Food Protection Trends - September/October 2020 - 321

have post-lethality exposure to the environment prior to
packaging, (ii) are capable of supporting growth of Listeria,
and (iii) will not be cooked or treated in any way to eliminate
Listeria prior to consumption (18, 23, 36). Among RTE
foods, consumption of contaminated delicatessen meats is
one of the main sources of listeriosis and outbreaks (6). In
an older study, it was estimated that almost 90% of human
listeriosis cases in the United States were caused by the
consumption of contaminated delicatessen meats (45).
Endrikat et al. (12) estimated that deadly L. monocytogenes
is five times more likely to contaminate delicatessen meats
sliced at retail as compared with prepackaged delicatessen
sliced meats. In addition, an older study by Gombas et al.
(16) found L. monocytogenes was nearly seven times more
likely to be found in delicatessen meats prepared in retail
establishments. The use of growth inhibitors appears to
effect Listeria-associated deaths from both manufactured and
retail sliced delicatessen meats (35). A more recent study
by Luchansky et al. (28) found that 15 (0.25%) of 5,917
deli meat samples were positive for L. monocytogenes and
Listeria-like organisms; chain stores had a slightly higher
percentage although not statistically significant. A metaanalysis by Churchill et al. (8) reviewed studies assessing L.
monocytogenes in deli meats and estimated prevalence to be
2.9% for those studies with larger samples.
Listeria monocytogenes may enter retail delis through various transmission routes, such as personnel, equipment, raw
product, and cross-contamination. The major transmission
route results from post-processing contamination that occurs
during additional product handling steps, such as slicing (24,
25, 35). Therefore, given that L. monocytogenes can be present
on both food contact and non-food contact surfaces, researchers have conducted sampling of each. Studies conducted by Hoelzer et al. (19) and Sauders et al. (38) in the United
States reported 55 to 56% of retail delis had L. monocytogenes
on food contact and non-food contact surfaces. In the study
by Sauders et al. (38), 151 (13.0%) of 1,161 environmental
samples tested were confirmed positive for L. monocytogenes,
125 (16.7%) non-food contact surface and 26 (6.3%) food
contact surface samples. Additionally, one longitudinal
study involving 30 retail delis in three U.S. states found
314 (6.8%) samples in the first phase of the study to be
confirmed positive for L. monocytogenes and 4,503 (9.5%)
samples in the second phase to be positive (40). Samples
were taken from food contact surfaces (e.g., deli case, slicer), non-food contact surfaces (e.g., drain, trash can), and
transfer points (e.g., slider knob, scale) in this longitudinal
study. Another study conducted at three butcher shops in
Brazil, found 87 (60.4%) samples positive for Listeria spp.
and 31 (21.5%) positive for L. monocytogenes. Samples were
taken from various sources, including the hands of employees, tables, knives, grinders, and displays. Tables, grinders,
and tenderizers were found to be the main contamination
areas for L. monocytogenes presence (10). Regardless of the

surface type, an ineffective cleaning program may support
the growth and persistence of L. monocytogenes for months
or years in a deli environment (40).
To explore potential reasons for these positive samples, researchers have studied retail deli environments and employee practices. One such study examined the frequency and
adequacy of cleaning retail deli slicers; half of 298 retail delis
in the study did not follow the recommended cleaning procedures of the U.S. Food and Drug Administration (FDA) to
prevent L. monocytogenes and other pathogenic bacteria. Also,
slicers at chain retail delis and in delis with more customers
were more adequately cleaned (5) as compared with slicers
at delis that were independent or had low customer volume.
To mitigate contamination of RTE foods in retail delis, employees must follow safe food handling practices, including
maintaining facilities, cleaning and sanitizing equipment, and
practicing good personal hygiene. Various policies related to
L. monocytogenes in RTE foods have been implemented (2),
and the U.S. Department of Agriculture (USDA) Food Safety
and Inspection Service (FSIS) has introduced L. monocytogenes guidelines containing eight recommendations (46).
For this study, and in line with previous research findings, the research objectives were as follows: (i) determine
the prevalence of L. monocytogenes in selected Iowa retail
delis and (ii) assess operational environmental aspects that
mitigate contamination of RTE products with L. monocytogenes by using eight recommendations in the USDA FSIS L.
monocytogenes guidelines.
MATERIALS AND METHODS
Two data collection techniques were used for this study:
(i) five to nine microbial samples were collected at each site
and (ii) an environmental assessment instrument was used
to determine environmental factors and practices. There
were two pilot data collection periods and then a final data
collection period. Pilot data are not reported.
Pilot study
For the first pilot, a Listeria self-assessment tool and Hygiena
InSite environmental Listeria species test swabs (screening test
for Listeria species, Hygiena LLC, Camarillo, CA) were used in
nine Iowa retail deli operations. For the Listeria self-assessment
tool, inspectors reviewed and recommended changes to questions on the assessment that were confusing; response options
were changed from "yes" to "in compliance" and "no" to "out
of compliance." In addition, some of the results from the test
kits produced false-positive results (culture confirmations
performed at the State Hygienic Laboratory, Coralville, Iowa).
Therefore, to ensure the accuracy of results, a new test kit was
used for the second pilot test.
For the second pilot, a revised version of the Listeria selfassessment and new Hygiena InSite L. monocytogenes Glo
(Hygiena LLC) test swabs (screening test for both Listeria
species and L. monocytogenes; non-commercial for research

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