title: Evaluation of selection media for the detection of borderline MRSA.
abstract: Recently, hospital-associated as well as community-acquired methicillin-resistant Staphylococcus aureus (MRSA) strains showing a low level of resistance to oxacillin have emerged worldwide, and as a result, a highly sensitive method to detect MRSA has become more important. To prevent MRSA being overlooked, some selection agar media have recently been developed. We evaluated six commercially available selection agar media in regard to the detection of 35 borderline MRSA (BOMRSA) strains which were mecA-positive but showed low resistance to oxacillin. The MIC values of oxacillin differed between the broth dilution method and the agar dilution method, and 11 of the 35 BOMRSA strains were judged as sensitive by the broth dilution method and 14 of the 35 strains were judged as sensitive by the agar dilution method. Thirty-two of the 35 strains were also judged as sensitive by an oxacillin disk diffusion test. Moreover, there was no consistent pattern of resistance to the tested beta-lactams among the BOMRSA strains. Some commercially available selection media designed for the detection of MRSA contain a beta-lactam antibiotic; oxacillin, ceftizoxime, or cefoxitin, and we evaluated media containing each of these agents. The detection sensitivities of cefoxitin-based agar media, such as CHROMagar MRSA and MRSA ID, for BOMRSA were 100% at 24-h culture. On the other hand, the media containing oxacillin or ceftizoxime gave lower results at 24 h, suggesting that, possibly, BOMRSA strains may not to be able to grow on these media. These results suggest that cefoxitin-based agar media should be recommended for the first-round screening of BOMRSA.
authors.affiliation: Department of Bacteriology, Nagoya University Graduate School of Medicine, Showa-ku, Nagoya, Japan.
authors.forename: Keiko
authors.initials: K
authors.lastname: Yamada
authors.forename: Kazuhisa
authors.initials: K
authors.lastname: Inuzuka
authors.forename: Norio
authors.initials: N
authors.lastname: Tatsumi
authors.forename: Ikuyo
authors.initials: I
authors.lastname: Sanzen
authors.forename: Teruko
authors.initials: T
authors.lastname: Ohkura
authors.forename: Akira
authors.initials: A
authors.lastname: Okamoto
authors.forename: Tadao
authors.initials: T
authors.lastname: Hasegawa
authors.forename: Michio
authors.initials: M
authors.lastname: Ohta
mesh_terms: D000362:Agar; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics; D001431:Bacteriological Techniques; D002440:Cefoxitin / Q000494:pharmacology*; D003470:Culture Media / Q000737:chemistry*; D006801:Humans; D055624:Methicillin-Resistant Staphylococcus aureus / Q000145:classification* / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests / Q000379:methods; D010068:Oxacillin / Q000494:pharmacology*; D046915:Penicillin-Binding Proteins; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D047090:beta-Lactams / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D003470:Culture Media; D046915:Penicillin-Binding Proteins; D012336:RNA, Ribosomal, 16S; D047090:beta-Lactams; C487789:mecA protein, Staphylococcus aureus; D002440:Cefoxitin; D000362:Agar; D010068:Oxacillin