The Eye Microbiome:
How Gut Bacteria Affect Vision Health

The ocular surface was long considered a virtually sterile environment — doctors monitored pathogens and didn’t particularly consider the possibility that something beneficial might live there. Now the picture is different: the conjunctiva, cornea, and tear film harbor their own microbial environment, albeit very limited in size. There’s no dense bacterial colony here, as in the intestine — and it’s precisely this paucity of numbers that makes these local microbes of interest to clinicians.

This environment exists alongside the local immune defenses and mucous barrier, constantly influencing them. When the balance is maintained, tissues more easily tolerate dust, dry air, prolonged visual stress, and contact with normal environmental flora. When this balance is altered, so does the quality of the tear film and the severity of the inflammatory response. For the patient, this manifests as a burning sensation, a gritty sensation, or unstable vision after using a screen.

Gut-eye axis

The connection between gut bacteria and the visual system has been given a specific name — the gut-eye axis. The eye and gut are anatomically distinct, yet they share immune signals, bacterial metabolic products, and common inflammatory mechanisms. This isn’t a metaphor: preclinical and clinical studies describe the mechanism with specific biochemistry.

A healthy gut microbiota produces short-chain fatty acids, indoles, and polyamines — substances directly linked to the immune response and ocular tissue health. When this metabolism is disrupted, the inflammatory environment becomes more persistent and persistent, and the clinical picture becomes unclear.

This is why complaints of dry eyes aren’t always limited to the ocular surface itself. If discomfort persists for weeks, a consultation with an ophthalmologist in St. Petersburg or another city can help differentiate between ordinary visual fatigue and dry eye syndrome, in which the composition of the ocular microbiota and tear proteins may deviate from the norm. Local causes remain, but a broader perspective is needed.

When the balance is disturbed

Microbial imbalance — dysbiosis — is considered a factor associated with dry eye syndrome, glaucoma, and age-related changes in the central retina. However, the connection shouldn’t be oversimplified:

Red eye doesn’t equal dysbiosis, and dysbiosis doesn’t equal infection. Self-medication based on a single symptom often prolongs the process — and by the time a person finally sees a doctor, the picture has become blurred.

The composition of the microbiota varies from person to person, influenced by age, region, and ethnicity. Microbial balance cannot be assessed using a single, universal approach.

What the patient notices

The most common complaints are mundane: dryness, burning, a foreign body sensation, a stinging sensation after using a screen, and intolerance to wind and air conditioning. Some people experience vision deterioration in the evening, then briefly improve after blinking. This combination of symptoms is often associated with a disruption of the tear film and superficial inflammation, rather than a deep retinal pathology.

The ocular surface is highly sensitive to minor, recurring irritants — dryness, infrequent blinking, smoke, and prolonged computer screen time. Against this backdrop, even a slight shift in the microbial environment is felt more acutely. The patient notices redness and fatigue, while the doctor notices a disruption in the protective film and signs of mild but persistent inflammation.

From a clinical perspective, the doctor’s task is to understand exactly where the problem is occurring. They evaluate the patient’s complaints, the condition of the eyelids, the quality of the tear film, the uniformity of its distribution, and the condition of the cornea and conjunctiva. Sometimes a routine examination and simple tests are sufficient, but sometimes a more comprehensive assessment is necessary — especially if the complaints are associated with gastrointestinal diseases or are resistant to topical treatment.

Daily measures and consulting a doctor

If discomfort persists, it’s best to start with simple measures: a regular visual activity schedule, breaks from screen time, humid indoor air, and gentle eyelid hygiene. Diet also plays a role — research on the gut-eye axis links a monotonous diet with an excess of ultra-processed foods to changes in inflammatory patterns. The ocular surface recovers slowly, so expect a gradual reduction in symptoms rather than an immediate effect.

If the burning sensation intensifies, pain, photophobia, a noticeable decrease in vision, or thick discharge occurs, an in-person examination is necessary without delay.

In such a situation, it’s not the time to discuss the microbiota: the doctor needs to quickly rule out infection, corneal damage, and other conditions that require specific treatment rather than observation. This is the point where self-diagnosis ends and the work of a specialist begins.