Agriculture Reference
In-Depth Information
pathogens that are arguably more likely to be hazards in fresh produce. For example,
bacteriocinogenic LAB tested against Gram-negative pathogens showed no antago-
nistic activity (Kelly and others 1998; Yildirim and Johnson 1998), although
Wilderdyke and others (2004) reported signifi cant reductions in populations of
Salmonella
and
E. coli
O157:H7 by two LAB cultures (
Lactococcus lactis
subsp.
lactis
and
Pediococcus acidilactici
), primarily through acid production. Non-LAB such as
pseudomonads and aeromonads have also been reported to inhibit Gram-negative
bacteria (Schuenzel and Harrison 2002) but the genus
Pseudomonas
includes a number
of spoilage and pathogenic species, and the status of
Aeromonas hydrophila
as a
foodborne pathogen is becoming more widely accepted. These considerations high-
light the need to assess carefully factors such as mechanism(s) of inhibition and safety
and suitability when choosing protective cultures for food applications.
While
in vitro
experiments are a useful starting point, it is often diffi cult to mimic
accurately conditions relevant to the food under typical storage conditions.
Consequently it is hard to predict how
in vitro
results will translate to
in vivo
condi-
tions. For example, in research by Bennik and others (1999), bacteriocinogenic
Enterococcus mundtii
demonstrated
in vitro
growth inhibition or reduction of
L.
monocytogenes
on sprout-derived vegetable agar stored at 8 °C over a 5-day period
but could not prevent growth on fresh mung bean sprouts held under similar condi-
tions, a situation attributed to proteolytic degradation. However the purifi ed bacterio-
cin (mundticin) was shown to have some potential either as a washing solution or in
a coating, although bacteriocin inactivation or resistance development was postulated
to occur during extended storage.
Many of the characterization papers cited previously did not consider activity
in
vivo
, and have therefore been excluded from the summary provided in Table 11.1
describing reported applications of bacterial antagonists in fresh produce. These pub-
lications have focused exclusively on salad vegetables, melons, and sprouted seeds.
Most studies have investigated activity against
L. monocytogenes,
which, although a
debatable cause of signifi cant fresh produce-associated disease, can grow on these
foods and is susceptible to the action of bacteriocins. Reported reductions in
L. mono-
cytogenes
levels using protective cultures only have been mixed—from no reduction
up to 4 log
10
CFU — refl ecting the experimental variability in terms of cultures exam-
ined, storage conditions studied, and produce items employed.
To improve effi cacy, the Agriculture Research Service in the U.S. has been inves-
tigating the use of a
Gluconobacter asaii
protective culture in parallel with a mixture
of six phages. This combination has been reported to reduce
L. monocytogenes
by up
to 5 log
10
CFU on melon, more than the individual treatments alone (Reynolds 2007).
The approach offers both immediate and longer-term control of one pathogen via the
combined action of phages and antagonistic bacteria, respectively, but could be modi-
fi ed to target other bacterial pathogens via the application of suitable phages.
Expanding the protective culture approach to Gram-negative pathogens by the
application of
Pseudomonas
cultures has also been reported
in vivo
by several groups
(Liao 2008; Liao and Fett 2001; Matos and Garland 2005; Fett 2006).
A summary of bacteriocin-specifi c applications to produce food safety has been
compiled in Table 11.2. As before, the majority of studies have evaluated antagonistic
activity against
L. monocytogenes
in association with salad vegetables, melons, and
