Evaluation of the bacterial ocular surface microbiome in clinically normal horses before and after treatment with topical neomycin-polymyxin-bacitracin.
Authors: Scott Erin M, Arnold Carolyn, Dowell Samantha, Suchodolski Jan S
Journal: PloS one
Summary
# Editorial Summary: Equine Ocular Surface Microbiome and Topical Antibiotic Response While the ocular surface microbiota in humans is now well characterised through next-generation sequencing (NGS), equivalent knowledge in horses remained absent until this 2019 study addressed the gap. Researchers sampled the inferior conjunctival fornix from 12 healthy horses at baseline, after one week of topical neomycin-polymyxin-bacitracin treatment (applied three times daily to one eye), and four weeks post-treatment, with the contralateral eye serving as an untreated control; bacterial DNA was identified through 16S rRNA sequencing. The equine ocular surface microbiome proved dominated by four phyla—Proteobacteria (46.1%), Firmicutes (24.6%), Actinobacteria (12.6%), and Bacteroidetes (11.2%)—with Pasteurellaceae being the most abundant family at 13.7%, followed by Sphingomonadaceae and Moraxellaceae; critically, neither alpha nor beta diversity measurements shifted significantly in treated or control eyes over the 35-day observation period. These findings suggest that routine topical antibiotic ointment produces remarkably limited disruption to the equine conjunctival microbiota, contrasting sharply with recognised effects in other species and implying either that equine ocular communities possess particular resilience or that the dosing regimen insufficient alters the microbial environment. For practitioners, this stability may offer reassurance regarding microbial dysbiosis from short-term topical therapy, though the clinical implications of maintaining this relatively static microbiome during infection versus allowing dynamic community shifts remain unanswered.
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Practical Takeaways
- •Topical antibiotic ointments appear to have minimal long-term impact on the stable ocular surface microbiome in healthy horses, suggesting the normal flora is resilient
- •Understanding the normal equine ocular microbiome composition provides a baseline for future studies investigating diseased states and may help identify dysbiosis-related eye conditions
- •Next-generation sequencing reveals a far more complex ocular microbiota than traditional culture methods can detect, which may be relevant for investigating recurrent or chronic ocular issues in horses
Key Findings
- •Equine ocular surface microbiome is dominated by Proteobacteria (46.1%), Firmicutes (24.6%), Actinobacteria (12.6%), and Bacteroidetes (11.2%)
- •Pasteurellaceae (13.7%) and Sphingomonadaceae (7.9%) are the most abundant bacterial families on the equine ocular surface
- •Alpha and beta diversity measurements remained unchanged in both treated and control eyes over 35 days
- •One week of topical neomycin-polymyxin-bacitracin treatment did not produce detectable shifts in major bacterial taxa composition