Encapsulation in fusogenic liposomes broadens the spectrum of action of vancomycin against gram-negative bacteria

Many antibacterial agents, among which glycopeptides, are inactive against gram-negative bacteria because of their incapacity to cross the outer membrane present in these cells. Different chemical and technological approaches have been described to circumvent such limitation. In this study, we tried to apply the strategy of fusogenic liposomes, up to now used to carry biological compounds and materials inside cells, to localize a glycopeptide antibiotic, vancomycin (VAN), in the perisplamic space, thus allowing it to exert its bactericidal activity. Small unilamellar liposomes were prepared using an extrusion procedure (SUVETs) from a phospholipid-cholesterol hemisuccinate mixture known for its fusogenic properties with eukaryotic cell membrane. VAN was loaded with high efficiency in these vesicles and, in the in vitro microbiological experiments, showed to be able to inhibit to a different extent the growth of wild and standard gram-negative bacterial strains. MIC values as low as 6 mg/L were registered, for instance, against clinical isolates of Escherichia coli and Acinetobacter baumannii. In comparison, neither the free antibiotic nor VAN-loaded ‘classical’ (non fusogenic) liposomes showed any activity against the same bacteria. Scanning and transmission electron microscopy studies allowed to confirm that the produced SUVETs were able to adhere to and fuse with the external membrane of E. coli. According to preliminary experiments, this technological strategy can be proposed as a potentially successful way to enlarge the spectrum of activity of VAN.