Agriculture Reference
In-Depth Information
to a biofi lm - forming defi cient mutant, the biofi lm-forming strain was better able to
survive a subsequent chlorine treatment with infi ltration of the tissue during dip-
inoculation likely enhancing survival. Following a 1999 California outbreak linked to
cilantro tainted with Salmonella Thompson (Campbell and others 2001), Brandl and
Mandrell (2002) used CLSM to confi rm that this same S . Thompson strain labeled
with GFP could infi ltrate natural lesions on cilantro leaves after the plants were dip-
inoculated and incubated for 24 h under high humidity. These fi ndings again stress the
importance of pre- and postharvest damage as a plausible route for pathogen uptake
into plant tissue.
Strategies for Minimizing Internalization
Strategies to minimize produce contamination should be in place long before inter-
nalization occurs. Since some plant pathogens enhance the internalization of human
pathogens, it is helpful to keep plants free of plant pathogens. Diseased fruits and
vegetables should be eliminated during storage and processing. In the fi eld, Good
Agricultural Practices should be followed that include proper soil management
(Beuchat 2002 ; Ibekwe and others 2007 ), use of certifi ed seed (Janse and Wenneker
2002) that has been decontaminated by chemical sanitizers (Annous and others 2001),
sublethal heating or irradiation (Erickson and Doyle 2007), careful selection of plant
varieties and crop rotation to minimize insect infestations and disease (Gniwotta and
others 2005 ; Kortekamp and Zyprian 1999 ), and careful harvesting to minimize micro-
bial contamination, mechanical damage and bruising (Altekruse and others 1997).
During processing, standard sanitizing procedures should be followed with special
attention given to the temperature of both the product and wash water as well as the
type of sanitizer, concentration and treatment time during processing, recognizing that
these sanitizer washes are unable to completely eliminate pathogenic and spoilage
organisms from the surface of fresh produce (Beuchat and Ryu 1997).
Several reports have also demonstrated the potential use of natural microbes isolated
from soil and plants as biological control agents for human pathogens both during and
after harvest (Whipps 2001). Bacillus spp., Pseudomonas aeruginosa , P. fl uorescens ,
and yeasts isolated from the native microfl ora of green bell peppers, romaine lettuce,
and prepeeled baby carrots were found to inhibit the growth of Salmonella
Chester and L. monocytogenes on green pepper 1 and 2 logs, respectively, (Liao and
Fett 2001). Some strains of Gluconobacter asaii , Candida sp., Discosphaerina fagi ,
and Metschnikowia pulcherrima were also reportedly able to decrease the growth and
survival of L. monocytogenes , E. coli O157:H7 and S. Poona on fresh-cut apples
(Janisiewicz and others 1999; Leverentz and others 2006). Although lytic bacterio-
phages also reduced Salmonella populations 3.5 logs on honeydew melon slices, these
same bacteriophages were ineffective against Salmonella on apple slices (Leverentz
and others 2001).
Bacterial endophytes could play an important role in inhibiting internalized human
pathogens, with two reports suggesting that these endophytes may trigger induced
systemic mechanisms that foster resistance against human pathogens (Zehnder and
others 2001; Sturz and others 2000). Hence, application of indigenous bacteria could
provide a potential biologically based strategy for preventing preharvest and posthar-
vest internalization of human pathogens in fresh produce.
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