Comparative antimicrobial activity of gatifloxacin tested against Streptococcus spp. including quality control guidelines and etest method validation

Gatifloxacin (formerly AM-1155 or CG 5501) is a new 8-methoxy fluoroquinolo ne with enhanced activity against Gram-positive cocci, especially Streptoco ccus pneumoniae and other streptococci. Recent clinical strains (599 isolat es) were tested against gatifloxacin, three comparison fluoroquinolones, an d penicillin by the reference broth microdilution, Etest (AB BIODISK, Solna , Sweden) and standardized disk diffusion methods (5 mu g gatifloxacin disk ). Gatifloxacin (MIC90, 0.5 mu g/ml) activity was generally comparable to t hat of trovafloxacin (MIC90, 0.25 mu g/ml), or sparfloxacin (MIC90, 0.5 mu g/ml) and markedly superior to ofloxacin (MIC90, 2-4 mu g/ml) against the s treptococci. Rates of penicillin non-susceptibility were 41.4, 38.0, and 16 .2% for S. pneumoniae (301 strains), viridans group streptococci (150 strai ns), and beta-haemolytic streptococci (148 strains). Etest results correlat ed well (95.7-100.0% +/- one log(2) dilution) with the reference MIC result s, but Etest tended to have elevated gatifloxacin MIC results compared to t he broth microdilution method for the highly resistant isolates (MICs, > 2 mu g/ml). Gatifloxacin disk zone diameters correlate well to reference MICs for all streptococci and proposed interpretive criteria (susceptible at le ss than or equal to 1 mu g/ml or greater than or equal to 18 mm, and resist ant at greater than or equal to 4 mu g/ml or less than or equal to 14 mm) d id not produce discords between method results (absolute agreement). A nine laboratory quality control (QC) study conforming to the National Committee for Clinical Laboratory Standards (NCCLS) Guideline M23-T3 studied S. pneu moniae ATCC 49619 and gatifloxacin. Proposed ranges for QC of NCCLS tests w ere 0.12-0.5 mu g/ml for the broth microdilution test and 24-31 mm for the disk diffusion method. These reported results indicate that gatifloxacin wa s a patent fluoroquinolone with extensive activity against streptococcal is olates. In vitro test methods to measure this activity appear accurate and comparable; and RC guidelines have been established for routine clinical la boratory use pending approval by the NCCLS and the Food and Drug Administra tion (FDA). (C) 1999 Elsevier Science Inc.