Agriculture Reference
In-Depth Information
Biofi lm formation may be prevented by QS disruption or by the use of phages able
to degrade biofi lm polymers, both of which are discussed below.
Biocontrol Technologies
The following section is divided into two main subsections: the use of organisms and
the use of products derived from them. Coverage of organisms includes bacterio-
phages, protective bacteria cultures (with the inclusion of bacteriocins), and yeast and
molds; derived products include essential oils and other plant-derived chemicals,
quorum-sensing molecules, siderophores, enzymes, and polylysine.
Organisms
Bacteriophages (Phages)
A substantial amount of research has been generated on the use of virulent phages as
biocontrol agents in foods (Greer 2005; Hudson and others 2005). This approach offers
some particular advantages over other treatments mentioned.
Virulent phages occur naturally in all environments where bacteria are found,
including produce (Kennedy and others 1986). All phages require a bacterial host for
replication, and virulent phages kill host cells by cell lysis. Host-phage interaction is
mediated by the specifi city of the phage, and the interaction can thus be specifi c at
genus, species, or even strain level. Lytic infection results in an increase in phage
numbers as they destroy their host, which is a desirable characteristic as is the lack
of interference with the growth of the natural competitive microbiota.
The use of a proprietary mixture of four virulent phages to control
Salmonella
Enteritidis on cut apple and melon surfaces stored at 5, 10, and 20 °C has been reported
(Leverentz and others 2001 ).
Salmonella
survived on both fruits at 5 °C and growth
occurred at 10 and 20 °C, with increases of up to 2 log
10
and 5 log
10
colony forming
units (CFU), respectively. The addition of phages reduced
Salmonella
populations on
melon by 3.5 log
10
at storage temperatures of 5 and 10 °C; a 2.5-log
10
reduction was
measured at 20 °C. These reductions were greater than those attributed to the use of
chemical sanitizers. Similar reductions in host numbers were not observed for apple
slices treated with phages. This was possibly due to inactivation of phages at the lower
pH of apple. The authors postulated that “ low - pH - tolerant phage mutants ” might be
needed to improve the effi cacy of treatment.
Subsequent work has focused on the biocontrol of
L. monocytogenes
on
fresh-cut produce (Leverentz and others 2003). This research described the use of
virulent phages in combination with the bacteriocin nisin and examined whether
application by spraying or pipetting had a signifi cant effect on host inactivation.
Growth of
L. monocytogenes
occurred most markedly on cut melon stored at 10 °C,
with little growth on apple slices stored at the same temperature. Reductions of up
to 4.6 log
10
were observed when melons were treated with phage, and improved to
up to 5.7 log
10
when combined with nisin. As before, the lower pH of apple may
have minimized the effectiveness of the phage treatment, and reductions in
Listeria
numbers were observed for only the combined treatment incorporating nisin. Spray
application was deemed to be at least as effective as pipetting, and in some cases
was better.
