TESTING
By David E. Gombas, Ph.D.
Environmental Monitoring for
Listeria: Getting Started
o one could have predicted that a major
listeriosis outbreak in 2011 would be linked
to a primary handling operation of fresh,
whole cantaloupes. Only two other listeriosis
outbreaks have been linked to commercially
prepared fresh or fresh-cut produce in the U.S.: the
watershed 1981 outbreak linked to coleslaw that first
brought Listeria monocytogenes to national attention,
and the 2010 outbreak linked to fresh-cut celery, where
the source of contamination was identified as poor
facility sanitation practices and a lack of environmental
monitoring in the fresh-cut operation.
Unlike with Salmonella and pathogenic Escherichia
coli—the two other bacteria of major importance in
fresh produce food safety—the most likely source of L.
monocytogenes contamination of fresh produce is not fecal
contamination. Rather, L. monocytogenes is considered to
be primarily a soil bacterium, expected to be found in
field environments and recoverable from fresh produce.
Why there have not been more listeriosis outbreaks
linked to fresh produce is unknown (and a subject ripe
for investigation), but studies of all three outbreaks
revealed that the operations had conditions that led to
significant postharvest growth of L. monocytogenes.
In;2008,;the;U.S.;Food;And;Drug;Administration
(FDA);published;a;draft;guidance;for;industry,1
recommending that operations for manufacturing
ready-to-eat products vulnerable to L. monocytogenes
growth implement environmental monitoring programs
to detect when and where Listeria may have become
entrenched in the environment, potentially becoming
a source of product contamination. While the draft
N
Tips for Listeria monitoring
in fresh produce operations
guidance was not written specifically
for fresh-cut products, and some of
the recommendations may not be
practical for those products, the draft
guidance does have some important
information. Because of their typically
wet and cold environments, fresh-cut
operations are vulnerable to Listeria-
finding harborage points. Fortunately,
while L. monocytogenes is one of the few
foodborne pathogens that can grow in
cold environments (> 32 ºF), it grows
slowly and does not compete well against
other cold-tolerant microorganisms,
which can cause spoilage but are not
pathogenic. However, as the celery
incident demonstrated, it is possible.
Therefore, every fresh-cut operation
should implement an environmental
monitoring program aimed at detecting
Listeria entrenchments in processed
product handling areas.
An effective Listeria monitoring
program cannot be fully described in
this short article, but examples can
be found in Tompkin et al. (1999), 2
who recommended a zone approach
to environmental monitoring, that
is, actively monitoring zones around
and on food contact equipment. Since
everything ultimately ends up in the
floor drains, they are a particularly good
location to test if Listeria has invaded the
processing area, such as swabbing the
drains after washing down equipment
but before sanitation. As Tompkin et al.
described, occasional transient findings
of Listeria in the processing environment
are not unexpected, especially when
working with fresh produce. However,
recurring positives are a reason for
concern and should trigger more
rigorous equipment testing and
sanitation to find and destroy Listeria
that may have found a harborage point.
Since L. monocytogenes is not
unexpected on raw produce, field and
raw produce testing for the pathogen is
not likely to yield useful information—it
