Conditions affecting the results of susceptibility testing for the quinolone compounds

The quinolone class of compounds was studied for conditions which might affect susceptibility results. These compounds included amifloxacin, ciprofloxacin, difloxacin, enoxacin, norfloxacin, ofloxacin, and RO 23-6240. Ciprofloxacin, a representative quinolone, was found to have rapid bactericidal activity, equivalent to that of gentamicin, in contrast to the slower activity of a cephalosporin, cefotaxime. Test conditions that might affect susceptibility test results included divalent magnesium and calcium cation concentrations and pH. For strains of Staphylococcus aureus, Enterobacteriaceae, and enterococcus, the effects were not large. A pH of 5.0 in general increased the minimum inhibitory concentrations (MICs) for the organisms against most carboxyquinolones, up to 8-fold, as compared to that at pH 7.4. In comparison, a similar lowering of pH caused an increased in MIC of 32-fold for gentamicin and no change for cefotaxime. Increasing the concentrations of divalent cations increased the MICs on the average of only 4-fold. Of the quinolones, difloxacin was the least affected by change in concentration of divalent cations and by pH. Such changes are not expected to greatly affect the efficacy of therapy of those members of Enterobacteriaceae which have MICs much less than 0.1 micrograms/ml, but might diminish therapeutic efficacy for those organisms such as Streptococcus aureus with MICs of 1.0 microgram/ml or higher.