Evaluation of a cefoxitin 30 microg disc on Iso-Sensitest agar for detection of methicillin-resistant Staphylococcus aureus.
OBJECTIVE
To evaluate the performance of a cefoxitin 30 microg disc on Iso-Sensitest agar, using a semi-confluent inoculum and overnight incubation at 35-36 degrees C, for detection of methicillin-resistant Staphylococcus aureus (MRSA).


METHODS
A total of 457 S. aureus, including 190 MRSA of several defined PFGE types and a number of low-level resistant isolates, were tested with a cefoxitin 30 microg disc on Iso-Sensitest agar, using a semi-confluent inoculum and overnight incubation at 35-36 degrees C. This method was compared with the standard SRGA (Swedish Reference Group for Antibiotics) method (oxacillin 1 microg disc on Iso-Sensitest agar supplemented with 5% defibrinated horse blood, confluent growth and 24 h incubation in ambient air at 30 degrees C).


RESULTS
The cefoxitin method was excellent, with a sensitivity of 100% and a specificity of 99% using an interpretative zone diameter of S > or = 29 mm and R < 29 mm. Its performance was much better than the SRGA method, which with this collection of difficult strains had a sensitivity of only 78% using the current breakpoint of S > or = 12 mm.


CONCLUSIONS
We suggest that the cefoxitin method should replace that currently recommended by the SRGA for the detection of MRSA, and that it would fit well into BSAC methodology.
Skov
R
R
National Center for Antimicrobials and Infection Control, Statens Serum Institut, Artillerivej 5, DK-2300 Copenhagen S, Denmark. rsk@ssi.dk
Smyth
R
R
Clausen
M
M
Larsen
A R
AR
Frimodt-Møller
N
N
Olsson-Liljequist
B
B
Kahlmeter
G
G
D000362:Agar / Q000494:pharmacology; D002440:Cefoxitin / Q000494:pharmacology*; D004353:Drug Evaluation, Preclinical / Q000379:methods; D006801:Humans; D016106:Methicillin Resistance / Q000502:physiology*; D008826:Microbial Sensitivity Tests / Q000379:methods / Q000706:statistics & numerical data; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification
D002440:Cefoxitin; D000362:Agar