Food Protection Trends - November/December 2021 - 563

Listeria, and (ii) to implement interventions and perform
sampling to determine the effectiveness of the interventions.
The following interventions were identified through RCA,
implemented, and tested: (i) increasing cleaning and
sanitation from once a week to twice a week, (ii) use of
quat powder around forklift stops and floor cracks, (iii)
a site-specific niche (a dead-end pipe) removal, and (iv)
implementation of a deep cleaning protocol in the drains. The
site-specific interventions (e.g., use of quat powder) appeared
to be more successful at eliminating Listeria from the apple
packinghouse compared to increasing the frequency of
cleaning and sanitation. This apple packinghouse case study
provides an example of how RCA could be performed to
eliminate or reduce persistent or persistent transient Listeria
populations from produce operations.
A persistent and persistent transient Listeria population
was present in the apple packinghouse investigated in the
current study
Overall, our findings show the facility used in the study
reported here had a number of sites with evidence for persistent
transient Listeria, as well as at least 1 site with evidence
of persistence. While most of the Listeria in the packinghouse
in the current study represents persistent transient Listeria, the
same sigB ATs were isolated ≥3 times from the square drain
and the dead-end pipe, indicating potentially persistent Listeria.
The dead-end pipe represented 3 sampling sites (the pipe
inlet, the pipe outlet, and the dead-end portion of the pipe). In
the dead-end pipe, sigB AT 9 was isolated on 3 sampling events
from the pipe inlet and on 1 sampling event from the dead
end and the pipe outlet. While a typing method with better
discriminatory power (e.g., pulsed field gel electrophoresis or
whole genome sequencing) would be needed to confirm these
isolates as truly the same, the repeat isolation of this AT within
one single pipe (and the lack of isolation of any other sigB ATs
from this site) is likely to indicate persistence. This pipe is a
likely site for persistence, as the dead-end portion can accumulate
apple debris and other organic matter that can support
the growth of Listeria. Consequently, cleaning and sanitation
can be a challenge because it is difficult to reach the dead-end
portion. In addition, the inlet to the pipe is taped into a catch
pan; the adhesive from the tape and the portion of the PVC
pipe that overlaps with the metal catch pan (on the inside of
the PVC pipe) can also act as harborage sites. While only one
study was identified which listed plastic tubing as a harborage
point for persistent Listeria (20), several studies have suggested
equipment that is difficult to clean is a risk factor for persistent
Listeria (6, 14, 21). Our findings provides further support of
the importance of sanitary design in controlling persistent
Listeria populations in processing environments and suggest
that complete root cause analyses should include a consideration
as to why equipment with poor sanitary design is present
in a given facility.
In addition, the site that included a square drain and
a connected inflow trench drain, showed evidence for
persistent and persistent transient Listeria populations. In the
square drain and the connected trench drain, sigB AT 57 was
isolated during 4 sampling events. While this may indicate
persistence of this subtype, sigB AT 57 is a common sigB AT
(15, 28, 30, 33, 40) and has been shown to be highly diverse
(3). As such, it is possible this does not truly represent a
persistent Listeria. In addition, sigB AT 20 was isolated from
at least 1 of the 3 drain sites during 3 sampling events, which
may indicate persistence; while not as common as sigB AT
57, sigB AT 20 was also isolated from a forklift stop on 1
sampling in the current study and was isolated in 2 previous
studies (3, 28). Drains have been listed as harborage points
for persistent Listeria in several previous studies (7, 19, 20,
29). However, additional subtyping (e.g., whole genome
sequencing) is still needed in the current study to confirm
persistence. In addition to the persistent population, the
isolation of a highly diverse population of Listeria from the
3 drain sites (i.e., 12 sigB ATs were isolated from the 3 sites
across the 5 samplings) indicates there is also a persistent
transient Listeria population present in the drain. The drain is
located under the dump tank and flume, which deposits large
amounts of organic matter and debris into the drain. Organic
matter and soils originating from outdoor environments
(e.g., similar to those environments apples are grown in) are a
known source of Listeria (28, 30, 31, 39, 40) and are therefore
a likely contributor of the diverse Listeria population present
in this drain. Furthermore, the drain is deep and has poor
drainage, which creates a large number of harborage points
that are difficult to clean and that have sufficient moisture
and nutrients to support Listeria growth; this can lead to both
the persistent and persistent transient Listeria populations.
A persistent transient Listeria population was isolated from
one of the forklift stops sampled in the current study, as 6
different sigB ATs were isolated from the forklift stop and no
AT was isolated more than once. The presence of a persistent
transient Listeria population at the forklift stops can be
expected, as the forklifts often go outside to pick up bins of
apples to be run on the packing line. In this packinghouse
there are no control measures (e.g., door foamers) for
the forklift wheels that would prevent Listeria transfer
into the packinghouse, and the outdoor environments in
the northeast have been shown to harbor diverse Listeria
populations (3, 28). As such, this could facilitate the transfer
of the observed persistent transient Listeria population in the
current study. Sullivan and Wiedmann (33) also identified
forklift stops as harborage sites for Listeria; 1 out of 2 of the
forklift stops positive for Listeria in this prior study was the
same forklift stop as discussed in the current study.
A persistent transient Listeria population was also isolated
from the floor crack samples in the current study, as Listeria
of 4 sigB ATs were isolated from the floor crack and no AT
was isolated more than once. This floor crack is directly
November/December Food Protection Trends 563
Food Protection Trends - November/December 2021

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