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Applied and Environmental Microbiology, March 2003, p. 1854-1857, Vol. 69, No. 3
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.3.1854-1857.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
A Pediocin-Producing Lactobacillus plantarum Strain Inhibits Listeria monocytogenes in a Multispecies Cheese Surface Microbial Ripening Consortium
Melanie Loessner, Susanne Guenther, Sandra Steffan, and Siegfried Scherer*
Zentralinstitut für Ernährungs- und Lebensmittelforschung, Wissenschaftszentrum Weihenstephan, Technische Universität München, D-85354 Freising, Germany
Received 8 July 2002/
Accepted 29 November 2002

ABSTRACT
The growth of
Listeria monocytogenes WSLC 1364, originating
from a cheese-borne outbreak, was examined in the presence and
in the absence of a pediocin AcH-producing
Lactobacillus plantarum strain on red smear cheese. Nearly complete inhibition was observed
at 10
2 CFU of
L. monocytogenes per ml of salt brine solution,
while contamination with
Listeria mutants resistant to pediocin
resulted in high cell counts of the pathogen on the cheese surface.
The inhibition was due to pediocin AcH added together with the
L. plantarum culture to the brine solution but not to bacteriocin
production in situ on cheese. Pediocin resistance developed
in vitro at different but high frequencies in all 12
L. monocytogenes strains investigated, and a resistant mutant remained stable
in a microbial surface ripening consortium over a 4-month production
process in the absence of selection pressure. In conclusion,
the addition of a
L. plantarum culture is a potent measure for
combating
Listeria in a contaminated production line, but because
of the potential development of resistance, it should not be
used continuously over a long time in a production line.

INTRODUCTION
Listeria monocytogenes, the causative agent of listeriosis,
has resulted in numerous major food-borne outbreaks worldwide.
Red smear cheeses are particularly sensitive to colonization
with this pathogen (
17,
27); 21 of 329 cheese samples have been
found to contain
L. monocytogenes, in one case more that 10
4 CFU per cm
2 of cheese surface (
24). A recall of 80 tons of
L. monocytogenes-contaminated soft and semisoft red smear cheeses
in Germany in March 2000 prompted renewed concern about the
presence of this bacterium in red smear cheese. It has been
shown that contamination with
L. monocytogenes and other species
of
Listeria occurs frequently in red smear cheese, even when
pasteurized milk has been used for cheese making (
24). Most
likely, this is due to postprocess contamination during the
traditional method of "old-young smearing," which includes frequent
handlings and washes required for proper development of the
complex, undefined microbial ripening consortium. Inhibition
of
L. monocytogenes after application of bacteriocinogenic cheese
smear coryneform bacteria (
5,
11) and
Staphylococcus equorum (
6) in situ has been reported and is of considerable interest
in order to enhance the hygienic quality of these products.
Many lactic acid bacteria, including members of the genera Lactococcus, Lactobacillus, Carnobacterium, Enterococcus, and Pediococcus, are known to secrete small, ribosomally synthesized antimicrobial peptides called bacteriocins (1, 14, 16, 21), many of which inhibit Listeria (7, 15, 20). Some bacteriocins have been used to inhibit this pathogen in food, either through bacteriocin-producing cultures (20, 29) or by the addition of pure or semipure bacteriocin preparations (8, 20, 28). Lactobacillus plantarum ALC 01 was reported to secrete the bacteriocin pediocin AcH (9), which is also produced by Pediococcus acidilactici (13, 19, 25). The activity spectrum of pediocin AcH is relatively wide, and it exhibits a bactericidal mode of action leading to lysis of cells (18) in three steps: (i) binding to the cytoplasmic membrane, (ii) insertion of bacteriocin molecules in the membrane, and (iii) formation of a poration complex which leads to dissipation of the proton motive force. A review on pediocin was recently published by Rodriguez et al. (23). The antilisterial action of L. plantarum ALC 01, which is commercially available (Danisco, Niebüll, Germany), was investigated on red smear cheese by using either complex wash-off cultures from commercial cheeses or a defined ripening culture distributed by a culture supplier.

Bacterial strains and determination of their inhibitory activity.
L. plantarum ALC 01, a pediocin AcH producer isolated from Munster
cheese (
10), and
L. plantarum ATCC 14917, a bacteriocin-negative
strain, were used as test bacteria to demonstrate bacteriocin-mediated
antilisterial activity. Both strains were cultured for 14 h
at 37°C in a special culture medium (VisStart TW ALC01;
Danisco) supplied by the manufacturer of the ALC 01 strain,
to reach a final pH of 3.9 and a maximum pediocin AcH activity.
A total of 12 different
L. monocytogenes strains (Table
1),
isolated from various foods, were used as indicator strains.
For detection of pediocin AcH released into the growth medium,
a sample of
L. plantarum ALC 01 was centrifuged (10,000
x g,
10 min, 4°C). The supernatant was neutralized, filtered
through a 0.45-µm-pore-size membrane filter, and used
in a "spot-on-the-lawn" assay (
2) by spotting 10 µl of
the samples onto a lawn of
L. monocytogenes indicator cells,
as specified in Table
1. Indicator plates contained 7 ml of
0.8% tryptose-soft agar (TB, with 8 g of agar/liter; Merck,
Darmstadt, Germany) and 100 µl of an overnight culture
of the
L. monocytogenes indicator strains. Activity was quantified
by serial twofold dilutions (
2) and expressed in activity units
(AU) per milliliter. Sensitivity tests were repeated twice.
ALC 01 produced clear zones of inhibition on solid media against
all
L. monocytogenes indicator strains (Table
1), whereas the
control strain,
L. plantarum ATCC 14917, showed no inhibition.
L. monocytogenes WSLC 1364 (serovar 4b), isolated from Vacherin
Mont d'Or cheese (
3), was used for contamination experiments
due to its origin from a listeriosis outbreak caused by red
smear soft cheese. A pediocin-resistant mutant was derived from
this strain (WSLC 1364R) by growing the wild type in the presence
of approximately 25,000 AU of pediocin per ml. After 24 h, inhibition
zones were examined under Henry's illumination for pinpoint
colonies, indicating resistant mutants, which were purified
on Palcam agar.
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TABLE 1. Sensitivity and resistance of various L. monocytogenes indicator strains against pediocin AcH produced by L. plantarum ALC 01
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Inhibition of L. monocytogenes on cheese.
To evaluate the antilisterial potential of the
L. plantarum ALC 01 strain in situ on soft cheese, ripening experiments of
model cheese were performed. An undefined wash-off flora from
commercially produced cheese and a defined commercial ripening
culture containing
Brevibacterium linens,
Geotrichum candidum,
and
Debaryomyces hansenii (OFR 9 and DH 2; Danisco) was used
as described by Eppert et al. (
11) under laboratory conditions
in glass desiccators. Smearing was applied five times at intervals
of 2 or 3 days under sterile conditions. The smear brine finally
contained approximately 10
8 CFU/ml of the ripening flora. For
contamination, an aliquot of a diluted overnight
Listeria culture
(see below) was added to the smear brine just before smearing
on day 1. For determination of
Listeria and
Lactobacillus cell
counts, two slices 3 to 4 mm thick were removed from the flat
surfaces of a round cheese (approximately 20 g, corresponding
to roughly 45 cm
2), homogenized in 180 ml of 1.75% trisodium
citrate-dihydrate solution with a stomacher, diluted, and plated
on Palcam and MRS agars (Merck). Cell counts were calculated
per square centimeter of cheese surface. When
Listeria cell
counts were expected to fall below 100 CFU/cm
2, 25 g of the
cheese surface was examined by an enrichment procedure according
to International Standard ISO 11290-1. To ensure that the ripening
processes in the laboratory were typical for red smear cheeses
produced in dairies, pH, aerobic plate counts, and yeast counts
on the cheese surface were determined throughout. In all experiments,
the development of these parameters was typical for the ripening
of industrial red smear cheese (
11) (data not shown).
For ripening experiment A (Fig. 1A), a stock culture of L. plantarum (containing approximately 50,000 AU of pediocin AcH/ml) was mixed 1:1 with 10% NaCl solution to reach a final NaCl concentration of 5%. The commercial undefined wash-off flora was added, yielding L. plantarum cell counts of 5 x 108 CFU/ml and L. monocytogenes cell counts of 2 x 102 and 4 x 103 CFU/ml. In this experiment, inhibition of the growth of L. monocytogenes was observed compared to control cheeses ripened with the bacteriocin-negative strain ATCC 14917 (Fig. 1A). The effect was dependent on the contamination level: when cheeses were challenged with 4 x 103 CFU of Listeria/ml, an inhibition of 1 to 2 log cycles could be demonstrated during the whole ripening period, whereas pronounced inhibition could be achieved with low initial contamination levels (2 x 102 CFU/ml of brine). Until day 14, no Listeria cells could be detected on the cheese surface. Between days 25 and 35, Listeria cells grew to approximately 3 x 103 CFU/cm2 on cheeses ripened with the addition of ALC 01 and to 6 x 105 CFU/cm2 on control cheese ripened with the bacteriocin-negative control strain. Although the control experiment using a pediocin-negative L. plantarum strain is in favor of the hypothesis that it is pediocin which inhibits L. monocytogenes, other inhibitory factors cannot be excluded, because the L. plantarum strains were not isogenic. In order to gain further data, a pediocin-resistant L. monocytogenes mutant, WSLC 1364R, was also used (Fig. 1B). As expected, this mutant was not inhibited at all by the pediocin-producing L. plantarum strain.
For ripening experiments using either the supernatant or the
cell pellet from an
L. plantarum culture (Fig.
1C), cells were
harvested by centrifugation (10,000
x g, 20 min, 4°C) and
the supernatant was collected. Cells were washed twice by centrifugation
and resuspended in fresh culture medium. A 10% NaCl solution
was added as described above to either the resuspended cell
pellet or the filter-sterilized supernatant, and the defined
ripening culture was added.
Listeria cells on cheeses inoculated
with the resuspended pellet reached final counts of approximately
4
x 10
3 CFU/cm
2, and the qualitative determination of
Listeria on the cheese surface was possible at days 7 and 24 of ripening.
Partial inhibition by the
L. plantarum cell pellet may be due
to pediocin produced before addition of the cells, to leakage
of intracellular pediocin from the producer cells, or to pediocin
produced in situ. On cheeses challenged with the filter-sterilized
supernatant of the culture containing the bacteriocin, listeriae
appeared to be eradicated from the cheeses. It is concluded
from this experiment that no growth or in situ production of
pediocin is necessary to achieve inhibition of
L. monocytogenes.
This is in agreement with the observation that
Lactobacillus cell counts on the cheese surface were approximately 4
x 10
7 CFU/cm
2 in all experiments and no significant growth of this
strain during cheese ripening could be observed.

Formation and stability of pediocin-resistant mutants.
Bacteriocins such as pediocin AcH act by means of a single-hit
mechanism (
26) and become inactivated at some step after binding
to the target cell. The amount of pediocin AcH added to the
brine solution is therefore insufficient to eliminate extremely
high initial levels of
L. monocytogenes, which will lead to
growth of the pathogen due to pediocin-sensitive survivors (Fig.
1). Alternatively, one could assume that pediocin-resistant
mutants would preferentially multiply during the ripening of
the cheeses. First, we determined the frequency of the appearance
of resistance in 12 different
L. monocytogenes strains. For
determination of the mutation frequency, a log-phase culture
of the
Listeria strain was incubated with pediocin AcH (approximately
25,000 AU/ml) in a 5% sodium chloride solution at 11°C.
After 1 h, 100 µl was spread on pediocin AcH-containing
PC agar plates (approximately 25,000 AU/plate). For calculation
of the mutation frequency, the number of CFU in the log-phase
culture was related to number of CFU on pediocin AcH-containing
PC agar plates. It was found that the frequency of pediocin-resistant
mutant was strongly strain dependent and varied between 4
x 10
-3 and 1
x 10
-6. This is in agreement with a recent study
of 20 strains of
L. monocytogenes reporting pediocin resistance
frequencies of 10
-4 to 10
-6 (reference
12 and references therein).
In order to check the stability of resistance of L. monocytogenes WSLC 1364R, a mutant was grown in brain heart infusion broth (BHI; Merck) for 24 h at 30°C. Subsequently, 10 µl of the culture was transferred to fresh, pediocin-free BHI broth for a growth cycle of 24 h. This procedure was repeated for a total of 10 transfers. At day 10, a sample was taken, diluted, and plated on BHI agar, and 50 colonies were picked randomly and tested for resistance to pediocin AcH. It was observed that after 10 transfers (approximately 100 generations), all isolates still were resistant to pediocin AcH. The high stability of pediocin resistance has also been reported by Rekhif et al. (22) and Duffes et al. (9).
In addition, a long-term ripening experiment was carried out to determine the stability of the pediocin-resistant mutant WSLC 1364R (which could grow in the presence of approximately 25,000 AU/ml) over a period of 16 weeks of ripening of the cheeses. For this experiment, the traditional method of "old-young smearing" was imitated under laboratory conditions, by transferring the ripening flora (including the listerial contamination) from batch 1 to batch 2 after 15 days of ripening and again from batch 2 to batch 3 after an additional 14 days of ripening (day 29). This procedure was continued until day 113 of ripening (day 15 of batch 7). Infection with L. monocytogenes WSLC 1364R (1.5 x 102 CFU/ml of brine solution) was applied only once, at day 1 of the first batch. Listeria cell counts on the cheese surface were found to exceed 108 CFU/cm2 (day 29 to 43) and then slightly decreased (approximately 107 CFU/cm2 at day 113) until the end of this ripening experiment. At various times, homogenized parts of the cheese surface were diluted and plated on Palcam agar, and 50 colonies were picked randomly and tested for resistance to pediocin AcH as described above. As seen in the in vitro serial transfer experiment, resistance was also stable in the complex microbial consortium until the end of the experiment (day 113).

Is resistance likely to occur in a production line?
Under our small-scale laboratory conditions, we found pediocin-resistant
mutants after applying pediocin to cheese infected with high
L. monocytogenes cell counts, 10
5 CFU/ml of brine solution,
in one of four experiments (data not shown). It is, of course,
not possible to perform a contamination experiment in a real
production line. Therefore, some estimates may help give an
idea of the potential occurrence of resistant mutants in a dairy.
If one assumes a titer of
Listeria cells in a dairy of 10 cells/ml
of brine solution and a frequency of 10
-4 for the emergence
of pediocin resistance, one would expect one mutant cell per
liter. From the brine solution, less than 1 ml is transferred
to the surface of an individual cheese. Even if a resistant
mutant is transferred to 1 out of 1,000 cheeses, this single
mutant cell needs to successfully compete with the microbial
cheese-ripening consortium. It has been reported that the fitness
cost of pediocin resistance of
Listeria can reduce the maximum
specific growth rate to 44% (
12). Therefore, we expect that
the establishment of a resistant cell line would be a rare event
in a cheese-making environment with a low average titer of
Listeria in the brine solution. However, it cannot be excluded that such
an event may happen, and resistance to class II bacteriocins
certainly is a potential obstacle to their application as food
preservatives (
23).

Conclusions.
Contamination levels of 10
2 CFU of
L. monocytogenes per ml of
brine solution are rather high compared to those found in brine
solutions of red smear cheese dairies. Nevertheless, complete
eradication of
L. monocytogenes was observed in our experiments.
Therefore, the supplementary use of pediocin-producing
L. plantarum strains appears to be a promising measure to combat
L. monocytogenes in an infected production line. However, resistant mutants are
frequently found in all
Listeria strains, multiply easily within
a food model system like soft cheese, and remain resistant over
a long period. Therefore, the continuous use of pediocin AcH
appears not to be suitable as a primary means of food preservation
(4). We recommend restricting its use to cases of acute contamination
of a dairy with
L. monocytogenes. Combination of pediocin AcH
with other bacteriocins as part of a hurdle concept may well
constitute an approach to avoid the outgrowth of resistant cells,
especially if bacteriocins such as nisin and lactacin, which
have a completely different structure from that of pediocin,
are used (
23).

ACKNOWLEDGMENTS
We thank Gertrud Huith and Evi Lang-Halter for excellent technical
assistance and Dieter Elsser for helpful discussions.
This research was supported by the Bayerische Staatsministerium für Ernährung, Landwirtschaft und Forsten.

FOOTNOTES
* Corresponding author. Mailing address: Zentralinstitut für Ernährungs- und Lebensmittelforschung, Wissenschaftszentrum Weihenstephan, Technische Universität München, D-85354 Freising, Germany. Phone: 49-8161-713516. Fax: 49-8161-714512. E-mail:
Siegfried.Scherer{at}lrz.tum.de.


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Applied and Environmental Microbiology, March 2003, p. 1854-1857, Vol. 69, No. 3
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.3.1854-1857.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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