Comparative in vitro studies with 4-quinolone antimicrobials

The minimal inhibitory concentrations (MICs) of nalidixic acid, pipemidic acid, cinoxacin, oxolinic acid, flumequine, pefloxacin, acrosoxacin, amifloxacin, norfloxacin, enoxacin, ofloxacin and ciprofloxacin were determined for a range of clinical isolates. MICs were determined using an agar dilution technique in Mueller-Hinton agar supplemented with 10% lysed horse blood. The inoculum used was approximately 10(4) colony forming units, contained in 10 microliters Mueller-Hinton broth, which was applied to the agar plates using a multipoint inoculator. Following inoculation, plates were incubated in conditions appropriate for the organisms under investigation. The MIC of each antimicrobial for each isolate examined was determined as the lowest concentration of the antimicrobial which completely inhibited growth of the inoculum. The minimum concentrations required to inhibit the growth of 50% (MIC50) and 90% (MIC90) of the organisms examined were also determined. All of the more recently synthesised 4-quinolones showed considerably greater activity than the parent compounds, nalidixic acid, pipemidic acid and cinoxacin, against the range of organisms used in this study. Ciprofloxacin and ofloxacin were the two most active of the 4-quinolones examined.