Effects of changes in pH, medium and inoculum size on the in vitro activity of different quinolone and fluoroquinolone antibiotics against urinary pathogens.

The antibacterial activity of quinolones and fluoroquinolones was evaluated against several strains of enterobacteria growing in Mueller-Hinton broth and in urine at different pH. The data obtained in alkaline urine and in Mueller-Hinton broth indicate that the MIC values of the fluoroquinolones are about 16-32 times inferior to those found in urine at pH 6. On the other hand, the activity of nalidixic acid and more particularly that of cinoxacin in urine appears to be optimum at this lower pH, showing MIC values only 2-4 times higher than those obtained in Mueller-Hinton media. Although the fluoroquinolones appear to have the highest degree of activity, a better MBC/MIC ratio was observed for cinoxacin and for nalidixic and oxolinic acids.