The research, led by Yong Wei and colleagues at Shenzhen Qianhai Shekou Free Trade Zone Hospital, together with Yuanhuan Wei of Shenzhen Nanshan People Hospital, focused on twenty clinical strains of carbapenem-resistant A. baumannii isolated at a tertiary hospital. These isolates were first characterized using multi-locus sequence typing, a genetic fingerprinting method that reads segments of several housekeeping genes to assign each strain to a sequence type. The typing revealed two dominant lineages circulating in the hospital, ST208 and ST195, both of which are recognized internationally as epidemic clones responsible for numerous hospital outbreaks of multidrug-resistant infection. Working with clinically relevant strains rather than laboratory-adapted reference bacteria gives the findings a weight that pure culture experiments often lack.

The core of the study was a systematic test of whether baicalein and ceftazidime-avibactam work better together than either drug alone. The team used checkerboard assays, a standard technique in which two antimicrobials are combined across a grid of serial dilutions to see whether their combined effect exceeds what would be expected from simple addition. The outcome is quantified by the fractional inhibitory concentration index, or FICI, a ratio in which values of 0.5 or below indicate synergism. When ceftazidime-avibactam was paired at 8/4 micrograms per milliliter with baicalein at 16 micrograms per milliliter, the combination achieved a FICI of 0.5, meeting the threshold for significant synergy against the resistant isolates.

That result is notable because ceftazidime-avibactam was never designed with A. baumannii in mind. The drug pairs ceftazidime, a third-generation cephalosporin that attacks the bacterial cell wall, with avibactam, a synthetic beta-lactamase inhibitor that shields the antibiotic from enzymatic destruction. Avibactam is effective against many of the beta-lactamases that shield Gram-negative pathogens, but A. baumannii deploys a distinct enzymatic arsenal, including OXA-type carbapenemases, that limits the drug’s utility against this organism. Baicalein appears to close part of that gap, weakening the bacterium’s defenses so that the antibiotic combination can do its work at concentrations that would otherwise fail.

Beyond killing planktonic, free-swimming bacteria, the combination also disrupted one of the most stubborn features of A. baumannii biology: its ability to form biofilms. Biofilms are organized bacterial communities encased in a self-produced matrix that adheres to catheters, ventilator tubes, and wound surfaces, and they shield resident bacteria from both antibiotics and immune attack. In the study, biofilm formation was significantly inhibited by the baicalein-antibiotic combination, with the reduction reaching statistical significance at a P value below 0.05. Because biofilm-associated infections are notoriously difficult to eradicate and often necessitate device removal, an adjuvant that suppresses biofilm development could have substantial practical value in hospital settings.

To understand how the synergy operates at the molecular level, the researchers turned to transcriptomic profiling using RNA sequencing, which measures the activity of every gene in the bacterial genome under different treatment conditions. Comparing gene expression across treatment groups, they identified 100 differentially expressed genes common to the treated populations. When these genes were mapped onto known biological pathways using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses, they clustered around sulfur metabolism, butanoate metabolism, and amino acid degradation pathways. The pattern suggests that the drug combination imposes a metabolic stress that forces the bacterium to reroute core biochemical processes, potentially compromising its ability to maintain resistance machinery while under antibiotic attack.

Two molecular themes stood out in the transcriptomic data. The first involved iron metabolism genes, which were differentially regulated as the concentration of baicalein in the combination changed. Iron is an essential nutrient that bacteria must scavenge from their host environment, and disrupting iron homeostasis is an emerging strategy in antimicrobial development. The second involved members of the major facilitator superfamily, a large class of transport proteins that includes many bacterial efflux pumps. Efflux pumps are among the most important mechanisms by which bacteria expel antibiotics before the drugs can reach lethal intracellular concentrations, and their modulation by baicalein offers a plausible mechanistic explanation for why the flavonoid potentiates ceftazidime-avibactam rather than acting as a simple independent killer.

Adding a further layer of regulatory complexity, the study examined small regulatory RNAs, short non-coding RNA molecules that fine-tune gene expression in bacteria by binding messenger RNAs. The team observed a negative correlation between the expression of a small RNA called AbsR25 and an MFS transporter gene, with a correlation coefficient of negative 0.689 and a P value below 0.001. In other words, when AbsR25 levels rose, transporter expression tended to fall, consistent with the small RNA repressing the pump. The researchers validated the changes in both small RNA and messenger RNA expression using quantitative reverse transcription PCR, a technique that confirms sequencing results with independent, highly sensitive measurements. This is, to the authors’ knowledge, among the first reports linking a specific small RNA to efflux-mediated resistance modulation in A. baumannii under flavonoid-antibiotic combination therapy.

The clinical context makes the work timely. A. baumannii thrives in hospital environments, survives desiccation on surfaces for weeks, and disproportionately infects ventilated patients, burn victims, and the immunocompromised. As carbapenem resistance spreads, clinicians have been left with increasingly toxic or ineffective options, and the pharmaceutical pipeline for Gram-negative pathogens has thinned. Adjuvant strategies, in which an existing antibiotic is paired with a resistance-breaking compound, offer a faster and cheaper route to new therapies than de novo drug discovery, because the antibiotic component is already approved and its safety profile is known. Baicalein, with its established history of human use in herbal formulations and its documented antimicrobial and anti-inflammatory properties, fits the profile of a candidate adjuvant worth developing.

Important caveats remain before the combination can reach patients. The findings are entirely in vitro, derived from checkerboard assays, time-kill kinetics, and biofilm experiments in the laboratory, and the authors themselves frame the mechanisms as putative, inferred from gene expression patterns rather than directly demonstrated by functional experiments. Effective concentrations, pharmacokinetics, toxicity, and dosing in animal models and ultimately human trials have yet to be established. Nevertheless, the study establishes a concrete molecular foundation for optimizing combination regimens: by showing that baicalein modulates efflux pumps, iron homeostasis, and central metabolic pathways, it gives medicinal chemists and clinicians a rational starting point for dose selection and for screening related flavonoids. As carbapenem-resistant A. baumannii continues its global advance, the humble skullcap root may prove to hold one of the more surprising tools in the effort to push back.

Subject of Research: Synergistic activity of baicalein combined with ceftazidime-avibactam against carbapenem-resistant Acinetobacter baumannii

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

Article References: Wei, Y., Wei, Y., Xin, X., Zhang, J., He, J., Zhong, Y., Zhao, M., Li, N., & He, S. (2026). Baicalein in combination with ceftazidime-avibactam exhibits synergism against carbapenem-resistant Acinetobacter baumannii in vitro. BMC Complementary Medicine and Therapies. https://doi.org/10.1186/s12906-026-05626-6

Image Credits: AI Generated

DOI: 10.1186/s12906-026-05626-6

Keywords: Acinetobacter baumannii, carbapenem resistance, baicalein, ceftazidime-avibactam, antibiotic synergy, biofilm inhibition, efflux pumps, iron homeostasis, transcriptomics, small regulatory RNA, antimicrobial adjuvants, hospital-acquired infection