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a) Acne (labeled whitehead) on a person’s cheek. B) A drawing of skin with a yellow bubble labeled pus. This is below a raised region on the skin.
(a) Acne is a bacterial infection of the skin that manifests as a rash of inflamed hair follicles (folliculitis). The large whitehead near the center of the cheek is an infected hair follicle that has become purulent (or suppurative), leading to the formation of a furuncle. (b) An abscess is a pus-filled lesion. (credit b: modification of work by Bruce Blaus)
A table labeled types of skin lesions. Crust is shown as a raised region on the surface of the skin. Cyst is shown as a large white sphere in the upper layers of the skin. Macule is shown as a dark mark on the surface. Papule is shown as a raised bubble on the surface. Pusture is shown as a large yellow sphere in the upper layers of the skin. Ulcer is a large cavity in the skin. Vesicle is a small blue bubble in the upper regions of the skin. Wheal is a small blue bubble on the surface of the skin.
Numerous causes can lead to skin lesions of various types, some of which are very similar in appearance. (credit: modification of work by Bruce Blaus)
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Anatomy and microbiota of the eye

Although the eye and skin have distinct anatomy, they are both in direct contact with the external environment. An important component of the eye is the nasolacrimal drainage system, which serves as a conduit for the fluid of the eye, called tears . Tears flow from the external eye to the nasal cavity by the lacrimal apparatus, which is composed of the structures involved in tear production ( [link] ). The lacrimal gland , above the eye, secretes tears to keep the eye moist. There are two small openings, one on the inside edge of the upper eyelid and one on the inside edge of the lower eyelid, near the nose. Each of these openings is called a lacrimal punctum . Together, these lacrimal puncta collect tears from the eye that are then conveyed through lacrimal ducts to a reservoir for tears called the lacrimal sac , also known as the dacrocyst or tear sac .

From the sac, tear fluid flows via a nasolacrimal duct to the inner nose. Each nasolacrimal duct is located underneath the skin and passes through the bones of the face into the nose. Chemicals in tears, such as defensins , lactoferrin , and lysozyme , help to prevent colonization by pathogens. In addition, mucins facilitate removal of microbes from the surface of the eye.

Diagram of an eye. Above the eye is the lacrimal gland. At the point nearest the nose is the punctums and tubes leading to the lacrimal sac and nasolacrimal duct.
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. 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. 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. 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.

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.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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