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Antimicrobial Agents and Chemotherapy, May 2008, p. 1635-1641, Vol. 52, No. 5
0066-4804/08/$08.00+0     doi:10.1128/AAC.01071-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Synergy between Polyethylenimine and Different Families of Antibiotics against a Resistant Clinical Isolate of Pseudomonas aeruginosa

Faculty of Life Sciences, Northwestern Polytechnical University, Xi'an, Shaanxi 710072, People's Republic of China,¹ Liposome Research Centre, 18 Gaoxin 2nd Road, Xi'an, Shaanxi 710075, People's Republic of China²

Received 14 August 2007/ Returned for modification 12 October 2007/ Accepted 5 February 2008

The in vitro activities of 10 families of antimicrobial agents alone and in combination with a synthetic polycationic polymer, polyethylenimine (PEI), against a resistant clinical isolate of Pseudomonas aeruginosa were investigated by MIC assays, checkerboard testing, and killing curve studies. At a concentration of 250 nM, PEI (10 kDa) was not directly bactericidal or bacteriostatic; but when it was used in combination with novobiocin, ceftazidime, ampicillin, ticarcillin, carbenicillin, piperacillin, cefotaxime, chloramphenicol, rifampin, or norfloxacin, it significantly reduced the MICs of these antibiotics by 1.5- to 56-fold. However, the MICs of aminoglycosides, polymyxins, and vancomycins were increased by 1.2- to 5-fold when these drugs were combined with PEI; and the MICs of tetracycline, erythromycin, ciprofloxacin, and ofloxacin were not affected when these drugs were combined with PEI. In the killing curve studies, combinations of PEI with novobiocin, ceftazidime, chloramphenicol, or rifampin resulted in 5- to 8-log₁₀ CFU/ml reductions in bacterial counts when 25% of the MIC of each antibiotic was used. These results indicate that infections due to resistant Pseudomonas strains could be treated by the use of a synergistic combination of PEI and antimicrobial drugs.

Published ahead of print on 19 February 2008.