OCULAR MICROBIOTA IN HEALTH AND DISEASE

Ocular microbiota refers community of commensal microorganisms living on ocular surface (OS). It has a role in maintaining the homeostasis of the ocular surface directly exposed to the external environment and endangered by various pathogenic microorganisms. The microbiota of ocular surface (OSM) can regulate, defend, or provoke and perpetuate the development of eye diseases in response to changes of this subtle ecosystem. The composition of the microbiome has not yet been completely defined, although the newer next-generation sequencing (NGS) technologies expanded the previously reported knowledge. Proteobacteria, Actinobacteria, and Firmicutes were the dominant phyla on the OS. The most common taxa at the genus level were Pseudomonas, Propionibacterium, Bradyrhizobium, Corynebacterium, Acinetobacter, Brevundimonas, Staphylococci, Aquabacterium, Sphingomonas, Streptococcus, Streptophyta, and Methylobacterium. However, there is no consensus on whether a “core” microbiome does exit because to age, sex, ethnicity effects demonstrated on ocular microbiome composition. Moreover, the composition varies from site to site. Eye biogeography of the microbiome is divided into four areas: ocular surface, fornix conjunctiva tissue, lid margin, and skin above the eyelid margin. Pseudomonas was often found on the fornix conjunctiva and lid margin, while Corynebacterium, Propionibacterium, and coagulase-negative Staphylococci were all routinely sampled from the conjunctiva. At the phylum level, Proteobacteria made up 90% of the OSM of the fornix conjunctival tissue, while Bacteroidetes appeared irregularly across all sites. Actinobacteria was more frequently found on the periocular skin than in other groups. The composition may also vary according to the pressure exerted with the swab at the time of sampling. A greater relative abundance of Staphylococcus and Corynebacterium was noted with soft pressure swabs; the presence of Proteobacteria increased in swabs applied with firm pressure. Moreover, differences in OSM can exist between each eye in a same person. The eye is a relatively hostile environment for potential colonizing microbes due to mucosal immune response, the continual flow of tears that wash the ocular surface, and the elevated levels of salt, antimicrobial peptides, antibodies, and other antimicrobial factors like lysozyme. Further, the action of blinking helps to prevent bacteria and other pathogens from adhering to the corneal surface. However, a drastic imbalance in the microbial equilibrium, referred as dysbiosis, can lead to pathogenic strains overgrowth and a reduction in microbial diversity. Ocular dysbiosis can be associated to several diseases and conditions including injuries, infections (blepharitis, conjunctivitis, keratitis, trachoma), allergies, Dry Eye Disease (DED), antibiotics therapy, contact lens risks, keratoprosthesis and more. Further studies are needed to improve the knowledge of the homeostatic microorganisms to better understand their effects on the ocular surface in health and disease.