Antibacterial activity of PD 131628 and proposed disc diffusion susceptibility test criteria

The antibacterial activity of PD 131628 was compared with that of ciprofloxacin against 401 clinical isolates. The two drugs had comparable MICs for most Gram-negative species. PD 131628 was two to eight times more active against Gram-positive species and against Xanthomonas maltophilia. Both drugs were bactericidal, Among Gram-positive isolates, resistant mutants were not detected in vitro. Among Gram-negative bacilli, spontaneously occurring mutants resistant to PD 131628 were readily demonstrated more frequently than for ciprofloxacin. Resistance to PD 131628 was more readily induced by serially transferring the Gram-negative species, and resistant organisms emerged more rapidly on prolonged incubation in time-kill studies. Correlating the PD 131628 5 micrograms disc diffusion zone diameters with PD 131628 MICs, the following breakpoints are tentatively proposed: susceptible, MIC < or = 1 mg/L or zone > or = 19 mm; intermediate, MIC 2.0 mg/L or zones 16-18 mm; and resistant, MIC > or = 4.0 mg/L or zones < or = 15 mm.