Identification of MRSA by various methods to detect borderline oxacillin-resistant Staphylococcus aureus

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) was identified by various methods, including the disk diffusion test, to predict mecA-mediated resistance by Staphylococcus with a cefoxitin disk, which was newly described in NCCLS (at present CLSI) M100-S14 (2004), among 79 strains of borderline oxacillin-resistant Staphylococcus aureus (BORSA) whose oxacillin MIC values were 1, 2, 4 and 8μg/mL.
The results of the susceptibility testing of strains containing mecA did not agree with the oxacillin MIC values, and mecA (+) was present in 1 of the 30 strains and in 5 of the 30 strains that were susceptible to oxacillin at 1 and 2μg/mL, respectively. There were 12 PBP2'-producing strains among the 54 mecA (-) strains, and since the reaction of 7 of these strains to latex particles was weak, their susceptibility to oxacillin was judged to be weakly positive. However, since these 12 strains were susceptible to oxacillin and cefoxitin, the results of the PBP2' test were presumed to be due to a nonspecific reaction. The test results with the cefoxitin disk indicated that 53 of the 60 strains susceptible to oxacillin were also susceptible to cefoxitin, but the remaining 7 strains were resistant to cefoxitin, and the diameters of the inhibition zone were around the borderline values for identification. The frequency of errors in the cefoxitin susceptibility test were low, i. e., 1.3% for very major error and 8.9% for major error, but since the diameters of the inhibition zone in case of BORSA may be around the borderline values for identification, it is concluded that it is necessary to confirm the susceptibility of these strains to oxacillin by other methods in addition.