The goal of this study was to clarify the synergistic antibacterial activity of the combination of tigecycline (TGC) and rifampicin (RIF). Additionally, the study sought to investigate the impact of this combination on the development of mutational resistance and to assess its efficacy in an in vivo model using Galleria mellonella. Through a checkerboard test, we found that the combination of TGC and RIF showed synergistic antibacterial activity against carbapenem-resistant Klebsiella pneumoniae (CRKP). The fractional inhibition concentration index (FICI) was found to be ≤0.5, confirming the potency of the combination. Additionally, this synergistic effect was further validated in vivo using the G. mellonella infection model. TGC-RIF treatment had a lower mutant prevention concentration (MPC) than that of monotherapy, indicating its potential to reduce the development of mutational resistance. We observed a substantial variation in the MPCs of TGC and RIF when they were measured at different proportions in the combinations. Furthermore, during the resistant mutant selection window (MSW) test, we noticed a correlation between strains with low FICI and low MSW. The expression of efflux-pump-related genes, namely rarA and acrB, is significantly decreased in the combination therapy group. This indicates that altered expression levels of certain efflux pump regulator genes are associated with a combined decrease in bacterial mutation resistance. In conclusion, the combination of TGC and RIF effectively suppresses antibiotic resistance selection in CRKP. This study establishes a paradigm for evaluating drug-resistant mutant suppression in antimicrobial combination therapy.
Keywords: carbapenem-resistant Klebsiella pneumoniae; mutant prevention concentration; synergistic effect; tigecycline.