Efficacy of rifampicin combination therapy for the treatment of enterococcal infections assessed in vivo using a Galleria mellonella infection model

Enterococci are a leading cause of healthcare-associated infection worldwide and display increasing levels of resistance to many of the commonly used antimicrobials, making treatment of their infections challenging. Combinations of antibiotics are occasionally employed to treat serious infections, allowing for the possibility of synergistic killing. The aim of this study was to evaluate the effects of different antibacterial combinations against enterococcal isolates using an in vitro approach and an in vivo Galleria mellonella infection model. Five Enterococcus faecalis and three Enterococcus faecium strains were screened by paired combinations of rifampicin, tigecycline, linezolid or vancomycin using the chequerboard dilution method. Antibacterial combinations that displayed synergy were selected for in vivo testing using a G. mellonella larvae infection model. Rifampicin was an effective antibacterial enhancer when used in combination with tigecycline or vancomycin, with minimum inhibitory concentrations (MICs) of each individual antibiotic being reduced by between two and four doubling dilutions, generating fractional inhibitory concentration index (FICI) values between 0.31 and 0.5. Synergy observed with the chequerboard screening assays was subsequently observed in vivo using the G. mellonella model, with combination treatment demonstrating superior protection of larvae post-infection in comparison with antibiotic monotherapy. In particular, rifampicin in combination with tigecycline or vancomycin significantly enhanced larvae survival. Addition of rifampicin to anti-enterococcal treatment regimens warrants further investigation and may prove useful in the treatment of enterococcal infections whilst prolonging the clinically useful life of currently active antibiotics.