Baicalein in combination with ceftazidime-avibactam exhibits synergism against carbapenem-resistant Acinetobacter baumannii in vitro

Abstract Background The escalating global threat of carbapenem-resistant Acinetobacter baumannii (CRAB), classified by the WHO as a critical priority pathogen, underscores the urgent need for innovative therapeutic strategies. Traditional antibiotics face diminishing efficacy due to rapid resistance evolution, necessitating exploration of adjuvant therapies. Baicalein (BCL), a flavonoid from Scutellaria baicalensis, exhibits broad antimicrobial and resistance-modulating properties. This study aims to evaluate the synergistic effects and mechanisms of BCL combined with ceftazidime-avibactam (CAZ/AVI, CZA) against CRAB to address therapeutic gaps in managing these infections. Methods Twenty CRAB clinical strains from a tertiary hospital were characterized using multi-locus sequence typing (MLST). Synergy between CZA and BCL was assessed via checkerboard assays (fractional inhibitory concentration index, FICI), time-kill kinetics, and biofilm inhibition assays. Transcriptomic profiling (RNA-seq) was performed to identify differentially expressed genes associated with synergistic effect. The change of sRNA and mRNA expression was validated by quantitative reverse transcription-PCR. Results MLST revealed two dominant sequence types: ST208 and ST195. Checkerboard assays demonstrated significant synergism between CZA (8/4 µg/mL) and BCL (16 µg/mL), with a FICI of 0.5. Biofilm formation was significantly inhibited by the combination ( P < 0.05). Transcriptomic analysis identified 100 common DEGs across treatment groups, enriched in sulfur metabolism, butanoate metabolism, and amino acid degradation pathways. Notably, iron metabolism genes and major facilitator superfamily (MFS) transporters were differentially regulated by changing the concentration of BCL in drug combination. A negative correlation was observed between AbsR25 and MFS-transporter ( r = -0.689, P < 0.001). Conclusion The combination of CZA and BCL exerts potent synergistic antibacterial and anti-biofilm activities against CRAB, with putative mechanisms involving the modulation of efflux pumps, iron homeostasis, and key metabolic pathways. The study not only establishes BCL as a promising adjuvant in the fight against CRAB infections, but also provides a molecular basis for optimizing combinatorial therapeutic regimens.