Liposomal encapsulation of vancomycin improves killing of methicillin-resistant Staphylococcus aureus in a murine infection model.

OBJECTIVES: Methicillin-resistant Staphylococcus aureus (MRSA) poses a major problem to public health worldwide. MRSA strains with increased resistance to vancomycin cause infections that are associated with greater morbidity and threaten the use of this once gold-standard antistaphylococcal drug. We investigated whether encapsulation of vancomycin within liposomes could improve its antistaphylococcal activity. METHODS: Two liposomal formulations of vancomycin were prepared using a rehydration-dehydration method. MICs and MBCs of the liposomal vancomycin for strains of MRSA were determined. The efficacy of one of the liposomal vancomycin formulations was also investigated in a time-kill assay in vitro and in a murine systemic infection model. RESULTS: Encapsulation in either liposome preparation decreased the vancomycin MICs and MBCs for MRSA strains by approximately 2-fold. Liposomal vancomycin increased killing of MRSA in vitro in a kinetic study. In a systemic murine infection model, treatment with a 50 mg/kg intraperitoneal injection of liposomal vancomycin improved kidney clearance of a USA300 strain by 1 log compared with an injection of 50 mg/kg of free vancomycin. CONCLUSIONS: Our findings suggest that entrapment within liposomes could improve the antistaphylococcal efficacy of vancomycin.

O-Acetylation of Peptidoglycan Limits Helper T Cell Priming and Permits Staphylococcus aureus Reinfection.

Humans do not usually develop effective immunity to Staphylococcus aureus reinfection. Using a murine model that mimics human infection, we show that lack of protective immunity to S. aureus systemic reinfection is associated with robust interleukin-10 (IL-10) production and impaired protective Th17 responses. In dendritic cell co-culture assays, priming with S. aureus promotes robust T cell proliferation, but limits Th cells polarization and production of IL-1ß and other cytokines important for Th1 and Th17 differentiation. We show that O-acetylation of peptidoglycan, a mechanism utilized by S. aureus to block bacterial cell wall breakdown, limits the induction of pro-inflammatory signals required for optimal Th17 polarization. IL-10 deficiency in mice restores protective immunity to S. aureus infection, and adjuvancy with a staphylococcal peptidoglycan O-acetyltransferase mutant reduces IL-10, increases IL-1ß, and promotes development of IL-17-dependent, Th cell-transferable protective immunity. Overall, our study suggests a mechanism whereby S. aureus modulates cytokines critical for induction of protective Th17 immunity.