In vitro synergistic activity of colistin and ceftazidime or ciprofloxacin against multidrug-resistant clinical strains of Pseudomonas aeruginosa.

Infections caused by multidrug resistant (MDR) Pseudomonas aeruginosa are difficult to treat. Antibiotic development is dwindling in recent years. In order to develop new alternate therapies antimicrobial activity of different antibiotic combinations are being studied in vitro and in vivo. Sub-inhibitory concentrations of colistin were tested in combination with ceftazidime or ciprofloxacin by the checkerboard method against 25 MDR strains of P. aeruginosa. Synergy was observed for ceftazidime or ciprofloxacin antibiotic combinations with colistin among 73.3% of MDR3 (R(AMK, GEN, TOB) R(CAZ) R(CIP)) strains and 100% of MDR4 (R(AMK, GEN, TOB) R(CAZ) R(CIP) R(TZP)) strains. 6.6% strains of MDR3 and 14.3% strains of MDR5 (R(AMK, GEN, TOB) R(CAZ) R(CIP) R(TZP) R(IPM)) phenotypes were inhibited by colistin and ceftazidime alone and 6.6% strains of MDR3 phenotypes were inhibited by colistin and ciprofloxacin alone. For the remaining strains, though synergy was not observed, significant reduction in minimum inhibitory concentration was evident. The results of this study are significant as sub-inhibitory concentrations of colistin have an advantage of reducing in vivo toxicity. These findings need further evaluation for clinical use.