Food Protection Trends - January/February 2023 - 35

TABLE 1. Number of samples negative or positive for Listeria spp. along with the percentage of
Listeria spp.-positive samples from the total number of samples collected per distribution
center; mean log(x + 1) transformed RLU is also given per distribution center
Distribution
center
Positive
Negative
Total
Percentage
positive
Average log
RLU
DC1 DC2 DC3 DC4 DC5 DC6 DC7 DC8 DC9 DC10 DC11 DC12 DC13 DC14 DC15 DC16 DC17
1
2
18
20
10.0
5.99
16
16
5.15
2
17
19
10.5
5.96
19
19
5.84
21
21
5.29
2
17
19
10.5
5.39
11
11
5.33
2
9
11
18.2
5.30
3
12
15
20.0
5.37
20
20
4.49
16
16
3.39
14
14
4.42
27
27
4.65
21
21
3.56
19
20
5.0
4.29
14
14
5.28
1
16
17
5.9
5.81
were returned to the original bag, sealed, stored on ice packs,
and shipped to the laboratory for microbiological analysis.
Upon receipt, insulated coolers (Uline, Pleasant Prairie,
WI) were checked with an infrared thermometer (Fluke,
Everett, WA) to confirm proper temperature was maintained
during shipping, and samples were processed within 48 h of
collection.
Each environmental sample was tested for Listeria species
by using a modified version of the FDA Bacteriological
Analytical Manual methodology for detection and isolation
of Listeria in environmental samples (15). Briefly, 90 ml
of buffered Listeria enrichment broth (Difco, BD, Sparks,
MD) was added to each bag with the sample, followed by
homogenization in a Stomacher 400 Circulator Lab Blender
(Seward, Worthing, UK) at 230 rpm for 60 s. A 360-μl
aliquot of Listeria selective enrichment supplement was
added to each sample bag after 4 h of incubation at 30°C.
After incubation at 30°C for 24 and 48 h, a 50-μl aliquot
of sample enrichment from each bag was streaked to BBL
CHROMagar Listeria (Difco, BD) and Oxford medium
base with modified Oxford antimicrobic supplement (Difco,
BD) agars and incubated at 30 and 35°C, respectively, for 48
h. Up to four presumptive Listeria-positive colonies (bluegreen
colonies surrounded by an opaque, white halo for
CHROMagar and brown-colored colonies with black zones
for Oxford medium base with modified Oxford antimicrobic
supplement) were selected for PCR confirmation by using
sigB as the target gene (31).
SuperSnap ATP swabs (Hygiena, Camarillo, CA) were
also collected adjacent to environmental sample locations.
For instance, if an area on a mop bucket was swabbed by
using a sponge, a directly adjacent area of the bucket was
used for ATP swabbing. Per the manufacturer's instructions,
swabs were used to sample an area (10 by 10 cm). Areas
were sampled by rotating the swab while sweeping across
the surface vertically, horizontally, and then vertically. The
swab was immediately placed in the Hygiena SystemSure
Plus ATP Measurement System (Hygiena) testing chamber
for measurement, and the displayed RLU level reading was
recorded. In total, 300 ATP swabs were collected, with
approximately 18 ATP swabs collected per facility.
Statistical analysis
RStudio (version 4.0.0) was used for statistical analyses
by using the package readxl (33). RLU data were log(x + 1)
transformed to normalize right-skewed data. The analysis
of variance function was used to determine the relationship
between the predictor variable (log-transformed ATP)
and outcome (Listeria spp. presence). Logistic regression
analysis was performed by using the general linearized model
function to determine if log(x + 1) transformed RLU levels
were a predictor of Listeria spp. presence. Figures were
created by using ggplot2 (32) and popbio (26).
RESULTS AND DISCUSSION
This study was conducted by using a subset of distribution
centers examined for prevalence of Listeria spp. (29, 31).
Listeria spp. were found in 12 of 17 distribution centers, with
a total prevalence of ca. 13 (4.3%) of 300 environmental
samples positive for Listeria. The prevalence of Listeria spp.
in samples collected per distribution center ranged from 0
to 20% (Table 1). Although six distribution centers did not
have any Listeria spp.-positive samples, it is still possible
that Listeria spp. were present in those facilities, as sampling
did not occur on all surfaces. Accordingly, there may be
differentiating factors influencing the likelihood or presence
of Listeria within the distribution center environment. These
factors may include facility characteristics, such as employee
population, facility size, and amount of product handled
annually. For instance, a larger employee population may lead
to increased Listeria presence from shoe soles contaminated
with naturally occurring Listeria. Several studies have
assessed Listeria contamination on shoes (17, 22) and
found that environmental soil is typically the main reservoir
leading to contaminated shoe soles. In addition, factors, such
as season, geography, and precipitation, were shown to be
January/February Food Protection Trends
35

Food Protection Trends - January/February 2023

Table of Contents for the Digital Edition of Food Protection Trends - January/February 2023