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Journal of Clinical Microbiology, June 2000, p. 2429-2433, Vol. 38, No. 6
Institute of Medical Microbiology and
Hygiene, University of Regensburg, D-93053 Regensburg, Germany
Received 22 October 1999/Returned for modification 31 January
2000/Accepted 23 March 2000
A duplex LightCycler PCR assay targeting the mecA gene
and a Staphylococcus aureus-specific marker was used to
test 165 S. aureus strains and 80 strains of other
bacterial species. Within an assay time of 60 min plus 10 min for
sample preparation, S. aureus as well as the presence or
absence of the mecA gene was correctly identified.
Staphylococcus aureus
represents one of the most significant pathogens causing nosocomial and
community-acquired infections. Beta-lactam antibiotics are the
preferred drugs for serious S. aureus infections
(11). Since the introduction of methicillin into clinical
use in 1961, the occurrence of methicillin-resistant S. aureus (MRSA) strains has increased steadily, and nosocomial infections have become a serious problem worldwide (1, 11, 14, 19,
21). Therefore, the detection of methicillin resistance has
important implications for therapy and management of patients. In the
clinical laboratory, S. aureus is identified by growth characteristics and the subsequent detection of catalase and coagulase activities or specific surface constituents. The DNase and thermostable endonuclease tests have been used as confirmatory tests for
inconclusive or negative coagulase tests. Conventional
susceptibility testing of S. aureus reliably detects
resistance to methicillin or oxacillin if agar dilution or agar
screening methods are used according to NCCLS standards (9).
Methicillin resistance is associated with the production of a
penicillin-binding protein, encoded by the mecA gene
(3, 5), or, in rare cases, with the hyperproduction of
Conventional identification methods are time-consuming and may yield
false-positive or false-negative results, and misclassifications with
automated susceptibility testing systems or commercially available
latex agglutination kits have been reported recently (16, 17, 18,
23). The rapid and sensitive PCR-based detection of the
mecA gene has evolved as the method of choice for the
definitive identification of MRSA and other methicillin-resistant
staphylococci (2, 4, 6, 7, 10, 13, 20, 22). Since S. aureus can resemble coagulase-negative staphylococci (CoNS) on
visual examination of agar plates and the coagulase status is not
always easily established in a timely fashion, the inclusion of a
species-specific marker is favorable (8).
Here we describe the development and evaluation of a duplex PCR assay
for the simultaneous detection of the mecA gene and a
recently described S. aureus-specific genomic fragment
(12). In order to meet the requirements of true rapid
diagnostics, the novel LightCycler device (Roche Molecular
Biochemicals, Mannheim, Germany) was used for ultrarapid thermal
cycling and online monitoring of the amount of specific PCR product
present in the amplification mixture.
All the strains used in this study were maintained on Columbia blood
agar and identified by colony morphology, Gram stain characteristics,
catalase reaction, coagulase production, and the results of the API
Staph System (bioMerieux Vitek, Inc., Hazelwood, Mo.). Oxacillin
susceptibility was determined by the agar screening method with
Mueller-Hinton agar containing 2% NaCl and 6 mg of oxacillin per liter
for S. aureus or 0.5 mg of oxacillin per liter for CoNS
(9).
Template DNA was prepared by a simple and rapid boiling procedure
(15). Briefly, portions of individual bacterial colonies were suspended in 200 µl of lysis buffer containing 1% Triton X-100,
0.5% Tween 20, 10 mM Tris-HCl (pH 8.0), and 1 mM EDTA and incubated in
a screw-cap reaction tube for 10 min in a boiling water bath. After
centrifugation for 2 min at 10,000 × g to sediment the
debris, a 2-µl aliquot of the clear supernatant was directly transferred to the PCR apparatus. Alternatively, a High Pure PCR Template Preparation Kit (Roche) was used to obtain pure genomic DNA,
which can be stored at Oligonucleotide primers and fluorescence-labeled hybridization probes,
designed for amplification and sequence-specific detection of a 408-bp
fragment within the mecA gene and a 179-bp fragment within
an S. aureus-specific genomic marker (12), were
obtained from Tib Molbiol (Berlin, Germany). Nucleotide sequences and
positions are listed in Table 1.
Amplification mixtures contained 2 µl of 10× LightCycler FastStart
DNA Master Hybridization Probes mixture (Roche), 5 mM MgCl2
(total concentration), 1 µM each Mec-F and Mec-R primer
oligonucleotide, 0.25 µM each Sa442-F and Sa442-R primer
oligonucleotide, 0.2 µM each hybridization probe oligonucleotide, and
2 µl of template DNA in a final volume of 20 µl. Following an
initial denaturation at 95°C for 10 min to activate the FastStart Taq DNA polymerase, the 40-cycle amplification profile
consisted of heating at 20°C/s to 95°C with a 10-s hold, cooling at
20°C/s to 50°C with a 10-s hold, and heating at 20°C/s to 72°C
with a 20-s hold. Fluorescence values of each capillary were measured at 640 and 705 nm (dual-color option).
0095-1137/00/$04.00+0
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Rapid Identification of Methicillin-Resistant
Staphylococcus aureus and Simultaneous Species
Confirmation Using Real-Time Fluorescence PCR
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-lactamase.
20°C for several months. The efficiencies of
the commercial kit and the boiling procedure were found to be
comparable for the extraction of amplifiable S. aureus DNA.
TABLE 1.
Oligonucleotide primers and LightCycler hybridization
probes used in the PCR assay
PCR experiments performed with MRSA strain ATCC 33592 (American Type
Culture Collection, Manassas, Va.), grown and serially diluted in
phosphate-buffered saline as described previously (12), revealed a detection limit of approximately 25 CFU (25 genome equivalents) per assay for the mecA-specific primer pair
Mec-F-Mec-R in combination with hybridization probes Mec-HP-1 and
Mec-HP-2 (Fig. 1A). For amplification and
detection of an S. aureus-specific genomic fragment
(12), a detection limit of approximately 5 CFU of S. aureus per assay was determined with primer pair Sa442-F-Sa442-R and hybridization probes Sa442-HP-1 and Sa442-HP-2 (Fig. 1B). The
duplex approach, containing four different primer
oligonucleotides and four different hybridization probes within a
single capillary, revealed identical detection limits (data not shown).
Significant formation of primer dimers or secondary structures or other
cross-reactions between oligonucleotide components, which frequently
interfere with the analytical sensitivity of multiplex PCR approaches,
are therefore unlikely in this particular assay.
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For specificity testing, type strains of methicillin-susceptible
S. aureus (MSSA) (ATCC 35696, ATCC 27704, ATCC 23361, ATCC 15752, and ATCC 27695) and MRSA (ATCC 43300, ATCC 33591, ATCC 33592, ATCC 33593, and ATCC 33594) and 155 clinical isolates of S. aureus, including 55 MRSA isolates with different patterns in
pulsed-field gel electrophoresis, were examined. Figure
2 shows the results for a representative
panel of strains. Visual examination of the plots generated by the
LightCycler software (cycle number versus fluorescence) allowed for a
clear discrimination between positive and negative samples. All 165 strains were correctly amplified and detected with the S. aureus-specific set of primers and hybridization probes, and all
60 methicillin-resistant strains were identified with the
mecA-specific set of primers and hybridization probes.
Compared with the results of conventional identification and
susceptibility testing, the PCR assay showed 100% specificity and
sensitivity for species identification and detection of methicillin resistance for the 165 S. aureus strains investigated.
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Specificity was further confirmed by testing a panel of 40 gram-negative and 40 gram-positive bacterial species, including methicillin-resistant isolates of Staphylococcus epidermidis (n = 3), Staphylococcus hominis (n = 2), and Staphylococcus pasteuri (n = 1) as well as methicillin-susceptible isolates of S. hominis (n = 1), Staphylococcus warneri (n = 1), Staphylococcus capitis (n = 1), and Staphylococcus haemolyticus (n = 1). Among the CoNS, methicillin-resistant isolates were positive for the mecA gene but negative for the S. aureus-specific marker (e.g., the negative curve in Fig. 2B representing a methicillin-resistant S. epidermidis strain). All other bacterial isolates were found negative for both parameters of the duplex PCR assay. These findings were in accordance with the results of Martineau et al. (12), who reported 100% sensitivity and specificity for this particular S. aureus target sequence.
Taq DNA polymerase inhibition was investigated with 64 samples, which were randomly selected from DNA preparations obtained with both extraction methods. Each of the samples was spiked with 300 pg of human DNA. PCR was performed using the components of the
LightCycler Control Kit (Roche), allowing for specific detection of the
human
-globin gene. Inhibition events were observed with none of the
DNA preparations tested.
Showing a detection limit of at least 25 MRSA genome equivalents, this PCR assay should also be sensitive enough for direct detection and identification of MRSA in clinical specimens. However, direct detection of MRSA is restricted to clinical specimens from normally sterile sites where the presence of only a single bacterial species is expected. If a clinical specimen contains, for example, a mixture of MSSA and methicillin-resistant CoNS (even in trace amounts in cases of contamination), PCR performed directly on this particular specimen will be positive for both S. aureus and mecA. Due to this dilemma, our PCR assay was routinely applied to bacterial colonies on agar plates judged to be pure.
Since procedures for sequence-specific detection of amplicons have evolved as the "gold standard" in the field of diagnostic PCR, we concentrated on the LightCycler HybProbe concept to avoid the application of time-consuming and laborious postamplification procedures, such as Southern blotting, solid-phase hybridization, or DNA sequencing. Due to its compact and reliable nature, our duplex PCR assay proved to be a valuable tool for the rapid identification of MRSA isolates in the environment of a routine microbiological laboratory setting. In combination with the simple boiling protocol for template DNA preparation, it can be easily integrated into the work flow of any diagnostic laboratory. Once the growth of staphylococci is observed on agar plates, a portion of the colony can be transferred to PCR. After 10 min of physical manipulation necessary for DNA extraction and completion of the reaction mixture, results are available within 60 min.
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ACKNOWLEDGMENTS |
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We thank Markus Heep and Stefan Lukas for active support and Vanessa Bennett and Jeffrey Emch for critically reading the manuscript. We gratefully acknowledge the excellent technical assistance of Michaela Hien and Xiaojun Wu during the study.
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FOOTNOTES |
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* Corresponding author. Mailing address: Institut für Medizinische Mikrobiologie und Hygiene, Universitätsklinikum Regensburg, Franz-Josef-Strauß-Allee 11, D-93053 Regensburg, Germany. Phone: 49-941-59609-50. Fax: 49-941-944-6402. E-mail: Udo.Reischl{at}klinik.uni-regensburg.de.
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