In-vitro bactericidal efficacy of sub-MIC concentrations of liposome-encapsulated antibiotic against Gram-negative and Gram-positive bacteria

It has been shown previously that tobramycin encapsulated in fluid liposomes (composed of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylglycerol (DMPG)) eradicated mucoid Pseudomonas aeruginosa in an animal model of chronic pulmonary infection. Exponential cultures of P. aeruginosa, Stenotrophomonas maltophilia, Burkholderia cepacia, Escherichia coli and Staphylococcus aureus were treated with (i) free tobramycin, (ii) sub-MIG tobramycin encapsulated in DPPC/DMPG liposomes, (iii) control liposomes without antibiotic or (iv) control liposomes combined with free tobramycin. Bacterial colonies were counted 0, 1, 3, 6 and 16 h after addition of antibiotic. After 3 h, the growth of B. cepacia, E. coli and S. aureus was reduced 129, 84 and 566 times respectively in cultures treated with encapsulated antibiotic compared with those treated with free antibiotic. Six hours and 16 h after treatment, the maximal reduction of growth between strains treated with liposome-encapsulated tobramycin and free tobramycin was 84, 129, 166, 10(5) and 10(4) times respectively for P. aeruginosa, B.cepacia, E. coli, S. maltophilia and S. aureus. The liposomes were stable at 4 degrees C and at room temperature for the whole period studied. At 37 degrees C, equivalent stability was observed for the first 16 h of the study. Administration of antibiotic encapsulated in DPPC/DMPG liposomes may thus greatly improve the management of resistant infections caused by a large range of microorganisms. The strong bactericidal activity of the encapsulated antibiotic at sub-MIG doses of the strains tested cannot be explained only as a result of prolonged residence time of liposome-encapsulated tobramycin and the resulting release of entrapped antibiotic at the bacterial site; rather, direct interaction of chemoliposomes and bacteria, probably by a fusion process, may explain the bactericidal effect of the sub-MIG antibiotic doses used.