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Applied and Environmental Microbiology, January 2006, p. 478-483, Vol. 72, No. 1
0099-2240/06/$08.00+0 doi:10.1128/AEM.72.1.478-483.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Real-Time Fluorogenic Reverse Transcription-PCR Assays for Detection of Bacteriophage MS2
Kevin P. O'Connell,*
Jennifer R. Bucher,
Patricia E. Anderson,
Cheng J. Cao,
Akbar S. Khan,
Mark V. Gostomski, and
James J. Valdes
Research and Technology Directorate, U.S. Army Edgewood Chemical Biological Center, Aberdeen Proving Ground, Maryland 21010
Received 11 April 2005/
Accepted 29 September 2005

ABSTRACT
Bacteriophage MS2 is used in place of pathogenic viruses in
a wide variety of studies that range from testing of compounds
for disinfecting surfaces to studying environmental transport
and fate of pathogenic viruses in groundwater. MS2 is also used
as a pathogen simulant in the research, development, and testing
(including open air tests) of methods, systems, and devices
for the detection of pathogens in both the battlefield and homeland
defense settings. PCR is often used as either an integral part
of such detection systems or as a reference method to assess
the sensitivity and specificity of microbial detection. To facilitate
the detection of MS2 by PCR, we describe here a set of real-time
fluorogenic reverse transcription-PCR assays. The sensitivity
of the assays (performed with primer pairs and corresponding
dye-labeled probes) ranged from 0.4 to 40 fg of MS2 genomic
RNA (200 to 20,000 genome equivalents). We also demonstrate
the usefulness of the primer pairs in assays without dye-labeled
probe that included the DNA-binding dye SYBR green. None of
the assays gave false-positive results when tested against 400
pg of several non-MS2 nucleic acid targets.

INTRODUCTION
Investigation and production of methods, reagents, and devices
for the detection of pathogens require a subject for testing
at each phase of technology development. For certain kinds of
tasks, such as the development of antibodies that are specific
for a pathogen of interest or the determination of assay specificity,
usually no organism other than the pathogen to be detected can
reasonably be substituted. However, there are phases of technology
development in which the use of live, virulent pathogens is
not ethically permissible, economically feasible, or politically
palatable. For example, it is not permissible now to produce
and disseminate aerosols of pathogens out of doors for the purposes
of testing a complete pathogen detection system or its components.
For such open air work, and in preliminary developmental benchwork
such as the engineering of particle collectors and fluidics
systems, the development of generalized PCR protocols, or the
testing of surface or water decontamination methods, nonpathogenic
organisms with generally similar physical and biological characteristics
are routinely substituted for more dangerous bacteria or viruses.
Such organisms or proteins are termed "simulants" or "surrogates"
in the biological defense research community. Examples of simulants
that are used widely in biological defense research include
nonpathogenic
Bacillus spp. (
B. atrophaeus,
B. subtilis, and
B. globigii) (
7) in place of
Bacillus anthracis,
Pantoea agglomerans (formerly
Erwinia herbicola) in place of pathogenic gram-negative
species (such as
Yersinia pestis and
Francisella tularensis),
hen egg ovalbumin in place of protein toxins such as ricin and
botulinum toxin, and the enteric bacteriophage MS2 in place
of pathogenic viruses.
Bacteriophage MS2 (family Leviviridae) is a small, icosahedral, male-specific bacteriophage of Escherichia coli (4). The MS2 genome is comprised of 3,569 bases of single-stranded RNA that encode four proteins (assembly, lysis, coat, and RNA replicase [ß chain] proteins) (Fig. 1). The small size of MS2 virions, their simple structure, their RNA genome, and harmlessness to humans, animals, plants, and other higher organisms have made them useful as simulants in place of small RNA viruses (such as Ebola virus, Marburg virus, and the equine encephalitis alphaviruses). Viral simulants in biological defense studies should approximate human pathogens in genome size, nucleic acid composition, and virion size and be nonpathogenic, easy and economical to culture, and easy to detect using common laboratory methods. Bacteriophage MS2 is not a perfect match in these regards to any of the small RNA viruses mentioned above, but has long been used in the biological defense community because it has proved to be a reasonable technical and practical compromise.
MS2 has been used in place of pathogens in a large number of
studies: for examining the survival of viruses on produce (
1,
11), developing systems and methods to remove viruses from water
and surfaces (
5,
9,
10,
12,
13,
17-
20,
23-
25,
29,
30,
32-
34,
42), and investigating the fate of pathogens in groundwater
(
2,
14,
15,
26,
31,
36-
39). It has also been used in the testing
and development of systems for the detection of biological warfare
agents (
3,
8,
22,
35,
40,
41).
Depending upon the pathogen detection technology or system under development, reagent sets for the detection of the simulants must be produced in addition to those specific to the pathogens themselves. For systems that rely upon antibody-antigen recognition to confer specificity and sensitivity (35), such reagents include polyclonal, monoclonal, and recombinant antibodies. The use of simulants in the testing of detection systems based on PCR or hybridization to nucleic acid microarrays requires oligonucleotides (primers and dye-labeled probes) whose sequences are complementary to unique sequences found in the genome of the simulant. While Belgrader et al. (3) reported the detection of bacteriophage MS2 in a hand-held real-time PCR thermocycler, they did not report the sequence of the PCR target or the sequences of the primers and probes used. To facilitate the use of bacteriophage MS2 in the development of PCR-based pathogen systems, we describe here five primer and probe sets for real-time fluorogenic reverse transcription (RT)-PCR assays for the detection of bacteriophage MS2.

MATERIALS AND METHODS
Strains, media, and culture conditions.
A seed stock of bacteriophage MS2 and
Escherichia coli host
strain A/

were obtained from the Biosensors Team, U.S. Army
Edgewood Chemical Biological Center, who obtained the original
cultures from the laboratory of D. Peabody, University of New
Mexico. To propagate bacteriophage MS2,
E. coli strain A/

was
grown in liquid Luria broth at 37°C with shaking, to an
optical density of 0.2 to 0.3 (measured spectrophotometrically
at 600 nm). MS2 virions were added at a multiplicity of infection
of approximately 20, and the culture was incubated a further
16 to 18 h. Cell debris was removed by centrifugation and subsequent
filtration through a 0.22-µm filter. Titers of lysates
were determined by mixing aliquots of diluted MS2 preparations
with suspensions of the host strain in molten Luria top agar
(Luria broth, 0.5% Bacto agar), pouring the mixtures onto solid
Luria medium (Luria broth plus 1.5% agar), and counting the
resulting plaques after overnight incubation at 37°C. Lysates
were stored at 4°C. RNA was isolated from virions with the
Trizol reagent (Invitrogen, Carlsbad, CA) according to the manufacturer's
instructions.
PCR assay design and conditions.
Real-time fluorogenic PCR assays were designed with PrimerExpress software (version 1.0; Applied Biosystems, Foster City, CA), using as a template GenBank accession no. NC_001417, the complete genome sequence of bacteriophage MS2. Primer sets were designed with the following constraints: a maximum amplicon length of 150 bp, a melting temperature (Tm) of approximately 58 to 60°C, and no more than two G or C bases among the last five bases on the 3' end (to minimize the likelihood of nonspecific priming at non-target sequences). Probe sequences were chosen with a Tm of approximately 68 to 70°C (10°C higher than the primer Tm), more C's than G's (and no more than 3 consecutive G's) in the overall probe sequence, and no 5'-terminal G. Primer and probe sequences in each assay are shown in Table 1. Primers were purchased from Integrated DNA Technologies (Coralville, IA). Probes (with 5' 6-carboxy-fluorescein [FAM] and 3'-carboxymethylrhodamine [TAMRA]) were purchased from Applied Biosystems. Following the computer-aided design of assays, candidate primer pairs were purchased and the success of PCRs was assessed by examining PCRs for the presence of amplified DNA by agarose gel electrophoresis (29a). Dye-labeled probes were ordered and tested only for those assays observed to yield abundant DNA.
To perform a PCR assay, TaqMan Universal PCR Master Mix, MultiScribe
reverse transcriptase, RNase inhibitor mix, and nuclease-free
water (Applied Biosystems, Foster City, CA) were added to a
reaction mixture (50-µl final volume) according to the
manufacturer's instructions. Primers were added to a final concentration
of 400 nM, and probe was added to a final concentration of 200
nM. To avoid contamination of the reagent stocks, reaction mixtures
were prepared in a hood separate from the hood in which template
samples were added to each reaction mixture. RNA was added in
a volume of 1.0 µl per 50-µl reaction mixture. For
negative (no template) control reactions, nuclease-free water
was substituted for virions or RNA. Assays were performed on
an ABI 7900HT sequence detection system, with a thermocycler
profile of 48°C for 30 min and 95°C for 10 min and then
45 cycles of 95°C for 15 s and 60°C for 60 s. In most
experiments, assays were prepared in duplicate 50-µl volumes,
from which 20 µl was pipetted into two wells of a 384-well
assay plate. Therefore, if 1 µl of a 1-ng/µl stock
solution of RNA was added to a 50-µl reaction mixture,
the amount of target generating a signal in a single well of
a 384-well plate was 400 pg. Assays were also tested without
probes using SYBR green PCR master mix, Multiscribe reverse
transcriptase, and RNase inhibitor according to the manufacturer's
instructions (Applied Biosystems). SYBR green assays were run
using a thermocycler profile of 48°C for 30 min, 95°C
for 10 min, 40 cycles of 95°C for 15 s, 62°C for 60
s, followed by incubations of 95°C for 15 s, 60°C for
15 s, and 95°C for 15 s.

RESULTS
Sensitivity of the RT-PCR assays for bacteriophage MS2.
We developed five separate assays for MS2, which among them
detect sequences in each of the four genes encoded by the phage
genome (assembly protein, coat protein, lysis protein, and RNA
replicase) (Table
1). The limits of detection for MS2 genomic
RNA for each assay (the lowest amount of template tested that
gave positive results in all experiments) were between 40 and
0.4 fg (20,000 to 200 genome equivalents) per 20-µl reaction
volume in assays that included the respective probes (Table
2). Representative traces of fluorescence development for assay
1 are shown in Fig.
2. The limit of detection for assay 1 was
less than 4 fg of purified genomic RNA (cycle threshold [
Ct]
of

33). (For the sake of brevity, data for only one of the five
MS2 assays are shown.)
We have also detected MS2 RNA in assays run with each of the
primer sets but without the dye-labeled probe oligonucleotides.
The assays were performed with reaction mixtures that included
SYBR green, a dye that fluoresces more strongly when bound to
double-stranded DNA (Table
3). Increases in the amount of double-stranded
DNA in a PCR in progress result in more bound dye; therefore,
the bulk fluorescence of the reaction mixture increases over
the time of the reaction in proportion to the amount of target
present. This assay format relieves the requirement for a specific
probe molecule and is sometimes more sensitive. Interestingly,
we found that the performance of a given primer pair with its
corresponding probe did not predict performance without the
probe in assays with SYBR green. The primer pairs for assays
4 and 5 gave similar limits of detection when used with either
dye-labeled probe or SYBR green. However, the primer pair for
assay 3 gave the most sensitive detection of MS2 RNA when used
with its corresponding dye-labeled probe, but the least sensitive
detection when used with SYBR green. It is possible that differences
in local secondary structure in the MS2 RNA template are responsible
for this phenomenon, but we have not performed experiments to
test this hypothesis.
View this table:
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TABLE 3. Performance of real-time fluorogenic RT-PCR assays for detection of bacteriophage MS2, performed without probe and including the DNA-binding dye SYBR green
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Specificity of the RT-PCR assays.
All five assays for MS2 were tested against a panel of DNA or
RNA from non-target organisms to determine the specificity of
the assays. Assay specificity was determined using assays that
incorporated the probe oligonucleotides. The panel included
plasmids pXO1 and pXO2 isolated from
Bacillus anthracis, and
whole genomic DNA isolated from
E. coli ATCC 43895,
B. cereus ATCC 14579,
B. subtilis ATCC 27370,
Francisella tularensis strain
Schu 4,
Clostridium perfringens ATCC 13124,
Clostridium tetani ATCC 19406,
Yersinia pestis 15-91,
Salmonella enterica serovar
Typhimurium LT2,
Staphylococcus aureus strains ATCC 13566 and
ATCC 14458,
Bacteroides fragilis ATCC 25285,
Bordetella pertussis ATCC 9797,
Pseudomonas aeruginosa PAO-1,
Streptococcus pyogenes ATCC 12384,
Burkholderia cepacia LSPQ-2217,
Corynebacterium diphtheriae LSPQ-3083, and
Homo sapiens (against the possibility
that human DNA, introduced from an operator, might contaminate
a sample). None of the non-target nucleic acids produced a signal
in any of the assays we developed when added to the assays at
a relatively large amount (400 pg DNA or RNA per assay; representative
results shown in Fig.
2B).

DISCUSSION
Studies that examine the environmental fate of organisms, the
effectiveness of antimicrobial compounds, or water treatment
protocols usually require tests for the viability of viruses.
However, in some settings, the rapid detection of the presence
of a pathogen is of greater importance than determinations of
viability. For that reason, many pathogen detection systems
currently under development employ either affinity (antibody)-based
methods or sequence recognition-based detection methods such
as PCR, backed up by sample archiving for confirmatory analysis
in a laboratory. PCR is a highly sensitive method for the detection
of specific nucleic acid sequences; it has been reported that
the use of PCR to detect viral simulants is at least as sensitive
as conventional plate assays for bacteriophage (
6). While culture-based
assays are sensitive and quantifiable, they are also time-consuming
and have limited use in industrial-scale settings that require
the rapid testing of large numbers of samples. PCR is faster
(from 1 to 3 h in the laboratory, but faster instruments are
now emerging in the marketplace), can be automated, and is more
sensitive per unit of sample. Plaque assays for the detection
of bacteriophage require the preparation and/or storage of media,
a laboratory for preparation of microbial cultures, incubation
periods of several hours, and a trained microbiologist to perform
the work, while new instrumentation is beginning to bring PCR
out of the laboratory and into the field.
Bacteriophages other than MS2 have also been used as simulants. Phage
X174, for example, has been used in studies examining the passage of virus through surgical and medical exam gloves made from latex and other elastomeric materials to assess their performance as microbial barriers (6, 21, 27). MS2 has been used as a simulant in the biological defense community, however, because, like many of the pathogens of greatest concern, it has a genome made of RNA.
The assays shown here were designed primarily to assist in the development of protocols for the laboratory testing of biological defense equipment (pathogen detection systems, air samplers, etc.). However, they are obviously applicable to studies in which the worker desires to detect bacteriophage MS2 from samples of essentially any origin, provided the samples do not contain substances that inhibit PCR, and contain MS2 above the limits of detection. Experiments in this study were performed only with purified RNA, rather than with titered phage lysates or environmental samples, to eliminate the potentially confounding factors of environmental contaminants or the presence of free MS2 RNA or intact but noninfective phage particles present in phage preparations. The data presented in this study are therefore "best-case" demonstrations of the assays' sensitivity. We acknowledge that the performance of the assays in environmental studies will necessarily vary with the contents and quality of a sample from a given environment, the efficiency of RNA or DNA extraction (or whether extractions take place at all), the possible presence of natural substances that may act as PCR inhibitors (humic acids from soils, for instance), or the presence of proteases and nucleases that may degrade virions in a sample. This study was not intended to demonstrate the optimization of detection of MS2 from a given set of environmental samples because of the limited usefulness of such demonstrations; optimizing the extraction of RNA or of methods for the removal of PCR inhibitors from environmental samples was beyond the scope of this work, and the choice of methods employed will necessarily vary with the needs of each worker.
The use of purified RNA in this study allowed the assays to be demonstrated at their limits of performance and precision. The assays were sufficiently repeatable and precise to allow straightforward discrimination of two-fold differences in target concentration (Fig. 3, Table 4). Furthermore, even phage titers in most laboratory preparations are known to decline with the passage of time, and free RNA and noninfective phages in lysates are difficult to quantify for obvious reasons. The lability of RNA and the presence of nucleases in lysates make it likely that the amount of free RNA in a phage lysate will decrease over time, requiring frequent recalibration of the ratio of viable PFU and detectable RNA targets in a given lysate. Such a calibration would be necessary in any given set of experiments in which quantitative PCR would be used to estimate the amount of viable MS2 in a given sample.
While the assays described here did not cross-react with DNA
from a wide variety of other species, we did not test directly
for cross-reactivity with other members of the
Leviviridae.
However, BLAST (blastn) searches using the five amplicons as
queries did reveal significant (and unsurprising) similarities
between the MS2 target sequences and sequences in the genomes
of other leviviruses (JP501, M12, R17, and fr) (data not shown).
It is therefore possible that under some conditions, the genomes
of these viruses (or other undescribed, related bacteriophages),
when present in environmental samples contaminated by human
or animal feces, could give positive signals in these assays.
However, methods for phage detection that rely on culture and
plaque counting are also susceptible to error introduced by
naturally occurring phages, because plaque morphology alone
is not a robust tool for phage identification. The RT-PCR assays
still provide an advantage over culture methods for phage detection
and enumeration, by restricting detection and identification
to MS2 and (potentially) its close relatives. The specificity
of the assays was also apparent in the lack of amplification
of non-target sequences from a large number of other species
including
H. sapiens and
Ricinus communis; we routinely perform
PCR assays of 35 to 45 cycles and do not see amplification of
non-target sequences from 1-µg quantities of DNA (Fig.
2B).
All of the assays described here are based on the 5'-to-3' nuclease activity of Taq polymerase, which cleaves a nonextendable, dual-labeled fluorogenic probe (16, 28). The chemistry of these assays is amenable to lyophilization and storage in a stable, dry form with a long shelf life and minimal required infrastructure. However, like all PCR assays, the determination of a positive result can also be obtained by direct observation of the PCR products in an agarose gel or through the use of dyes (such as SYBR green) that do not require the 5'-to-3' exonuclease activity of a thermostable polymerase. There are currently on the market, or under development, several platforms, including laboratory instruments such as the ABI 7900HT, ABI 7700, and ABI 7300, the LightCycler (Cepheid), so-called "field-ruggedized" instruments (such as the R.A.P.I.D. from Idaho Technologies), and hand-held equipment (for example, the BioSeeq from Smiths-Environmental Technologies Group, Inc.) that are designed to run assays using real-time fluorogenic chemistries, giving workers in public health and the military and first responders a growing selection of rapid pathogen detection and identification capabilities that can be used in a variety of settings. Assays for nonpathogenic microorganisms, such as those described in this study, provide a safer, cost-effective means of testing such equipment in advance of validation with live pathogens.

FOOTNOTES
* Corresponding author. Mailing address: US Army Edgewood Chemical Biological Center, AMSRD-ECB-RT-BM, 5183 Blackhawk Rd., APG, MD 21010. Phone: (410) 436-5999. Fax: (410) 436-5999. E-mail:
kevin.oconnell1{at}us.army.mil 
Present address: Defense Threat Reduction Agency, 8725 John J. Kingman Road, MS 6201, Fort Belvoir, VA 22060-6201. 

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Applied and Environmental Microbiology, January 2006, p. 478-483, Vol. 72, No. 1
0099-2240/06/$08.00+0 doi:10.1128/AEM.72.1.478-483.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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