Evaluation of cefoxitin 5 and 10 μg discs for the detection of methicillin resistance in staphylococci

Abstract

Objective: To evaluate cefoxitin 5 and 10 μg 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 μg cefoxitin disc and overnight incubation in ambient air at 35–37°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 μg, R < 14 mm (99.5% and 98.1%); ISA 10 μg, R < 22 mm (99.5% and 98.1%); MH 5 μg, R < 12 mm (99.7% and 98.1%); and MH 10 μg, 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. Conclusion: For S. aureus, cefoxitin 5 and 10 μg 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.