| Description |
On the whole, 128 staphylococcal isolates (Staphylococcus aureus 65, Staphylococcus epidermidis 42, and Staphylococcus haemolyticus 21 strains) from patients of ten Czech hospitals were classified as susceptible or resistant to oxacillin by the standard broth dilution micro-method (using the MIC break-point of 4.0 mg/L for resistance). The MRSA-Screen test (Denka Seiken Co., Japan), a latex agglutination for the detection of penicillin binding protein PBP2a in oxacillin/methicillin-resistant staphylococci, was compared with other commonly used phenotypic susceptibility tests. The PCR detection of the mecA gene, the presence of which is equaled to oxacillin resistance, was used as a reference genotypic method. In addition, the determination of the femA gene was performed also by this procedure. All 68 isolates (S. aureus 26, S. epidermidis 21, and S. haemolyticus 21 strains), determined as oxacillin-resistant, contained the mecA gene. The femA gene was present in all these S. aureus strains. The other 60 isolates (S. aureus 39, and S. epidermidis 21 strains) were characterized as oxacillin-susceptible. All the S. aureus strains lacked the mecA gene, however, all but two were femA gene-positive. The examination of S. epidermidis strains demonstrated the mecA gene in six (29%) of them. Most of these discrepancies between the mecA PCR tests and determination of the oxacillin MICs could be avoided by decreasing of the resistance breakpoint (0.5 mg/L) according to the new recommendation of the NCCLS from 1999. In this connection, however, other phenotypic susceptibility methods (MRSA-Screen, oxacillin-agar screen, and disk difusion tests) should also be modified, as most of them give false negative results in the scope of the oxacillin MICs from 0.5 to 2.0 mg/L. When evaluating the MRSA-Screen test by 3 min, only 46 (68%) oxacillin-resistant strains agglutinated, however, other 20 (29%) strains (mostly coagulase-negative ones) were positive after prolongation of the reaction time from 3 to 9 min. The sensitivities and specificities for MRSA-Screen latex agglutination in comparison with the broth dilution micro-method, oxacillin agar screen assay, and disk difusion test were as follows: 98.5 and 100%, 98.5 and 100%, 100 and 98%, 100 and 97%, respectively. These results showed that the MRSA-Screen test has a very good sensitivity and specificity for the detection of oxacillin resistance as compared with other conventional phenotypic procedures, however, its priorities are simplicity and speed as required for routine microbiological laboratories.
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