The significance of targeting biofilms cannot be overstated. Biofilms are structured communities of bacteria encased in a self-produced matrix of polysaccharides, proteins, and extracellular DNA. Within this matrix, bacteria adopt gene expression profiles that differ dramatically from their free-floating, planktonic counterparts, altering their morphology, surface properties, and susceptibility to antibiotics. Cells buried in a biofilm are shielded by enzymatic degradation of antimicrobials, active efflux pumps, and reduced permeability, which is why biofilm-associated infections on catheters, ventilators, and implants so often defy standard treatment regimens. Among the virulence factors that govern this process in A. baumannii, the biofilm-associated protein known as Bap, encoded by the bap gene, plays a critical role in adhesion to bronchial cells, structural integrity of the biofilm, and the development of water channels that distribute nutrients through the community. Disrupting bap has been shown to reduce biofilm thickness, volume, and interbacterial adhesion, making it an attractive molecular target.

In the new study, published in MicrobiologyOpen, the team collected 100 clinical isolates of A. baumannii from patients at a burn hospital affiliated with Mazandaran University of Medical Sciences. The isolates came from wounds, urine, and blood samples of 50 male and 50 female patients ranging in age from six months to 88 years, distributed across adult burn, intensive care, surgical, and pediatric burn units. Wound samples accounted for the majority of isolates at 73 percent, followed by urine at 15 percent and blood at 12 percent. Identification was confirmed through conventional microbiological and biochemical methods as well as polymerase chain reaction targeting the blaOXA-51 beta-lactamase gene, a molecular signature of the species. The researchers then used a microtiter plate crystal violet staining assay to classify each isolate as a weak, moderate, or strong biofilm producer.

The nanoparticles themselves were produced through green synthesis, an approach in which plant extracts serve as both reducing and capping agents. Phytochemicals such as flavonoids, terpenoids, and polysaccharides donate electrons to reduce silver ions into metallic silver nanoparticles while simultaneously stabilizing their surfaces. This method is cost-effective, environmentally sustainable, and avoids the toxic solvents associated with conventional chemical synthesis. Nepeta pogonosperma, a member of the Lamiaceae family identified in 1984, has a long history in traditional medicine across Iran and neighboring regions, where it has been used to treat conditions ranging from pneumonia and influenza to stomach disorders and asthma. Astrodaucus persicus, a member of the Apiaceae family native to Asia, has traditionally been consumed as a food additive in Iran and Turkey, and plants in this family exhibit antibacterial, hepatoprotective, antitumor, and apoptosis-inducing activities.

When the researchers measured the minimum inhibitory and minimum bactericidal concentrations of the two nanoparticle formulations, the results were striking. MIC and MBC values against the 100 clinical isolates ranged from 0.1 to 40 micrograms per milliliter, but for more than 94 percent of isolates these values fell between 0.1 and 3 micrograms per milliliter. For AgNP@Ap, the Astrodaucus persicus formulation, 80 of the isolates were inhibited at concentrations of 0.1 to 0.5 micrograms per milliliter, while the Nepeta formulation inhibited 52 isolates at that same low range. A significant correlation emerged between biofilm formation intensity and susceptibility: over 90 percent of strong and moderate biofilm producers exhibited MIC and MBC values between 0.1 and 3 micrograms per milliliter, suggesting that the most dangerous biofilm-forming strains were also the most vulnerable to the nanoparticles.

At sub-inhibitory concentrations of 1 microgram per milliliter for AgNP@Ap and 1.171 micrograms per milliliter for AgNP@Np, both formulations interfered with biofilm development in the 20 strongest producers. For AgNP@Ap, 40 percent of isolates showed 41 to 60 percent inhibition of new biofilm formation and 10 percent showed inhibition exceeding 80 percent. AgNP@Np performed comparably, with 30 percent of isolates showing 41 to 60 percent inhibition and 10 percent showing greater than 80 percent inhibition. Crucially, the nanoparticles did not merely prevent new biofilms from forming; they also dismantled established ones. When pre-formed biofilms were treated, 45 percent of isolates exposed to AgNP@Ap showed 41 to 60 percent disruption, and 50 percent of those exposed to AgNP@Np fell into the same category, with some isolates experiencing up to 80 percent or greater biomass loss.

The molecular analysis revealed why these effects matter. Among the 100 isolates, 93 exhibited elevated bap gene expression, with fold-changes spanning from 2-fold to as high as 20-fold relative to the reference strain. Every isolate showing more than a 5-fold increase was a strong biofilm producer, and statistical testing confirmed that strong biofilm formers expressed bap at significantly higher levels than weaker producers. When the strong producers were treated with sub-MIC concentrations of the nanoparticles, bap expression dropped significantly compared with untreated controls at both 24 and 48 hours, with the reduction becoming more pronounced at the longer exposure. Quantitative real-time PCR with the 16S rRNA gene as an internal control, analyzed using the comparative 2^-ΔΔCt method, documented these changes precisely. Notably, isolates with higher baseline bap expression tended to have lower MIC and MBC values, indicating greater susceptibility to the nanoparticles.

The authors emphasize that these green-synthesized particles are not simply bits of metallic silver. Prior characterization showed that AgNP@Np particles are spherical, with an average crystallite size of 31.68 nanometers, a hydrodynamic diameter of 233 nanometers, and a zeta potential of −35.1 millivolts indicating good colloidal stability. Fourier-transform infrared spectroscopy confirmed that polyphenols, carbohydrates, and amides from the plant extract form an organic corona around the silver core. These capping molecules are not inert; many phytochemicals possess intrinsic antibacterial, anti-adhesion, and quorum-sensing inhibitory activities. The anti-biofilm efficacy observed is therefore likely the product of synergy between the silver core, which disrupts bacterial membranes, generates reactive oxygen species, and interferes with transcriptional machinery, and the bioactive plant-derived surface layer, which may modulate the sustained release of bactericidal silver ions. Differences in the phytochemical profiles of the two plants—polyphenols and flavonoids in Nepeta versus monoterpenes and benzodioxole compounds in Astrodaucus—likely explain the subtle variations in performance between the two formulations.

The findings arrive at a moment when the clinical urgency of A. baumannii infections has never been greater, particularly for burn patients and intensive care populations, where biofilm-mediated persistence drives mortality. Because silver nanoparticles target multiple biological pathways simultaneously, the likelihood of resistance developing is reduced compared with single-target antibiotics, and prior studies have documented similar effects, including reductions in bap expression with chemically synthesized silver nanoparticles at higher concentrations. The consistency of the present results, achieved at lower concentrations in multidrug-resistant clinical isolates rather than laboratory strains, strengthens the case for clinical relevance. The authors caution, however, that translating these nanoparticles into practice requires further work: precise mechanisms of action, long-term safety profiles, potential side effects, and integration into medical devices and treatment protocols all remain to be established. Still, the study offers compelling evidence that an eco-friendly, low-cost synthesis route rooted in traditional medicinal plants can disarm one of the hospital’s deadliest pathogens at the level of its own genes.

Subject of Research: Plant-based silver nanoparticles as anti-biofilm agents against Acinetobacter baumannii

Article Title: Effect of Green‐Synthesized Silver Nanoparticles From Nepeta pogonosperma and Astrodaucus persicus on the Reduction of Bap Gene Expression in Strong Biofilm‐Producing Acinetobacter baumannii Clinical Isolates

Article References: Kakavan, M., Gholami, M., Ahanjan, M., Ebrahimzadeh, M. A., Hossein Nataj, A., Mousavi, T., & Goli, H. R. (2026). Effect of Green‐Synthesized Silver Nanoparticles From Nepeta pogonosperma and Astrodaucus persicus on the Reduction of Bap Gene Expression in Strong Biofilm‐Producing Acinetobacter baumannii Clinical Isolates. MicrobiologyOpen, 15(5), Article e70391. https://doi.org/10.1002/mbo3.70391

Image Credits: AI Generated

DOI: 10.1002/mbo3.70391

Keywords: Acinetobacter baumannii, silver nanoparticles, green synthesis, biofilm, bap gene, antimicrobial resistance, Nepeta pogonosperma, Astrodaucus persicus, qRT-PCR, nosocomial infections, nanomedicine, ESKAPE pathogens