Ocular Surface Microbiome in Dry Eye Patients

The ocular surface is the first line of defence of the eye, it is therefore where external threats are sensed, and potential insults neutralised. Over the course of evolution, various microbes, especially bacteriae, have come to colonise the ocular surface as commensals. The commensals have a role to maintain the homeostasis of the ocular surface. 1 The innate immunity of the ocular surface is very active, and consists of active mechanisms to suppress inflammation 2. For example, there exist macrophages, dendritic cells, suppressor cells, regulatory cells, B cells, IgA, lysozyme, anti-microbial peptides and barriers against external agents. The normal commensals of the ocular surface maintain a basal level of activation of innate defence by stimulating the pattern recognition receptors on ocular surface epithelial cells. This normal composition of microbes is important since inflammation and infection will result if there is introduction of a pathogenic strain that overcomes the flora, or if a dominant strain secretes excessively immunogenic products, such as the exotoxin A of Staphylococcus which triggers marginal keratitis, a form of type IV hypersensitivity. The flora load of microbiome could also influence tear function as a higher flora load was found to be associated with increased mucin degradation 3 and reduced globet cell densitiy 4. Previous studies \[I'm not sure which studies these are\] at SERI/SNEC also point to the importance of microbes. For example, in dry eye patients, there is increased lysophospholipids in the tear, and this may contribute to inflammatory mediators such as arachidonic acid and other metabolites. The lysophospholipids are formed by phospholipase A2 reactions, and the latter may be microbial in origin. Since dry eye is a known inflammatory disease of the ocular surface, this is one way that microbes can contribute to the pathology.