1. 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
- Author
-
Håkan Miörner, Carl Kamme, and Ann Cathrine Petersson
- Subjects
Microbiology (medical) ,Staphylococcus aureus ,food.ingredient ,Meticillin ,Micrococcaceae ,Staphylococcus ,Microbial Sensitivity Tests ,Sodium Chloride ,medicine.disease_cause ,Microbiology ,Methicillin ,Minimum inhibitory concentration ,food ,polycyclic compounds ,medicine ,Agar ,Oxacillin ,Antibacterial agent ,biology ,Osmolar Concentration ,Bacteriology ,biochemical phenomena, metabolism, and nutrition ,bacterial infections and mycoses ,biology.organism_classification ,Culture Media ,bacteria ,Methicillin Resistance ,Methicillin Susceptible Staphylococcus Aureus ,Bacteria ,medicine.drug - Abstract
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 μg of oxacillin and inocula of approximately 5 × 10 5 CFU/spot, respectively, provided that agar with 0 to 0.5% NaCl and incubation at 30°C were employed. The 5-μg 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 μg/ml were obtained for strains lacking mecA . Significantly higher levels of β-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.
- Published
- 1999