Biomedical Engineering Reference
In-Depth Information
cattle fecal samples (Nightingale et al.
2004
,
2005
).
L. monocytogenes
also can be
isolated from a number of nonruminant species' feces such as poultry (Weber et al.
1995
), wild birds (Fenlon
1985
), swine (Hayashidani et al.
2002
; Yokoyama et al.
2005
), horses (Weber et al.
1995
; Gudmundsdottir et al.
2004
), farmed fi sh
(Miettinen and Wirtanen
2005
), and some domestic animals.
L. monocytogenes
in
ruminants and on farms contributes directly to human disease (e.g., consumption
of contaminated raw milk (Ryser
1999
)) and indirectly by introduction into food
processing plants or onto vegetables through contaminated manure (e.g., Fenlon
et al.
1996
; Rorvik et al.
2003
). In a recent study on produce farms,
L. monocyto-
genes
was detected in 17.5 % of fi elds. Soil cultivation, irrigation, and presence of
wildlife within a given number of days prior to sampling, all increased the likeli-
hood of a soil sample testing positive for the presence of
L. monocytogenes
(Strawn
et al.
2013
).
5.2.2
Cross-Contamination and Growth
of
L. monocytogenes
in Food
The common occurrence of
L. monocytogenes
in nature and agricultural systems
contributes to the frequent introduction of the pathogen into foods.
L. monocyto-
genes
is a salt- and acid-tolerant organism and can grow at and below refrigeration
temperatures with little oxygen (McLauchlin and Rees
2009
). It is, however, sensi-
tive to extreme acidity, pressure, and high temperature (McLauchlin and Rees
2009
).
Cooking kills
L. monocytogenes
, thus preventing disease. As
L. monocytogenes
can
be killed by heat, contaminated raw ingredients rarely cause illness directly when
food is heat treated. The more likely source of
L. monocytogenes
on foods is cross-
contamination during processing after heating (e.g., slicing, casing removal, or
packaging), which transfers the pathogen onto already cooked, RTE products
(Lawrence and Gilmour
1995
; Pradhan et al.
2011
). Departments that handle raw
meat products and RTE foods must pay particular attention to prevent cross-
contamination. For example, 15-34 % of raw chicken sampled at retail was positive
for
L. monocytogenes
(Cook et al.
2012
). Poor food handling practices could result
in products such as deli meat becoming inadvertently contaminated with
L. monocy-
togenes
. Cross-contamination alone does not create a risk of listeriosis. The infec-
tious dose or dose response (the number of cells required to cause illness) varies and
is not conclusive (McLauchlin et al.
2004
). In general, it is thought to be high so the
few cells transferred to foods during cross-contamination are not typically enough to
cause illness (Vazquez-Boland et al.
2001
). However, if the food product supports
the growth of
L. monocytogenes
, the few transferred cells may multiply during stor-
age (even at refrigeration temperature) to potentially infectious levels before
consumption.
