Which part of the eye contains no normal microbiota?

• A. Conjunctiva
• B. Sclera
• C. Vitreous humor
• D. Retina

Answer: (C)

Microbes live on surfaces that contact the outside environment, like the conjunctiva, where skin- and mucous-associated bacteria such as Staphylococcus and Corynebacterium can be found. In contrast, the interior spaces of the eye are protected by multiple barriers and are normally free of resident microbes. The vitreous humor sits inside the eye, surrounded by the retina and lens, and is separated from the external world by these tissues and the blood-ocular barrier. Because it has no mucosal surface and no direct exposure to nutrients that would support a stable microbial community, it remains sterile in health. If microbes appear there, it indicates an infection, not a normal microbiota.