Use of an oxacillin disk screening test for detection of penicillin- and ceftriaxone-resistant pneumococci

In a contort of worldwide emergence of resistance among Streptococcus pneum oniae strains, early detection of strains with decreased susceptibility to beta-lactam antibiotics is important for clinicians. If the 1-mu g oxacilli n disk diffusion test is used as described by the National Committee for Cl inical Laboratory Standards, no interpretation is available for strains sho wing zone sizes of less than or equal to 19 mm, and there is presently no d isk diffusion test available for screening cephalosporin resistance, The zo nes obtained by the diffusion method by using the 1-mu g oxacillin disk wer e compared with penicillin MICs for 1,116 clinical strains and with ceftria xone MICs for 695 of these strains. Among the 342 strains with growth up to the 1-mu g oxacillin disk margin, none were susceptible (MIC, less than or equal to 0.06 mu g/ml), 62 had intermediate resistance (MIC, 0.12 to 1.0 m u g/ml), and 280 were resistant (MIC, greater than or equal to 2.0 mu g/ml) to penicillin. For ceftriaxone, among 98 strains with no zone of inhibitio n in response to oxacillin, 68 had intermediate resistance (MIC, 1.0 mu g/m l), and 22 were resistant (MIC, greater than or equal to 2.0 mu g/ml). To o ptimize the use of the disk diffusion method, we propose that the absence o f a zone of inhibition around the 1-mu g oxacillin disk be regarded as an i ndicator of nonsusceptibility to penicillin and ceftriaxone and recommend t hat such strains be reported as nonsusceptible to these antimicrobial agent s, pending the results of a MIC quantitation method.