Detection of methicillin resistance in Staphylococcus aureus

Abstract number: r1991

Alepopoulou  E., Grapsa  A., Panopoulou  M., Kantzanou  M., Kartali  S.


Methicillin-resistant Staphylococcus aureus (MRSA) is among the major pathogens. The most common methods currently used for identifying methicillin (oxacillin) resistance in many clinical laboratories are susceptibility tests. The performance of these tests has been erratic because the expression of resistance is variable and commonly heterogeneous within strains.


A retrospective laboratory-based study carried out with clinical isolates of S. aureus from active infections in patients of intensive care units in a tertiary University hospital during 2001–2002. Sixty one strains of S. aureus recovered from various specimens, as blood cultures, tracheal aspirates, wound swabs and venous catheters. Methicillin (oxacillin) susceptibility tested by five different methods:

(1) agar screening test [MH- oxacillin ( 6 mg/ml ) agar supplemented with 4% NaCl], (2) susceptibility determination by microdilution method (Vitek 2, BioMerieux), (3) MIC determination by E-test (AB Biodisk), (4) Penicillin-binding protein (PBP) 2a detection by latex agglutination test (BioMerieux), (5) mecA gene detection by real-time PCR (Light Cycler, Roche), using specific primers and probes. The strains evaluated by using the presence of mecA gene detected by PCR, as definitive criteria for MRSA and non-MRSA. The susceptibility tests carried out as recommended by the NCCLS.


Among all the isolates, 31 (51%) identified as mecA positive and the remaining 30 (49%) as mecA-negative. The percentages of sensitivity were: oxacillin agar screen 98%, latex agglutination test 95%, E-test 93%, and Vitek2 93%. Three isolates, negative for the mecA gene by real-time PCR, recognized by at least one phenotyping method as oxacillin resistant. Two strains, mecA positive, incorrectly identified as oxacillin sensitive by the oxacillin agar screening test.


As shown in this and other studies, no phenotypic method is completely reliable for detection of oxacillin resistance in S. aureus. The sensitivity was higher with the agar-screening test than with the other conventional methods. In particular, the oxacillin agar screening test and PBP2a latex agglutination test were the most accurate methods and they approached the accuracy of PCR, so they should be applied in association with the other susceptibility methods, to improve the ability to accurately detect oxacillin susceptibility in S. aureus.

Session Details

Date: 01/08/2007
Time: 00:00-00:00
Session name: XXIst ISTH Congress
Location: Oxford, UK
Presentation type:
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