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Antimicrobial Agents and Chemotherapy, June 2006, p. 2016-2022, Vol. 50, No. 6
0066-4804/06/$08.00+0     doi:10.1128/AAC.01547-05
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Mechanism of Enhanced Activity of Liposome-Entrapped Aminoglycosides against Resistant Strains of Pseudomonas aeruginosa

The Novel Drug and Vaccine Delivery Systems Facility, Department of Chemistry and Biochemistry, Laurentian University, Sudbury, Ontario P3E 2C6, Canada,¹ University of Texas at Tyler, Department of Biology, 3900 University Blvd, Tyler, Texas 75799,² Division of Tumour Biology, Northeastern Ontario Regional Cancer Centre, Sudbury, Ontario, Canada P3E 5J1, Canada³

Received 5 December 2005/ Returned for modification 31 January 2006/ Accepted 4 April 2006

Pseudomonas aeruginosa is inherently resistant to most conventional antibiotics. The mechanism of resistance of this bacterium is mainly associated with the low permeability of its outer membrane to these agents. We sought to assess the bactericidal efficacy of liposome-entrapped aminoglycosides against resistant clinical strains of P. aeruginosa and to define the mechanism of liposome-bacterium interactions. Aminoglycosides were incorporated into liposomes, and the bactericidal efficacies of both free and liposomal drugs were evaluated. To define the mechanism of liposome-bacterium interactions, transmission electron microscopy (TEM), flow cytometry, lipid mixing assay, and immunocytochemistry were employed. Encapsulation of aminoglycosides into liposomes significantly increased their antibacterial activity against the resistant strains used in this study (MICs of 32 versus 8 µg/ml). TEM observations showed that liposomes interact intimately with the outer membrane of P. aeruginosa, leading to the membrane deformation. The flow cytometry and lipid mixing assays confirmed liposome-bacterial membrane fusion, which increased as a function of incubation time. The maximum fusion rate was 54.3% ± 1.5% for an antibiotic-sensitive strain of P. aeruginosa and 57.8% ± 1.9% for a drug-resistant strain. The fusion between liposomes and P. aeruginosa significantly enhanced the antibiotics' penetration into the bacterial cells (3.2 ± 2.3 versus 24.2 ± 6.2 gold particles/bacterium, P 0.001). Our data suggest that liposome-entrapped antibiotics could successfully resolve infections caused by antibiotic-resistant P. aeruginosa through an enhanced mechanism of drug entry into the bacterial cells.

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