Ocular Microbiology and Immunology
Ocular microbiology and immunology
This domain tests normal ocular flora, common pathogens, host defence, and immune-mediated disease. Exam items often use a clinical pattern to point to an organism or an immune mechanism.
Core rules
- Normal conjunctival flora includes coagulase-negative staphylococci, diphtheroids, and some streptococci. The ocular surface is not sterile.
- Contact lens keratitis is commonly bacterial, especially Pseudomonas aeruginosa; fungal keratitis is associated with trauma and agricultural exposure; Acanthamoeba is linked to contact lens use and water exposure.
- Endophthalmitis after cataract surgery is usually caused by gram-positive skin flora, especially Staphylococcus epidermidis.
- The eye has immune privilege: anterior chamber-associated immune deviation, blood-ocular barriers, and local immunosuppressive factors reduce inflammation.
- Hypersensitivity types matter: type I allergic conjunctivitis, type III immune complex disease, type IV contact dermatitis and graft rejection.
Worked example
A contact lens wearer presents with a corneal ring infiltrate and severe pain out of proportion to findings. What organism should be suspected? Acanthamoeba is the classic concern, especially with water exposure and a ring infiltrate. Pseudomonas can also cause severe contact lens keratitis, but it more often produces a rapidly progressive ulcer with mucopurulent discharge. The reasoning combines exposure history, lesion pattern, and severity; culture or PCR confirms, but treatment should not wait.
Traps
- Calling the normal conjunctiva sterile.
- Assuming all contact lens ulcers are bacterial.
- Confusing immune privilege with absence of immune responses.
- Missing that fungal and amoebic keratitis can mimic herpes simplex.