Disk with high oxacillin content discriminates between methicillin-resistant and borderline methicillin-susceptible Staphylococcus aureus strains in disk diffusion assays using a low salt concentration

Summary, in English

A separation between mecA+ strains of Staphylococcus aureus and strains lacking mecA was achieved by the disk diffusion assay and the agar dilution method, utilizing disks containing 5 microg of oxacillin and inocula of approximately 5 x 10(5) CFU/spot, respectively, provided that agar with 0 to 0.5% NaCl and incubation at 30 degrees C were employed. The 5-microg oxacillin disks clearly discriminated between borderline methicillin-susceptible and mecA+ strains. The oxacillin MICs were more affected by the inoculum density and salt concentration than were the methicillin MICs, and oxacillin MICs of 4 to 16 microg/ml were obtained for strains lacking mecA. Significantly higher levels of beta-lactamase production and reduced oxacillin susceptibilities were recorded for strains lacking mecA, in particular strains of phage group V, when agar with >/=2% NaCl was used than when agar with 0 to 0.5% NaCl was employed. The results indicate that the borderline methicillin-susceptible phenotype is a salt-dependent in vitro phenomenon of questionable clinical relevance.

• Author: Ann Cathrine Petersson; Carl Kamme; Håkan Miörner
• Department/s: Division of Medical Microbiology
• Publishing year: 1999
• Language: English
• Pages: 2047-2050
• Publication/Series: Journal of Clinical Microbiology
• Volume: 37
• Issue: 6
• Links: http://jcm.highwire.org/cgi/content/full/37/6/2047
• Document type: Journal article
• Publisher: American Society for Microbiology
• Topic: Microbiology in the medical area
• Status: Published
• ISBN/ISSN/Other: ISSN: 1098-660X