Plant Flavonoid Sophoraflavanone G Cracks Staphylococcal Biofilms but Spares Pseudomonas

A prenylated flavonoid extracted from a shrub in the Sophora genus is drawing fresh attention to the search for antimicrobial weapons that can do what many conventional antibiotics cannot: penetrate and dismantle the slimy microbial fortresses known as biofilms. In a study published in BMC Complementary Medicine and Therapies, researchers at Mashhad University of Medical Sciences, working with a colleague at Shahid Beheshti University in Tehran, isolated sophoraflavanone G from Sophora pachycarpa and put it through a battery of tests against two clinically notorious bacteria, Pseudomonas aeruginosa and Staphylococcus epidermidis. The results were strikingly lopsided, and that asymmetry is precisely what makes the work informative for anyone trying to understand how plant-derived molecules might be engineered into the next generation of anti-biofilm therapies.

The team began with classical natural-products chemistry. Sophoraflavanone G was isolated and purified using chromatographic techniques, and its identity was confirmed by proton nuclear magnetic resonance spectroscopy, the standard method for verifying the hydrogen framework of an organic molecule. Prenylated flavonoids like sophoraflavanone G carry a hydrophobic isoprenoid side chain attached to a flavonoid core, a structural feature long associated with enhanced membrane activity and, consequently, stronger antimicrobial effects than their non-prenylated relatives. That lipophilic appendage is thought to help the molecule insert itself into bacterial lipid bilayers, disturbing membrane integrity and function. The researchers then evaluated both the antibacterial and the anti-biofilm properties of the purified compound across a panel of bacterial strains, including the well-characterized P. aeruginosa strain PAO1 and Staphylococcus epidermidis DSMZ 3270.

Against Gram-positive bacteria, the compound performed impressively. It exhibited potent inhibitory and bactericidal activity against all tested Gram-positive organisms, with Listeria monocytogenes emerging as the most sensitive, inhibited at a minimum inhibitory concentration of just 0.98 micrograms per milliliter. For a plant-derived molecule, sub-microgram-per-milliliter potency is noteworthy and places sophoraflavanone G among the more active flavonoid antibacterials described in the literature. The finding reinforces a pattern that microbiologists have observed repeatedly: prenylated flavonoids tend to be far more effective against bacteria that lack the protective outer membrane characteristic of Gram-negative organisms. That outer membrane, with its tightly packed lipopolysaccharide layer, acts as a molecular sieve that excludes many hydrophobic compounds, including most flavonoids, before they can reach their targets.

The Gram-negative results bore this out in blunt fashion. Sophoraflavanone G showed no meaningful activity against P. aeruginosa in its planktonic, free-swimming form, with a minimum inhibitory concentration exceeding 1000 micrograms per milliliter. In other words, even at the highest concentration tested, the compound could not stop the growth of suspended Pseudomonas cells. This is not a surprise to anyone familiar with the barriers Gram-negative pathogens present, but it matters for the study’s central question: could the compound still do something useful against P. aeruginosa once that pathogen had settled into a biofilm, the matrix-encased community mode in which bacteria are notoriously tolerant of antibiotics?

The answer was nuanced. At low doses, sophoraflavanone G failed to inhibit P. aeruginosa biofilm formation, and it did not enhance the penetration of tobramycin, an aminoglycoside antibiotic commonly deployed against Pseudomonas infections, into the biofilm matrix. Biofilms resist antibiotics through several cooperating mechanisms: an extracellular polymeric substance matrix that physically retards diffusion, metabolic dormancy of cells deep within the community, and the accumulation of resistance determinants. A compound that cannot breach the planktonic outer membrane barrier was unlikely to loosen this fortress on its own at subinhibitory concentrations. The low-dose combination experiments with tobramycin therefore came back essentially negative, a result the authors report transparently rather than overinterpret.

But at a substantially higher concentration of 1 milligram per milliliter, the picture changed. Sophoraflavanone G significantly improved antibiotic penetration into the established P. aeruginosa biofilm. The compound, in other words, appears to act on the biofilm’s permeability barrier in a dose-dependent manner, even though it cannot kill the organism outright. This kind of adjuvant activity, where a molecule with weak intrinsic antibacterial power potentiates a co-administered antibiotic by opening access routes through the biofilm matrix, is an actively pursued strategy in anti-biofilm drug development. The practical caveat is the concentration required: 1 milligram per milliliter is high for systemic therapy, which is why the authors frame this as a proof of principle and call for further development to improve efficacy against Gram-negative pathogens rather than an immediate clinical recommendation.

Where the compound truly shone was against Staphylococcus epidermidis, a Gram-positive coccus that is a leading cause of device-associated infections. S. epidermidis colonizes catheters, prosthetic joints, heart valves, and other implanted materials, encasing itself in biofilms that shield it from both host defenses and antibiotics. The study found that sophoraflavanone G demonstrated strong inhibitory, disruptive, and biofilm-penetrating effects against this organism, with a clear dose-dependent response. It did not merely prevent new biofilm from forming; it also attacked established biofilms and enhanced the penetration of antibiotics into them. For an organism whose pathogenicity depends almost entirely on its ability to form resilient biofilms on medical devices, a molecule that hits biofilm structure and integrity at multiple stages is a genuinely interesting lead.

The contrast between the two organisms encapsulates a central tension in natural-product antimicrobial research. Sophoraflavanone G is, on the evidence of this study, a potent Gram-positive antibacterial and anti-biofilm agent with a plausible secondary role as a biofilm-disrupting adjuvant against Gram-negative pathogens at high concentrations. Its membrane-active chemistry, conferred by the prenyl group, likely explains both strengths: Gram-positive bacteria expose their cytoplasmic membranes directly to the environment, making them vulnerable, while the Gram-negative outer membrane excludes the compound until concentrations are high enough to force some degree of penetration. Understanding this structure-activity relationship could guide medicinal chemists toward derivatives that retain Gram-positive potency while acquiring the ability to cross the Gram-negative barrier, perhaps by tuning lipophilicity or adding metal-chelating or efflux-pump-inhibiting features.

The work also fits into a broader and increasingly urgent context. Antimicrobial resistance, particularly among biofilm-forming pathogens, poses a serious threat to public health, and the World Health Organization has repeatedly flagged carbapenem-resistant P. aeruginosa as a critical-priority pathogen. Biofilm-associated infections are especially intractable because standard susceptibility testing measures activity against planktonic cells, which dramatically underestimates the drug concentrations needed to clear a biofilm on an implanted device. Plant-derived compounds have historically been a rich source of antimicrobial scaffolds, and prenylated flavonoids in particular have attracted attention for their potency and their capacity to act on bacterial membranes and biofilm matrices in ways that complement conventional antibiotics. Studies like this one, which test both planktonic and biofilm modes and include combination experiments with existing drugs, provide exactly the kind of data needed to prioritize candidates for further development.

The authors, led by Niloofar Zanganeh and Vahid Soheili at Mashhad University of Medical Sciences together with Milad Iranshahy and colleagues, are appropriately measured in their conclusions. They underscore the potential of sophoraflavanone G as a candidate for managing Gram-positive and biofilm-associated infections, particularly those involving S. epidermidis, while explicitly highlighting the need for further development to improve its efficacy against Gram-negative pathogens. The study was funded by the Research Council of Mashhad University of Medical Sciences and formed part of a student thesis, illustrating how academic natural-products laboratories continue to feed the antimicrobial pipeline. Whether sophoraflavanone G itself ever reaches the clinic will depend on pharmacokinetics, toxicity, formulation, and the chemistry needed to overcome the Gram-negative barrier, but as a molecular probe it has already delivered a valuable lesson: the same membrane-disrupting feature that makes a flavonoid lethal to staphylococci can, at the right dose, help antibiotics slip past the defenses of one of medicine’s most stubborn Gram-negative adversaries.

Subject of Research: Antibacterial and anti-biofilm activity of the plant-derived prenylated flavonoid sophoraflavanone G against Pseudomonas aeruginosa and Staphylococcus epidermidis

Article Title: Antibacterial evaluation of sophoraflavanone G from Sophora pachycarpa against Pseudomonas aeruginosa and Staphylococcus epidermidis biofilm

Article References: Zanganeh, N., Hatamian, G., Shakeri, A., Bazzaz, B. S. F., Iranshahy, M., & Soheili, V. (2026). Antibacterial evaluation of sophoraflavanone G from Sophora pachycarpa against Pseudomonas aeruginosa and Staphylococcus epidermidis biofilm. BMC Complementary Medicine and Therapies. https://doi.org/10.1186/s12906-026-05564-3

Image Credits: AI Generated

DOI: 10.1186/s12906-026-05564-3

Keywords: sophoraflavanone G, Sophora pachycarpa, biofilm, Pseudomonas aeruginosa, Staphylococcus epidermidis, antimicrobial resistance, prenylated flavonoid, natural products, tobramycin, Listeria monocytogenes, antibiotic adjuvant, Gram-positive bacteria