Evaluation of different methods for detecting methicillin (oxacillin) resistance in Staphylococcus aureus.
OBJECTIVE
To evaluate the performance of oxacillin, cefazolin, cefoxitin, cefotaxime and imipenem discs; Etest for oxacillin; microdilution; agar screening plates with 2 and 6 mg/L of oxacillin; and PBP2' agglutination for detection of methicillin-resistant Staphylococcus aureus (MRSA).


METHODS
A total of 102 clinical S. aureus isolates, including 51 MRSA isolates, tested by PCR for the presence or absence of the mecA gene (gold standard method), isolated from different patients and at different times, were tested with: oxacillin (1 microg), cefazolin, cefoxitin, cefotaxime and imipenem (all 30 microg) discs; Etest for oxacillin; microdilution with oxacillin; agar screening tests (ORSAB medium) with 2 mg/L or 6 mg/L of oxacillin; and PBP2' agglutination with two different kits for detection of MRSA strains.


RESULTS
The cefoxitin disc, ORSAB medium and PBP2' detection all showed 100% sensitivity. The cefoxitin, cefazolin and imipenem discs, Etest for oxacillin, microdilution and agar screening method with 6 mg/L at 24 h showed the highest specificity (100%), although variable degrees of sensitivity. The cefoxitin disc, which showed negative and positive predictive values of 100% and 98%, respectively was the best method for detecting MRSA isolates.


CONCLUSIONS
In the absence of availability of molecular biology techniques, the cefoxitin disc was the best predictor of methicillin resistance in S. aureus from among the techniques tested.
Velasco
David
D
Servicio de Microbiologia, and Unidad de Investigacion, Complejo Hospitalario Universitario Juan Canalejo, As Xubias 84, 15006 La Coruña, Spain.
del Mar Tomas
Maria
M
Cartelle
Monica
M
Beceiro
Alejandro
A
Perez
Astrid
A
Molina
Francisca
F
Moure
Rita
R
Villanueva
Rosa
R
Bou
German
G
D001426:Bacterial Proteins / Q000235:genetics; D006801:Humans; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests; D046915:Penicillin-Binding Proteins; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics
D001426:Bacterial Proteins; D046915:Penicillin-Binding Proteins; C487789:mecA protein, Staphylococcus aureus