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Home / Equine Research Bank / Vet Ophthalmol / In Vitro Activity of Manuka Honey, Either Alone or in Combin...
Veterinary ophthalmology2025; 29(2); e70111; doi: 10.1111/vop.70111

In Vitro Activity of Manuka Honey, Either Alone or in Combination With Topical Antibiotics, Against Bacteria Commonly Found in Equine Ulcerative Keratitis.

Abstract: To assess the antibacterial activity of manuka honey against bacterial isolates commonly associated with infected corneal ulcerations in horses, and to investigate possible combined effects of manuka honey and commonly prescribed topical antibiotics. Methods: Four Staphylococcus aureus, including three methicillin-resistant (MRSA), two methicillin-resistant coagulase-negative staphylococci (S. sciuri and S. haemolyticus), and two Streptococcus equi subspecies zooepidemicus isolates from horses with stromal ulcerative keratitis were selected. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of manuka honey were determined with microdilution assays and spectrophotometric analyses. Potential combined antibacterial effects were explored by assessing bacterial growth inhibition using disk diffusion and E-tests, both with and without a sub-inhibitory concentration of manuka honey. Tested antibiotics included chloramphenicol, tetracycline, gentamicin, ofloxacin, tobramycin, cloxacillin, and fusidic acid. Results: The spectrophotometric MIC of manuka honey was 12% (w/v) for all tested isolates. MBC values ranged between 20% (w/v) and 28% (w/v) for all isolates, except S. sciuri. Combining 5% (w/v) manuka honey with tetracycline, chloramphenicol, or fusidic acid enhanced the antibacterial effect against Staphylococcus spp. (including methicillin-resistant isolates). The antibacterial effect of ofloxacin against S. aureus and streptococci and of cloxacillin against S. aureus appeared slightly reduced when combined with 5% manuka honey. No consistent difference was observed when manuka honey was combined with gentamicin or tobramycin. Conclusions: Manuka honey has in vitro inhibitory and bactericidal activity against equine corneal surface pathogens including multi-resistant isolates. Further studies are required to assess potential synergistic and antagonistic effects of manuka honey in combination with antibiotics.
© 2026 The Author(s). Veterinary Ophthalmology published by Wiley Periodicals LLC on behalf of American College of Veterinary Ophthalmologists.
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Publication Date: 2025-11-12 PubMed ID: 41229077PubMed Central: PMC12963514DOI: 10.1111/vop.70111Google Scholar: Lookup
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Summary

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Overview

  • This study evaluates the antibacterial properties of manuka honey against bacteria causing eye infections in horses and examines how manuka honey interacts with commonly used topical antibiotics.

Background and Purpose

  • Equine ulcerative keratitis is a serious corneal infection in horses often involving bacterial pathogens.
  • Common bacteria involved include Staphylococcus aureus (including methicillin-resistant strains, MRSA), coagulase-negative staphylococci (such as S. sciuri and S. haemolyticus), and Streptococcus equi subspecies zooepidemicus.
  • Manuka honey is known for antibacterial effects, but its effectiveness specifically against equine ocular bacteria had not been well studied.
  • The study aims to determine the minimum concentrations of manuka honey needed to inhibit and kill these bacteria in vitro and to explore whether combining manuka honey with different topical antibiotics alters their antibacterial effectiveness.

Methods

  • Bacterial Isolates:
    • Four Staphylococcus aureus strains, including three MRSA isolates.
    • Two methicillin-resistant coagulase-negative staphylococci (S. sciuri, S. haemolyticus).
    • Two Streptococcus equi subspecies zooepidemicus isolates.
    • All strains were obtained from horses with stromal ulcerative keratitis.
  • Antibacterial Testing of Manuka Honey:
    • Minimum inhibitory concentrations (MICs) were measured to determine the lowest concentration of manuka honey that prevents visible bacterial growth.
    • Minimum bactericidal concentrations (MBCs) were determined to find the lowest concentration that kills bacteria.
    • Microdilution assays combined with spectrophotometric analysis were used for precise measurement of bacterial growth.
  • Combination Testing:
    • Disk diffusion and E-test methods assessed the inhibition of bacterial growth using antibiotics with and without a sub-inhibitory concentration (5% w/v) of manuka honey.
    • Tested antibiotics included chloramphenicol, tetracycline, gentamicin, ofloxacin, tobramycin, cloxacillin, and fusidic acid.

Results

  • Manuka Honey Alone:
    • The MIC for manuka honey was consistently 12% (weight/volume) across all isolates, indicating it effectively inhibits bacterial growth at this concentration.
    • MBC varied from 20% to 28% (w/v) for most bacterial strains, except S. sciuri, which differed.
  • Honey-Antibiotic Combinations:
    • At 5% manuka honey, enhancement of antibacterial activity was seen with tetracycline, chloramphenicol, and fusidic acid against Staphylococcus species, including MRSA strains, suggesting synergistic effects.
    • The antibacterial effect of ofloxacin on S. aureus and Streptococcus spp., and cloxacillin on S. aureus was slightly reduced with manuka honey, indicating possible antagonistic interactions.
    • No significant changes were observed when manuka honey was combined with gentamicin or tobramycin.

Conclusions and Implications

  • Manuka honey demonstrates powerful in vitro inhibitory and bactericidal effects against typical bacteria causing equine corneal ulcers, including antibiotic-resistant strains.
  • There is potential for manuka honey to be used alongside certain topical antibiotics to enhance antibacterial therapy in equine eye infections.
  • However, some antibiotics may have reduced efficacy when combined with manuka honey, highlighting the complexity of these interactions.
  • Further research is necessary to better understand the mechanisms behind these synergistic and antagonistic interactions and to evaluate safety and effectiveness in clinical equine cases.

Cite This Article

APA
Barvelink M, Brok B, Djajadiningrat-Laanen SC, Vernooij JCM, Broens EM, Slenter IJM. (2025). In Vitro Activity of Manuka Honey, Either Alone or in Combination With Topical Antibiotics, Against Bacteria Commonly Found in Equine Ulcerative Keratitis. Vet Ophthalmol, 29(2), e70111. https://doi.org/10.1111/vop.70111

Publication

Veterinary ophthalmology
ISSN: 1463-5224
NlmUniqueID: 100887377
Country: England
Language: English
Volume: 29
Issue: 2
Pages: e70111
PII: e70111

Researcher Affiliations

Barvelink, M
  • Department of Clinical Sciences, Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands.
Brok, B
  • Department of Clinical Sciences, Equine Sciences, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands.
Djajadiningrat-Laanen, S C
  • Department of Clinical Sciences, Surgery of Companion Animals, Veterinary Ophthalmology Section, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands.
Vernooij, J C M
  • Department of Population Health Sciences, Farm Animal Health, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands.
Broens, E M
  • Department of Biomolecular Health Sciences, Veterinary Microbiological Diagnostic Centre, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands.
Slenter, I J M
  • Department of Clinical Sciences, Surgery of Companion Animals, Veterinary Ophthalmology Section, Faculty of Veterinary Medicine, Utrecht University, Utrecht, the Netherlands.

MeSH Terms

  • Animals
  • Honey
  • Horses
  • Anti-Bacterial Agents / pharmacology
  • Anti-Bacterial Agents / administration & dosage
  • Anti-Bacterial Agents / therapeutic use
  • Corneal Ulcer / veterinary
  • Corneal Ulcer / microbiology
  • Corneal Ulcer / drug therapy
  • Horse Diseases / microbiology
  • Horse Diseases / drug therapy
  • Microbial Sensitivity Tests / veterinary
  • Staphylococcus / drug effects
  • Staphylococcal Infections / veterinary
  • Staphylococcal Infections / microbiology
  • Staphylococcal Infections / drug therapy

Conflict of Interest Statement

The authors declare no conflicts of interest.

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