[Detection of mecA in strains with oxacillin and cefoxitin disk tests for detection of methicillin-resistant Staphylococcus].
BACKGROUND
Cinical and Laboratory Standards Institute (CLSI) recommends the use of cefoxitin disks instead of long-used oxacillin disks for screening methicillin-resistant isolates of staphylococci. The frequency of discrepant results and accuracy of the tests were evaluated by detecting mecA gene.


METHODS
A total of 3,123 Stapylococci isolates from patients in Severance Hospital were tested during September 2005 to August 2006 by the CLSI-recommended test using both cefoxitin and oxacillin disks. The mecA gene was detected by PCR and the oxacillin minimum inhibitory concentration (MIC) was determined by using agar dilution method for the isolates with discrepant tests.


RESULTS
Among 1,915 S. aureus islolates tested, one isolate was resistant to oxacillin disk but susceptible to cefoxitin disk; the isolate did not have mecA gene. Another isolate susceptible to oxacillin but resistant to cefoxitin had mecA gene. Among 1,208 coagulase-negative staphylococcal isolates, 15 isolates were resistant to oxacillin disk but susceptible to cefoxitin disk; the isolates did not have mecA genes. Two isolates susceptible to oxacillin disk but resistant to cefoxitin disk had mecA genes. Among the 16 Staphylococcus isolates that did not have mecA gene, 15 isolates had the oxacillin MICs of </= 2 microg/mL and were considered as methicillin-susceptible, while 1 isolate with the MIC of 4 microg/mL was considered as methicillin-resistant.


CONCLUSIONS
Overall, 1.9% of staphylococcal isolates showed discrepant results when the screening tests were performed by using oxacillin and cefoxitin disks. None of the isolates resistant to oxacillin disk but susceptible to cefoxitin disk had mecA gene. In conclusion, the cefoxitin disk test is more reliable than oxacillin disk test in screening methicillin-resistant staphylococcal isolates.
Lee
Yangsoon
Y
Department of Laboratory Medicine and Research Institute of Bacterial Resistance, Yonsei University College of Medicine, Seoul, Korea.
Kim
Chang Ki
CK
Kim
Myungsook
M
Yong
Dongeun
D
Lee
Kyungwon
K
Chong
Yunsop
Y
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000032:analysis / Q000235:genetics*; D002440:Cefoxitin / Q000494:pharmacology*; D052978:Disk Diffusion Antimicrobial Tests; D006801:Humans; D008712:Methicillin / Q000494:pharmacology; D016106:Methicillin Resistance; D010068:Oxacillin / Q000494:pharmacology*; D046915:Penicillin-Binding Proteins; D016133:Polymerase Chain Reaction; D013203:Staphylococcal Infections / Q000382:microbiology; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D046915:Penicillin-Binding Proteins; C487789:mecA protein, Staphylococcus aureus; D002440:Cefoxitin; D008712:Methicillin; D010068:Oxacillin