Agriculture Reference
In-Depth Information
incognita
, a plant nematode, were associated with internalization of
Salmonella
in
cantaloupe, with
Salmonella
protected after being ingested by the nematode (Caldwell
and others 2003). Insects can also harbor human pathogens both externally and inter-
nally (Waterfi eld and others 2004). However, according to Hora and others (2005),
the root-knot nematode,
Meloidogyne hapla
, did not enhance internalization of
E. coli
O157:H7 in spinach.
Evidence for Internalization of Foodborne Pathogens in Produce
Compared to the internalization of plant pathogens, studies addressing the infi ltration
of fruits and vegetables by foodborne pathogens are of relatively recent origin, with
much of this work having been conducted in response to the growing number of high
profi le outbreaks involving fresh produce, the fi rst of which received widespread
attention in the mid-1990s. Since this time, well over 50 studies addressing internal-
ization of primarily
Salmonella
and
E. coli
O157:H7 in various fruits and vegetables
have been reported, the results from which are summarized in the next sections.
Apples
A series of
E. coli
O157:H7 outbreaks during the mid-1990s involving fresh apple cider
prompted concern over the use of dropped apples and potential internalization of patho-
gens. In response, Buchanan and others (1999) were fi rst to show that
E. coli
O157:H7
could penetrate apples during washing. Initial dye immersion tests demonstrated dye
uptake into the inner core of warm (
22 ° C) but not cold (4 ° C) apples. When warm or
cold apples were subsequently immersed for 20 min in a cold (4 °C) aqueous suspen-
sion containing
∼
7.5 log
E. coli
O157:H7 CFU/ml, highest average populations were
found in the outer core near the calyx and blossom ends (5.43 log CFU/g) followed by
the skin (3.91 log CFU/g), inner core (3.40 CFU/g) and pulp (3.08 CFU/g), with similar
results obtained for four different varieties: Golden Delicious, McIntosh, Red Delicious,
and Braeburn. These fi ndings were subsequently confi rmed by Burnett and others
(2000) who used CLSM to visualize a GFP-labeled strain of
E. coli
O157:H7 inside the
fl oral tubes of Red Delicious apples. Once internalized, Buchanan and others (1999)
also showed that a 1-min rinse in a 2000 mg/l sodium hypochlorite solution followed
by a 1-min water rinse decreased
E. coli
populations only ten- to thousandfold, with
highest numbers present in the outer core.
When apples strike the ground, bruising and other forms of damage also increase
the likelihood for internalization, with bacteria more readily infi ltrating cracks in the
wax layer, lenticels, and stomata. Using a GFP-labeled strain of
E. coli
O157:H7,
Kenney and others (2001) reported that bruising increased pathogen migration into
lenticels and decreased penetration into wax platelets compared to unbruised apples
when apples at 30 °C were immersed in a 4 °C bacterial suspension. Rubbing these
apples forced the pathogen from the edges of the wax platelets into crevices. Although
subsequent washing decreased
E. coli
populations about 2 logs, those cells embedded
in the wax layer were not removed. After infi ltrating bruised tissue or wounds, the pH
and °Brix within this microenvironment as infl uenced by the native microfl ora par-
tially dictates the ability of
E. coli
O157:H7 to survive and/or grow (Dingman 2000;
Janisiewicz and others 1999; Riordan and others 2001). When Seeman and others
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