Agriculture Reference
In-Depth Information
sprouted seeds. Results published by Molinos and others (2005) in particular demon-
strate the variability in achieved pathogen reductions in relation to produce type,
storage temperature, and concentration of bacteriocin employed.
The application of defi ned antimicrobials does lend itself more easily to the option
of combined strategies, either to improve effi cacy or extend the biocontrol approach
to Gram-negative pathogens. Various chelating agents and other antimicrobials have
been successfully employed in combination with nisin to enhance activity against
Salmonella
spp. (Ukuku and Fett 2004),
E. coli
O157:H7, and
L. monocytogenes
(Ukuku and others 2005). Alternatively, using colicins produced by
E. coli
has been
shown to be effective in treating alfalfa seeds contaminated with
E. coli
O157:H7,
with reductions of 4 to
6 log
10
CFU observed within 24 to 48 hours of treatment
(Nandiwada and others 2004).
As previously mentioned, the development of resistance to bacteriocins during
extended exposure is a potential problem. However, reported means of dealing with
this include combining bacteriocins (Bari and others 2005) or alternatively applying
bacteriocins in combination with bacteriocin-producing cultures (reviewed by Settanni
and Corsetti 2008). The latter approach has also been shown to enhance overall activ-
ity and reduce the amount of nisin required.
The ideal protective culture or bacteriocin must be able to target a range of patho-
gens at concentrations that do not infl uence the organoleptic properties or shelf life
of the product, without susceptibility to phage attack (in the case of protective cultures)
or the development of resistance (to bacteriocins). Given that protective cultures and
antimicrobial metabolites are not in their own right a complete biocontrol approach,
additional hurdles must be considered. The application of multiple bacteriocinogenic
strains (both LAB and non-LAB) in combination with defi ned bacteriocins and other
antimicrobial agents appears to offer one means of achieving biocontrol targets.
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Yeast and Fungi
Isolates of yeast and fungi have been extensively evaluated for the biocontrol of
postharvest diseases of produce. The yeast
Candida oleophilia
Montrocher, isolated
from tomato peel, has also been registered with the EPA under the trade name Aspire
since 1995 for the control of fungal citrus postharvest diseases (Richards and others
2004). Although there are relatively few reports of fungi or yeast being used for the
biocontrol of human pathogens, they have many features that make them good can-
didates for this purpose. Yeasts are frequent colonizers of vegetables and fruits, with
up to 10
6
cells per apple reported (Deak and Beuchat 1997), have simple nutritional
requirements, grow easily in fermenters, and survive in a wide range of environments
(Bleve and others 2006).
Yeast and fungal biocontrol of plant pathogens has been found to be complex and
involve multiple mechanisms, including competition with the pathogen for space and
limiting nutrients; induction of host plant responses; and production of killer toxins,
lytic enzymes, or iron-chelating siderophores (Calvente and others 1999; Chan and
Tian 2005; Ippolito and others 2000; Lowes and others 2000). Some of these mecha-
nisms have also been shown to be involved in the control of human pathogens.
Leverentz and others (2006) tested yeast biocontrol of
L. monocytogenes
and
Salmonella
Poona on fresh-cut apples.
Metschnikowia pulcherrima
,
Candida
, and
