Antibacterial effect and mechanisms of action of forsythoside B, alone and in combination with antibiotics, against Acinetobacter baumannii and Pseudomonas aeruginosa

BACKGROUND: Antibiotic resistance complicates infection treatments. Natural products, such as phenylethanoid glycosides, including forsythoside B (FB), are gaining attention in clinical use as alternative treatments, either alone or in combination with antibiotics. PURPOSE: To investigate the antibacterial effects and mechanisms of FB alone and in combination with antibiotics against Acinetobacter baumannii and Pseudomonas aeruginosa. METHODS: To elucidate the underlying antibacterial mechanism of FB, we assessed intracellular ATP concentration, pH levels, membrane potential, and cell membrane integrity. We also observed bacterial morphology and conducted biofilms eradication assay. FB toxicity was evaluated using the cell counting kit-8 assay. The in vivo pharmacodynamics of FB was explored using a P. aeruginosa systemic infection mouse model. The study also examined the potential synergistic effects of FB with commonly used antibiotics by the checkerboard dilution method and time-kill assay. RESULTS: The findings indicate that the mechanism of antibacterial activity of FB is through the disruption of bacterial cell membranes, thereby increasing cell membrane permeability, particularly in gram-negative bacteria. Synergistic effects of FB combined with meropenem were demonstrated against resistant strains. FB demonstrated low toxicity in both in vitro and in vivo models, supporting its safety and efficacy for use alone or as an antibiotic adjuvant. CONCLUSIONS: FB expands the antibacterial spectrum and enhances the effectiveness of existing antibiotics against resistant bacterial strains, making it a promising adjuvant for treating gram-negative bacterial infections. This study highlights the potential of FB in combating antibiotic resistance and suggests further research into its mechanisms and drug development applications. It provides a framework for studying the interaction between natural products and microorganisms, revealing new biological mechanisms.