Demonstration of a fusion mechanism between a fluid bactericidal liposomal formulation and bacterial cells

It was previously demonstrated that fluid liposomal-encapsulated tobramycin, named Fluidosomes, was successful in eradicating mucoid Pseudomonas aeruginosa in an animal model of chronic pulmonary infection, whereas free antibiotic did not reduce colony-forming unit (CFU) counts (C, Beaulac et al., Antimicrob. Agents Chemother. 40 (1996) 665-669; C. Beaulac et al., J. Antimicrob. Chemother. 41 (1998) 35-41), These liposomes were also shown to be bactericidal in in vitro tests against strong resistant P, aeruginosa strains. To define the mechanism by which Fluidosomes work, negative staining, immunoelectron microscopy, fluorescence activated cell sorting (FACS) and lipid-mixing studies were performed. All the lines of evidence suggest that Fluidosomes fuse with bacterial cells leading to a marked increase of tobramycin in the cytoplasm of a resistant bacteria (minimal inhibitory concentration (MIC) > 64 mu g/ml). The time needed to reach the maximal fusion rate was about 5 h for the resistant strain comparatively to much shorter time for the sensitive strain. The specific characteristics of Fluidosomes could help overcome bacterial resistance related to permeability barrier and even enzymatic hydrolysis considering the importance of synergy in the whole process of antibiotic resistance. (C) 2000 Elsevier Science B.V. All rights reserved.