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Frontiers in microbiology2025; 16; 1535095; doi: 10.3389/fmicb.2025.1535095

The effect of topical antibiotic or antibiotic-corticosteroid treatment on the ocular surface microbiota of healthy horses.

Abstract: Information regarding the impact of topical antibiotics with or without corticosteroids on the microbiota of the horses' eyes is limited. This study aimed to describe the bacterial ocular surface microbiota in healthy horses and evaluate the effect of topical antibiotics or antibiotic-corticosteroid medication on the ocular surface microbiota. Unassigned: This was a prospective, randomized, longitudinal, blinded study in which one eye of 12 horses was treated 3 times daily for 7 days with neomycin, polymyxin B and bacitracin ophthalmic ointment ( = 6) or neomycin, polymyxin B and dexamethasone ophthalmic ointment ( = 6). The contralateral eyes operated as untreated controls. The inferior conjunctival fornix of both eyes was sampled at baseline before antibiotic administration (day 0), on days 3, 7, 9, 14, and 30. The ocular surface microbiota was characterized by amplifying the V4 region of the 16S ribosomal RNA gene. Unassigned: Alpha- (richness and diversity) and beta-diversity (weighted and unweighted UniFrac distances) measurements of the ocular surface microbiota varied similarly after treatments starting on day 1, returning to baseline measurements by day 30. At baseline, the main phyla detected in the ocular microbiota was Proteobacteria, representing 75% relative abundance, followed by Firmicutes and Bacteroidetes. After treatments, Proteobacteria declined in all groups, and Firmicutes and Bacteroidete's relative abundance increased, returning to baseline levels on day 30. The main genera detected on the ocular surface on day 0 were , , and members of the family Moraxellaceae. After treatment, the relative abundance of this bacteria declined in all groups, returning to baseline levels on day 30, although some alterations were still present. Unassigned: Here we show that topical antibiotics administered with or without corticosteroids induce changes in the ocular surface of horses' eyes, and the microbiota appears to return to baseline approximately three weeks after treatment discontinuation.
Copyright © 2025 Martin de Bustamante, Plummer, Caddey and Gomez.
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Publication Date: 2025-08-04 PubMed ID: 40831630PubMed Central: PMC12359475DOI: 10.3389/fmicb.2025.1535095Google Scholar: Lookup
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  • Journal Article

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study investigated how applying topical antibiotics, with or without corticosteroids, affects the bacterial communities on the surface of healthy horses’ eyes.
  • The research found that although antibiotic treatments temporarily altered the eye microbiota, the bacterial balance generally returned to normal about three weeks after treatment stopped.

Background and Purpose

  • The ocular surface microbiota consists of bacteria that naturally inhabit the eye, playing a role in eye health.
  • Understanding how topical medications like antibiotics and corticosteroids affect this microbiota is important, especially since such treatments are commonly used for eye conditions in horses.
  • There was limited knowledge about the impact of these medications on the horse’s eye microbiome before this study.
  • The study aimed to describe the normal bacterial communities present on healthy horses’ eyes and evaluate the effects of topical antibiotic or antibiotic-corticosteroid ointments on these communities.

Methods

  • Study Design: Prospective, randomized, longitudinal, and blinded.
  • Subjects: 12 healthy horses.
  • Treatment Groups:

    • One group (6 horses) received topical antibiotic ointment containing neomycin, polymyxin B, and bacitracin.
    • The other group (6 horses) received topical antibiotic-corticosteroid ointment containing neomycin, polymyxin B, and dexamethasone.
  • Procedure:

    • One eye of each horse was treated 3 times daily for 7 days.
    • The untreated opposite eye served as the control.
    • The inferior conjunctival fornix (a part inside the lower eyelid) was sampled for bacterial DNA.
    • Sampling occurred on multiple days: before treatment (day 0), during treatment (days 3 and 7), and after treatment cessation (days 9, 14, and 30).
  • Microbiota Analysis:

    • Used 16S ribosomal RNA gene sequencing, specifically targeting the V4 region, to identify and quantify bacterial populations.
    • Assessed bacterial richness and diversity (alpha-diversity) along with differences in bacterial community composition (beta-diversity) using UniFrac distances.

Results

  • Baseline Microbiota:

    • The ocular microbiota was dominated by the phylum Proteobacteria (~75% relative abundance).
    • Other common phyla included Firmicutes and Bacteroidetes.
    • At the genus level, prominent bacteria belonged to Moraxellaceae and other genera (names missing in the abstract but indicating common ocular bacteria).
  • Effect of Treatments:

    • Both antibiotic alone and antibiotic-corticosteroid treatments resulted in similar changes beginning as early as day 1 after treatment started.
    • There was a decline in Proteobacteria and a relative increase in Firmicutes and Bacteroidetes during treatment.
    • The relative abundance of key genera, especially those in the Moraxellaceae family, decreased during treatment.
    • Alpha-diversity (richness and diversity) and beta-diversity measurements changed during treatment but showed a trend back toward baseline values over time.
  • Recovery Post-Treatment:

    • By day 30, most bacterial community aspects returned to baseline levels, indicating resilience in the ocular microbiota.
    • Some minor alterations persisted even at day 30, but overall the microbiota recovered.

Conclusions

  • Topical administration of antibiotics, whether alone or combined with corticosteroids, causes temporary shifts in the composition of ocular surface bacteria in healthy horses.
  • The ocular microbiota exhibits resilience, generally returning to its original state approximately three weeks after stopping treatment.
  • This suggests that short-term topical antibiotic treatments for eye conditions in horses may not cause long-lasting disruption to normal ocular bacterial communities.
  • Understanding this recovery timeline can inform veterinary clinical decisions regarding ocular therapies and microbiome impacts.

Implications for Future Research

  • Further studies could explore the functional consequences of these microbiota changes on eye health and disease resistance.
  • Longer-term effects and impacts on diseased or compromised eyes need investigation.
  • Potential benefits of probiotic or microbiota-supportive therapies during or after antibiotic treatment could be assessed.

Cite This Article

APA
Martin de Bustamante MG, Plummer CE, Caddey B, Gomez DE. (2025). The effect of topical antibiotic or antibiotic-corticosteroid treatment on the ocular surface microbiota of healthy horses. Front Microbiol, 16, 1535095. https://doi.org/10.3389/fmicb.2025.1535095

Publication

Frontiers in microbiology
ISSN: 1664-302X
NlmUniqueID: 101548977
Country: Switzerland
Language: English
Volume: 16
Pages: 1535095
PII: 1535095

Researcher Affiliations

Martin de Bustamante, M G
  • Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL, United States.
Plummer, C E
  • Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL, United States.
Caddey, B
  • Faculty of Veterinary Medicine, University of Calgary, Calgary, AB, Canada.
Gomez, D E
  • Department of Internal Medicine, Abu Dhabi Equine and Camel Hospital (ADECH), Abu Dhabi, United Arab Emirates.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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