Agriculture Reference
In-Depth Information
shown to interact (Liao and others 2003). For example,
Salmonella
Poona penetrated
3-4 cm below a wound site into the edible part of cantaloupe when inoculated alone,
but the penetration was enhanced by the presence of two plant pathogenic fungi,
particularly
Cladosporium cladosporoides
(Richards and Beuchat 2005). The growth
of
Escherichia coli
O157:H7 in apple wounds was prevented by the application of
Pseudomonas syringae
, which is used in products to control postharvest decay of
apples and pears (Janisiewicz and others 1999).
E. coli
O157:H7 grew when incubated
at room temperature on punctured apples coinoculated with the plant pathogen
Glomerella cingulata
but at a rate similar to that obtained in its absence, and the pres-
ence of
Penicillium expansum
resulted in a reduction in pathogen numbers (Riordan
and others 2000). Similar fi ndings were made for
L. monocytogenes
(Conway and
others 2000 ).
It has been observed that fruits and vegetables affected by bacterial soft rot are
more frequently contaminated by
Salmonella
(Wells and Butterfi eld 1997 ), although
a similar relationship was not shown clearly for produce affected by fungal rot or
physical injury (Wells and Butterfi eld 1999 ).
These examples show that plant pathogens, or their associated biocontrol agents,
can affect the establishment and/or growth of human pathogens on produce. Research
on the biocontrol of plant pathogens can offer options for the control of human patho-
gens, as illustrated later in this chapter.
Location of Pathogens in Produce
Pathogens may be introduced to the external surfaces of produce via the application
of improperly composted manure or contaminated water. Furthermore they may be
physically introduced via a piercing wound (Burnett and others 2000) or from cut
edges (Takeuchi and Frank 2000). It has been proposed that pathogens may become
internalized into produce via the root system during cultivation, taken up during pro-
cessing (US FDA 1999) or by a number of other routes (Cooley and others 2003).
The possible presence of human pathogens in internal tissues means that they may
not be amenable to treatment by chemical sanitizers (Takeuchi and Frank 2000).
Under these circumstances biocontrol may prove advantageous over postprocessing
chemical treatments by preventing initial colonization. For example, inhibition of
plant colonization by
E. coli
O157:H7 and
Salmonella
has been demonstrated by an
Enterobacter asburiae
isolate from the phyllosphere (Cooley and others 2003). In
addition, reduction of
E. coli
O157:H7 and
L. monocytogenes
on lettuce has been
shown after coinfection with
Enterobacter cloacae
(Jablasone and others 2005).
The formation of biofi lms may be of signifi cance for both the survival of human
pathogens on produce and the ability of biocontrol organisms to control them. Naturally
occurring biofi lms may provide human pathogens with safe harbor and protect them
from the action of sanitizers (Fett 2000). This has been shown in a
Salmonella
biofi lm
formed on parsley (Lapidot and others 2006), although other unknown factors were
also involved.
Salmonella
can form biofi lms on tomatoes after 4 days storage at 20 °C and 95%
relative humidity (Iturriaga and others 2007 ) and autoinducer - 2 - like activity, i.e.,
quorum sensing (QS), is suggested as having a role in biofi lm formation by
E. coli
O157:H7 on the same fruit (Lu and others 2005).
