2

International Journal of Food Microbiology 81 (2003) 73 – 84
www.elsevier.com/locate/ijfoodmicro

Applicability of a bacteriocin-producing Enterococcus faecium
as a co-culture in Cheddar cheese manufacture
M.R. Foulquie Moreno a,b, M.C. Rea b, T.M. Cogan b, L. De Vuyst a,*
´
a
Research Group of Industrial Microbiology, Fermentation Technology and Downstream Processing (IMDO),
Department of Applied Biological Sciences, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050, Brussels, Belgium
b
Dairy Products Research Centre, Moorepark, Teagasc, Fermoy, Cork, Ireland
Received 25 November 2001; received in revised form 28 March 2002; accepted 25 April 2002

Abstract

Two strains, Enterococcus faecium RZS C5 and E. faecium DPC 1146, produce listericidal bacteriocins, so-called enterocins.
E. faecium RZS C5 was studied during batch fermentation in both a complex medium (MRS) and in milk to understand the
influence of environmental factors, characteristic for milk and cheese, on both growth and bacteriocin production. Fermentation
conditions were chosen in view of the applicability of in situ enterocin production during Cheddar cheese production. Enterocin
production by E. faecium RZS C5 in MRS started in the early logarithmic growth phase, and enterocin activity decreased during
the stationary phase. The effect of pH on enterocin production and decrease of activity was as intense as the effect on bacterial
growth. Higher enterocin production took place at pH 5.5 compared with pH 6.5. The use of lactose instead of glucose increased
the production of enterocin, and at higher lactose concentration, production increased more and loss of activity decreased. The
production in skimmed milk compared to MRS was lower and was detected mainly in the stationary phase. When casein
hydrolysate was added to the milk, enterocin production was higher and started earlier, indicating the importance of an additional
nitrogen source for growth of E. faecium in milk. For co-cultures of E. faecium RZS C5 with the starters used during Cheddar
cheese manufacture, no enterocin activity was detected during the milk fermentation. Furthermore, the applicability of E. faecium
RZS C5 and E. faecium DPC 1146 strains was tested in Cheddar cheese manufacture on pilot scale. Enterocin production took
place from the beginning of the cheese manufacturing and was stable during the whole ripening phase of the cheese. This
indicates that both an early and late contamination of the milk or cheese can be combated with a stable, in situ enterocin
production. The use of such a co-culture is an additional safety provision beyond good manufacturing practices.
D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Enterococcus faecium; Bacteriocin; Enterocin; Cheddar; Cheese

1. Introduction consumers’ demand for healthy, safe and fresh food
(Smith, 1993). However, chemicals such as potassium
In the last decade, the interest in natural preserva- nitrate are still used in cheesemaking to prevent late
tives is increasing, which is in accordance with the gas formation by Clostridium tyrobutyricum during
cheese ripening. On the other hand, the risk of Listeria
*
Corresponding author. Tel.: +32-2-6293245; fax: +32-2-
spp. contamination of milk and cheese must be mini-
6292720. mised to achieve a zero tolerance policy. The use of
E-mail address: ldvuyst@vub.ac.be (L. De Vuyst). bacteriocinogenic lactic acid bacterial strains as starter

0168-1605/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 1 6 0 5 ( 0 2 ) 0 0 1 6 7 - 8

74 ´
M.R. Foulquie Moreno et al. / International Journal of Food Microbiology 81 (2003) 73–84

or co-cultures is a promising alternative in food fer- pounds (Jensen et al., 1975a; 1975b; 1975c; Trovatelli
mentation processes like cheese manufacture, both to and Schiesser, 1987; Coppola et al., 1988, 1990;
prevent late blowing and to combat Listeria spp. (De Parente et al., 1989; Ledda et al., 1994; Villani and
Vuyst, 2000). Coppola, 1994; Centeno et al., 1996, 1999). On the
Bacteriocins are ribosomally synthesised, extrac- other hand, enterococci are less proteolytic compared
ellularly released, antibacterial peptides (De Vuyst to lactococci and lactobacilli (Jensen, et at., 1975b).
and Vandamme, 1994). They display a limited inhib- Hence, they may play an important role as co-culture
itory spectrum encompassing Gram-positive bacteria, in milk fermentations for cheese manufacture, provid-
in particular closely related strains. Large numbers of ing the cheese with both an organoleptic and safety
bacteriocins are produced by all genera of the lactic advantage.
acid bacteria (De Vuyst and Vandamme, 1994; Jack et E. faecium RZS C5 is a natural cheese isolate, which
al., 1995). In general, bacteriocins produced by Enter- produces an enterocin with high antilisterial activity
ococcus faecium and E. faecalis are small, hydro- (Vlaemynck et al., 1994). In this paper, enterocin
phobic and thermostable peptides with an interesting production by E. faecium RZS C5 during batch fer-
technological potential (Giraffa, 1995). Indeed, an mentation is studied in both a complex medium and in
increasing number of enterococcal bacteriocins milk. The aim of the study is to investigate the
against foodborne pathogens such as Listeria spp. influence of environmental factors, characteristic of
and Clostridium spp. has been reported during the milk and cheese (Cheddar), on the kinetics of both
´
last decade (Ben Embarek et al., 1994; Farıas et al., growth and bacteriocin production. Therefore, fermen-
1994; Vlaemynck et al., 1994; Giraffa et al., 1994, tation conditions have been chosen in view of the
1995a,b; Maisnier-Patin et al., 1996; Vlaemynck, applicability of in situ enterocin production by E.
1996; Bennik et al., 1999; McAuliffe et al., 1999; faecium RZS C5 during Cheddar cheese production.
Mendoza et al., 1999). Even activity towards Gram- Furthermore, this bacteriocin-producing strain is tested
negative bacteria like Vibrio cholerae has been shown as well as the bacteriocin-producing strain E. faecium
(Simonetta et al., 1997). DPC 1146 in Cheddar cheese manufacture on pilot
Studies on in vitro enterocin production through scale.
fermentation by E. faecium are scarce, except for
enterocin 1146, whose production has been studied
in detail (Parente and Hill, 1992a,b,c; Parente and 2. Materials and methods
Ricciardi, 1994; Parente et al., 1997). Enterocin 1146
has a rapid bactericidal effect on Listeria monocyto- 2.1. Bacterial strains and media
genes in buffer systems, broth and milk.
The application of several enterocins, either as food E. faecium RZS C5 (deposited as E. faecium FAIR
additive or through in situ production by an appropri- E-171 in the Laboratory of Microbiology (LMG) Gent
ate starter culture or co-culture during fermentation, Culture Collection, Gent, Belgium) and E. faecium
has been studied in cheese production (Sulzer and DPC 1146 were used as bacteriocin-producing strains.
Busse, 1991; Giraffa, 1995; Giraffa et al., 1995b; E. faecium FAIR E-171 has been shown to be a safe
´˜ ´
Joosten et al., 1995; Nunez et al., 1997; Farıas et al., strain, i.e. vancomycin-sensitive and cytolysin-nega-
1999), and in other food products (Laukova and ´ tive (unpublished results). The enterocin-sensitive
´
Czikkova, 1999; Aymerich et al., 2000; Callewaert et Listeria innocua LMG 13568 strain was used as an
al., 2000). It turned out that enterocins are able to indicator for bacteriocin production. The bacteriocin-
reduce the Listeria count by 2 to 9 log cycles, depend- producing and the indicator strains were propagated at
ing on the product and enterocin tested. However, less 37 (or 21) and 30 jC, respectively, in de Man-Rogosa-
is known about the kinetics of bacteriocin production Sharpe (MRS) and Brain Heart Infusion (BHI) broth,
in food ecosystems. Furthermore, it has been postu- respectively (Oxoid, Basingstoke, United Kingdom).
lated that enterococci play an important role in the Lactococcus lactis subsp. cremoris 223 and L. lactis
ripening of different cheeses, due to their proteolytic subsp. cremoris 227 from Christian Hansen Labora-
and lipolytic activities and production of flavour com- tories (Hørsholm, Denmark) were used as commercial

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M.R. Foulquie Moreno et al. / International Journal of Food Microbiology 81 (2003) 73–84 75

cheese starter cultures. Stock cultures of all strains sterilised in situ at 121 jC for 20 min. The energy
were kept in the appropriate medium with 25% (v/v) source was sterilised separately, and aseptically added
glycerol, and stored at À80 jC. Strains were propa- to the fermentor. The fermentor was inoculated with
gated twice in the appropriate broth before use as 1% (v/v) of a log phase culture of the enterocin-
fresh bacteriocin-producing or indicator culture. Cus- producing strain. The fermentor was run at a constant
tomised MRS and milk media were used to study the temperature of 21 or 37 jC, without aeration. Slow
influence of physical and chemical factors on cell agitation (50 rpm) was maintained to keep the fer-
growth and bacteriocin production by E. faecium RZS mentation broth homogeneous. The pH value (5.5 or
C5. The composition of these media is presented in 6.5) was kept constant by automatic addition of 10 M
Table 1. Agar media were prepared by addition of NaOH. The fermentation conditions tested are listed
1.5% granulated agar (Oxoid) to the broth medium; in Table 1. At regular intervals, samples were asepti-
overlay agar media contained 0.7% granulated agar. cally withdrawn from the fermentor. Cell number
(colony forming units or CFU mlÀ1), biomass (grams
2.2. Bacteriocin production kinetics in batch fermen- of cell dry mass or CDM lÀ1), and bacteriocin activity
tations (activity units or AU mlÀ1) were determined for each
sample. Biomass was determined gravimetrically after
The kinetics of cell growth and bacteriocin pro- membrane filtration (0.45-Am pore-size filters, type
duction by E. faecium RZS C5 were studied in a 15 L HA; Millipore, Bedford, MA) of a known volume of
BiostatR C fermentor (B. Braun Biotech International, fermentation liquor followed by washing the filter
Melsungen, Germany), containing 10 l of medium. with demineralized water and drying it overnight at
The medium was adjusted to pH 5.5 or 6.5, and 105 jC. The maximum specific growth rate (lmax)

Table 1
Fermentation conditions tested for bacteriocin production by E. faecium RZS C5
Fermentation conditions lmax Xmax tstationary Bmax tBmax
(hÀ1)a (g lÀ1)b (h)c (AU mlÀ1)d (h)e
MRS (glucose, 2.0%), 37 jC, uncontrolled pH 1.0 0.8 7 1200 2–8
MRS (glucose, 2.0%), 37 jC, controlled pH 6.5 0.9 2.2 6 2400 3–4
MRS (glucose, 2.0%), 37 jC, controlled pH 5.5 0.3 1.1 14 3200 12 – 32
MRS (glucose, 2.0%), 21 jC, controlled pH 5.5 0.1 1.2 20 2400 48 – 52
MRS (lactose, 2.0%), 37 jC, controlled pH 6.5 0.8 2.3 7 4800 4 – 11
MRS (lactose, 2.0%) without peptone and Lab Lemco, 0.8 1.8 9 4800 4–6
casein hydrolysate (1.8%), 37 jC, controlled pH 6.5
MRS (lactose, 5.0%) without peptone and Lab Lemco, 0.8 2.5 10 9600 15 – 99
casein hydrolysate (1.8%), 37 jC, controlled pH 6.5
MRS (lactose, 2.0%) without peptone and 0.9 2.2 9 6400 7 – 11
Lab Lemco, casein hydrolysate (1.8%), NaCl (2.0%),
37 jC, controlled pH 6.5
Skim milk, casein hydrolysate (1.8%), nm nm nm 1200 49
37 jC, controlled pH 6.5
Skim milk, 37 jC, controlled pH 6.5 nm nm nm 600 71
Skim milk, 37 jC, free pH nm nm nm 200 56
Skim milk, Cheddar conditions nm nm nm 150 61
Skim milk, Cheddar conditions, starter cultures (L. lactis) nm nm nm 0 nr
nm=not measured. nr=not relevant.
a
lmax=maximum specific growth rate.
b
Xmax=maximum biomass obtained (g of cell dry mass or CDM lÀ1).
c
tstationary=time at which was reached the stationary phase.
d
Bmax=maximum enterocin activity measured.
e
tBmax=time at which the Bmax was obtained.

76 ´

was calculated as the derivative of the linear regres- temperature for cheesemaking was 32 jC. Rennet was
sion obtained for the logarithm of the biomass as a added (Chr. Hansen Laboratories’ Chymax Plus, 0.18
function of time. The bacteriocin activity was quanti- ml lÀ1 diluted in 400 ml of sterile water) 30 min after
fied through a twofold serial dilution by the agar spot the starter addition, followed by stirring of the milk
technique (see below), using L. innocua LMG 13568 for 3 min. The rennet was allowed to set for 40 min
as indicator strain. For reproducibility reasons, four before cutting, and the curd was allowed to rest for 10
fermentations (those with glucose) were carried out min after cutting. The curd was then stirred and
twice. cooked to a maximum of 38.5 jC (maximum scald)
at a rate of 1 jC every 5 min. Growth of the starter
2.3. Bacteriocin production during fermentation si- was monitored by pH. At pH 6.18, the whey was
mulating Cheddar cheese production drained. The curd was stacked and allowed to Ched-
dar until the pH reached a value of approximately
To prepare a preculture, 0.1% (v/v) of a fresh E. 5.35, at which time it was milled and salted at a rate of
faecium RZS C5 culture was inoculated in separate 2.7% NaCl (m/m). After salting, the curd was kept at
lots of 10% heat-treated (30 min at 90 jC) reconsti- room temperature for 15 min; the curd was then
tuted skimmed milk powder (RSM), which were packed in two moulds of 23 kg and pressed overnight
incubated at 21 jC for approximately 16 h. The at 412 kPa. Finally, the cheese was cut in two pieces,
fermentation broth was inoculated with 0.75% (v/v) vacuum-packed, and ripened at 8 jC.
of the previous fermented milk. The initial pH of the Microbiological analyses of milk and cheese sam-
fermentation broth was 6.52. The initial temperature ples were performed as follows. Raw milk and pas-
was 32 jC, and it was kept constant for 1.5 h. The teurised milk of each vat were analysed for aerobic
temperature was then increased at a rate of 1 jC every plate count (Milk Plate Count Agar, MPCA, Merck,
5 min until the temperature reached 38.5 jC. The Darmstadt, Germany; 3 days at 30 jC) and coliforms
incubation continued until the pH reached a value of (Violet Red Bile Agar, VRBA, Merck; 24 h at 30 jC).
6.15. Then the temperature was decreased to 33 jC After addition of the starter and bacteriocin producer
over the next 10 min (1 jC/2 min), and maintained strains, all milks were analysed for coliforms
constant until a pH of 5.2 was achieved. After this (VRBA), enterococci (Kanamycin Aesculin Azide
time, the pH was kept constant with 10 M NaOH. agar, KAA, Merck; 24 h at 37 jC), and lactobacilli
(LactoBacillus Selective agar, LBS agar, Beckton,
2.4. Cheddar cheese production on pilot scale Dickenson and Company, Cockeysville, USA; 5 days
at 30 jC). Curd samples were also analysed at max-
Two Cheddar cheese trials were undertaken. The imum scald, pressing and day 1 for starters (MRS and
first trial consisted of two vats, one control and one L-M17 (M17 with lactose) agar, Difco Laboratories,
with the enterocin producer E. faecium RZS C5, and Detroit, MI, USA; 3 d at 30 jC), enterococci (KAA),
the second trial consisted of four vats, one control, one lactobacilli (LBS) and coliforms (VRBA). Cheese was
with the enterocin producer E. faecium RZS C5 and sampled after 3 days, 1, 2 and 4 weeks, and 3, 6, 9 and
two with the enterocin producer E. faecium DPC 12 months for starters (MRS and L-M17), enterococci
1146. The starter cultures L. lactis subsp. cremoris (KAA) and lactobacilli (LBS).
223 and L. lactis subsp. cremoris 227 were inoculated
into heat-treated (90 jC for 30 min) reconstituted 2.5. Quantitative determination of bacteriocin titres in
(10%, m/v) skim milk, and incubated at 21 jC for liquid samples
16 h. For cheesemaking, the pregrown cultures were
inoculated at 0.75% (m/v) into 450 l of pasteurised Bacteriocin activity in the liquid was determined
(72 jC for 15 s) milk, which had been cooled to 32 by an adaptation of the critical dilution method,
jC. E. faecium RZS C5 and DPC 1146 were subcul- currently used for the assay of bacteriocins, as
tured in 500 ml of MRS broth, grown overnight at 37 reported by De Vuyst et al. (1996b). Briefly, serial
jC, and then inoculated (0.02%, v/v) into the cheese twofold dilutions of cell-free culture supernatant con-
milk at the same time as the starter. The initial milk taining bacteriocin were spotted (10 Al) on agar plates

´

containing overlayers of fresh cultures of a sensitive ´
method (Lopez-Lara et al., 1991). Also, uniform
strain. These overlayer cultures were prepared by cheese pieces were cut using a cork borer (number
propagating fresh cultures to an optical density at 12). Agar plates were prepared with the indicator
600 nm of 0.45, and adding 100 Al of the cell strain L. innocua LMG 13568, and before the agar
suspension to 3.5 ml of overlay agar. Overlaid agar solidified, the cheese pieces were placed on the agar.
plates were incubated for 18 h at the appropriate After incubation at 37 jC for 12 h, the plates were
temperature. The bacteriocin activity was defined as checked for zones of inhibition.
the reciprocal of the highest dilution that demonstra-
ted complete inhibition of the indicator lawn, and was
expressed in activity units (AU) per millilitre of 3. Results
culture medium.
3.1. Enterocin production in modified MRS broth and
2.6. Bacteriocin detection in cheese samples skimmed milk

Bacteriocin activity was measured in frozen cheese The influence of temperature and pH on the
samples of maximum scald, pressing, day 1 and 3, kinetics of growth and bacteriocin production by E.
week 1 and 2, and month 1, 3, 6, 9 and 12. Different faecium RZS C5 was studied during different batch
protocols were used for the bacteriocin extraction and fermentations. The results are summarised in Table 1.
detection from cheese. Trisodium citrate (2.0%, m/v) During batch fermentation at uncontrolled pH and at a
or tetrasodium EDTA (2.0%, m/v) at dilutions of 1:1, controlled pH of 6.5, both at 37 jC, lmax was ap-
1:2, 1:3, 1:4, 1:5 and 1:10 (m/v) were tested as proximately the same. In both cases, bacteriocin pro-
extraction liquors. These sample dilutions were trea- duction started in the early logarithmic growth phase
ted in a stomacher (Seward Medical, London, UK) for (Fig. 1a and 1b). At controlled pH, the biomass in-
15 min. The resulting suspension was either centri- crease was more than twice the value at uncontrolled
fuged 5 min or heat treated for 10 min at 80 jC. The pH, and the bacteriocin activity also increased con-
extracted bacteriocin was tested by the well-diffusion siderably (Table 1). In both fermentations, the max-

Fig. 1. Biomass (n; CDM, g lÀ1), bacteriocin activity (Â; AU mlÀ1), number of cells (E; CFU mlÀ1), and pH (continuous line) during batch
fermentation of E. faecium RZS C5 at 37 jC in MRS containing (a) 2.0% glucose at uncontrolled pH; (b) 2.0% glucose at constant pH 6.5; (c)
2.0% lactose and 1.8% casein hydrolysate (instead of Lab Lemco and peptone) at constant pH 6.5; (d) 5.0% lactose and 1.8% casein hydrolysate
(instead of Lab Lemco and peptone) at constant pH 6.5.

78 ´

Fig. 2. Bacteriocin activity (Â; AU mlÀ1), number of cells (E; E. faecium RZS C5, cfu mlÀ1—n; starters, cfu mlÀ1), pH (continuous line), and
temperature (discontinuous line; jC) during batch fermentation of (a and c) E. faecium RZS C5, and (b and d) E. faecium RZS C5 and starter
cultures, in skim milk with the pH and temperature profiles of Cheddar cheese production.

imum activity was obtained early in the exponential pH value and at 21 jC, a similar amount of biomass
growth phase (after about 3 h of fermentation). How- was produced, and after 48 h of fermentation the
ever, a higher decrease of bacteriocin activity was maximum bacteriocin activity (2400 AU mlÀ1) was
observed during the stationary phase at controlled pH, detected, followed by a considerable decrease at the
compared with uncontrolled pH (Fig. 1a and b). At 37 end of the fermentation (data not shown).
jC and at a lower controlled pH of 5.5, lmax was Two other fermentations at controlled pH 6.5 and
lower, while the biomass amount was approximately 37 jC were performed to mimic milk fermentations
the same as in the former fermentation. The maximum (Table 1). One was carried out with lactose (2.0%, m/v)
bacteriocin activity (3200 AU mlÀ1) was obtained instead of glucose as the energy source (data not
after 12 h of fermentation, and was constant until the shown), and the other was performed with lactose
end of the fermentation (data not shown). At the same and casein hydrolysate instead of meat extract (Lab

Fig. 3. pH profile during Cheddar cheese production of Trial 2: x, control; n, vat-2: E. faecium DPC 1146; E, vat-3: E. faecium RZS C5; Â,
vat-4: E. faecium DPC 1146.

´

Fig. 4. Numbers of enterococci during cheese production. x, control-vat, Trial 1; n, control-vat, Trial 2; E, E. faecium RZS C5, Trial 1; Â, E.
faecium DPC 1146, Trial 2; B, E. faecium RZS C5, Trial 2; , E. faecium DPC 1146, Trial 2.

Lemco) and peptone as complex nitrogen source (Fig.
1c). In both fermentations, the maximum enterocin
activity (4800 AU mlÀ1) was seen after 4 h of fermen-
tation, and in both cases the activity decreased during
the stationary phase. The maximum biomass was 2.3
and 1.8 g CDM lÀ1, respectively, indicating the
importance of a complex nitrogen source for cell
synthesis.
To mimic cheese manufacture much better, salt was
added and the lactose concentration was increased in
two independent fermentations (Table 1). When 2.0%
NaCl was added, a higher decrease in the enterocin
activity was observed as compared with the same
medium without salt. When 5.0% lactose was fer-
mented (Fig. 1d), a higher maximum enterocin acti-
vity (9600 AU ml À1 ) was seen after 15 h of
fermentation compared to the same medium with
2.0% lactose, and it remained constant until the end
of the fermentation.
For the fermentations carried out in skimmed milk,
bacteriocin activity was higher (1200 vs. 600 AU
mlÀ1) and started earlier (after 49 and 71 h, respec-
tively), when casein hydrolysate was added to the
milk. Maximum bacteriocin production was observed
in the late stationary phase, and no decrease of the
bacteriocin activity was seen. At uncontrolled pH,
very low activity was detected (200 AU mlÀ1).

3.2. Bacteriocin production in fermentations simulat-
ing Cheddar cheese production conditions
Fig. 5. Bacteriocin activity of cheese (Trial 1) visualised on plates
containing L. innocua LMG 13568 as indicator strain: (a) following
Two fermentations in 10% skimmed milk were per- the well-diffusion method; (b) placing a piece of cheese on the agar
formed with the temperature and pH profile of Ched- (A, control-vat; B, bacteriocin producer E. faecium RZS C5).

80 ´

dar cheese production. During the first fermentation, These last two fermentations were repeated and the
only the bacteriocin producer strain E. faecium RZS same results were obtained.
C5 was added to the milk (Fig. 2a and c), while in the
second fermentation, the bacteriocin producer strain 3.3. Cheddar cheese production on pilot scale
and the commercial starter cultures for Cheddar cheese
production were included (Fig. 2b and d). When the The follow-up of the cheese production on pilot
temperature was modified during the fermentation scale is represented in Figs. 3 –6. The pH profile of
following the temperature profile of Cheddar cheese cheese production was the same for all vats in both
production, no significant differences were detected in Cheddar cheese production trials. The results for the
the enterocin production, which reached its maximum second trial are shown in Fig. 3. In the first trial,
(150 AU mlÀ1) after 38 h of incubation. However, no 4Â101 CFU/g of enterococci were found in milk of
enterocin activity could be measured when the bacter- the control vat after pasteurisation, which increased to
iocin producer strain E. faecium RZS C5 was grown as 5Â103 CFU/g during cheese manufacture. This num-
co-culture with the commercial starter cultures L. lactis ber remained constant during the ripening. No enter-
subsp. cremoris 223 and L. lactis subsp. cremoris 227. ococci were found in milk of the control vat of the

Fig. 6. Bacteriocin activity of cheese (Trial 2) visualised on plates containing L. innocua LMG 13568 as indicator strain. A, control vat; B,
enterocin producer E. faecium DPC 1146; C, bacteriocin producer E. faecium RZS C5; C, bacteriocin producer E. faecium DPC 1146.

´

second trial, however 4Â101 CFU/g of enterococci produced, and losses were more pronounced than at
were counted after 14 days. The counts of enterococci constant pH 5.5. The specific bacteriocin activity was
in both trials of the vats inoculated with the enterocin higher at pH 5.5 than at pH 6.5, indicating that
producers E. faecium RZS C5 or E. faecium DPC bacteriocin production is enhanced at pH 5.5. These
1146 were approximately the same, increasing from results are in accordance with those obtained previ-
105 to 107 CFU/g during cheese manufacture (Fig. 4), ously for enterocin 1146 by Parente and Ricciardi
after which they remained constant during ripening (1994), where bacteriocin activity displayed a de-
(data not shown). The counts of the starters were the crease in the early stationary phase at pH values
same in the control vat as in the vats inoculated with higher than 4.5. The decrease of activity was ascribed
enterococci, remaining constant during the ripening to the adsorption of the bacteriocin molecules on the
(108 CFU/g). The number of lactobacilli increased cell surface of producer cells and depends on the pH
during ripening (data not shown). of the cell environment, being more pronounced at
Enterocin production was determined by two meth- higher pH (Yang et al., 1992; Parente and Ricciardi,
ods, either bacteriocin extraction of cheese samples 1994; De Vuyst et al., 1996a; Leroy and De Vuyst,
with 2.0% tetrasodium EDTA or 2.0% trisodium 1999). However, with decreasing temperature, the
citrate followed by a well-diffusion method, or plac- adsorption increased. Thus, one can assume that the
ing a piece of cheese on agar plates containing L. enterocin activity measured in the liquid is lower than
innocua LMG 13568 as indicator strain. However, the the total amount really produced. Therefore, this so-
indicator strain was sensitive to tetrasodium EDTA, called bioavailable bacteriocin activity will be most
which was not the case with trisodium citrate. Bacter- probably the determining factor for in situ antibacte-
iocin extraction of cheese samples with trisodium rial action. The use of lactose instead of glucose
citrate, followed by heating or centrifugation of the increased the production of enterocin, and at higher
samples revealed clear inhibition zones for a 1:4 lactose concentrations, enterocin production increased
dilution, indicating a good extraction of the heat- more and adsorption decreased. At present, it is
stable bacteriocin. No inhibition zones were observed difficult to explain this phenomenon. The enterocin
in the product from the control vat (Fig. 5a). When the production in skimmed milk was lower and took place
cheese samples were placed on agar plates containing mainly in the stationary phase, a phenomenon that
the indicator strain, clear inhibition zones were seen could be very interesting in view of the applicability
around the cheese pieces with the bacteriocin-produc- of enterococci in cheese production. When casein
ing strains (Figs. 5b and 6). The antilisterial activity hydrolysate was added to the milk, bacteriocin pro-
was detected from the maximum scald to after 12 and duction was higher and started earlier in the growth
6 months for the first and second trial, respectively, phase, indicating the importance of an additional
indicating production of the enterocin during fermen- nitrogen source for growth of E. faecium in milk.
tation and its persistence upon further cheese manu- This can be explained by the low proteolytic activity
facturing and ripening. of enterococci. For co-cultures of E. faecium RZS C5
and the starters, no enterocin activity was detected
during the milk fermentation, nevertheless, the acid-
4. Discussion ifying activity of the starters did not inhibit the growth
of E. faecium RZS C5 significantly. However, it may
E. faecium RZS C5 and E. faecium DPC 1146 be that the amount of cells (1 log less) resulted in a
produce enterocins with anti-Listeria activity. Enter- nondetectable amount of enterocin produced (see
ocin production by E. faecium RZS C5 in a complex below). This effect was also observed by Giraffa et
medium (MRS) started in the early logarithmic al. (1994, 1995a), who found that the final levels of
growth phase, and bacteriocin activity decreased dur- bacteriocin produced in milk by E. faecium 7C5 in co-
ing the stationary phase. The effect of pH on bacter- culture with starters were lower than in pure cultures.
iocin production and apparent degradation was as One can assume either reduced growth, degradation of
intense as the effect on bacterial growth. At constant the enterocin by proteases derived from the starter
pH 6.5 growth was higher, but less enterocin was cultures, or loss of bacteriocin adsorbed to coagulating

82 ´

milk proteins with the curd. In contrast, when enter- production and stability during cheese manufacturing
ococci were used as co-cultures during the cheese in the case of E. faecium RZS C5 and E. faecium DPC
production, a low but clear inhibition was detected in 1146, because of their high bacteriocin production in
samples taken when the maximum scald was reached laboratory media. Second, during laboratory fermen-
(38.5 jC). Cheese samples placed on agar plates tations, enterocin production took place in the early
containing the indicator strain also showed the pres- exponential growth phase in a complex medium
ence of enterocin in the cheese. Therefore, it is (MRS) and in the late stationary phase in milk. It
assumed that the bacteriocin is produced during could very well be that the bioavailable enterocin
cheese manufacturing and that it is stable during the activity level in milk is only measured when a certain
whole ripening phase of the cheese. Giraffa et al. amount is already bound to milk particles like pro-
´˜
(1995b) and Nunez et al. (1997) observed that enter- teins. During the cheese processing, enterocin produc-
ocin 7C5 and enterocin 4 are stable during the ripen- tion took place in the early logarithmic phase as well,
ing of Taleggio and Manchego cheese, respectively. and it was carried over into the cheese. This indicates
This is in contrast with other studies where a degra- that both an early and late contamination of the milk
dation of the bacteriocin by peptidases present in the or cheese can be combated with a stable, in situ
cheese was seen (Sulzer and Busse, 1991; Nettles and enterocin production. The use of such a co-culture is
Barefoot, 1993). Sulzer and Busse (1991) found that an additional safety provision beyond good manufac-
different enterocins are not stable during the ripening turing practices in cheesemaking.
of Camembert because of the proteases produced by
the cheese mould. In the latter case, Listeria is only
suppressed when the contamination occurs in the early Acknowledgements
´
stage of ripening. Farıas et al. (1999) could not detect
enterocin CRL35 during ripening, although suppres- This work was supported by the FAIR Programme
sion of Listeria took place during the whole period of of the European Commission (grants CT97-3078 and
ripening. Since in Cheddar manufacture, only limited ´ ´
CT97-5013). Marıa Remedios Foulquie Moreno was a
autolysis of starters occurs, and since lysis is then a recipient of a Marie Curie Fellowship from the Com-
gradual process during ripening, the peptidase poten- mission of the European Communities (grant FAIR-
tial in Cheddar is initially low (these being intra- CT97-5013).
cellular enzymes). The stability of enterocins in our
system is therefore as expected and enterocin activity References
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