Journal of Applied Microbiology 2005, 98, 203–209
doi:10.1111/j.1365-2672.2004.02439.x
Survival of viruses on fresh produce, using MS2 as a surrogate
for norovirus
D.J. Dawson1, A. Paish1, L.M. Staffell1, I.J. Seymour1 and H. Appleton2
1
Department of Microbiology, Campden and Chorleywood Food Research Association, Chipping Campden, Gloucestershire, UK, and
Enteric, Respiratory and Neurological Virus Laboratory, Health Protection Agency, Colindale, London, UK
2
2004/0207: received 24 February 2004, revised and accepted 30 July 2004
ABSTRACT
D . J . D A W S O N , A . P A I S H , L . M . S T A F F E L L , I . J . S E Y M O U R A N D H . A P P L E T O N . 2004.
Aims: To study the survival and removal of viruses from fresh fruit and vegetables using the bacteriophage MS2 as
a potential surrogate for noroviruses.
Method and Results: Survival of MS2 in buffer and on fresh produce was studied at 4, 8 and 22C. At 4 and 8C
a reduction of <1 log10 was observed after 50 days in buffer; however a reduction in excess of 1 log10 occurred
within 9 days at 22C. Similar results were obtained with fresh produce with virus survival times exceeding the shelf
life of the produce. In washing experiments, using a chlorine wash (100 ppm), in all but one case <1Æ5 log10 MS2
bacteriophage was removed from fruit and vegetables. The mean across all produce types was 0Æ89 log10. With
potable water, reduction was lower (0Æ3 log mean across all produce types).
Conclusions: MS2 survived for prolonged periods, both in buffer and on fresh produce, at temperatures relevant
to chilled foods. It was not removed effectively by chlorine washing.
Significance and Impact of the Study: Bacteriophage MS2 has been evaluated as a potential surrogate for
noroviruses on fresh produce. Experimental results together with current knowledge of norovirus resistance and
survival indicate that MS2 could be used as an effective surrogate in future evaluations.
Keywords: bacteriophage MS2, chlorine washing, fruit and vegetables, norovirus, virus survival.
INTRODUCTION
Although many viruses can be implicated in gastroenteritis,
it is noroviruses (formerly known as Norwalk-like viruses
or small round structured viruses) that are most commonly
identified in foodborne outbreaks. The Health Protection
Agency (HPA) Communicable Disease Surveillance Centre
has reported that noroviruses account for one-third of all
gastroenteritis outbreaks in England and Wales, mainly by
person-to-person transmission, but with 6% being foodborne (O’Brien et al. 2000). Long et al. (2002) reviewed
outbreaks associated with salad vegetables and fruit in
England and Wales between 1992 and 2000. Of 83
outbreaks, 13 (15Æ7%) were caused by noroviruses.
Correspondence to: David. J. Dawson, Campden and Chorleywood Food Research
Association, Chipping Campden, Gloucestershire GL55 6LD, UK
(e-mail: [email protected]).
ª 2004 The Society for Applied Microbiology
Twenty-three outbreaks (28%) were caused by unknown
agents, but in these the clinical and epidemiological
features suggested that the majority were also caused by
noroviruses. Twenty per cent of these outbreaks were
associated with fruit and vegetables. In the USA, noroviruses are associated with ca 40% of foodborne outbreaks
and 96% of all reported outbreaks of viral gastroenteritis
(Gulati et al. 2001).
Noroviruses are highly infectious and are associated with
both sporadic gastroenteritis in the community and outbreaks in hospitals, residential homes, schools, hotels,
restaurants and cruise ships. Norovirus outbreaks have been
linked to fresh and frozen produce (Herwaldt et al. 1994;
Ponka et al. 1999). Such produce can become contaminated
with noroviruses either during handling and preparation,
often by infected food handlers, or at source in the growing
and harvesting area.
204 D . J . D A W S O N ET AL.
Noroviruses belong to the Caliciviridae family (Green
et al. 2000). There are many different strains with a few
predominant strains circulating at any one time. Detection is
routinely by molecular methods such as PCR or by EIA.
However, because of the great genomic diversity and the
frequent emergence of new strains, these methods do not
necessarily detect all noroviruses. Noroviruses may also be
detected by electron microscopy, but this method is
relatively insensitive. When examined in the electron
microscope the 30–35 nm virus particles appear to have an
amorphous surface with a ragged outline that gave rise to the
earlier name of small round structured viruses (Caul and
Appleton 1982).
Because of the potential for fresh produce to be a vehicle
for the transmission of noroviruses, it is important to
understand the survival characteristics of these viruses on
fresh produce and whether they are effectively removed by
washing/sanitization procedures. A review by Seymour and
Appleton (2001) found that definitive information on the
survival and removal of viruses from fruit and vegetables
was lacking.
It is not possible to grow these viruses in cell culture,
and hence their study is problematic. An alternative to
using the virus itself is to use a model organism or
surrogate which is likely to respond in a similar way to
environmental conditions or sanitizing treatments. One
option is to use a related virus such as feline calicivirus
(FCV) (Slomka and Appleton 1998; Doultree et al. 1999;
Taku et al. 2002) which may have similar properties. An
alternative approach is to use a bacteriophage that does not
require mammalian cell culture facilities for growth and
viability determination. A study by Dore et al. (2000)
showed that an F-specific RNA bacteriophage worked
successfully as an indicator organism for noroviruses in a
study on oyster contamination. However, although bacteriophages have long been used as indicators of water
pollution, similar behaviour in ecological terms does not
necessarily indicate similarities with respect to survival on
or decontamination of fresh produce.
MS2 phage belongs to group I of the RNA coliphages
within the family Leviviridae (Calender 1988). The bacterial
host for MS2 is Escherichia coli, and therefore this
bacteriophage is found most frequently in sewage and
animal faeces. Like noroviruses, MS2 is adapted to the
intestinal tract, it is a positive sense single-stranded RNA
virus with icosahedral symmetry and is in the same size
range at 26 nm diameter. Bacteriophages have been used
previously to study sensitivity to disinfectants with a view to
modelling properties of human pathogens (Maillard et al.
1994; Shin and Sobsey 2003).
This paper describes a study on the survival properties of
MS2 in buffer and on fresh fruit and vegetable surfaces, and
its removal from fresh produce by washing.
MATERIALS AND METHODS
Methods for culture and purification of MS2 were based
upon those for bacteriophage k described by Sambrook et al.
(1989).
Preparation of bacterial host
The host bacterial strain E. coli K-12 (ATCC 12435) was
grown at 37C in 100 ml of NZYCM broth (10 g NZ amine,
5 g NaCl, 5 g bacto-yeast extract, 1 g casamino acids, 2 g
MgSO4Æ7H2O in 1 l deionized water adjusted to pH 7) to an
optical density at 595 nm which represented a cell density of
108 ml)1. The culture was centrifuged (4000 · g, 10 min)
and resuspended in 0Æ01 mol l)1 MgSO4 to give a cell
density of ca 109 ml)1 and stored at 4C. The suspensions
were used for up to 8 weeks.
Preparation of plate lysate stocks
This technique was routinely used to boost stocks of MS2
bacteriophage.
Thirty to fifty plates were prepared as follows: MS2
bacteriophage (ATCC 15597-B1) >108 PFU ml)1 was
mixed 1 : 1 with 0Æ1 ml of host bacterial suspension for
20 min at 37C. Molten NZCYM agar (3 ml, 0Æ7% w/v)
was added, immediately vortexed and poured into a 90 mm
Petri dish containing set NZCYM agar (1Æ5% w/v). After
overnight incubation at 37C, the plates were removed from
the incubator and 5 ml SM buffer (5Æ8 g NaCl, 2 g
MgSO4Æ7H2O, 50 ml 1 mol l)1 Tris–Cl (pH7Æ5), 5 ml 2%
gelatine in 1 l deionized water) was added. The plates were
stored at 4C for several hours with intermittent, gentle
shaking. A pasteur pipette was then used to harvest as much
SM buffer as possible which was then transferred to a
polypropylene tube (13 · 100 mm). Fresh SM buffer (1 ml)
was added to each plate which was then stored for 15 min in
a tilted position. The remaining SM buffer was then
removed and added to the previous harvest and the plate
discarded. Chloroform (0Æ1 ml) was added to the SM buffer
and following brief vortexing, the tube was centrifuged
(4000 · g, 10 min, 4C). The supernatant was removed and
a drop of chloroform added to it prior to purification.
Purification of MS2 stocks
NaCl was added to the clarified SM buffer harvest to a
concentration of 1 mol l)1 and dissolved by shaking prior to
being left for 1 h on ice. Bacterial debris was removed by
centrifugation at 11 000 · g for 10 min at 4C.
Polyethylene glycol (PEG8000; Aldrich, Gillingham, UK)
was added to pooled supernatants in a clean flask to a final
concentration of 10% (w/v) and allowed to dissolve slowly.
ª 2004 The Society for Applied Microbiology, Journal of Applied Microbiology, 98, 203–209, doi:10.1111/j.1365-2672.2004.02439.x
MS2 AND FRESH PRODUCE
The solutions were then cooled on ice for at least 1 h (or
alternatively overnight at 4C) to allow precipitation of the
bacteriophage particles.
The precipitated bacteriophage particles were concentrated by centrifugation (11 000 · g, 10 min, 4C). The
supernatant was discarded and the centrifugation bottle left
in a tilted position to allow remaining fluid to drain away
from the pellet. This fluid was then removed.
The pellet was resuspended in SM buffer at a dilution rate
of 10 ml per 500 ml of the supernatant being purified. The
centrifuge bottle was additionally rinsed with 2 ml of SM
buffer.
The PEG and cell debris were separated from the
bacteriophage by addition of an equal volume of chloroform
followed by vortexing for 30 s. The aqueous and organic
phases were separated by centrifugation at 3000 · g for
15 min. The aqueous phase (containing the bacteriophage
particles) was recovered and centrifuged again (3000 · g,
15 min). This process was repeated until only the aqueous
phase remained, which was recovered and stored at 4C.
Plaque assay technique – MS2 bacteriophage
From each MS2 bacteriophage sample to be assayed, a 10fold dilution series was created using SM buffer. From each
dilution, 0Æ1 ml was dispensed into sterile polypropylene
tubes (13 mm · 100 mm). To each tube, 0Æ1 ml of bacterial
host (E.coli) was added. Each tube was then vortexed and
placed in an incubator at 37 ± 1C for 20 min to allow the
bacteriophage to adsorb the bacteria.
Following this, 3 ml molten NZCYM (0Æ7% w/v) top agar
was added to each tube and immediately vortexed. The
resulting mixture was poured onto pre-prepared NZCYM
(1Æ5% w/v) base plates and swirled without delay to ensure
total coverage. Once set, the plates were placed into an
incubator at 37 ± 1C for 16–24 h. Plaques were counted
and the original density of PFU in the sample was calculated.
205
produce (100 g or 10 g for the survival work) was placed
in a polypropylene bag and inoculated with 1 ml of
bacteriophage with an inoculum titre of 108 PFU ml)1.
The bag was then heat sealed and shaken 30 times before
being stored overnight at 4C. Gentle shaking (more of a
rolling action for soft fruit) ensured that the produce was
not damaged. The produce was left in the virus
containing diluent but in every case except for tomatoes
all the liquid was absorbed into the produce overnight.
This inoculation technique had been shown to give good
uptake of virus in preliminary studies with recoveries after
inoculation similar for the different produce types (data
not shown).
Recovery/enumeration of bacteriophage from
fresh produce
Samples (10 g) were removed to determine the levels of
bacteriophage. The samples were processed with 100 ml SM
buffer in a stomacher (Seward 400, Thetford, UK) at
normal speed for 60 s. Bacteriophage levels in the buffer
were then enumerated as described above.
Decontamination of MS2 after washing with
chlorinated/unchlorinated water
Five replicate 100 g samples of produce (spiked as described
above) were washed in a beaker containing 1 l of autoclaved
tap water with or without an initial concentration of
100 ppm free chlorine, measured using a DPD based
chlorine measurement kit (Palintest, Gateshead, UK)
according to the manufacturer’s instructions. The wash
water was agitated by manual stirring at moderate speed and
the wash period was 5 min in all cases. The initial
bacteriophage titre was estimated using 2 · 10 g samples
removed from separate bags.
This whole process was repeated with a separate batch of
vegetables spiked on a different occasion (wash 2).
Preparation of vegetables and fruit
Fresh produce items (iceberg lettuce, baton carrot, cabbage,
spring onion, curly leaf parsley, capsicum pepper, tomato,
cucumber, raspberries and strawberries) were purchased
from local supermarkets. Raspberries of suitable quality
were only available for the survival work at 22C. Fruits and
vegetables were either left whole, or trimmed and sliced by
hand using aseptic techniques. All produce was stored at
4C prior to treatment.
Vegetable inoculation technique
The bacteriophage inoculation method followed one
developed for bacteria by Zhang and Farber (1996). The
Survival of MS2 in buffer at different temperatures
From the MS2 stock, a 10-fold dilution series using SM
buffer was made to obtain a concentration of ca
106 PFU ml)1. These dilutions of 106 PFU ml)1 were
dispensed separately in 1 ml amounts into three sets of
20 · 2 ml polypropylene microcentrifuge tubes. One set
was maintained at 4C, one at 8C and the final set at
22C.
In order to assess the viability of MS2 phage, one vial
from each set was removed at predetermined intervals and
enumerated using four replicate determinations. The initial
sampling was conducted at frequent intervals, but as a trend
emerged the frequency was reduced.
ª 2004 The Society for Applied Microbiology, Journal of Applied Microbiology, 98, 203–209, doi:10.1111/j.1365-2672.2004.02439.x
206 D . J . D A W S O N ET AL.
Survival of MS2 on fresh produce
Produce was inoculated as described above and stored at 4, 8
and 22C. Ten gram samples were used, with the exception
of parsley where only 1 or 2 g per bag was used, because of
the large volume. At intervals 2 · 10 g amounts were used
for determination of the bateriophage counts.
RESULTS
Log 10 reduction (PFU)
3·0
2·5
2·0
1·5
1·0
0·5
Survival in buffer
Bacteriophage stored at 4 and 8C showed similar survival
characteristics with <1 log10 decline in the first 50 days.
However, at 22C there was almost a 5 log10 reduction
within the same period and more than 1 log10 decline in the
first 9 days. No bacteriophage was detected after day 50
from an initial titre of nearly 107 (Fig. 3).
Strawberry
Parsley
Cabbbage
Carrot
Spring onion
Lettuce
Pepper
Fig. 2 Reduction of MS2 on fresh produce in the presence of
100 ppm free chlorine on two separate occasions each based upon five
replicate washings
8
7
Log10 PFU ml–1
Figures 1 and 2 show reduction with washing on separate
occasions using unchlorinated or chlorinated water. The
means and standard deviations of reductions in five replicates after washing are shown.
Reduction at laboratory scale with water (Fig. 1) was in
the range 0Æ08–0Æ79 log10. Apart from two sets of washes of
tomatoes and one set of strawberries, reductions were all
below 0Æ4 log10. The mean reduction across all produce
types and both washes was 0Æ3 log10.
Reduction at laboratory scale with chlorine (Fig. 2) was
higher than that with water. Mean reduction across all
produce types and both washes was 0Æ89 log10. The range of
reductions with 100 ppm free chlorine was 0Æ30–2Æ14 log10.
Tomato
Cucumber
0·0
Decontamination experiments
6
5
4
3
2
1
0
0
20
40
60
80
100 120 140 160 180 200
Time (days)
Fig. 3 Survival of MS2 in buffer at three temperatures: 4C (¤) 8C
(j) and 2C (m)
Log 10 reduction (PFU)
3·0
2·5
Survival on fruits and vegetables
2·0
1·5
1·0
Strawberry
Parsley
Cabbbage
Carrot
Spring onion
Lettuce
Pepper
Tomato
0·0
Cucumber
0·5
Fig. 1 Reduction of MS2 on fresh produce using potable water on two
separate occasions each based upon five replicate washings
Figures 4–6 show survival of MS2 on fresh produce at 4, 8
and 22C.
At 4C, all produce types showed a reduction of <1 log10
over the first 7 days. By day 87 the reduction remained
<2 log10 in cabbage and carrots (which were in sufficiently
good condition to allow continued sampling). The response
is similar to that in buffer at 80 days.
At 8C a slight drop in viral titre was observed by day 7
(apart from carrots). Approximately 1 log10 decline in
tomato, cabbage, carrots and lettuce was observed by day
39. Parsley showed ca 1Æ5 log10 reduction in this period.
At 22C, all produce types tested showed a decline in
bacteriophage count after 7 days. On lettuce the reduction
ª 2004 The Society for Applied Microbiology, Journal of Applied Microbiology, 98, 203–209, doi:10.1111/j.1365-2672.2004.02439.x
MS2 AND FRESH PRODUCE
was 1 log10 while on tomato and parsley it was minimal.
Further decline in titre was observed on some produce types
but other produce became spoiled. Data for cabbage and
carrots were available up to day 43 and show a similar rate of
decline to that of the bacteriophage titre in buffer.
8
Log10 PFU g–1
7
6
DISCUSSION
5
4
3
0
20
40
60
80
100
Time (days)
Fig. 4 Survival of MS2 on various produce types at 4C. Tomato
(¤), cabbage (j), carrot (m), lettuce (·), parsley ( ), pepper (d) and
strawberry (+)
8
Log10 PFU g–1
7
6
5
4
3
0
20
40
60
80
100
Time (days)
Fig. 5 Survival of MS2 on various produce types at 8C. Tomato
(¤), cabbage (j), carrot (m), lettuce (·), parsley ( ), pepper (d) and
strawberry (+)
8
Log10 PFU g–1
7
6
5
4
3
207
0
20
40
60
Time (days)
80
100
Fig. 6 Survival of MS2 on various produce types at 2C. Tomato
(¤), cabbage (j), carrot (m), lettuce (·), parsley ( ), pepper (d),
strawberry (+) and raspberry ())
This study represents an attempt to determine the retention
and survival properties of a potential analogue for noroviruses on fresh produce.
Noroviruses are known to be highly resistant to inactivation in the environment (Sellwood et al. 1999; Appleton
2000). MS2 was shown in the buffer survival work to be
stable for prolonged periods, particularly at refrigeration
temperatures. The data obtained for MS2 survival on fresh
produce show similar patterns to the studies in buffer, but
are more variable, probably because of the problems in
recovering virus from the various fruits and vegetables. In
addition, the deterioration in the plant material, particularly
at 22C, meant that these trials did not last so long, but
generally virus survival exceeded the shelf life of the
product. Bidawid et al. (2001) suggested that on plant
surfaces, compounds such as phenolics, ethanol and acetaldehyde could also accelerate loss of infectivity.
Survival studies of noroviruses on fresh produce are
lacking because of the difficulties in working with the virus,
but data have been produced for other viruses. Seymour and
Appleton (2001) reviewed earlier published information
relating to survival of viruses on fruits and vegetables. They
concluded that most studies report viability in excess of
product shelf life. Kurdziel et al. (2001) studied survival of
poliovirus type 1 on lettuce, green onions, white cabbage,
strawberries and raspberries. Most studies were performed
for a period of ca 2 weeks at refrigeration temperatures.
Using linear regression analysis, the decline in viral titre was
quantified and the time for 1 log10 (90%) reduction
calculated. No decline was determined for green onions or
fresh raspberries, while 90% reduction had occurred at
11Æ6 days on lettuce, 14Æ2 days on white cabbage and
8Æ4 days on frozen strawberries. The results from the
present study appear to be similar.
The degree of reduction of MS2 in the experimental
decontamination work described here from raw vegetables is
either similar to or less than that achieved for bacteria (1–
2 log10) as described by Beauchat (1998). Within the five
washing replicates the coefficient of variation was in general
relatively small (<10%), illustrating that virus attached at
the same time to the same produce batch did not give rise to
variable results. It was noted, however, that in some repeat
experiments with different vegetable batches there could be
marked variation: this probably reflects that vegetables
cannot be completely standardized, and the surface condi-
ª 2004 The Society for Applied Microbiology, Journal of Applied Microbiology, 98, 203–209, doi:10.1111/j.1365-2672.2004.02439.x
208 D . J . D A W S O N ET AL.
tions may have been different in the different batches,
resulting in stronger or weaker viral attachment. This factor
must be taken into account when considering decontamination on an industrial scale. Decontamination of naturally
attached viruses is if anything likely to be more difficult and
more variable than was shown by our data. In the natural
environment, viruses may be complexed with soil particles
or protein material, which may also affect their adsorption
and attachment.
Reduction of MS2 appears to be more efficient using
chlorinated water than washing with water alone. However,
considerable numbers of virus particles remain after what
might be considered a standard washing regime. Reduction
in virus counts in the washing experiments is partly the
result of physical removal and partly because of the presence
of free chlorine, as evidenced by the greater reduction when
free chlorine was used. Re-contamination of produce with
viable virus from the wash water is a possibility in the
absence of free chlorine.
Planktonic MS2 bacteriophage was shown by Stagg et al.
(1977) to be inactivated (99% kill) by 0Æ6 ppm chlorine in
7 s. Clearly disinfection is less effective than this on surfaces
as is well established in the case of bacteria (Sommer et al.
1999). In our work, some MS2 inactivation was achieved
with a 100 ppm chlorine wash for 5 min but not to the
degree that would be expected in suspension.
Susceptibility of FCV, another member of the Caliciviridae, to two hypochlorite based disinfectants is described by
Doultree et al. (1999). According to these authors, 100 ppm
concentrations of available chlorine gave 1Æ75 log10 reduction in 1 min. It is not clear whether this was the final
concentration of free chlorine that the virus was exposed to
because proteins present in the tissue culture medium would
have significantly reduced the free chlorine level. Gulati
et al. (2001) examined the effect of disinfectants on FCV
seeded onto strawberry, lettuce and stainless steel. To obtain
an effect on stainless steel surfaces above that achieved with
water, 200 ppm free chlorine was required with 10 min
contact time: this gave an additional 0Æ3 log10 reduction
above water alone. On strawberries and lettuce an additional
effect of chlorine (1 log10 and 1Æ5 log10 respectively) could
only be seen using 800 ppm with 10 min contact time.
However, the reduced efficacy compared with results for
stainless steel can probably be explained by the high ratio of
produce to water in the wash, i.e. 100 g produce per 100 ml
of water. The amount of organic material present would
have reduced the free chlorine level to a greater degree than
in the work we describe, where the ratio is 100 g of produce
per litre.
MS2 and FCV show similar susceptibility to each other
with other disinfectants. Maillard et al. (1994) showed that a
4 log reduction of MS2 titre could be achieved in 20 min
with 0Æ5% glutaraldehyde while 70% ethanol removed over
3 log in the same time period. In the case of FCV, in 1 min a
5 log inactivation was achieved with 0Æ5% glutaraldehyde
and a 1Æ25 log reduction with 75% ethanol (Doultree et al.
1999). It is not known, however, whether MS2 and FCV in
the environment behave in a similar way or whether they
adequately predict survival of noroviruses. Although FCV is
related to noroviruses, it is a respiratory virus and is not
adapted to the gastrointestinal tract (unlike MS2), therefore
it may well be more fragile than norovirus. However,
survival of MS2 both in buffer and on fresh produce was
similar to that in other parallel experiments with human
enteric viruses (poliovirus and rotavirus – data not shown).
Like noroviruses, both poliovirus and rotavirus survive for
prolonged periods in the environment (Sellwood et al.
1999), as does MS2 in the current study.
Maillard (1998) reviewed the viral sites targeted by
chlorine compounds. The specific critical effect of the
chlorine is probably dependent upon the particular virus
concerned. Non-enveloped viruses such as MS2 bacteriophage are less readily inactivated by disinfectants than
enveloped viruses (Madigan et al. 2000). In non-enveloped
viruses structural alteration of the capsid is one possibility,
including oxidation of SH groups within protein sequences.
Another possibility is direct alteration of the viral genome.
Shin and Sobsey (2003) used quantitative reverse transcription (RT)-PCR alongside infectivity to assess reduction of
Norwalk virus, MS2 and poliovirus in water by ozone
disinfection. All three viruses showed similar reduction to
each other by RT-PCR assays and, in the case of MS2 and
poliovirus, the reductions were similar to those seen using
infectivity assays. There is concern in relying on PCR assays
for measuring survival because PCR does not necessarily
detect viable infectious virus. However, these authors
concluded from their experiments that RT-PCR is a reliable
surrogate assay for both culturable and nonculturable viruses
disinfected with ozone.
In the absence of a method for culturing noroviruses, it
will be impossible to determine whether surrogate techniques such as RT-PCR or model viruses such as MS2
adequately predict survival of noroviruses under all conditions. The present study only gives useful information
relevant to noroviruses if they have similar properties to
MS2. Currently it is only feasible to make direct observations about MS2 and other culturable viruses. It is possible
that under certain conditions, a model virus predicting the
response of noroviruses will be a better option than RTPCR. The mechanisms by which a virus degrades and loses
infectivity are uncertain. The infection process of the
bacteriophage uses enzymes, which may lose stability over
time. In addition the capsid proteins, important in recognition of the host and protection of the nucleic acid, could
degrade during the same period. Either of these changes
could result in a decline in infectivity.
ª 2004 The Society for Applied Microbiology, Journal of Applied Microbiology, 98, 203–209, doi:10.1111/j.1365-2672.2004.02439.x
MS2 AND FRESH PRODUCE
ACKNOWLEDGEMENTS
The authors acknowledge the financial support of the Food
Standards Agency for this study (Project: B02014).
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