Authored by Mohammad AA Al-Najjar*
Eye Microbiota
The human eye is a very complex organ which consists of three layers: tear film, lacrimal glands, and eyelids [1–3]. The outer section consists of the cornea and the sclera, where cornea protects the eye against infection [3]. The ocular surface is frequently exposed to the environment and to different types of microbes starting from birth time and throughout life. If a swab was taken from different parts of the ocular surface, many types of microbes can be isolated and examined by culture [4,5]. Interestingly, the microbial composition could be (i) gram-positive bacteria such as Staphylococcus aureus and staphylococciepidermidis, (ii) gram-negative bacteria such as Pseudomonas aeruginosa, and [4,5], (iii) fungi such as Aspergillus spp, Curvularia sp., Penicillium sp., Helminthosporium sp., Candida albicans, C. guilliermondii, C. parapsilosis, Saccharomyces cerevisiae, Hormodendrum sp., and Rhodotorula rubra [6–9] (Table 1 & 2).Effect of The Gut Microbiome on Ocular Disease
Relevant to ophthalmologist’s studies, gut bacteria can influence immunity at distant sites, including the eye. Some ocular conditions have been associated with gut microbiome abnormalities including Sjögren’s associated dry eye, glaucoma, and macular degeneration. Patients have infectious keratitis, Patients with bacterial keratitis had higher relative abundances of Proteobacteria and Firmicutes in the gut comparing them with healthy individuals [11].Dry eye
Sjögren’s is a disease caused by lymphocytic infiltration of the lacrimal and salivary glands disorder which as consequence will cause dry eye and mouth [12]. Studies demonstrated that commensal bacteria variation in the gut provokes a worse dry eye phenotype, while the normalization of the improvement of the microbiome can cause improvement in the dry eye phenotype [13,14].Autoimmune uveitis
The results of studies were done on patients with posterior segment uveitis have shown that these patients have gut microbiome disturbance [15,16]. Compared to healthy individuals there were increased abundances of Fusobacterium and Enterobacteriaceae [17]. This alteration in the commensal gut bacteria believed to alter the immune privilege status of the eye and thus prompted toward uveitis. A series of actions potentially relate the microbiome to uveitis; consist of loss of immune tolerance to commensal microbiomes increased inflammation and permeability of the gastrointestinal tract and translocation of microbial antigens to extraintestinal sites such as the eye [15,16,18].Glaucoma
Studies investigated the link between the microbiome and different sub-types of glaucoma, as they revealed that alterations of gut microbiome were related to primary open angle glaucoma (POAG). This infection was correlated with the raise abundance of Bacteroides and Prevotella [19,20].Age-related macular degeneration (AMD)
Age-related macular degeneration (AMD) is a deactivating eye condition that affects the slow decrease of central visualization [21,22]. This eye disorder influences a central part of the retina well-known as the macula, which controls critical central vision that is necessary for daily actions as reading, watching television, driving and facial recognition. Even though AMD does not normally indicate to absolute loss of sight (peripheral vision is retained), failure of central vision possibly will have the main effect to an individual’s conventionality and value of natural life [23]. Although in this disorder the exact pathogenesis of AMD is poorly understood, it is believed to be an inflammatory component related to innate immunity. These inflammatory pathways consist of complement and Toll-like receptors, pathways adjusted by the microbiome [17].Stages of AMD: AMD has three stages all of which will appear in the eye retina. The early AMD in which the size of drusen is medium and the patient vision will not be lost. In the intermediate AMD, the drusen size will be larger; this stage is asymptomatic although the patient may have an abnormality in the retinal pigment. The last stage is the late AMD, which is characterized by having two types the wet AMD and the dry AMD, and at this stage, the size of drusen is large too but the patient may loss his central vision [23]. Patients with neovascular AMD had to gut microbiota that was supplemented with Anaerotruncusspp., Oscillibacter spp., Ruminococcus torques, and Eubacteria ventriosum, whereas the microbiota of a healthy person was dominated by Bacteroideseggerthii. The latter microorganism could be a defensive anti-immune-mediated disease, which could be due to the variations in bacterial genes associated with type of metabolic pathways. Especially those involving the diminished amount of genes included in fatty acid elongation and supplementation of genes associated with L-Alanine fermentation, glutamate degradation, and Arginine biosynthesis [24].
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