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Journal of Clinical Microbiology, March 2006, p. 1141-1144, Vol. 44, No. 3
0095-1137/06/$08.00+0 doi:10.1128/JCM.44.3.1141-1144.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Novel Multiplex PCR Assay for Detection of the Staphylococcal Virulence Marker Panton-Valentine Leukocidin Genes and Simultaneous Discrimination of Methicillin-Susceptible from -Resistant Staphylococci
Jo-Ann McClure,1
John M. Conly,1,2,3,4,5
Vicky Lau,1
Sameer Elsayed,2,3,5
Thomas Louie,3,4
Wendy Hutchins,3 and
Kunyan Zhang1,2,3,4,5*
Centre for Antimicrobial Resistance, Calgary Health Region/Calgary Laboratory Services/University of Calgary, Calgary, Alberta, Canada,1
Departments of Pathology & Laboratory Medicine,2
Microbiology and Infectious Diseases,3
Medicine, University of Calgary, Calgary, Alberta, Canada,4
Calgary Laboratory Services, Calgary, Alberta, Canada5
Received 3 October 2005/
Returned for modification 28 November 2005/
Accepted 4 January 2006

ABSTRACT
We developed a new multiplex PCR assay for detection of Panton-Valentine
leukocidin virulence genes and simultaneous discrimination of
methicillin-susceptible from -resistant staphylococci. This
assay is simple, rapid, and accurate and offers the potential
for prompt detection of newly emerging community-associated
methicillin-resistant
Staphylococcus aureus.

TEXT
Historically, methicillin-resistant
Staphylococcus aureus (MRSA)
has been associated with nosocomial infections (hospital-acquired
MRSA [HA-MRSA] strains). However, these organisms have recently
emerged as an important cause of community-associated staphylococcal
infections (
1,
2,
25,
28,
29). Although diversity and variation
in their genomic and antibiogram backgrounds exist, virtually
all of these newly emerging community-associated MRSA (CA-MRSA)
strains carry the Panton-Valentine leukocidin (PVL) virulence
genes and possess a novel small mobile staphylococcal cassette
chromosome
mec (SCC
mec) type IV or V genetic element which harbors
the methicillin resistance (
mecA) gene and which is more easily
transferred to other strains of
S. aureus than the larger SCC
mec types (types I to III) that are prevalent in HA-MRSA strains
(
25,
28,
30). Panton-Valentine leukocidin is a bicomponent leukocidin
encoded by two cotranscribed genes, namely,
lukS-PV and
lukF-PV (
lukS/
F-PV), which reside on a prophage and which cause leukocyte
destruction and tissue necrosis (
6,
22,
27). Until recently,
genes coding for PVL were infrequently encountered, being noted
in <5% of
S. aureus isolates worldwide (
10,
12,
15,
28).
However, they are found in a very high proportion of newly emerging
CA-MRSA strains, with rates of 77% to 100%, as reported in various
studies (
19,
20,
26). The presence of PVL in
S. aureus appears
to be associated with increased disease severity, ranging from
cutaneous infection requiring surgical drainage to severe chronic
osteomyelitis and deadly necrotizing pneumonia (
7,
10,
11,
14,
17,
18). In the future, screening for the PVL virulence factor
in
S. aureus may become a routine laboratory procedure (
5).
Previously described PCR methods for the detection of PVL genes
and the methicillin resistance (
mecA) gene have required the
use of separate assays (
15,
24) or involved real-time PCR (
4,
13,
16,
21,
23). We recently developed a new conventional multiplex
PCR assay for the simultaneous detection of PVL and methicillin
resistance (
mecA) genes, which could allow the discrimination
of MRSA from methicillin-susceptible
S. aureus (MSSA) strains
if isolates are phenotypically identified as
S. aureus prior
to PCR testing.
The new multiplex PCR assay targets the Staphylococcus genus-specific 16S rRNA gene (which serves as an internal control) with primers Staph756F (5'-AACTCTGTTATTAGGGAAGAACA-3') and Staph750R (5'-CCACCTTCCTCCGGTTTGTCACC-3') (31), the lukS/F-PV genes (which encode the PVL S/F bicomponent proteins) with primers Luk-PV-1 (5'-ATCATTAGGTAAAATGTCTGGACATGATCCA-3') and Luk-PV-2 (5'-GCATCAAGTGTATTGGATAGCAAAAGC-3') (15), and the mecA gene (a determinant of methicillin resistance) with primers MecA1(5'-GTAGAAATGACTGAACGTCCGATAA-3') and MecA2 (5'-CCAATTCCACATTGTTTCGGTCTAA-3') (31). To ensure that the individual primer pairs were adequate for the amplification of all three gene fragments, a single-target PCR protocol with each individual primer pair was conducted, prior to the multiplex PCR optimization, with 13 control strains (Table 1). Each individual PCR yielded fragments of the expected sizes, i.e., 756, 433, and 310 bp for the 16S rRNA, lukS/F-PV, and mecA genes, respectively (Fig. 1A). The optimized multiplex PCR conditions were obtained by assaying different primer concentrations and other PCR components, as follows: 2 µl of template DNA prepared by a previously described boiling method (31) in a 25-µl final reaction volume containing 0.07, 0.08, and 0.24 µM for the primers specific for the 16S rRNA, lukS/F-PV, and mecA genes, respectively, with the thermocycling conditions set at 94°C for 10 min, followed by 10 cycles of 94°C for 45 s, 55°C for 45 s, and 72°C for 75 s and 25 cycles of 94°C for 45 s, 50°C for 45 s, and 72°C for 75 s. Amplification in a single multiplex PCR produced distinct bands, corresponding to their respective molecular sizes, that were easily recognizable in agarose gels stained with ethidium bromide (Fig. 1B).
To determine assay sensitivity, limiting dilution experiments
were performed as described previously (
31) with five representative
control strains, including one PVL-negative MSSA strain (ATCC
51811), one PVL-positive MSSA strain (ATCC 49775), one PVL-negative
MRSA strain (N315), and two PVL-positive MRSA strains (N02-590
and CA05). This assay was capable of detecting, with reproducibility,
a band in ethidium bromide-stained gels at dilutions corresponding
to 6
x 10
4 CFU per PCR for all the appropriate genes in each
strain, which was quite compatible with the level of detection
of single-target PCR assays (1
x 10
4 to 3
x 10
4 CFU per PCR).
Clinical staphylococcal isolates were obtained from Calgary Laboratory Services (Alberta, Canada) and were characterized by phenotypic and genotypic methods as described previously (31). Multiplex PCR assay validation was performed by simultaneous comparison with the results of single-target PCR assays, as reported previously for PVL (15) and mecA (31), and with the antibiotic susceptibility phenotypes of 178 well-characterized clinical strains, including 30 PVL-negative MSSA strains, 13 PVL-positive MSSA strains, 30 PVL-negative MRSA strains, 45 PVL-positive MRSA strains, 30 PVL-negative methicillin-susceptible coagulase-negative staphylococci (MS-CoNS), and 30 PVL-negative methicillin-resistant CoNS (MR-CoNS) (Table 2). We found a 100% concordance among the results of these tests (Table 2).
To address applicability and accuracy, we further applied our
multiplex PCR assay to test a total of 287 MRSA, 280 MSSA, 220
MS-CoNS, and 228 MR-CoNS local clinical isolates randomly selected
from our Calgary frozen isolate stock collection (the isolates
were recovered from 1999 to 2003) and found PVL positivity rates
of 1.9%, 2.1%, 0% and 0%, respectively, confirming the feasibility
and practicality of our new assay. The low PVL positivity rates
encountered with our local general
S. aureus isolates are comparable
to those encountered with isolates from other locales (<5%
of
S. aureus isolates worldwide) (
10,
12,
15,
28).
Our new multiplex assay for the detection of PVL and methicillin resistance (mecA) genes represents a new tool to aid with the early identification of CA-MRSA strains. A rapidly increasing prevalence of serious CA-MRSA infections and deaths has been reported worldwide (7, 10, 11, 14, 17, 18), and a simple and rapid method of screening for the identification of S. aureus isolates carrying PVL genes is a crucial first step in controlling the dissemination of this potentially virulent pathogen. Very recently, several groups have developed effective real-time PCR assays for the detection of the PVL genes, alone or in combination with the mecA, spa, or nuc gene (4, 13, 16, 21, 23). Due to the high costs of equipment and reagents, use of the real-time PCR technology has generally been limited to larger microbiology laboratories. To the best of our knowledge, our assay represents the first conventional multiplex PCR assay capable of detecting both the PVL and the mecA genes and was shown to be 100% accurate and reliable. Moreover, this assay is easily amenable to routine clinical use in any molecular biology laboratory with PCR capabilities. Our multiplex PCR assay was used in our regional MRSA surveillance program and helped identify and confirm the emergence of an outbreak of CA-MRSA infection in Calgary, Alberta, Canada in 2004, with the resultant expedient implementation of prevention and control measures (3, 8, 9).

ACKNOWLEDGMENTS
We thank K. Hiramatsu and T. Ito for the kind gift of the control
strains JCSC4469, CA05, 8/6-3P, MR108, and N315; F. Tenover
for the gift of strain HIP 5827 (GISA); and M. Mulvey for the
gift of strain N02-590.
This work was partially supported by a CHR/CLS Research Grant (grant 17975) to K.Z. and an Operating Grant from the Centre for Antimicrobial Resistance (CAR), Calgary Health Region/CLS/University of Calgary. V.L. was a 2004 summer student.

FOOTNOTES
* Corresponding author. Mailing address: Department of Pathology & Laboratory Medicine, University of Calgary, #9-3535 Research Road N.W., Calgary, Alberta T2L 2K8, Canada. Phone: (403) 770-3583. Fax: (403) 770-3347. E-mail:
kunyan.zhang{at}cls.ab.ca.


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Journal of Clinical Microbiology, March 2006, p. 1141-1144, Vol. 44, No. 3
0095-1137/06/$08.00+0 doi:10.1128/JCM.44.3.1141-1144.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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