Ondine Biomedical: Testing shows no MB-aPDT Resistance in VISA, MRSA

9 November 2026

ONDINE BIOMEDICAL INC.

(“Ondine Biomedical”, “Ondine”, or “the Company”)

Laboratory Study Demonstrates No Observed Resistance to Methylene Blue Photodisinfection (MB-aPDT) Across Multidrug-Resistant Staphylococcus aureus Strains, Including VISA, LRSA and MRSA

Evaluation of 20 consecutive cycles of sub-lethal exposure showed no build-up of resistance or loss of susceptibility in 13 antibiotics tested unlike control (ciprofloxacin)

Ondine Biomedical Inc. (AIM: OBI), the leader in light-activated antimicrobial therapies to prevent and treat hospital infections, announces new laboratory findings demonstrating no development of resistance to methylene blue photodisinfection (“MB-aPDT”) following 20 repeated sub-lethal exposure cycles against multidrug-resistant Staphylococcus aureus ("S. aureus”) strains including high-priority pathogens, MRSA, VISA and LRSA. The data were presented as an oral presentation, titled ‘Assessment of Resistance and Cross-Resistance following Sequential Exposures to MB-aPDT’, at the 6th International Photodynamic Therapy and Photodiagnosis Symposium (“PDT 2026”) in Nancy, France.

Key Findings:

• No Photodisinfection Resistance: Across all six evaluated strains, including VISA and LRSA, no decrease in susceptibility to MB-aPDT was detected after 20 repeated selective cycles.
• Preserved Antibiotic Susceptibility: Serial exposure to MB-aPDT produced no consistent reduction in susceptibility across the 13 antibiotics tested, indicating an absence of cross-resistance induction of these strains treated with MB-aPDT.
• Methodological Validation: The parallel antibiotic used as control (ciprofloxacin) rapidly developed resistance under identical culture conditions, confirming the test system's sensitivity.

The complete methodology and findings will be submitted for publication in a peer-reviewed journal.

S. aureus is a leading infectious cause of morbidity and mortality worldwide, causing infections ranging from moderately severe to fatal, including skin and soft-tissue infections, pneumonia, infective endocarditis, osteoarthritis infections, central nervous system and bloodstream infections, and toxic shock syndrome [1]. Penicillin initially treated S. aureus infections effectively after its introduction in the 1940s, but resistant strains soon emerged and have persisted since then, with methicillin-resistant S. aureus (“MRSA”) becoming a particularly significant resistant strain [2].

While MRSA remains a widespread hospital concern with 30% one-year mortality rates [3], new emerging strains such as VISA (vancomycin-intermediate S. aureus) and LRSA (linezolid-resistant S. aureus) pose a growing challenge for clinicians. Given their resistance to many antibiotics, treatment options are limited and often carry significant toxicity risks. These constraints underscore the urgent need for new antimicrobial therapies, such as photodisinfection, to address these difficult-to-treat infections resulting from the growing antibiotic resistance.

The in vitro study, conducted at Ondine Research Laboratories in Bothell, Washington, evaluated six well-characterised strains of S. aureus. The panel encompassed clinically challenging, multidrug-resistant (“MDR”) phenotypes [4] and resistance determinants, including:

• Methicillin-resistant S. aureus (“MRSA”)
• Vancomycin-intermediate S. aureus (“VISA”)
• Linezolid-resistant S. aureus (“LRSA”)
• Strains harboring multidrug efflux-pump (norA) and biocide/antiseptic-tolerance (qacA) genes

Conventional antibiotics generally act on single, specific molecular targets, which can facilitate the rapid evolution of bacterial resistance. In contrast, MB-aPDT delivers a multi-targeted physical mechanism of action, generating reactive oxygen species that rapidly disrupt multiple cell structures simultaneously, a mechanism long hypothesised to prevent the emergence of resistance.

To rigorously challenge this hypothesis, researchers subjected each strain to 20 serial sub-lethal exposure cycles (at 0.5x minimum inhibitory concentration [MIC], or sub-MIC). Following each cycle, surviving bacteria were recovered, cultured, and re-exposed under selective pressure. Bacterial susceptibility was evaluated using MIC assays benchmarked against epidemiological cut-off values (ECOFFs), alongside cross-resistance testing against a panel of 13 clinical antibiotics spanning 11 distinct classes representing most of the antibiotics in use today. A conventional antibiotic comparator (ciprofloxacin) was evaluated in parallel to validate the assay’s ability to detect resistance development.

Antimicrobial resistance (“AMR”) represents a critical global health priority. A comprehensive evaluation by the GRAM Project (The Lancet, 2024) estimated that drug-resistant bacterial infections were directly responsible for 1.14 million deaths globally in 2021.

Topical decolonisation of nasal carriage prior to surgery is a recognised infection prevention strategy [5], as S. aureus carriage in the nares is a primary reservoir for downstream surgical site infections. However, standard topical antibiotic regimens (such as mupirocin) typically require twice-daily self-administration over five days prior to admission. Published real-world data indicate that patient compliance with such multi-day home regimens is frequently suboptimal, with full adherence rates reported at approximately 40% [6]. Incomplete adherence risks inadequate decolonisation and contributes to selective pressure; recent genomic studies have identified mupirocin resistance in up to 34.4% of evaluated MRSA clinical isolates [7].

While Methicillin-Resistant S. aureus (“MRSA”) is traditionally managed with vancomycin or linezolid, repeated selective exposure has driven the emergence of Vancomycin-Intermediate (“VISA”) and Linezolid-Resistant (“LRSA”) strains. VISA develops abnormally thickened peptidoglycan walls that physically trap vancomycin and confer cross-resistance to daptomycin. MB-aPDT bypasses these biochemical drug defenses entirely. By generating localised reactive oxygen species that physically disrupt bacterial membranes and internal targets upon light activation, MB-aPDT has demonstrated in this study rapid bactericidal activity in vitro against multi-drug-resistant strains, including VISA and LRSA, without observing the selection of resistant phenotypes in repeat-exposure cycles.

Dr. Caetano P. Sabino, Director of Research and Development at Ondine Research Laboratories, commented:

“We designed this investigation involving high-priority pathogens to assess whether repeated sub-lethal exposure would select for reduced susceptibility in these multidrug-resistant strains under controlled laboratory conditions. Under the same serial-exposure protocol, bacteria exposed to the comparator antibiotic, ciprofloxacin, developed resistance to that antibiotic. In contrast, after 20 consecutive MB-aPDT exposure cycles, we detected no reduction in susceptibility to MB-aPDT and no consistent reduction in susceptibility to the antibiotics tested.

“Because photodisinfection acts via immediate, multi-target oxidative disruption rather than a single enzymatic pathway, the genetic barrier to resistance appears remarkably high. These findings build on Ondine’s earlier research (Pedigo et al, 2009) and reinforce the potential of photodisinfection to play an important role in infection prevention while helping preserve the effectiveness of existing antibiotics.”

Ondine's nasal photodisinfection technology is commercially available outside of the USA under the brand name Steriwave®. Steriwave is a 5-minute, single-application procedure administered directly by healthcare professionals at the point of care, removing reliance on multi-day patient self-administration at home. Steriwave is authorised for nasal decolonisation in Canada, the UK, the EU (CE mark), Mexico, and Australia, where hospitals use it as part of perioperative infection-prevention programmes.

Enquiries:

About Ondine Biomedical Inc.

Ondine Biomedical Inc. is a Canadian life sciences company focused on the development of light-activated antimicrobial therapies (“photodisinfection”) for the prevention and treatment of infections, including those associated with multidrug-resistant organisms.

Ondine's lead product, Steriwave®, is authorized for nasal decolonisation in Canada and certain international markets. In the United States, the technology has been granted Qualified Infectious Disease Product designation and Fast Track status by the U.S. Food and Drug Administration and recently completed enrolment in its Phase 3 clinical trial.

The Company has a pipeline of additional photodisinfection products in various stages of development for application in areas including chronic sinusitis, ventilator-associated pneumonia, burns and other indications.

U.S. Regulatory Status and Clinical Data Disclaimer

Ondine’s light-activated antimicrobial technology, commercially available as Steriwave® in certain jurisdictions, is not approved, cleared, or commercially available for sale or clinical use in the United States and is limited by U.S. law to investigational use only.

References to clinical outcomes in this release should be interpreted in the context of the specific studies or real-world evaluations described and do not imply regulatory approval for any particular indication.

References

1. Cheung GYC, Bae JS, Otto M. Pathogenicity and virulence of Staphylococcus aureus. Virulence. 2021;12:547–69.
2. Turner NA, Sharma-Kuinkel BK, Maskarinec SA, Eichenberger EM, Shah PP, Carugati M, et al. Methicillin-resistant Staphylococcus aureus: an overview of basic and clinical research. Nat Rev Microbiol. 2019;17:203–18.
3. Bai AD, Lo CKL, Komorowski AS, et al. Staphylococcus aureus bacteraemia mortality: a systematic review and meta-analysis. Clin Microbiol Infect 2022; 28:1076–84.
4. Centers for Disease Control and Prevention. Antibiotic Resistance Threats in the United States, 2019. Atlanta, GA: U.S. Department of Health and Human Services, CDC; 2019.
5. Bode, L. G., Kluytmans, J. A., Wertheim, H. F., et al. Preventing surgical-site infections in nasal carriers of Staphylococcus aureus. N Engl J Med. 2010;362(1):9-17. doi:10.1056/NEJMoa0808939.
6. Schweizer, M. L., Chiang, H., Septimus, E., et al. Association of a Bundled Intervention With Surgical Site Infections Among Patients Undergoing Cardiac, Hip, or Knee Surgery. JAMA. 2015;313(21):2162–2171. doi:10.1001/jama.2015.5387.
7. Virgillio, A. M., Felton, E. A., Jackson, J. K., et al. Genomic and Phenotypic Characterization of Mupirocin-Resistant Staphylococcus aureus Clinical Isolates. Open Forum Infect Dis. 2025;12(7):ofaf374. doi:10.1093/ofid/ofaf374.
8. Pedigo LA, Gibbs AJ, Scott RJ, Street CN. Absence of bacterial resistance following repeat exposure to photodynamic therapy. Proc SPIE. 2009;7380:73803H. doi:10.1117/12.822834.

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