Agriculture Reference
In-Depth Information
Discospaerina fagi
showed activity against
L. monocytogenes
, and
D. fagi
and
M.
pulcherrima
activity against
S.
Poona. For
L. monocytogenes
at 10 ° C, a 2.1 - 2.8 - log
10
reduction after 5 days was observed compared to the control, and at 25 °C up to a
6 - log
10
reduction occurred in 7 days. No antagonists were effective against
Salmonella
at 10 ° C, but
D. fagi
(4.8 - log
10
reduction after 7 days incubation) and
M. pulcherrima
(2.4 log
10
) reduced pathogen numbers at 25 °C. Preliminary evidence also suggested
that combining phage cocktails, which have a quick knockdown, with this slower
effect might be benefi cial for controlling pathogens over the long term, reinforcing
similar observations made in the previous section.
Application of mixed cultures of bacteria and yeasts, or yeasts and molds, may also
present a useful strategy. During a survey for antagonistic microfl ora on produce, yeast
strain D1 was isolated from prepeeled baby carrots and inhibited
L. monocytogenes
,
E. coli
,
S.
Chester, and
Erwinia carotovora
in vitro (Liao 2007). However, the com-
bination of
P. fl uorescens
A3 and yeast D1 was found to be a more effective treatment,
reducing
S.
Chester by 1 log
10
and
L. monocytogenes
by 2 log
10
compared to controls
when tested on green pepper discs over 3 days. Richards and others (2004) found that
Candida oleophila
and
Rhodotorula glutinis
at 4 ° C (
1 - log
10
reduction), and
C.
albidus
,
Debaryomyces hansenii
and
Pichia guilliermondii
at 20 ° C (1.2 - 2.7 - log
10
reduction) could be effective antagonists to
S.
Poona in cantaloupe wounds when they
were coinfected by the plant pathogen
Geotrichum candidum
.
Some species of plant pathogenic molds produce enzymes that release breakdown
products into the surrounding plant tissue causing localized changes in pH. This can
signifi cantly change the suitability of the niche for enteric pathogens and therefore
may be useful for biocontrol purposes. It is known that some molds increase the pH
of fruits and promote the growth of pathogens (Draughon and others 1988; Riordan
and others 2000). However, on fresh-cut apple slices
Penicillium expansum
was dem-
onstrated to lower the pH from 4.7 to 3.7 and cause a signifi cant decline, up to 4 log
10
,
in numbers of coinoculated
Listeria
(Conway and others 2000). Another study revealed
that yeasts could negate increases in the pH of cantaloupe fruit wounds but that this
was insuffi cient to control
S.
Poona when coinoculated (Richards and others 2004).
There have been some indications (Izgu and Altinbay 1997) that yeast killer toxins,
generally only thought to target yeasts and some fungi, might also inhibit Gram-
positive bacteria including
Listeria
but these results remain largely unsubstantiated.
More recently, Goerges and others (2006) screened 304 yeasts and found 11 that could
signifi cantly inhibit two or more
L. monocytogenes
isolates in vitro. Ten of these yeasts
were
Candida intermedia
and the other was
Yarrowia lipolytica
. A selection of 14
yeasts produced between 1 - and 4 - log
10
reduction compared to the control in a 24-hour
cocultivation assay, the strongest inhibition being caused by a
C. intermedia
isolate.
An assay for killer toxins detected only very small clearing zones, and their presence
did not correlate well with results from other assays. So, although it remains possible
that killer toxins may be infl uential in yeast-pathogen interactions in produce, sub-
stantially more work needs to be done before any conclusions can be drawn.
In grapes and other produce, the production of ochratoxins by some
Aspergillus
and
Penicillium
species is of particular concern because these compounds are neph-
rotoxic and carcinogenic to animals. Bleve and others (2006) collected epiphytic yeast
from the berry surface of
Vitis vinifera
cv. Negroamaro and tested 144 unique isolates
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