— March 5, 2026 · Silver Nanoparticles vs Meningitis

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Silver Nanoparticles Tested Against Meningitis-Causing Bacteria in the Laboratory
Featured Study: Actinobacteria-Derived Silver Nanoparticles Against Meningitis Microbes
Biofilms: What the Laboratory Tests Measured
DNA-Templated Silver Nanoclusters: A Cell-Culture Study
Silver Nanoparticles and Antibiotics: What Reviews Report
Reading the Evidence: Lab Findings, Silver Products and Safety
Key Research Papers
Connections
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Silver Nanoparticles Tested Against Meningitis-Causing Bacteria in the Laboratory

A laboratory (in vitro) study published in Scientific Reports (Nature) tested biologically synthesized silver nanoparticles against three bacteria that cause bacterial meningitis — Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis — on culture plates and in microtiter wells. The authors describe the nanoparticles’ inhibition activity as “nearly identical” to that of the standard antibiotic gentamicin. No animals or patients were treated in the study.

The research, by Bano et al. (2023), used silver nanoparticles synthesized from two actinobacteria strains isolated from the rhizosphere soil of medicinal plants around Lucknow, India: Microbacterium proteolyticum LA2(R) and Streptomyces rochei LA2(O). These biogenic nanoparticles, designated RAgNPs and OAgNPs respectively, were tested against all three meningitis-causing pathogens using multiple antibacterial assays.

Key Findings

Minimum Inhibitory Concentrations (MICs): RAgNPs showed MIC values of 109.4 μg/ml against S. pneumoniae, 120.60 μg/ml against H. influenzae, and 138.80 μg/ml against N. meningitidis. OAgNPs had somewhat lower MICs of 105.80, 114.40, and 129.06 μg/ml respectively.
Compared with an antibiotic: The authors describe the inhibition activity of the biogenic silver nanoparticles as nearly identical to that of gentamicin; in their table the nanoparticle zones of inhibition (23–32 mm) were smaller than gentamicin’s (33–34 mm).
Green synthesis: The nanoparticles were produced with metabolites from actinobacteria rather than chemical reducing agents; the authors describe the method as inexpensive, eco-friendly and non-toxic. The study did not test the nanoparticles on human cells, so toxicity to human cells was not reported.
Proposed mechanism: The paper’s introduction cites earlier work that silver nanoparticles produce reactive oxygen species (ROS) such as hydrogen peroxide, superoxide anion and hydroxyl radicals; the study itself did not measure them. A 2025 review notes concerns about bacterial adaptation and emerging resistance patterns to silver nanoparticles.

The World Health Organization’s meningitis fact sheet states that around 1 in 6 people who get bacterial meningitis die and 1 in 5 have severe complications. The study’s authors give drug-resistant bacteria and long-term treatment with different medications as the reasons new meningitis drugs are needed.

Featured Study: Actinobacteria-Derived Silver Nanoparticles Against Meningitis Microbes

Paper: Bano, N., Iqbal, D., Al Othaim, A., Kamal, M., Albadrani, H.M., Algehainy, N.A., Alyenbaawi, H., Alghofaili, F., Amir, M., and Roohi. "Antibacterial efficacy of synthesized silver nanoparticles of Microbacterium proteolyticum LA2(R) and Streptomyces rochei LA2(O) against biofilm forming meningitis causing microbes." Scientific Reports, 2023; 13: 4150. PubMed PMID: 36914689 | PMC10011373

Background and Motivation

The authors describe bacterial meningitis caused by Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis as able to cause death within hours, with hearing, learning and memory problems in some survivors; the WHO fact sheet states that 1 in 5 survivors of bacterial meningitis may have long-lasting after-effects. The paper cites multidrug-resistant (MDR) bacteria and biofilm formation as reasons for seeking new antimicrobial agents. It explored actinobacteria — soil bacteria known for producing secondary metabolites — as a biological source for synthesizing silver nanoparticles.

Methodology

The researchers isolated two actinobacteria strains from soil samples in India: Microbacterium proteolyticum LA2(R) and Streptomyces rochei LA2(O). Secondary metabolites from each strain were extracted and used to biologically reduce silver nitrate (AgNO₃) into silver nanoparticles at 37°C over a seven-day incubation period. The resulting nanoparticles — designated RAgNPs (from LA2(R)) and OAgNPs (from LA2(O)) — were characterized using four analytical techniques:

UV-Vis spectroscopy: Confirmed nanoparticle formation, with surface plasmon resonance absorption maxima at 412 nm (RAgNPs) and 413 nm (OAgNPs).
Transmission Electron Microscopy (TEM): Revealed spherical, polydispersed nanoparticles with diameters from 11 to 42 nm and average sizes of 27 ± 1 nm (RAgNPs) and 29 ± 2 nm (OAgNPs).
FTIR spectroscopy: Identified the functional groups from the actinobacterial metabolites that served as capping and stabilizing agents on the nanoparticle surface.
HPLC analysis: Characterized the secondary metabolite profiles of each actinobacteria strain.

Antibacterial testing was performed against meningitis-causing reference strains from the Microbial Type Culture Collection (MTCC), Chandigarh, India: S. pneumoniae (MTCC 655), H. influenzae (MTCC 3826), and N. meningitidis (MTCC 7981; the methods section lists this strain as ATCC 13090-0454L). Three assays were used: agar well diffusion (zone of inhibition), broth microdilution (MIC), and crystal violet biofilm assay.

Antibacterial Results

In these plate and well tests, both sets of nanoparticles produced larger zones of inhibition than the crude secondary metabolites alone:

Zone of Inhibition (mm) — Agar Well Diffusion Method

The table reproduces the paper’s Table 1 (pure bacteria, secondary metabolites and silver nanoparticles, with the antibiotic gentamicin as the comparison drug):

Pathogen	Pure Bacteria LA2(R) / LA2(O)	Secondary Metabolites LA2(R) / LA2(O)	AgNPs LA2(R) / LA2(O)	Gentamicin (Control)
H. influenzae (MTCC 3826)	9 mm / 6 mm	16 mm / 14 mm	32 mm / 29 mm	34 mm
S. pneumoniae (MTCC 655)	10 mm / 8 mm	19 mm / 15 mm	26 mm / 23 mm	33–34 mm
N. meningitidis (MTCC 7981)	11 mm / 10 mm	17 mm / 15 mm	31 mm / 29 mm	34 mm / 33 mm

Gentamicin produced the largest zones against all three organisms. RAgNPs produced larger zones than OAgNPs against all three.

Minimum Inhibitory Concentration (MIC) and IC50 Values

MIC values for RAgNPs: 109.4 μg/ml (S. pneumoniae), 120.60 μg/ml (H. influenzae), 138.80 μg/ml (N. meningitidis).
MIC values for OAgNPs: 105.80 μg/ml (S. pneumoniae), 114.40 μg/ml (H. influenzae), 129.06 μg/ml (N. meningitidis).

The IC50 values (concentration required to inhibit 50% of bacterial growth) were lower for the AgNPs than for the crude secondary metabolites, and higher than for gentamicin:

IC50 for RAgNPs: 54.70 μg/ml (N. meningitidis), 60.30 μg/ml (S. pneumoniae), 69.40 μg/ml (H. influenzae).
IC50 for OAgNPs: 52.90 μg/ml (N. meningitidis), 57.20 μg/ml (S. pneumoniae), 64.53 μg/ml (H. influenzae).
IC50 for secondary metabolites: 90.3, 100.3, and 109.1 μg/ml for LA2(R) and 82.90, 87.20, and 99.50 μg/ml for LA2(O) (N. meningitidis, S. pneumoniae, H. influenzae, respectively).
IC50 for gentamicin (control): 37.90 μg/ml (N. meningitidis), 49.20 μg/ml (S. pneumoniae), 58.40 μg/ml (H. influenzae).

The authors conclude that Microbacterium proteolyticum LA2(R) showed better results than Streptomyces rochei LA2(O), citing its larger zones of inhibition and its antibiofilm activity; by MIC and IC50, the OAgNP values were slightly lower.

Biofilm Disruption Results

In a crystal violet microtiter-plate assay, the nanoparticles reduced biofilm formation by the three organisms (figures as reported in the abstract):

S. pneumoniae: 73.14% biofilm inhibition
H. influenzae: 71.89% biofilm inhibition
N. meningitidis: 64.81% biofilm inhibition

Mechanism of Action

The study did not test how the nanoparticles act. Its introduction summarizes mechanisms reported in earlier literature:

Reactive oxygen species (ROS) generation: The nanoparticles catalyze the production of free radicals including hydrogen peroxide (H₂O₂), superoxide anion (O₂⁻), hydroxyl radical (OH•), hypochlorous acid (HOCl), and singlet oxygen (¹O₂),
Interaction with cell components: Silver nanoparticles may interact with proteins, lipids, and DNA when they enter microbial cells.
Silver ion release: Silver nanoparticles emit silver ions that can destroy bacteria. The same passage notes that the mechanisms and associated toxicity are unclear.
Penetration: The authors cite work that nano-formulations can overcome the limited penetration and retention of conventional antibiotics in cells or biofilm.

The Authors’ Conclusion and Its Limits

The authors conclude that RAgNPs and OAgNPs “can be used as a potent antibacterial agent” against meningitis-causing bacteria and that the findings provide insight into developing new antimicrobial agents. The work was done entirely in culture: the paper reports no animal or human testing, no toxicity testing on human cells, and no measurement of whether the particles reach the brain or cross the blood-brain barrier.

Full text: PMC10011373 | DOI: 10.1038/s41598-023-30215-9

Biofilms: What the Laboratory Tests Measured

The Bano et al. study also measured biofilms — the slimy matrices bacteria form on surfaces. The authors write that biofilm-embedded bacteria are more resistant to antimicrobials than free-floating (planktonic) cells and therefore harder to treat.

Biofilm Inhibition Results

S. pneumoniae: 73.14% biofilm inhibition, the largest of the three.
H. influenzae: 71.89% biofilm inhibition.
N. meningitidis: 64.81% biofilm inhibition.

These were reductions in biofilm formation in microtiter wells, not in tissue. The nanoparticles measured 11–42 nm by TEM (averages 27 and 29 nm); the paper did not test how they act on the biofilm matrix.

A separate laboratory study by Ma et al. in the AIChE Journal, aimed at middle-ear infection (otitis media), reported that polyvinylpyrrolidone-coated silver nanoparticles (AgNPs-PVP) completely eradicated S. pneumoniae and nontypeable H. influenzae at 1.04 and 2.13 μg/ml. The authors attribute the greater efficacy against S. pneumoniae to its hydrogen-peroxide-producing ability and the known synergy between hydrogen peroxide and silver nanoparticles. Their hydrogel formulation eradicated the pathogens in vitro without cytotoxicity.

DNA-Templated Silver Nanoclusters: A Cell-Culture Study

A separate study by a team at the University of North Carolina at Charlotte tested DNA-templated silver nanoclusters (AgNCs) — very small silver structures stabilized on DNA. By energy-dispersive X-ray spectroscopy, the authors measured 8.59 ± 0.64 silver atoms on each hairpin structure.

The study, published online in February 2026 in ACS Applied Bio Materials, evaluated AgNCs templated on single hairpin (HP) or fibrous hairpin structures (HP-F) against Neisseria meningitidis and Streptococcus pneumoniae. The fibrous HP-F configuration provided higher local concentrations of silver nanoclusters with stable physicochemical properties and demonstrated potent antimicrobial activity against both pathogens.

Reported Findings

Concentrations: Growth of both organisms was reduced from 0.8–1.6 μM silver; complete growth inhibition (MIC) was reached at 52 μM silver for N. meningitidis and 104 μM silver for S. pneumoniae, with both constructs. The study did not compare AgNCs with conventional silver nanoparticles.
Inflammation in cultured cells: In human microglia (brain immune cells) grown in culture and infected with either organism, prophylactic and therapeutic AgNC treatment reduced IL-6 production and bacterial viability.
Cytotoxicity: The authors observed no toxic effects on the human microglia by MTS assay, and the constructs alone did not trigger IL-6 production.
Stability: Both HP and HP-F AgNCs kept comparable antimicrobial activity after 7 days at 37 °C. The authors list tunable size and fluorescence properties among the advantages of DNA-templated AgNCs.

The authors note that these bacteria trigger detrimental neuroinflammatory responses in the central nervous system, and write that their results support further development of AgNCs as therapeutics for meningitis. The work used bacterial cultures and cultured human cells; no animal or human testing was reported.

Silver Nanoparticles and Antibiotics: What Reviews Report

A 2022 review in Antibiotics (Mateo and Jiménez) summarizes laboratory reports of silver nanoparticles combined with antibiotics. In the studies it covers, a mix of several antibiotics with AgNPs (8–12 nm) made Acinetobacter baumannii susceptible to the antibiotics except cephalosporins in vitro, and AgNPs showed synergy with polymyxin B and rifampicin and an additive effect with tigecycline against carbapenem-resistant A. baumannii. The review concludes that researchers do not always agree and that more work is needed before AgNPs can be applied in clinical therapy against multidrug-resistant bacteria.

Mechanisms Described in the Reviews

Membrane effects: Both reviews list interactions at the bacterial plasma membrane among the mechanisms of AgNP action.
Protein and enzyme inactivation: The reviews describe inactivation of necessary proteins and enzymes, and interference with bacterial metabolic pathways.
ROS generation: Formation of reactive oxygen species leading to cell death.
DNA effects: Interactions affecting DNA replication.

A 2025 review in Frontiers in Cellular and Infection Microbiology (Khalifa et al.) discusses targeted delivery approaches — surface functionalization, biopolymer encapsulation, liposomal carriers and stimuli-responsive nanoplatforms — prompted by concerns about bacterial adaptation, cytotoxicity and non-specific interactions, and states that current limitations must be addressed for clinical translation.

A laboratory study in the Journal of Microbiology, Immunology and Infection (Huang et al., 2022) tested 50 clinical isolates each of six respiratory pathogens. Silver nanoparticles showed lower MICs than selenium nanoparticles for all species. The MIC50 of silver nanoparticles was <3.125 ppm for H. influenzae but >50 ppm for S. pneumoniae; the authors concluded that metal nanoparticles had good in vitro activity against Gram-negative bacteria.

Reading the Evidence: Lab Findings, Silver Products and Safety

Every study described above was laboratory work: bacteria grown on plates or in wells, and in one case human microglia grown in culture. None treated an animal or a person with meningitis, and none tested the silver products sold to the public. The concentrations reported (for example, MICs of 105.80–138.80 μg/ml in Bano et al., or 52–104 μM silver in the nanocluster study) are concentrations in a test well, not doses.

Colloidal silver products: Silver products sold over the counter are often promoted with claims that they treat or prevent infections. In a 1999 final rule (21 CFR 310.548), the U.S. Food and Drug Administration stated that there is a lack of adequate data to establish general recognition of the safety and effectiveness of colloidal silver ingredients or silver salts for OTC use in the treatment or prevention of any disease. Under the rule, any OTC product containing them that is labeled, represented or promoted for treating or preventing disease is regarded as a new drug requiring FDA approval, and is otherwise misbranded; it applies to products introduced after September 16, 1999.
Argyria: Case reports describe argyria — a gray-blue discoloration of the skin caused by chronic exposure to silver salts — in people who took colloidal silver by mouth, including an 81-year-old man diagnosed incidentally in an emergency department (Simon and Buchanan, 2020) and three cases of systemic argyria after ingestion of a colloidal silver solution (Chung et al., 2010).
Toxicity of nanoparticles: The 2022 Mateo and Jiménez review reports that the toxic activity of silver nanoparticles depends on dose, size, shape and electrical charge, and that a silver-coated wound dressing tested in burn patients produced reversible liver toxicity, argyria-like discoloration of treated skin, raised blood and urine silver levels and raised liver enzymes.

Bacterial meningitis is a medical emergency; decisions about its treatment belong with clinicians.

Key Research Papers

Bano, N. et al. "Antibacterial efficacy of synthesized silver nanoparticles of Microbacterium proteolyticum LA2(R) and Streptomyces rochei LA2(O) against biofilm forming meningitis causing microbes." Scientific Reports, 2023; 13: 4150. — doi:10.1038/s41598-023-30215-9
Ma, X. et al. "Silver nanoparticles as an effective antimicrobial against otitis media pathogens." AIChE Journal, 2021; 67(12): e17468. — doi:10.1002/aic.17468
Mateo, E.M. and Jiménez, M. "Silver Nanoparticle-Based Therapy: Can It Be Useful to Combat Multi-Drug Resistant Bacteria?" Antibiotics, 2022; 11(9): 1205. — doi:10.3390/antibiotics11091205
Dakal, T.C. et al. "Mechanistic Basis of Antimicrobial Actions of Silver Nanoparticles." Frontiers in Microbiology, 2016; 7: 1831. — doi:10.3389/fmicb.2016.01831
Yin, I.X. et al. "The Antibacterial Mechanism of Silver Nanoparticles and Its Application in Dentistry." International Journal of Nanomedicine, 2020; 15: 2555–2562. — doi:10.2147/IJN.S246764
Salleh, A. et al. "The Potential of Silver Nanoparticles for Antiviral and Antibacterial Applications: A Mechanism of Action." Nanomaterials, 2020; 10(8): 1566. — doi:10.3390/nano10081566
Liao, C. et al. "Bactericidal and Cytotoxic Properties of Silver Nanoparticles." International Journal of Molecular Sciences, 2019; 20(2): 449. — doi:10.3390/ijms20020449
Huang, Y.S. et al. "Metal nanoparticles and nanoparticle composites are effective against Haemophilus influenzae, Streptococcus pneumoniae, and multidrug-resistant bacteria." Journal of Microbiology, Immunology and Infection, 2022; 55(4): 708–715. — PubMed PMID: 35718718
Sánchez-López, E. et al. "Metal-Based Nanoparticles as Antimicrobial Agents: An Overview." Nanomaterials, 2020; 10(2): 292. — doi:10.3390/nano10020292
Gomaa, E.Z. "Silver nanoparticles as an antimicrobial agent: a case study on Staphylococcus aureus and Escherichia coli as models for Gram-positive and Gram-negative bacteria." Journal of General and Applied Microbiology, 2017; 63(1): 36–43. — PubMed PMID: 28123131
Majithia, K.J., Skelly, E., Afonin, K.A., Johnson, M.B. "DNA-Templated Silver Nanoclusters Demonstrate Potent Antimicrobial Activity Against the Clinically Relevant Pathogens, Neisseria meningitidis and Streptococcus pneumoniae." ACS Applied Bio Materials, 2026; 9(5): 2359–2365. — PubMed PMID: 41707080
Khalifa, H.O. et al. "Silver nanoparticles as next-generation antimicrobial agents: mechanisms, challenges, and innovations against multidrug-resistant bacteria." Frontiers in Cellular and Infection Microbiology, 2025; 15: 1599113. — PubMed PMID: 40895297
Simon, M., Buchanan, J.A. "Argyria, an Unexpected Case of Skin Discoloration From Colloidal Silver Salt Ingestion." Journal of Emergency Medicine, 2020; 59(2): e39–e41. — PubMed PMID: 32591303
Chung, I.S. et al. "Three systemic argyria cases after ingestion of colloidal silver solution." International Journal of Dermatology, 2010; 49(10): 1175–1177. — PubMed PMID: 20883406
U.S. Food and Drug Administration. "Drug products containing colloidal silver ingredients or silver salts offered over-the-counter (OTC) for the treatment and/or prevention of disease." 21 CFR 310.548; 64 FR 44658, August 17, 1999. — eCFR: 21 CFR 310.548

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