General and Ocular Microbiology and Immunology
This domain tests host defenses, ocular surface flora, common pathogens, and hypersensitivity mechanisms. Expect organism-to-syndrome matching and immune-privilege concepts.
Core rules
- Normal conjunctival flora includes coagulase-negative staphylococci, diphtheroids, and Propionibacterium; culture positivity does not always equal infection.
- Contact lens keratitis: Pseudomonas aeruginosa is a classic gram-negative cause and can perforate; Acanthamoeba causes ring infiltrate and severe pain; Fusarium is linked to lens-related fungal keratitis.
- Postoperative endophthalmitis after cataract surgery is most often coagulase-negative Staphylococcus; Bacillus is associated with trauma; Candida with indwelling lines or immunosuppression.
- Immune privilege occurs in the anterior chamber, cornea, and subretinal space, mediated by factors such as TGF-beta and Fas ligand; it is not absolute.
- Hypersensitivity: type I IgE immediate; type II antibody-mediated; type III immune complex; type IV T-cell delayed, as in contact dermatitis.
Worked example
A soft contact lens wearer presents with severe pain, corneal infiltrate, and hypopyon. Reasoning: the highest-yield association is Pseudomonas aeruginosa, a gram-negative rod. Treatment should cover gram-negative organisms, typically with a fluoroquinolone, and contact lens use must stop. Steroids alone are inappropriate because they do not treat the infection and may worsen it.
Traps that cost marks
- Calling all staphylococcal disease Staph aureus; coagulase-negative Staph is a common commensal and postoperative pathogen.
- Confusing HSV dendritic keratitis with bacterial ulcer; antiviral therapy differs.
- Assuming immune privilege means no immune response can ever occur.