Abstract

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 μg disc diffusion zone diameters with PD 131628 MICs, the following breakpoints are tentatively proposed: susceptible, MIC ≤ 1 mg/L or zone ≽ 19 mm; intermediate, MIC 2⋅0 mg/L or zones 16–18 mm; and resistant, MIC ≽ 4⋅0 mg/L or zones ≤ 15 mm.

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