Microbiology, 2021

21.1 • Anatomy and Normal Microbiota of the Skin and Eyes 859
defensins, lactoferrin, and lysozyme, help to prevent colonization by pathogens. In addition, mucins facilitate
removal of microbes from the surface of the eye.
Figure 21.6
The lacrimal apparatus includes the structures of the eye associated with tear production and drainage. (credit: modification
of work by “Evidence Based Medical Educator Inc.”/YouTube)
The surfaces of the eyeball and inner eyelid are mucous membranes called conjunctiva. The normal
conjunctival microbiota has not been well characterized, but does exist. One small study (part of the Ocular
Microbiome project) found twelve genera that were consistently present in the conjunctiva. 4 These microbes
are thought to help defend the membranes against pathogens. However, it is still unclear which microbes may
be transient and which may form a stable microbiota. 5
Use of contact lenses can cause changes in the normal microbiota of the conjunctiva by introducing another
surface into the natural anatomy of the eye. Research is currently underway to better understand how contact
lenses may impact the normal microbiota and contribute to eye disease.
The watery material inside of the eyeball is called the vitreous humor. Unlike the conjunctiva, it is protected
from contact with the environment and is almost always sterile, with no normal microbiota (Figure 21.7).
Figure 21.7
Some microbes live on the conjunctiva of the human eye, but the vitreous humor is sterile.
4 Abelson, M.B., Lane, K., and Slocum, C.. “The Secrets of Ocular Microbiomes.” Review of Ophthalmology June 8, 2015.
http://www.reviewofophthalmology.com/content/t/ocular_disease/c/55178. Accessed Sept 14, 2016.
5 Shaikh-Lesko, R. “Visualizing the Ocular Microbiome.” The Scientist May 12, 2014. http://www.the-scientist.com/?articles.view/
articleNo/39945/title/Visualizing-the-Ocular-Microbiome. Accessed Sept 14, 2016.