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Journal of Clinical Microbiology, September 1999, p. 2789-2792, Vol. 37, No. 9
Department of Clinical Microbiology,
Received 8 March 1999/Returned for modification 13 April
1999/Accepted 20 May 1999
The MRSA screen test (Denka Seiken Co., Ltd.), a commercially
available, rapid (20-min) slide latex agglutination test for the
determination of methicillin resistance by detection of PBP 2a in
Staphylococcus aureus, was compared with the oxacillin agar screen test and PCR detection of the mecA gene. A total of
563 S. aureus isolates were tested. Two hundred ninety-six
of the isolates were methicillin-susceptible isolates from cultures of blood from consecutive patients. Also, 267 methicillin-resistant isolates that comprised 248 different phage types were tested. Methicillin resistance was defined as the presence of the
mecA gene. Of the 267 mecA gene-positive
isolates, 263 were positive by the MRSA screen test (sensitivity,
98.5%), and all the mecA-gene negative strains were
negative by the MRSA screen test (specificity, 100%). The oxacillin
agar screen test detected methicillin resistance in 250 of the
mecA gene-positive isolates (sensitivity, 93.6%). The
sensitivity of the MRSA screen test was statistically significantly higher than the sensitivity of the oxacillin agar screen test (P < 0.05). The MRSA screen test is a highly
sensitive and specific test for the detection of methicillin
resistance. Also, it offers results within half an hour and is easy to
perform, which makes this test a valuable tool in the ongoing battle
against methicillin-resistant S. aureus.
Over the last three decades
methicillin-resistant Staphylococcus aureus (MRSA) has
caused major problems in hospitals throughout the world
(29). In The Netherlands the prevalence of MRSA is low
( The mechanism of methicillin resistance in S. aureus is
based on the production of an additional low-affinity
penicillin-binding protein (PBP; PBP 2a), which is encoded by the
mecA gene (1, 9, 21). Many strains are
heterogeneous in their phenotypic expression of methicillin resistance,
despite their genetic homogeneity. Typically, only a few cells
within the total population of cells express resistance, which makes
detection of MRSA by conventional susceptibility testing methods
difficult. Several factors are known to influence phenotypic expression
of methicillin resistance (1, 9, 21). Commonly used methods
for the detection of methicillin resistance, such as the oxacillin agar
screen test, disk diffusion, or broth microdilution, rely on modified
culture conditions to enhance the expression of resistance.
Modifications include the use of oxacillin, incubation at 30 or
35°C instead of 37°C, and the addition of NaCl to the growth
medium. Furthermore, for accurate detection by these methods, a
prolonged incubation period of 24 h instead of 16 to 18 h is
required. Rapid methods with acceptable (>96%) sensitivity for
detection of methicillin resistance include automated microdilution
systems such as the Vitek GPS-SA card (bioMérieux Vitek, Inc.,
Hazelwood, Mo.), the Rapid ATB Staph system (bioMérieux, La
Balme-Les Grottes, France), and the Rapid Microscan Panel system
(Baxter Microscan, West Sacramento, Calif.), which provide results
after 3.5 to 15, 5, and 5 to 11 h, respectively (12, 24,
30). The Crystal MRSA ID system (Becton Dickinson, Cockeysville,
Md.) is a rapid method based on detection of growth of S. aureus in the presence of 4 mg of oxacillin per liter and 2% NaCl
with an oxygen-sensitive fluorescence sensor. Reported sensitivities
range from 91 to 100% after 4 h of incubation (13, 20,
32). The limitation of all the methods mentioned above is that
they are phenotypic methods, and their accuracies can be influenced by
the prevalence of strains that express heterogeneous resistance.
Therefore, the "gold standard" for the detection of methicillin
resistance is PCR or DNA hybridization of the mecA gene
(1). At present, these methods are becoming more feasible
for some laboratories, but most clinical laboratories do not have the
resources to efficiently perform these techniques on a routine basis.
Furthermore, they take several hours to perform. Methods for the
detection of the mecA gene product, PBP 2a, could be used to
determine resistance and might be more clinically reliable than
standard test methods (7). Until now the techniques
described for the detection of PBP 2a were not feasible outside a
research laboratory (7, 23). In a recent publication
Nakatomi and Sugiyama (16) describe the successful
development of a slide latex agglutination assay for the direct
detection of PBP 2a from isolates of S. aureus after a rapid
extraction procedure.
The MRSA screen test (Denka Seiken Co., Ltd.) is a commercially
available, rapid (20-min) slide latex agglutination test for the
detection of PBP 2a. This study compared the MRSA screen test with the
oxacillin agar screen test and PCR detection of the mecA gene for the detection of methicillin resistance in S. aureus.
Bacterial isolates.
The methicillin-susceptible S. aureus (MSSA) isolates used in the study were from cultures of
blood collected between January 1995 and December 1998 from consecutive
patients at St. Elisabeth Hospital and Tweesteden Hospital, Tilburg,
The Netherlands; Pasteur Hospital, Oosterhout, The Netherlands;
Tweesteden Hospital, Waalwijk, The Netherlands; and St. Ignatius
Hospital and Hospital de Baronie, Breda, The Netherlands. Only one
isolate per patient was included. Isolates were identified by a latex
agglutination test (Staphaurex Plus; Murex Diagnostics Ltd., Dartford,
England), by the detection of free coagulase by the tube coagulase test
with rabbit plasma (10), and by the detection of DNase
(DNase agar; Oxoid Unipath Ltd., Basingstoke, England). If the results
of the tests were discordant, an AccuProbe culture identification test
(Gen-Probe; San Diego, Calif.) was performed according to the
manufacturer's instructions (14). The AccuProbe test was
considered the gold standard. Isolates were classified as methicillin
susceptible (MIC, Multiplex PCR for the mecA and coagulase genes.
A 298-bp fragment of the mecA gene was amplified with the
primers 5'-GTT GTA GTT GTC GGG TTT GG-3' (upstream) and 5'-CTT CCA CAT
ACC ATC TTC TTT AAC-3' (downstream) specific for the mecA gene (GenBank accession no. X52593). A second set of primers was
included in each reaction mixture to amplify a polymorphic region of
the coagulase gene that varied between approximately 350 and 600 bp.
The coagulase primers specific for the coagulase gene (GenBank
accession no. X17679) were 5'-CTG GTA CAG GTA TCC GTG AAT A-3'
(upstream) and 5'-TTG TAT TGA CTG TAT GTC TTT GGA-3' (downstream). The
latter primers provided an internal control to check for the presence
of S. aureus DNA and for the absence of PCR inhibitors. MSSA
isolates yield only one PCR product (the coa amplicon),
while MRSA isolates yield two PCR products: the coa amplicon
and the 298-bp mecA amplicon. A streak obtained with a
1-µl loop from a blood agar plate culture of each S. aureus isolate to be tested was resuspended in 50 µl of TE
buffer (0.01 M Tris-HCl, 0.001 M EDTA [pH 8.0]) containing 100 µg
of lysostaphin per ml, the mixture was incubated for 30 min at 37°C,
and the cells were lysed by heating for 10 min at 99°C. This crude
lysate was either used directly in the PCR or stored at
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Rapid Slide Latex Agglutination Test for Detection
of Methicillin Resistance in Staphylococcus aureus
![]()
ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
![]()
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
1.5%) (2, 28). MRSA isolates are usually found in
patients who have been treated in foreign hospitals and who are
transferred to hospitals in The Netherlands. Because of the multitude
of sources, these isolates show a wide variety of phage types (4,
26). All isolates of MRSA are sent to the National Institute of
Public Health and Environmental Protection (RIVM; Bilthoven, The
Netherlands) for phage typing and confirmation of susceptibility test
results. The low prevalence of MRSA in The Netherlands can be
attributed to a stringent national policy. The mainstays of this policy
are strict isolation of patients who carry MRSA, active search for carriers by screening, and treatment of those who are carriers (26). Accurate and rapid detection of methicillin resistance in S. aureus is essential for the success of this policy.
Moreover, it is of great importance for the institution of appropriate
antimicrobial therapy for patients with infections caused by these organisms.
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
2 µg/ml) by broth microdilution susceptibility
testing. Furthermore, no growth was observed by the oxacillin agar
screen test (as described below).
20°C for
later use.
Oxacillin agar screen test. All MRSA isolates were spot inoculated onto a Mueller-Hinton agar plate (Difco Laboratories, Detroit, Mich.) supplemented with 6 µg of oxacillin per ml and 4% NaCl by using a cotton swab dipped into a 0.5 McFarland standard suspension of each test isolate. The plates were incubated at 35°C for 24 h. If any growth was detected, the isolate was considered oxacillin resistant (17).
MRSA screen test. The MRSA screen test is a latex agglutination test based on the reaction of latex particles sensitized with monoclonal antibodies against PBP 2a of S. aureus and PBP 2a extracted from tested colonies. The test was performed according to the manufacturer's instructions. Briefly, isolates were subcultured onto Columbia agar supplemented with 5% sheep blood (Oxoid Unipath Ltd.) at 37°C for 18 h to obtain fresh growth. To extract PBP 2a from the tested colonies, a loopful of cells was suspended in 4 drops of extraction reagent 1. This suspension was placed in a heating block (>95°C) for 3 min. After allowing the suspension to cool to room temperature (±10 min), 1 drop of extraction reagent 2 was added and the mixture was vortexed thoroughly. The suspension was then centrifuged at 1,500 × g for 5 min. The actual latex agglutination test was performed with the supernatant; 50 µl of the supernatant was mixed with 1 drop of sensitized latex. For the negative control, 50 µl of the supernatant was mixed with 1 drop of negative control latex. Mixing for 3 min was performed with a shaker. The investigators that performed the tests were blinded to the results of the susceptibility tests and the results of the PCR detection of the mecA gene.
MIC of oxacillin (E test). The MIC of oxacillin was determined by using the E-test system (AB Biodisk, Solna, Sweden). The E-test was performed with isolates which were mecA positive and MRSA screen test or oxacillin agar screen test negative. An E-test strip was placed onto a Mueller-Hinton agar plate supplemented with 2% NaCl. These plates were inoculated by swabbing the surfaces with a direct colony suspension of the tested strain equivalent to a 0.5 McFarland standard. After incubation at 35°C for 24 h, the MIC was read at the point of intersection between the zone edge and the E-test strip.
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RESULTS |
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A total of 296 MSSA and 267 MRSA isolates were included in the
evaluation. All 296 MSSA isolates tested negative by the
mecA gene PCR, MRSA screen, and oxacillin agar screen tests.
The 267 MRSA strains were all mecA gene PCR positive; 4 tested negative by the MRSA screen test and 17 did not grow by the
oxacillin agar screen test (Table 1).
This resulted in a sensitivity of 98.5% and a specificity of 100% for
the MRSA screen test. The sensitivity and specificity of the oxacillin
agar screen test were 93.6 and 100%, respectively. Upon retesting, the
results for all samples with discordant results were confirmed.
The MICs determined by the E test for the 19 discordant
strains are presented in Table 2.
According to the National Committee for Clinical Laboratory Standards
breakpoint (
2 µg/ml) (17), the E test identified 11 mecA gene-positive isolates as oxacillin susceptible.
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DISCUSSION |
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This study shows that detection of PBP 2a by the MRSA screen test is a highly sensitive and specific means for the detection of methicillin resistance in S. aureus. In this evaluation MRSA isolates comprising 248 different phage types were included. In fact, at least one isolate of each phage type identified among the MRSA strains isolated in The Netherlands between 1989 and 1998 was included in the study. Since MRSA strains in The Netherlands are usually recovered from patients who have been hospitalized in other countries, this collection can be considered a reflection of MRSA strains from throughout the world. Most isolates are of European origin (4, 26). No phage typing was performed with the methicillin-susceptible blood culture isolates. These isolates were collected from patients admitted to six different hospitals in The Netherlands and to many different wards during a 4-year period. Therefore, it is most likely that this collection includes many different isolates as well. For evaluations of tests for the detection of S. aureus it is essential to define the collection of isolates tested. S. aureus is a prime example of a microorganism which spreads clonally in the environment (11). Consequently, many collections will contain many isolates of the same strain. This leads to over- or underestimation of the true value of the test under evaluation. This evaluation is the first which includes such a large, polyclonal collection of MRSA strains for detection purposes. Therefore, it provides a valid estimation of the potential value of the MRSA screen test for the detection of MRSA. The high sensitivity of the MRSA screen test makes this test suitable for detection purposes.
Only 4 of the 267 mecA-positive isolates tested negative by
the MRSA screen test. For all four strains the oxacillin MIC was 8 µg/ml or lower. This may indicate that only small amounts of PBP 2a
are present and that the amounts are too small to be detected by the
MRSA screen test. However, other isolates for which MICs were low and
which did not grow on the oxacillin agar screen test tested positive by
the MRSA screen test (Table 2, isolates 1, 2, 4 to 6, 9 to 12, 14 and
17 to 19). S. aureus strains that are mecA
positive but that do not produce PBP 2a have been reported previously
(15, 23, 25). These strains were all methicillin susceptible
phenotypically. It has been suggested that testing of those kind of
strains by PCR or DNA probe techniques can lead to false-positive
results for resistance and that detection of PBP 2a may be more
appropriate for the detection of MRSA (16). Others have
stated that these strains should be classified as MRSA, despite their
phenotypic susceptibility to
-lactam antibiotics. This is because of
the possibility that methicillin resistance appears during therapy with
-lactam antibiotics (15, 18). Therefore, it is
recommended that detection of the mecA gene remain the gold
standard for the detection of methicillin resistance in S. aureus.
For borderline MRSA strains, MICs are at or just above the
susceptibility breakpoint (e.g., oxacillin MICs, 4 to 8 µg/ml). Strains with borderline resistance do not contain the mecA
gene and resistance is not based on the production of PBP 2a but has been hypothesized to result from modification of normal PBP genes, overexpression of normal PBPs, or overproduction of staphylococcal
-lactamases (1). Differentiation of
borderline-resistant mecA-negative strains from
heterogeneous mecA-positive, PBP 2a-producing strains is
important in choosing the correct antimicrobial treatment. In vitro
susceptibility data, experimental data from studies with animals, and
some clinical data indicate that treatment with
-lactam antibiotics
is effective for infections caused by these mecA
gene-negative, non-PBP 2a-producing borderline resistant strains
(1, 7). Furthermore, non-PBP 2a-producing strains of
S. aureus may not require expensive and inconvenient
patient isolation procedures (8). The MRSA screen test could
probably be useful for the identification of these strains. In this
study, however, no borderline-resistant strains were included.
Methicillin resistance in coagulase-negative staphylococci (CoNS) is also based on the mecA gene product PBP 2a; therefore, thorough identification of the tested strain is necessary. Detection of methicillin resistance in CoNS by conventional susceptibility tests is even more difficult than detection of methicillin resistance in S. aureus. The oxacillin agar screen test is reported to be very reliable but requires 48 h of incubation for CoNS (31). It is possible that the MRSA screen test could also successfully detect methicillin resistance in CoNS. The manufacturer does not recommend use of the MRSA screen test for the detection of methicillin resistance in CoNS, and this study did not include CoNS. Further testing for this purpose is warranted.
Five mecA-positive strains showed only weak agglutination after 3 min of rotation of the test card, as recommended by the manufacturer's instructions. When rotated for another 3 min the agglutination pattern became strongly positive. It is important to check carefully for any sign of agglutination. If a weak agglutination pattern is seen, one can rotate the test card for another 3 min, which can clarify how one should interpret the test result. To evaluate the chance of false-positive results as a result of an increase in the duration of rotation, 100 MSSA isolates were rotated for 6 min. No agglutination was observed.
The oxacillin screen agar test is recommended by the National Committee for Clinical Laboratory Standards (17) as one of the most reliable phenotypic tests for the detection of oxacillin resistance. In this evaluation the sensitivity was only 93.6%, which was statistically significantly (P < 0.01) lower than the sensitivity of the MRSA screen test. The risk of misclassification of an MRSA isolate as methicillin susceptible was 4.3 times higher by the oxacillin agar screen test (95% confidence interval, 1.5 to 12.5).
The E test is also considered a very reliable method for the detection of methicillin resistance and is recommended by the National Committee for Clinical Laboratory Standards as well (5). In this evaluation only the 19 strains with discordant results were tested by the E test. Of this subset, 11 strains were found to be suspectible. This results in a sensitivity which is maximally 95.9%. The true value can be estimated only when all strains are tested, but it is definitely lower than the sensitivity of the MRSA screen test.
In conclusion, the MRSA screen test is a rapid, easy-to-perform, and highly reliable test for the detection of methicillin resistance in S. aureus. Results are available in approximately 20 min, whereas PCR detection of the mecA gene takes several hours. Therefore, the MRSA screen test offers a new, valuable tool in the ongoing battle against MRSA.
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ACKNOWLEDGMENTS |
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We thank Gerlinde Pluister, Jules Rost, Karel Vellheuer, and Wendy van Rijckevorsel for technical assistance. We thank Bipharma b.v., Weesp, The Netherlands, for supplying the MRSA screen tests.
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FOOTNOTES |
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* Corresponding author. Mailing address: Department of Clinical Microbiology, St. Elisabeth Hospital, P.O. Box 747, 5000 AS Tilburg, The Netherlands. Phone: (31) 13-5392650. Fax: (31) 13-5441264. E-mail: a.vgriethuysen{at}worldonline.nl.
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