Aims: To compare mutant prevention concentration (MPC) of ciprofloxacin and time-killing curve with regards to 11 genotyped Escherichia coli.
Method: MICs were determined using the E-test method. Time-killing studies were performed in accordance with the NCCLS guidelines. The genes gyrA, gyrB, parC, parE and marR were amplified by PCR and sequenced. The MPC was defined as the lowest antibiotic concentration preventing the growth of resistant colonies when 10(10) CFU/mL were spread on a solid medium.
Results: Strains with no genes gyrA, gyrB, parC, parE and marR mutation presented MIC less or equal to 0.023 mg/L and MPC less or equal to 0.25 mg/L. Strains with two mutations (gyrA and parC) presented MIC equal to 1.5 mg/L and MPC equal to 4 mg/L. Strains with one mutation (gyrA) presented MIC less or equal to 0.75 mg/L, but MPC ranged from 0.5 to 6 mg/L depending of the MIC of ciprofloxacin. The time-killing curves for ciprofloxacin showed a bactericidal activity of 0.25 mg/L in 1h for strains without mutation, compared with a bactericidal activity of 2 and 4 mg/L in 4h for strains with one and two mutations, respectively.
Conclusion: For strains of E. coli resistant to nalidixic acid, it was necessary to evaluate the MIC of ciprofloxacin in order to asses the optimal dosage of ciprofloxacin.