Food Safety Magazine - June/July 2013

SANITATION
ceilings; the main reason for this is L.
monocytogenes’ ability to form biofilms
on many surfaces. 5 It is also considered
a highly pathogenic facultative intracellular
organism, which actively invades
hosts and induces listeriosis; infection
is normally associated with ingestion of
large numbers of the bacteria in healthy
adults, but likely much
smaller numbers can
affect immunocompromised
individuals.
Many of these factors
are unique and differ
from those of other
foodborne pathogens
such as Escherichia
coli, Salmonella and C.
perfringens. Generic
pathogen reduction
programs may be insufficient
to take into
account the pathogen’s
ability to grow at
low temperatures and
its persistence within
biofilms. Another issue
is that there may
be a continual source
of the pathogen via
raw material entering a
processing facility, carrying
with it both transient
and persistent
strains. Some strains
have been shown to persist for months
or even years in such environments. 6
Surveillance
All countries should consider making
listeriosis a notifiable disease and have
in place both active and passive surveillance
systems for noninvasive gastrointestinal
infections. In 2005, the U.S.
Centers for Disease Control and Prevention
launched its Listeria Initiative for
state and local health departments to
follow up on all listeriosis patients as
soon as their cases are reported and to
collect relevant data using an extended
questionnaire. This enhanced surveillance
system contributed substantially
to the relatively quick resolution of a
Listeria outbreak linked to cantaloupes
“...L. monocytogenes
has distinctive
characteristics that
must be recognized
before implementing
prevention and control
strategies applied to
the food processing
and food preparation
environments.”
in the summer of 2011. One of the big
questions is why there is an increased
proportion of elderly people suffering
from listeriosis in developed countries.
A recent survey showed an increase in
risk of listeriosis among older persons,
pregnant women and Hispanics in the
U.S. 7 Comparative analyses of strains
worldwide are essential
to the identification of
novel outbreak strains
and epidemic clones.
For instance, sequence
typing confirmed that
a predominant L.
monocytogenes clone
caused human listeriosis
cases and outbreaks
in Canada from 1988
to 2010. 8
Food Vehicles
Even though most
attention has been
directed to controlling
contamination of meat
and poultry products,
a variety of products
have recently been
implicated as vehicles
of L. monocytogenes.
These include several
RTE foods, such as
hard-boiled eggs,
sandwiches and hummus,
as well as lettuce, celery, cabbage
and walnuts. Produce is clearly now a
major vehicle for risks of outbreaks of
listeriosis, and these have been linked
to poor storage conditions and environmental
cross-contamination after
processing. However, different types of
the implicated produce seem to have
different growth rates and maximum L.
monocytogenes population densities on
subsequent experimentation. This may
depend partly on the surface structure
of each vegetable or fruit and the way
it is harvested. For instance, lettuce and
spinach leaves are fragile and can leak
contents, and cantaloupes’ rough surface
allows bacterial harborage. A number
of predictive models for the growth
kinetics on different items have been
carried out on different products such as
white cabbage, lettuce and cantaloupe.
Much attention has been placed on
reducing risks in deli meat production
facilities because of the many outbreaks
that have been linked to these foods
over many years; the last major outbreak
was in Canada in 2008 with 58
confirmed cases and 22 deaths. The U.S.
Department of Agriculture (USDA) conducted
a risk assessment for deli meats
but focused on risk management questions.
9 They found that 1) food contact
surfaces (FCS) positive for Listeria
spp. greatly increased the likelihood of
finding RTE product lots positive for
L. monocytogenes; 2) the frequency of
contamination of FCS with Listeria species
encompasses a broad time frame,
and the duration of a contamination
event lasts approximately a week; 3)
minimal testing was estimated to result
in a small reduction in the levels of L.
monocytogenes on deli meats at retail, and
an increased frequency of FCS testing
and sanitation was estimated to lead to
a proportionally lower risk of listeriosis;
and 4) combinations of interventions
(e.g., testing and sanitation of FCS, preand
postpackaging interventions and the
use of growth inhibitors/product reformulation)
appeared to be much more
effective than any single intervention in
mitigating the potential contamination
of RTE product with L. monocytogenes
and reducing the subsequent risk of
illness or death. A subsequent USDA
Food Safety Inspection Service study
showed that deli meats prepared at retail
had a far greater risk of causing listeriosis
than prepackaged deli meats. 10
Many cheeses made from unpasteurized
or even pasteurized milk allow
growth of L. monocytogenes, and
surveys of artisanal cheeses in different
countries have shown them to be contaminated.
For instance, the pathogen
can grow in Mexican-style queso fresco at
both 10 °C and 4 °C to reach a maximum
population density greater than 7
logs. 11 The use of antimicrobials and/or
postprocessing interventions is recommended
to prevent the growth of the
organism. However, in some artisanal
30 F o o d S a f e t y M a g a z i n e