Characterization of the normal microbiota of the ocular surface.

The ocular surface is continually exposed to the environment and as a consequence to different types of microbes, but whether there is a normal microbiota of the ocular surface remains unresolved. Using traditional microbial culture techniques has shown that <80% of swabs of the conjunctiva yield cultivable microbes. These usually belong to the bacterial types of the coagulase-negative staphylococci, Propionibacterium sp., with low frequency of isolation of bacteria such as Staphylococcus aureus, Micrococcus sp., Gram-negative bacteria or fungi. Even when these are grown, the numbers of colony forming units (cfu) per swab of the conjunctiva is usually much less than 100 cfu. Swabs of the lid more commonly result in microbial growth, of the same species as from the conjunctiva and slightly higher cfu. Contact lenses have also been cultured, and they yield similar microbial types. Microbes can be isolated from the ocular surface almost immediately after birth. The advent of molecular techniques for microbial identification based on 16S rRNA sequencing has opened up the possibility of determining whether there are non-cultivable microbes that can colonise the ocular surface. Additionally, use of these techniques with cross-sectional and longitudinal studies may help to understand whether the ocular surface harbours its own unique microbiota, or whether the microbiota are only transiently present.