General Comments (1) For disk diffusion, test a maximum of 12 disks on a 150-mm plate and up to 6 disks on a 100-mm plate; disks should be placed no less than 24 mm apart, center to center (M02, Section 9.2 will be updated during its next scheduled revision to include this recommendation). Each zone diameter should be clearly measurable; overlapping zones prevent accurate measurement. Measure the diameter of the zones of complete inhibition (as judged by the unaided eye), including the diameter of the disk. Hold the Petri plate a few inches above a black background illuminated with reflected light, except for linezolid, which should be read with transmitted light (plate held up to light source). The zone margin should be considered the area showing no obvious, visible growth that can be detected with the unaided eye. Ignore faint growth of tiny colonies that can be detected only with a magnifying lens at the edge of the zone of inhibited growth. With trimethoprim and the sulfonamides, antagonists in the medium may allow some slight growth; therefore, disregard slight growth (20% or less of the lawn of growth) and measure the more obvious margin to determine the zone diameter. For linezolid, any discernible growth within the zone of inhibition is indicative of resistance to the respective agent. (2) Historically, resistance to the penicillinase-stable penicillins (see Glossary I) has been referred to as “methicillin resistance” or “oxacillin resistance.” MRSAs are those strains of S. aureus that express mecA or another mechanism of methicillin resistance, such as changes in affinity of penicillin-binding proteins for oxacillin (modified S. aureus strains).