Evidence that the National Committee for Clinical Laboratory Standards disk test is less sensitive than the screen plate for detection of low-expression-class methicillin-resistant Staphylococcus aureus

Mackenzie, A. M. R.; Richardson, Harold; Lannigan, R.; Wood, D.E.

doi:10.1128/jcm.33.7.1909-1911.1995

Cited by 30 publications

(13 citation statements)

References 15 publications

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“…Standard susceptibility testing of MRSA, as recommended by the National Committee for Clinical Laboratory Standards, is performed as a disc diffusion test using oxacillin discs or an agar screen test using oxacillin in the culture medium, while MIC determination is performed by a microdilution or an agar dilution test (65). Agar screen methods are considered preferable to disc diffusion methods in order to unveil methicillin resistance (56). Agar screen media should contain elevated concentrations of NaCl (41, 65), and should be incubated at temperatures in the range 30-35°C since heteroresistance may not be expressed at 37°C (35).…”

Section: Detection O F Methicillin Resistancementioning

confidence: 99%

Mechanisms of methicillin resistance in staphylococci

Brakstad

Mæland

1997

APMIS

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The continuously high prevalence of methicillin‐resistant staphylococci (MRS) throughout the world is a constant threat to public health, owing to the multiresistant characteristics of these bacteria. Methicillin resistance is phenotypically associated with the presence of the penicillin‐binding protein 2a (PBP2a) not present in susceptible staphylococci. This protein has a low binding affinity for β‐lactam antibiotics. It is a transpeptidase which may take over cell wall synthesis during antibiotic treatment when normally occurring PBPs are inactivated by ligating β‐lactams. PBP2a is encoded by the mecA gene, which is located in mec, a foreign DNA region. Expression of PBP2a is regulated by proteins encoded by the plasmid‐borne blaR1‐blaI inducer‐repressor system and the corresponding genomic mecR1‐mecI system. The blaR1‐blaI products are important both for the regulation of β‐lactamase and for mecA expression. Methicillin resistance is influenced by a number of additional factors, e.g. the products of the chromosomal fem genes which are important in the synthesis of normal peptidoglycan precursor molecules. Inactivation of fem‐genes results in structurally deficient precursors which are not accepted as cell wall building blocks by the ligating PBP2a transpeptidase during antibiotic treatment. This may result in reduced resistance to β‐lactam antibiotics. Inactivation of genes affecting autolysis has shown that autolytic enzymes are also of importance in the expression of methicillin resistance. Methicillin resistance has evolved among earth microorganisms for protection against exogenous or endogenous antibiotics. Presumably the mec region was originally transferred from coagulase negative staphylococci (CNS) to Staphylococcus aureus (SA). A single or a few events of this kind with little subsequent interspecies transfer had been anticipated. However, recent data suggest a continuous horizontal acquisition by S. aureus of mec, being unidirectional from CNS to SA. Methicillin resistance may also be associated with mechanisms independent of mecA, resulting in borderline methicillin resistance. These mechanisms include β‐lactamase hyperproduction, production of methicillinases, acquisition of structurally modified normal PBPs, or the appearance of small colony variants of SA. Most MRS are multiresistant, and the mec region may harbour several resistance determinants, resulting in a clustering of resistance genes within this region.

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Section: Detection O F Methicillin Resistancementioning

confidence: 99%

Mechanisms of methicillin resistance in staphylococci

Brakstad

Mæland

1997

APMIS

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“…Several studies have raised concerns over the failures of the conventional methods to detect such resistance and have led to various recommendations to enhance the expression of the resistance in vitro (2, 4, 8-11, 17, 20-23, 25, 27, 31, 33, 37). At present, the detection of the mecA gene, which is responsible for methicillin resistance in practically all clinical methicillin-resistant staphylococcal strains (9,24,28), is considered the reference test (2,4,7,17,23,(25)(26)(27)(28)32). In spite of the growing consensus in the literature for this method, it is not yet available in all clinical laboratories, and the alternative reference test remains the oxacillin agar screen plate test (1).…”

mentioning

confidence: 99%

“…Both mecA detection and agar screening have been used as "gold standards" for the evaluation of commercial methods (12-14, 16, 22, 25, 26, 33-35, 38). Automated systems are widely used in clinical laboratories, but they may lack accuracy for the detection of heterogeneously resistant isolates (9,17,22,25). However, in the past few years, several reports have emphasized the performance characteristics of different rapid methods, such as the Rapid ATB Staph (bioMérieux, la Balme-Les Grottes, France) system (26), the Rapid MicroScan panel (Baxter Microscan, West Sacramento, Calif.) (25,35), and the Vitek system (bio-Mérieux Vitek, Inc., Hazelwood, Mo.)…”

mentioning

confidence: 99%

Comparison of ATB Staph, Rapid ATB Staph, Vitek, and E-Test Methods for Detection of Oxacillin Heteroresistance in Staphylococci Possessing mecA

et al. 1998

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The performance characteristics of the E-test (AB Biodisk, Solna, Sweden), the ATB Staph, the Rapid ATB Staph, and the Vitek GPS-503 card (bioMérieux, La Balme Les Grottes, France) methods for the detection of oxacillin resistance in a collection of staphylococci with a high proportion of troublesome strains were evaluated. Sixty-fourStaphylococcus aureus strains and 76 coagulase-negative staphylococcal strains were tested. All strains were mecApositive and were characterized by the oxacillin agar screen plate test; 75 (53.6%) were found to be heterogeneous by a large-inoculum oxacillin disk diffusion assay, and oxacillin MICs for 89 (63.6%) were ≤32 μg/ml. Three (4.7%) S. aureus strains and 25 (32.9%) coagulase-negative strains were classified as susceptible by the E-test, as defined by the National Committee for Clinical Laboratory Standards (NCCLS) oxacillin breakpoint (MIC ≤ 2 μg/ml). The ATB Staph method failed to detect oxacillin resistance in 7 (11%) S. aureus isolates and 32 (42.1%) coagulase-negative isolates. The MICs for all but six of these discrepant isolates were ≤16 μg/ml. The Rapid ATB Staph method was tested against S. aureus strains only and yielded 15 (23.4%) false-susceptible results for strains for which the MICs were ≤32 μg/ml. The Vitek system was the best-performing system, since it failed to detect oxacillin resistance in only 3 (4.7%) S. aureus strains and 15 (19.7%) coagulase-negative strains, the MICs for all of which were ≤2 μg/ml. These data indicate that (i) the performance of the two ATB Staph systems can be limited when the prevalence of borderline-heteroresistant staphylococci is high and (ii) the unreliability of the E-test and the Vitek methods for detecting resistant coagulase-negative strains might be reduced by the potential revision of the oxacillin breakpoint currently recommended by the NCCLS.

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“…Achieving accurate and reproducible antimicrobial susceptibility testing results with enterococci and staphylococci con-tinues to be a problem for many clinical microbiology laboratories (2,6,8,17,18,21). Testing of staphylococci against oxacillin and enterococci against vancomycin and the quinolones can be particularly frustrating since for many of these organisms MICs appear to be at or near the breakpoints for resistance (2,9,18).…”

Section: Discussionmentioning

confidence: 99%

“…Given the discrepancies in previous studies and our continuing concerns about the ability of conventional antimicrobial susceptibility methods to detect emerging antimicrobial resistance (2,6,8,9,19,21), we undertook an evaluation of the Alamar system to determine its accuracy when testing enterococci and staphylococci. We challenged the Alamar test system with 26 clinical isolates and 93 gram-positive bacteria from the Centers for Disease Control and Prevention (CDC) challenge set (5), which is designed to test the limits of accuracy of new antimicrobial susceptibility testing systems.…”

mentioning

confidence: 99%

Evaluation of Alamar colorimetric broth microdilution susceptibility testing method for staphylococci and enterococci

Baker

Tenover

1996

J Clin Microbiol

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We compared the results of the Alamar broth microdilution susceptibility testing method with the results of the National Committee for Clinical Laboratory Standards reference broth microdilution method for 119 gram-positive organisms. The strains were tested for their susceptibilities to 20 antimicrobial agents. Only appropriate antimicrobial agents were evaluated for each species of bacteria. Absolute categorical agreement between the reference method and the test method was 91.5% for enterococci, 99.8% for oxacillin-susceptible staphylococci, and 97.4% for oxacillin-resistant staphylococci. Essential agreement (percent complete agreement plus percent minor errors) was >99% for all organisms tested. The results for enterococci showed no very major errors, one major error with ofloxacin, and numerous minor errors with the quinolones. However, all except one of the minor errors were within ؎1 log 2 dilution of the reference result. For staphylococci, only 2 very major errors (one each with chloramphenicol and oxacillin), 1 major error (chloramphenicol), and 15 minor errors (multiple drugs) were observed. The Alamar colorimetric system was easy to use and the results were easy to read. It appears to be an acceptable method for antimicrobial susceptibility testing of staphylococci and enterococci.

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Evidence that the National Committee for Clinical Laboratory Standards disk test is less sensitive than the screen plate for detection of low-expression-class methicillin-resistant Staphylococcus aureus

Cited by 30 publications

References 15 publications

Mechanisms of methicillin resistance in staphylococci

Mechanisms of methicillin resistance in staphylococci

Comparison of ATB Staph, Rapid ATB Staph, Vitek, and E-Test Methods for Detection of Oxacillin Heteroresistance in Staphylococci Possessing mecA

Evaluation of Alamar colorimetric broth microdilution susceptibility testing method for staphylococci and enterococci

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