Tentative interpretive criteria for testing the susceptibility of Streptococcus pneumoniae to eight fluoroquinolones

Broth microdilution and disk diffusion methods were used to test ciprofloxacin, clinafloxacin, fleroxacin, grepafloxacin, ofloxacin, PD131628, sparfloxacin, and trovafloxacin against Streptococcus pneumoniae isolates. With each fluroquinolone tested, there was a striking unimodal population. With the exception of fleroxacin, over 96% of pneumococci tested were inhibited by each drug's modal MIC +/-1 twofold concentration. Over 92% of pneumococci produced inhibitory zone diameters equal to the mode +/-4 mm for each drug. For most fluoroquinolones, the interpretive criteria approved or proposed for nonfastidious organisms appear to be appropriate for pneumococci. For clinafloxacin and trovafloxacin, however, the proposed MIC breakpoints for nonfastidious organisms are four-to eightfold greater than the highest MIC encountered with pneumococci. For future surveillance efforts, low concentrations must be evaluated to detect any subtle shift in pneumococcal populations that might be undetected by testing only breakpoint concentrations.