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.

Cefoxitin is increasingly recommended for detection of methicillin resistance in Staphylococcus aureus (MRSA) when using disk diffusion testing. In this study, 95 mecA-negative S. aureus isolates and a highly genetically diverse collection of mecA-positive S. aureus types (n ‫؍‬ 50) were used to investigate the influence of technical factors such as disk potency, incubation time, and temperature on Mueller-Hinton agar. The use of cefoxitin MIC testing by Etest for the same purpose was investigated under similar conditions. For disk diffusion, the accuracy was high at both 35°C and 36°C using overnight incubation, while incubation at 30°C or 37°C was associated with slightly lower accuracy. Increasing incubation times from 18 to 24 h did not improve accuracy at either temperature. Cefoxitin Etest MICs for mecA-positive strains were 6 mg/liter or higher, while cefoxitin Etest MICs for mecA-negative strains were <4 mg/liter. Our findings suggest that the current CLSI zone diameter breakpoints should be adjusted from resistance (R) < 19 mm to R < 21 mm. In conclusion, cefoxitin disk diffusion testing and Etest MIC testing can accurately predict the presence of the mecA gene in S. aureus. Testing can be reliably performed using incubation temperatures of 35 to 36°C and incubation times of 18 to 22 h. We suggest MRSA interpretive criteria of susceptible (S) < 4 mg/liter and R > 4 mg/liter, corresponding to S > 22 mm and R < 21 mm for the 30-g disk and S > 17 mm and R < 16 mm for the 10-g cefoxitin disk. These criteria resulted in only one mecA-positive isolate being misclassified as susceptible.

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.

No abstract

In disk diffusion tests, cefoxitin is now considered a better indicator than oxacillin for the presence of the mecA gene in Staphylococcus aureus. A logical extension of this work is the incorporation of cefoxitin into media selective for methicillin-resistant Staphylococcus aureus (MRSA). This paper describes the development and subsequent testing of mannitol salt agar containing 4 mg/liter cefoxitin with a unique collection of wellcharacterized MRSA strains, including low-level methicillin-resistant strains and an equal number of known mecA-negative S. aureus strains. The agar supported the growth of 96.6% of the mecA-positive strains in the collection and inhibited the growth of 100% of the mecA-negative strains. These results suggest that selective media based on cefoxitin are superior to those based on oxacillin for the detection of MRSA.Culture media selective for methicillin-resistant Staphylococcus aureus (MRSA) have traditionally been based on blood agar, mannitol salt agar (MSA), or Baird-Parker agar containing methicillin or oxacillin alone or in combination with other antibiotics. An enrichment step consisting of culture in nutrient broth with up to 7.5% sodium chloride prior to inoculation of one of the selective agars mentioned above has also been advocated. Countless reports in the literature (2, 3, 5, 7, 10) have described screening media for MRSA, and this possibly reflects the fact that phenotypic methods based on methicillin or oxacillin have never achieved levels of sensitivity and specificity which have been totally acceptable. However, we have continued to base our screening media on these agents simply because we have had nothing better. Recent reports (4,8,9) have shown that cefoxitin is a better agent for prediction of methicillin resistance in S. aureus, and disk susceptibility testing with cefoxitin now replaces disk susceptibility testing with methicillin and oxacillin in an increasing number of centers. We describe the use of a collection of well-characterized MRSA strains in the development of a selective medium which replaces oxacillin with cefoxitin. MATERIALS AND METHODSMSA (DM160D) was from Mast Diagnostics (Merseyside, United Kingdom), cefoxitin sodium salt was from Sigma-Aldrich, and Etest was from AB Biodisk (Solna, Sweden).A total of 89 mecA-negative and 87 mecA-positive Staphylococcus aureus strains, described below, and three S. aureus control strains (two mecA-positive strains, ATCC 33591 and ATCC 43300, and one mecA-negative strain, ATCC 29213) were used in the study. Pulsed-field gel electrophoresis and/or multilocus sequence typing (MLST) had previously been performed with all mecA-positive strains (9, 10).The following groups of strains were tested in this study: (i) a 100-strain collection of 75 mecA-positive S. aureus (MRSA) strains and 25 mecA-negative S. aureus (methicillin-susceptible S. aureus [MSSA]) strains described previously (10), which were used in the initial stages in order to set an appropriate concentration of cefoxitin for use in the medium; (ii) 64 S...