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Your Eyes and Microbiomes

Many of you have heard of the "enteric nervous system."

The enteric nervous system (ENS) is a quasi-autonomous part of the nervous system and includes several neural circuits that control motor functions, local blood flow, mucosal transport and secretions, and modulates immune and endocrine functions.

According to Harvard Medical School:

The enteric nervous system that regulates our gut is often called the body’s “second brain.” Although it can’t compose poetry or solve equations, this extensive network uses the same chemicals and cells as the brain to help us digest and to alert the brain when something is amiss. The gut and brain are in constant communication.

“There is immense crosstalk between these two large nerve centers,” says Braden Kuo, MD, MMSc ’04, co-executive director of the Center for Neurointestinal Health at Massachusetts General Hospital (MGH) and assistant professor of medicine at Harvard Medical School. “This crosstalk affects how we feel and perceive gastrointestinal (GI) symptoms and impacts our quality of life.”

Normally, when we see something tasty, the brain signals the gut to prepare for incoming food. When we feel anxious or stressed, we might experience as abdominal pain, diarrhea, nausea, or “butterflies.” Messages travel from the gut to the brain, too. This helps explain why, when we eat something that makes us sick, we instinctively avoid the food and even the place we found it.

These everyday activities can go awry when gut nerves are damaged or malfunctioning. The Center for Neurointestinal Health treats patients with life-altering conditions such as chronic constipation, extreme bloating, and irritable bowel syndrome (IBS). Center physician-scientists contribute to the exciting basic, clinical, and translational research across HMS to understand the gut-brain connection.


Your eyes are part of your brain and thus respond to the ebbs and flows of the enteric nervous system. For those interested in a deep read into this connection, here is a book by a "chaired" professor at Harvard.

John Dowling’s The Retina, published in 1987, quickly became the most widely recognized introduction to the structure and function of retinal cells. In this Revised Edition, Dowling draws on twenty-five years of new research to produce an interdisciplinary synthesis focused on how retinal function contributes to our understanding of brain mechanisms.


But the eye's health is not just dictated by the gut microbiome. The eye also has a microbiome. Eyeworld suggests that probiotics may be applied topically to eyes.

Ophthalmology News May 2009

by Vanessa Caceres EyeWorld Contributing Editor

How oral or topical use of “friendly” bacteria may affect eye health

Probiotics are being hailed to help treat gastrointestinal tract problems, counteract the side effects of antibiotics, and offer general immune support. In fact, their benefits have turned them into a nutritional marketing tool as food manufacturers add them to products ranging from yogurt to cereal to granola bars.

Probiotics also may have an intriguing place in ophthalmology, particularly to help treat allergic conjunctivitis and inflammation from ocular rosacea or inflammatory dry eye.

“There’s possibly a very interesting role for probiotics in ophthalmology, either orally or topically,” said Robert Latkany, M.D., founder and director, Dry Eye Clinic, New York Eye and Ear Infirmary, New York. “Probiotics may represent a valuable and safe support to standard anti-allergic treatment in any form of allergic conjunctivitis, possibly decreasing on a long-term basis the use of topical steroids,” said Alfonso Iovieno, M.D.,

Interdisciplinary Center for Biomedical Research, Laboratory of Ophthalmology, the University of Rome Campus Bio-Medico, Rome. Dr. Iovieno recently completed a study on probiotics used to treat vernal keratoconjunctivitis.


Connection to ophthalmology

The connection between probiotics and the eye may seem far removed, but investigators are actually discovering a relationship. A study led by Dr. Iovieno and published last year in Graefes Archives of Clinical and Experimental Ophthalmology tested using a probiotic eye drop with Lactobacillus acidophilus for a month to treat seven patients with signs and symptoms of vernal keratoconjunctivitis. Investigators measured conjunctival hyperemia, chemosis, secretion, Trantas dots, and superficial punctuate keratitis and their symptoms. They also performed impression cytology to evaluate the expression of ICAM-1 and TLR-4.

Investigators diluted Lactobacillus acidophilus into a saline solution to create the eye-drop solution.

Symptoms in all six patients who completed the study (one patient dropped out) were significantly improved at two weeks and four weeks. Clinical signs were also significantly better after four weeks (although not at the two-week mark). The impression cytology, which was performed in three patients, showed downregulation of ICAM-1 and TLR-4 in two patients. “We were fairly surprised by the fact that every patient, regardless of statistically significant clinical improvement, described a positive effect of Lactobacilli eye drops on their symptoms, and none of them reported discomfort or the occurrence of side effects,” Dr. Iovieno said. He and fellow investigators are now analyzing the effects of prolonged oral consumption of Lactobacilli for more mild forms of eye allergies.

A number of allergy studies, some of which also track eye symptoms, are finding good results associated with probiotic use. A review article published in the December 2008 issue of Annals of Allergy, Asthma, and Immunology analyzed 12 randomized controlled trials that studied the effects of probiotics in allergic rhinitis and asthma. “Nine of the 12 randomized controlled trails that evaluated clinical outcomes in allergic rhinitis showed an improvement due to the use of probiotics,” wrote lead investigator Harissios Vliagoftis, M.D., Department of Medicine, University of Alberta, Edmonton, Alberta. “All the [trials] that studied perennial allergic rhinitis showed lower symptom scoring with the use of probiotics compared with placebo.

Also, five of the eight [trials] that referred to seasonal allergic rhinitis suggested an improvement in clinical outcomes,” he wrote. Four trials reviewed also tracked ocular allergy symptoms; all four studies reported decreased eye symptoms after probiotic use. Although Dr. Vliagoftis and co-investigators wrote that the mechanism of action for probiotics in allergy is not entirely clear, they may have an effect on immune regulation by altering the composition of gut flora. Others have postulated a connection to the so-called “hygiene hypothesis,” which is based on research that finds children exposed to more microbes earlier in life have a lower incidence of allergy and asthma than children exposed to fewer microbes.


Here are a few references that explain this localized microbiome and how the gut microbiome influences the eye.


Dr. Trempe, my Ophthalmology partner at Harvard Medical School, tested for and treated chronic stealth infections in people with eye disease. Many improved.

How do probiotics work: The safest antibiotic is a probiotic.


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