ANTIMICROBIAL SUSCEPTIBILITY STUDIES Comparative Antimicrobial Activity of Gatifloxacin Tested Against Streptococcus spp. Including Quality Control Guidelines and Etest Method Validation Ronald N. Jones, David M. Johnson, Meredith E. Erwin, Mondell L. Beach, Douglas J. Biedenbach, Michael A. Pfaller, and The Quality Control Study Group Gatifloxacin (formerly AM-1155 or CG 5501) is a new 8-methoxy fluoroquinolone with enhanced activity against Gram-positive cocci, especially Streptococcus pneumoniae and other streptococci. Recent clinical strains (599 isolates) were tested against gatifloxacin, three comparison fluoroquino- lones, and penicillin by the reference broth microdilution, Etest (AB BIODISK, Solna, Sweden) and standardized disk diffu- sion methods (5 g gatifloxacin disk). Gatifloxacin (MIC 90 , 0.5 g/ml) activity was generally comparable to that of trova- floxacin (MIC 90 , 0.25 g/ml), or sparfloxacin (MIC 90 , 0.5 g/ ml) and markedly superior to ofloxacin (MIC 90 , 2–4 g/ml) against the streptococci. Rates of penicillin non-susceptibility were 41.9, 38.0, and 16.2% for S. pneumoniae (301 strains), viridans group streptococci (150 strains), and -haemolytic streptococci (148 strains). Etest results correlated well (95.7– 100.0%  one log 2 dilution) with the reference MIC results, but Etest tended to have elevated gatifloxacin MIC results compared to the broth microdilution method for the highly re- sistant isolates (MICs,  2 g/ml). Gatifloxacin disk zone diameters correlate well to reference MICs for all streptococci and proposed interpretive criteria (susceptible at  1 g/ml or  18 mm, and resistant at  4 g/ml or  14 mm) did not produce discords between method results (absolute agree- ment). A nine laboratory quality control (QC) study conform- ing to the National Committee for Clinical Laboratory Stan- dards (NCCLS) Guideline M23-T3 studied S. pneumoniae ATCC 49619 and gatifloxacin. Proposed ranges for QC of NCCLS tests were 0.12–0.5 g/ml for the broth microdilution test and 24–31 mm for the disk diffusion method. These re- ported results indicate that gatifloxacin was a potent fluoro- quinolone with extensive activity against streptococcal isolates. In vitro test methods to measure this activity appear accurate and comparable; and QC guidelines have been established for routine clinical laboratory use pending approval by the NCCLS and the Food and Drug Administration (FDA). © 1999 Elsevier Science Inc. From the Medical Microbiology Division, University of Iowa College of Medicine, Iowa City, IA 52242. Includes the University of Iowa College of Medicine, Iowa City, IA (G.V. Doern); VA Medical Center, Portland, OR (D. Sewell); VA Medical Center, Iowa City, IA (T.H. Haugen); Clini- cal Microbiology Institute, Inc., Wilsonville, OR (S. Brown); Ac- cuMed, Cleveland, OH (C. Knapp); University of Texas-LBJ, Houston, TX (A. Wanger); Washington University-Barnes Hos- pital, St. Louis, MO (P. Murray); The Cleveland Clinic Founda- tion, Cleveland, OH (J.A. Washington); and the University of Alberta Medical Centre, Edmonton, Alberta, Canada (R. Ren- nie). Address reprint requests to Dr. Ronald N. Jones, Medical Mi- crobiology Division, C606 GH, Department of Pathology, Uni- versity of Iowa College of Medicine, Iowa City, IA 52242. Received 1 July 1998; revised and accepted 10 July 1998. DIAGN MICROBIOL INFECT DIS 1999;34:91–98 © 1999 Elsevier Science Inc. All rights reserved. 0732-8893/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0732-8893(99)00085-6