Agriculture Reference
In-Depth Information
CLSM confi rming internalization up to 45
m below the surface. When mature 50-
day-old plants were given a one-time exposure to 200 ml of irrigation water or manure
slurry inoculated at 10
7
to 10
8
CFU/ml,
E. coli
O157:H7 was detected in irrigated and
manure-treated plants at least 5 and 3 days after exposure, respectively. However,
using a somewhat similar study design, Johannessen and others (2005) later reported
no infi ltration of a non-GFP labeled antibiotic resistant strain of
E. coli
O157:H7 into
crisp head lettuce plants that were fertilized once with a manure slurry inoculated at
10
4
CFU/g. These later fi ndings, which suggest that
E. coli
populations of at least
10
6
CFU/g or ml are needed during a contamination event for subsequent internaliza-
tion through the root system, are also supported by the results of a related study
(Wachtel and others 2002b) in which a mixture of nonpathogenic root-, shoot- and
seed coat-adhering strains of
E. coli
naturally present in improperly treated sewage
wastewater and a cabbage fi eld colonized the roots but did not infi ltrate edible portions
of cabbage plants.
Several additional publications also attest to the ability of
E. coli
O157:H7 and
Salmonella
to become internalized in lettuce plants via the root system. Using strains
of
E. coli
O157:H7 labeled with GFP and nonpathogenic strains of
E. coli
in a simple
seed and seedling adherence model, Wachtel and others (2002a) observed similar
adherence of both organisms to the shoots and seed coats of hydroponically grown
leaf lettuce seedlings. However,
E. coli
O157:H7 adherence rates to roots and root
hairs were almost tenfold higher compared to the nonpathogen. Although similar fi nd-
ings were obtained when inoculated seeds were later grown in soil, GFP-labeled cells
of
E. coli
O157:H7 were also seen on cotyledons in close proximity to stomatal pores,
suggesting another entry route that has been confi rmed by others (Seo and Frank 1999;
Takeuchi and Frank 2000). Using GFP-labeled strains of
E. coli
O157:H7,
Salmonella
and
L. monocytogenes
, Jablasone and others (2005) subsequently reported similar
colonization and growth on dip-inoculated lettuce and other seed types that were
gnotobiotically cultivated, with
E. coli
O157:H7 growth confi ned to the seedling roots
and root junctions. Although
E. coli
O157:H7 was quantifi able in internal tissue from
surface-sterilized lettuce as well as cress, radish, and spinach plants after 9 but not 49
days,
S
. Typhimurium counts could be obtained only from 9-day-old lettuce and radish
seedlings with no evidence of
L. monocytogenes
internalization in any of the nine seed
types examined. Finally, when lettuce seeds were germinated in pathogen-inoculated
soil, Franz and others (2007) reported greater internalization of
E. coli
O157:H7 as
opposed to
Salmonella
into lettuce plants, with average
E. coli
O157:H7 and
S
.
Typhimurium populations of 3.95 and 2.47 log CFU/g, respectively, recovered from
AgNO
3
surface-sterilized leaves. When grown hydroponically, only
S
. Typhimurium
was recovered from surface-sterilized leaves, suggesting that root damage in soil-
grown lettuce plants may play an important role in
E. coli
O157:H7 internalization.
Using
S. enterica
, Bernstein and others (2007) subsequently confi rmed this theory,
reporting that 33-day-old lettuce leaves from lettuce plants transplanted to pots
with and without prior root decapitation harbored mean
Salmonella
populations of 3.7
and 2.7 log CFU/g, respectively, with
E. coli
O157:H7 internalization into the aerial
portions of maize plants also enhanced when the root system is damaged.
In addition to the aforementioned preharvest routes of internalization, certain post-
harvest handling practices (e.g., fi eld coring, shredding, and washing) may also
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