Chapter Three - The “normal” ocular microbiome and human eye health

Abstract

The ocular surface contains a unique microbiome that is exposed to the environment and is involved in the maintenance of ocular surface health and defense from disease. While only a few microorganisms were originally identified on the ocular surface using traditional culture methods, newer techniques, such as 16S rDNA genome sequencing, have expanded our knowledge regarding the composition of the “normal” ocular surface microbiome. This chapter discusses the composition of the healthy ocular surface microbiome, which has been found to be paucibacterial microbiome, compared to other areas of the body, such as the skin and oral mucosa. Considering bacteria, the three phyla most commonly recovered from the ocular surface are Proteobacteria, Actinobacteria, and Firmicutes. Viruses (e.g., MS-associated retrovirus (MSRV), human endogenous retrovirus K (HERV-K), and torque teno virus) and fungi (e.g., Basidiomycota and Ascomycota) have also been recovered from the ocular surface, although at levels much lower than bacteria. This chapter also examines how intrinsic and extrinsic factors influence the ocular surface microbiome. In short, age appears to influence the composition of the ocular surface microbiome more so than sex (with a noted increase in the phyla Actinobacteria and Proteobacteria and a reduction of Firmicutes with increasing age). Furthermore, geographic and environmental conditions appear to influence the microbiome. For example, high altitudes and certain geographical locations have been noted to have increased Proteobacteria compared to low altitude and other geographic locations. Finally, this chapter discusses how the ocular surface microbiome interacts with the immune system to maintain ocular surface health.