Ocular Microbiome

Dengyi Zhou

Introduction

The human eye, once thought to be a nearly sterile environment, harbours a distinct microbial community known as the ocular microbiome (1). This diverse population of bacteria, fungi, and viruses plays a crucial role in maintaining ocular surface health and preventing infections (1). Recent advances in next-generation sequencing and metagenomics have provided deeper insights into the composition and function of the ocular microbiome, revealing its involvement in both health and disease states (2). This article explores the ocular microbiome’s composition, its role in ocular health, its involvement in disease and potential therapeutic implications.

Composition of the Ocular Microbiome

The ocular surface is home to a relatively low microbial biomass, containing 0.06 bacteria per conjunctival cell versus the 10 bacteria per gut epithelial cell (3). Culture-based methods previously identified a limited set of commensal bacteria, but modern sequencing techniques have expanded our understanding of the ocular microbiome (2). The predominant bacterial taxa found on the ocular surface include:

  • Corynebacterium (the most abundant)
  • Simonseilla
  • Streptococcus
  • Propionibacterium (now reclassified as Cutibacterium)
  • Bacillus
  • Staphylococcus (4)

A landmark study by Dong et al. (5) using 16S rRNA sequencing confirmed that the ocular microbiome is highly individualised but maintains a core set of bacteria across different individuals. Additionally, there are fungal and viral communities, although their roles in ocular health and disease remain less well understood (3).

Role of the Ocular Microbiome in Eye Health

A balanced ocular microbiome plays a critical role in maintaining homeostasis by:

  1. Competing with Pathogens: Commensal bacteria help prevent the colonisation of pathogenic microbes by competing for nutrients and space.
  2. Modulating Immune Responses: Beneficial bacteria contribute to immune regulation, ensuring that inflammatory responses are appropriately controlled.
  3. Producing Antimicrobial Peptides: Some members of the ocular microbiome produce antimicrobial substances that inhibit the growth of harmful pathogens (3).

St Leger et al. (6) demonstrated that Corynebacterium mastitidis induces mucosal immunity in the eye by driving an interleukin-17 response, highlighting the protective role of commensal microbes.

The Ocular Microbiome in Disease

Disruptions in the ocular microbiome, termed dysbiosis, have been implicated in various ophthalmic conditions.

DiseaseMicrobial AssociationsReference
Bacterial ConjunctivitisHaemophilus, Streptococcus and MoraxellaEpling (7)
BlepharitisStaphylococcus, Streptophyta, Corynebacterium and EnhydrobacterLee et al. (8)
Bacterial KeratitisPseudomonas, Staphylococcus, Streptococcus, Moraxella and NocardiaSingh et al. (9)
Fungal KeratitisAspergillus, Fusarium and CandidaSingh et al. (9)
Dry Eye DiseaseProteobacteriaSchlegel et al. (10)
EndophthalmitisStaphylococcus, Streptococcus and BacillusDurand (11)
Contact Lens-induced DysbiosisMethylobacterium, Lactobacillus, Acinetobacter and PseudomonasShin et al. (12)

Table 1. Key diseases linked to microbial imbalances

Emerging Therapeutic Approaches

Given the crucial role of the ocular microbiome in health, novel therapeutic strategies aim to restore microbial balance. Promising approaches include:

1. Probiotics

A pilot study conducted by Chisari et al. (13) suggests that supplementation with Bifidobacterium lactis and Bifidobacterium bifidum alongside artificial tears significantly improves tear film stability and reduces bacterial overgrowth in patients with dry eye syndrome, highlighting a potential role for probiotics in managing ocular surface health.

2. Microbiome Transplantation

Emerging research suggests that ocular microbiome transplantation, inspired by the success of faecal microbiota transplantation, may offer therapeutic potential for ocular surface diseases by restoring microbial balance and modulating local immune responses (14).

3. Bacteriophage Therapy

Targeting specific bacterial pathogens with bacteriophages has been proposed as an alternative to traditional antibiotics. This approach may help in cases of antibiotic-resistant infections, such as keratitis (15).

Conclusion

The ocular microbiome is a complex and dynamic ecosystem that plays a fundamental role in eye health. Advances in sequencing technologies have deepened our understanding of its composition and functions, revealing its influence on various ocular diseases. As research progresses, microbiome-based therapies hold significant promise for preventing and treating ophthalmic conditions. Further research is needed to establish microbiome-modulating interventions to improve ocular health.

References

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  3. Zilliox MJ, Bouchard CS. The microbiome, ocular surface, and corneal disorders. The American Journal of Pathology. 2023 Nov 1;193(11):1648-61.
  4. Zhou Y, Holland MJ, Makalo P, Joof H, Roberts CH, Mabey DC, Bailey RL, Burton MJ, Weinstock GM, Burr SE. The conjunctival microbiome in health and trachomatous disease: a case control study. Genome medicine. 2014 Nov;6:1-0.
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  7. Epling J. Bacterial conjunctivitis. BMJ clinical evidence. 2012;2012.
  8. Lee SH, Oh DH, Jung JY, Kim JC, Jeon CO. Comparative ocular microbial communities in humans with and without blepharitis. Investigative ophthalmology & visual science. 2012 Aug 1;53(9):5585-93.
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  11. Durand ML. Bacterial and fungal endophthalmitis. Clinical microbiology reviews. 2017 Jul;30(3):597-613.
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  14. Watane A, Cavuoto KM, Rojas M, Dermer H, Day JO, Banerjee S, Galor A. Fecal microbial transplant in individuals with immune-mediated dry eye. American journal of ophthalmology. 2022 Jan 1;233:90-100.
  15. Fukuda K, Ishida W, Uchiyama J, Rashel M, Kato SI, Morita T, Muraoka A, Sumi T, Matsuzaki S, Daibata M, Fukushima A. Pseudomonas aeruginosa keratitis in mice: effects of topical bacteriophage KPP12 administration.

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