Food Protection Trends - November/December 2021 - 564

adjacent to a trench drain and was created when the drain
was installed in the packinghouse. This is consistent with the
findings of Murugesan et al. (25), who repeatedly isolated
Listeria from a floor crack next to a trench drain. Therefore,
while all floor cracks are likely harborage points, a floor
crack's proximity to the drain may increase the likelihood
of Listeria being present in the floor crack itself (i.e., due
to potential splash from the drain into the floor crack).
However, more extensive sampling is required to determine if
floor cracks adjacent to other high-risk areas (e.g., drains) are
at a higher likelihood of becoming contaminated.
As such, PVC pipes (especially those with dead ends),
drains, forklift stops, and floor cracks area a few examples of
sites that should be included in environmental monitoring
programs in produce operations. The contamination
patterns (i.e., the diverse Listeria population and repeat
Listeria isolation from the same sites) in the packinghouse
investigated in the current study represent " persistent
Listeria " and " persistent transient Listeria " populations.
Persistent Listeria poses public health and business risks
(e.g., recalls) because as the Listeria survives in the packing
environment over time it can grow; as the Listeria grows
it is more likely to be transferred to other areas in the
packing environment (e.g., by employees or mobile pieces
of equipment) and eventually contaminate product. Since
persistent Listeria represent a single strain of Listeria,
finished product contamination can be traced back to the
packing environment through environmental and product
testing and subsequent subtyping of isolates (e.g., via whole
genome sequencing). While it is more difficult to link
final product contamination to the environment when a
persistent transient Listeria population is present (compared
to persistent Listeria), it can represent an instance of
continuous introduction of Listeria from the same sources.
As such, a persistent transient population may indicate more
stringent supplier verification programs are required, and
in serious cases the identification of alternative suppliers
may be needed. Control measures may also be necessary
to prevent transfer of Listeria from employees, forklifts,
distribution trucks, or storage crates, among other routes,
into the packing environment (e.g., captive boot programs,
compartmentalization of forklifts).
It is important to note a highly diverse persistent transient
Listeria population may hide a persistent population present in
the packing environment (i.e., if there are a large number of Listeria
subtypes present at any given site, the chance of identifying
the persistent subtype is less likely as compared to if only the
persistent subtype was present). Regardless, identifying a persistent
Listeria that is covered up by a persistent transient Listeria
population is still possible through large sampling efforts, such as
" swab-a-thons " that subtype multiple isolates from each positive
site. However, as we characterized up to 16 isolates from each
positive sample, it is unlikely that true persistence was " covered
up " by persistent transient strains in this case.
564 Food Protection Trends November/December
Root cause analysis can be utilized to identify
interventions to eliminate or reduce Listeria populations
in the apple packing environment; however, multiple
iterations of testing and intervention implementation
may be required to reduce Listeria populations
RCA was utilized to identify likely root causes of frequent
repeat Listeria detection in the apple packinghouse in the
current study; a previous study as well as this study reported
these issues represent a combination of (i) persistent Listeria
(e.g., in a catch pan with an outflow pipe with a dead-end)
as well as (ii) persistent transient Listeria (e.g., at the forklift
stops). RCA provided a formal process for identification
of possible root causes associated with (i) overall high
frequency of Listeria detection and (ii) different areas where
repeat isolation of Listeria was an issue. The identified root
causes were then used to identity interventions deemed
likely to reduce overall frequent Listeria isolation as well as
frequent site specific isolation of Listeria. Overall, the facility
implemented one plant-wide intervention and four site
specific interventions, which are discussed in detail below.
The RCA and subsequent interventions were successful for the
forklift stops and floor crack, but further iterations of the RCA
are required to control the Listeria populations in the drain
and dead-end pipe, and to account for the increase in Listeria
positives in the second half of the week between cleaning and
sanitation events. In addition, a multipronged approach that
targets multiple interventions at the same time is likely needed
to effectively reduce or eliminate persistent and persistent
transient Listeria populations. In addition, further root cause
analyses that explore the reasons for why certain practices were
not implemented (e.g., selection of equipment with sanitary
design) will be needed for long-term successful Listeria control.
The plant-wide intervention tested in the current study
was to increase the cleaning and sanitation frequency from
once to twice a week; this was implemented to address the
increase in percentage of Listeria positive samples observed
when sampling was conducted at the end of the week. No
significant change in the log odds of isolating Listeria after
intervention implementation was observed in the 2020
sampling results. Increasing the frequency of cleaning and
sanitation can also increase the amount of moisture present
in the packinghouse. The increase in moisture can allow
for an increase in the growth of Listeria and may explain
the lack of a significant reduction in Listeria following
implementation of this intervention. In addition, while
the adequacy of the cleaning and sanitation protocol in
the packinghouse was assessed, no changes in the protocol
were made besides increasing the frequency. It is possible
improvements in how cleaning and sanitation are performed
(e.g., use of a foamer to apply cleaners and sanitizers) may
be needed to further reduce the percent of Listeria positive
sites in the wet area of the packinghouse. However, as only
one sampling was conducted prior to increased cleaning
and sanitation in the 2020 sampling events, the lack of a
Food Protection Trends - November/December 2021

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