Curcumin reverses high-level tigecycline resistance mediated by different mechanisms in Gram-negative bacteria

Tigecycline is one of the few effective treatments for multidrug-resistant bacteria. However, the recent emergence and spread of high-level tigecycline resistance in Enterobacteriaceae have significantly limited its clinical use. To combat this challenge, combining antibiotics with adjuvants has emerged as a promising strategy. Curcumin, known for its antibacterial properties and ability to enhance antibiotic efficacy, presents a viable option for reversing tigecycline resistance.

This study aimed to evaluate the in vitro and in vivo synergistic effects of curcumin and tigecycline in gram-negative bacteria with different tigecycline resistance mechanisms and to elucidate the molecular mechanisms by which curcumin reverses tigecycline resistance.

The checkerboard assay was used to evaluate the synergistic effects of tigecycline and curcumin, while the time-killing curves were used to assess their antibacterial activity. The study also examined their impact on biofilm eradication, the development of tigecycline resistance, and the conjugative transfer of tigecycline-resistant plasmids. The molecular mechanisms underlying the combined effect were investigated. Additionally, the in vivo efficacy of the tigecycline-curcumin combination against tigecycline-resistant Escherichia coli was assessed using a mouse peritonitis infection model.

This study revealed that curcumin and tigecycline exhibited synergistic effects against tigecycline-resistant gram-negative bacteria with various resistance mechanisms in vitro. Notably, the addition of curcumin delayed the development of tigecycline resistance in E. coli and impeded the horizontal transfer of tet(X4)-positive IncX1 plasmid. The curcumin-tigecycline combination significantly disrupted cell membrane integrity and reduced efflux pump activity by modulating proton dynamics and inhibiting ATP synthesis. Transcriptomic and proteomic analyses supported these findings, revealing disruptions in central carbon metabolism and substantial effects on the electron transport chain. In vivo experiments using a peritonitis model induced by the tet(X4)-carrying E. coli ZZ9DT16R demonstrated that the curcumin-tigecycline combination improved survival rates, reduced bacterial counts, and enhanced liver and spleen histopathology. Furthermore, the expression of the tet(X4) gene was reduced, and molecular docking studies indicated that curcumin binds to diverse tigecycline resistance proteins.

This study is the first to reveal the synergistic action of curcumin and tigecycline, highlighting its potential as a combined therapeutic strategy against tigecycline-resistant gram-negative pathogens with various resistance mechanisms.