Evaluation of cefoxitin 5 and 10 microg discs for the detection of methicillin resistance in staphylococci.
OBJECTIVE
To evaluate cefoxitin 5 and 10 microg discs for detection of methicillin resistance in staphylococci.


METHODS
Six hundred and forty-one Staphylococcus aureus (261 mecA-negative and 380 mecA-positive) and 344 coagulase-negative staphylococci (CoNS) (132 mecA-negative and 212 mecA-positive) were investigated. The CoNS represented nine species, Staphylococcus epidermidis being the most frequent (n = 231). All isolates were tested using semi-confluent growth on Iso-Sensitest agar (ISA), and Mueller-Hinton agar (MH) using a 5 and a 10 microg cefoxitin disc and overnight incubation in ambient air at 35-37 degrees C.


RESULTS
For S. aureus, both cefoxitin discs performed with high accuracy on both media. The sensitivity and specificity for the following proposed interpretive zone diameters were: ISA 5 microg, R < 14 mm (99.5% and 98.1%); ISA 10 microg, R < 22 mm (99.5% and 98.1%); MH 5 microg, R < 12 mm (99.7% and 98.1%); and MH 10 microg, R < 18 mm (99.5% and 98.9%), respectively. All four variants were superior to oxacillin using the former SRGA methodology. In CoNS, a substantial overlap was seen for all variants. However, by avoiding primary interpretation in the overlapping interval, highly accurate results could be obtained for 81%, 80%, 91% and 97% of the isolates, respectively.


CONCLUSIONS
For S. aureus, cefoxitin 5 and 10 microg discs performed with high accuracy on both ISA and MH using semi-confluent growth and standard incubation conditions. With the introduction of a defined interval in which primary interpretation should be avoided, the method could also be used for CoNS.
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
Larsen
A R
AR
Frimodt-Møller
N
N
Kahlmeter
G
G
D002440:Cefoxitin / Q000494:pharmacology*; D004341:Drug Evaluation / Q000379:methods; D006801:Humans; D016106:Methicillin Resistance / Q000187:drug effects* / Q000502:physiology; D008826:Microbial Sensitivity Tests / Q000379:methods; D013210:Staphylococcus / Q000187:drug effects* / Q000254:growth & development* / Q000302:isolation & purification
D002440:Cefoxitin