, and 2%, respectively. 

Results based on percentages of establishments with Lm-positive results. Four establishments, or 

about 2% of the 204 establishments in which samples were collected, had positive product samples, while 

about 1 in 25 (4.4%) had positive contact surface samples. About one in seven establishments (14.1%) 

had environmental samples that were positive for L. monocytogenes. A key aspect of the RLm program is 

in the identification of establishments that have L. monocytogenes somewhere in the environment and, 

thus, have a potential for product contamination. The data indicate that collecting multiple contact surface 

and environmental samples during a particular production shift increases the likelihood of finding L 

monocytogenes in an establishment. Thus, the RLm program serves as a proactive sampling project in 

identifying establishments in which the risk of product contamination may be elevated before 

contamination is actually identified in the actual products. The use of 1 to 3 sampling units per 

establishment (to collect 3 product, 10 contact, and 5 environmental samples per unit, respectively) 

contributes to the detection of positive contact surface and environmental samples. 

Results based on type of Lm-positive sample. The five positive product samples were composed of 

three poultry products (two chicken franks, one chicken teriyaki), one beef product (brisket), and one type 

of shredded or sliced pork product. Contact surface samples that were positive for L. monocytogenes 

included containers, blades, tables, and trays, which accounted for 12 of the 19 positive samples of this 

type. Of the 35 Lm-positive environmental samples, drains, wheels, floors, floor mats and squeegees were 

isolated on more than one occasion. Of particular interest is that although only 3 floor mat samples were 

positive, this was out of a total of 15 floor mat samples, or 20%. As similar results were obtained in 2006 

and 2007, floor mats may prove to be an important source of Lm contamination. In contrast, while there 

were 11 positive drain samples, this was from 240 collected drain samples, or less than 5% of all drain 

samples. It should be noted that the detection of L. monocytogenes in the environment is not, in and of 

itself, an indicator that a control problem exists. However, under FSIS Directive 10,240.5, Lm-positive 

environmental samples may be considered evidence that an establishment’s products are produced under 

unsanitary conditions. 

Results based on PFGE pattern. The collection and testing of multiple product, contact surface, and 

environmental samples from a given establishment has the potential to demonstrate how strains of L. 

monocytogenes could move from the environment to product contact surfaces and eventually contaminate 

a given product. Determining the PFGE pattern for each positive isolate aids in analyzing the likelihood 

of such occurrences. Finding the same PFGE pattern types (or subtypes) in isolates from different 

locations would provide evidence of cross-contamination. An analysis of the PFGE subtyping data for 

2008 revealed multiple instances of cross-contamination between isolates from Lm-positive food product, 

contact surface, and/or environmental samples. Thus, transfer of L. monocytogenes between the 

environment and contact surfaces and/or products was a more common finding in 2008 than in 2006 or 

2007. Four instances involved cross-contamination between contact surfaces and products, one instance 

was cross-contamination between environmental sources and contact surfaces, and there was one instance 

of multiple environmental samples having the same PFGE pattern. The RLm PFGE data for recovered 

isolates, if done in conjunction with an analysis of IVT “for cause” PFGE data, could permit the 

evaluation of harborage (presence of specific PFGE pattern types over time within a given establishment) 

in addition to cross-contamination. IVT results from establishments with positive RLm samples have, in 

fact, yielded instances of matching PFGE patterns for isolates from multiple sources (product, contact 

surface, and/or environmental) within a given establishment. This information is a factor in the 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

development of regulations that would mandate IVT in response to Lm-positive product and/or contact 

surface samples. 

Results based on Lm control alternative. Results based on Lm control alternatives employed by the 

establishments showed that most of the positive samples were obtained from Lm control Alternative 2b 

and 3. These data appear to reinforce the concept that Alternative 3 (sanitation only/highest risk) and 

Alternative 2b (antimicrobial treatment/higher-risk) establishments are more at risk than Alternative 1 

(and possibly even category 2a) establishments with respect to detecting Lm-positive samples. This 

observation was more applicable to positive product and contact samples than to positive environmental 

samples, which were found in establishments that were classified as using mixed Alternatives 1 and 2 

and, in particular, mixed Alternatives 1 and 3. The analysis of data related to Lm control alternatives has 

benefited from improvements in information entered on FSIS 10,240-1 forms. 

Results based on size, product category, production volumes, District, geographic region, month, 

and season. Results based on establishment HACCP sizes indicated that in 2008, most positive product 

and contact surface samples came from small or very small establishments. However, environmental 

samples were in establishments in all HACCP categories. Results based on food product categories 

showed that most of the Lm-positive samples were from establishments that produced deli meats, hot 

dogs, and cooked products. Results based on establishment production volumes (which, as with control 

alternative data, were from FSIS 10,240-1 forms) indicated that most positive product and contact surface 

samples were from establishments producing between 10,000 and 10,000,000 pounds of product per year. 

Presentation of results by FSIS District and geographic region appears to be mainly descriptive in nature, 

but may be more meaningful on a comparative basis. Analysis of results by month and season indicated 

that Lm-positive environmental samples were obtained at all times of the year (as was the case in 2006 

and 2007). In contrast, positive product and contact surface samples were obtained at specific times of the 

year, mainly between the months of May and September. 

Results based on year-to-year trends. Trends in the percentages of Lm-positive results from 2006 to 

2008 permitted an expanded view of trends. In particular, the percentages of Lm-positive product and 

contact surface samples trended higher between 2006 and 2008, and these increases were statistically 

significant. Trends for environmental samples were essentially flat. The percentages of establishments 

with at least one Lm-positive sample trended down slightly between 2006 and 2008, though there were 

upward trends for establishments with positive product and contact surface samples. However, the net 

effect for the RLm program between 2006 and 2008 was that while the percentage of Lm-positive 

establishments appeared to be decreasing over time, the percentages of positive samples within those 

establishments were increasing. Still, these results should be interpreted with caution because even 3 

years’ worth of RLm data may not be sufficient for an objective evaluation of the effectiveness of the 

RLm sampling program. Changes observed between 2006 and 2008 may reflect true trends or may reflect 

normal variations in sampling results. Furthermore, the data represent results from only about 400 of over 

2,000 establishments that are subject to regulation 9 CFR 430. Because of the nature of scheduling of 

establishments within the RLm sampling program (i.e., which establishments are sampled when), these 

year-to-year results are not truly representative of all establishments sampled in the RLm program. It is 

expected that additional data over multiple years will provide for an expanded evaluation of the data 

trends. In fact, effective August 2009, FSIS is committed to performing RLms (in conjunction with 

routine FSAs) in each of the 2000-plus 9 CFR 430 establishments over a 4-year period. This represents a 

significant expansion of the RLm sampling program. 

Next steps. The following is a list of actions that are being implemented subsequent to the completion of 

this report and its presentation to, and acceptance by, the appropriate FSIS offices and Management 

Council: 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

An ongoing examination of the data from this report with respect to applying them to preventing
 

foodborne outbreaks of L. monocytogenes. (This includes modifications of existing compliance 

guidelines and other regulatory practices that help protect public health.)
 

Initiation of RLm sampling in all (approximately 2100) 9 CFR 430 establishments over a fouryear period, which commenced in August 2009.
 

Reporting of RLm test results on a quarterly, in addition to an annual, basis.
 

IVT for those establishments in which positive RLMPROD and/or RLMCONT samples are 

obtained.
 

Publication of these data analysis in a peer-reviewed scientific journal.
 

Re-initiation of hands-on training for RLm/IVT sample collection.
 

Analysis of 2009 RLm results. 

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Results of Data Analysis for the Listeria monocytogenes Risk-based Sampling Program, Calendar Year 2008 

Appendix A:	 FSIS Scheduling Criteria for Routine Lm Risk-based (RLm) 

Sampling Program 

Before the month when samples are to be collected, FSIS uses a statistical algorithm to generate a risk 

ranking of establishments producing post-lethality exposed RTE meat and poultry products. The 

following criteria are then used to identify establishments from the risk ranking to be tested for Listeria 

monocytogenes in food product, contact surface, and environmental samples under the Routine Lm Riskbased (RLm) Sampling Program: 

1.	 Once RLm sampling has been conducted in an establishment, that establishment will not be 

eligible for scheduling again for a 24-month period. 

2.	 If there is a current-month positive result from any FSIS Lm sampling project, the Agency 

conducts FSAs and IVT at the establishment: 

a.	 If positive results are found during the IVT, the RLm will not be scheduled until 6 months 

after the IVT and FSA and any accompanying regulatory actions are complete. 

b.	 If the IVT test results were negative, RLm sampling would revert back to the 24-month 

sampling cycle. 

3.	 RLm sampling at an establishment will also not be scheduled for 6 months after closeout of an 

Lm-related NOIE, suspension, or other enforcement action. 

4.	 Previously, FSIS did not schedule RLm testing in more than one establishment operated by the 

same corporation in the same month. This restriction will not apply in FY 2008. 

5.	 Collecting RLm samples will no longer take precedence over the other RTE sampling programs 

(i.e., ALLRTE and RTE001). If FSIS Lm sampling projects are scheduled at the same 

establishment over the same time period, all samples will be collected as scheduled. 

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