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Microbial keratitis
Microbial keratitis is invasion of the stroma by an organism through an epithelial defect — with pain, a red eye, a white infiltrate and a defect that stains with fluorescein — and it is urgent: a scraping is taken for culture in a significant ulcer and frequent antibiotic drops are started before the result arrives.
Why it matters in the exam
The most important risk factor is contact lens wear, especially sleeping in lenses, swimming and rinsing with tap water. Pseudomonas aeruginosa is the typical contact lens organism: a fast ulcer with purulent discharge and sometimes hypopyon. Acanthamoeba — pain far out of proportion to the findings, a ring infiltrate and infiltrates along the corneal nerves, after water exposure; it is often misdiagnosed as herpes. Fungal keratitis (Fusarium, Aspergillus) — after trauma with plant matter, with "feathery" margins and satellite lesions; Candida — in an already diseased cornea.
When to culture: an ulcer larger than 1–2 mm, central, deep, with hypopyon, an ulcer not responding to treatment or suspicion of an atypical organism. A small peripheral ulcer can be treated with a single fluoroquinolone (moxifloxacin), and a large or central ulcer with fortified drops (cefazolin and tobramycin) every hour, through the night too. In filamentous fungus — natamycin 5% is better than voriconazole according to MUTT I. In Acanthamoeba — PHMB or chlorhexidine for many months.
Steroids: the SCUT trial showed no overall benefit from adding a steroid in bacterial ulcers, and one is considered only after the infection is under control and known not to be fungal. An eye with a contact lens is not patched, because that encourages Pseudomonas. An ulcer thinning towards perforation needs tissue glue or a corneal graft. Very acute conjunctivitis with profuse discharge in a newborn or an adult raises suspicion of gonorrhoea, which penetrates even an intact cornea.
Key points
- Pain, red eye, white infiltrate and an epithelial defect.
- Contact lenses = the key risk factor; Pseudomonas.
- Large or central ulcer → culture and fortified drops.
- Filamentous fungus → natamycin; Acanthamoeba → PHMB.
- No patch with contact lenses; steroid only after control.
Summary table
| Organism | Setting | Identifying sign | Treatment |
|---|---|---|---|
| Pseudomonas | Contact lenses | Fast ulcer, purulent discharge | Fortified drops or a fluoroquinolone hourly |
| Staphylococcus, Streptococcus | Ocular surface disease, blepharitis | Well-defined infiltrate | Fortified cefazolin or a fluoroquinolone |
| Filamentous fungus (Fusarium) | Trauma with plant matter | Feathery margins, satellite lesions | Natamycin 5% |
| Acanthamoeba | Contact lenses and water | Severe pain, ring, perineural infiltrates | PHMB or chlorhexidine |
| Neisseria gonorrhoeae | Newborn or sexually active adult | Profuse discharge, rapid perforation | Systemic ceftriaxone |
A free exam question
קרנית ומקטע קדמי איזו משפחת תרופות הוכיחה יעילות קלינית נגד ציסטות וטרופוזואידים( TROPHOZOITES ) של אקנטמבה( ACANTHAMOEBA ) ? (the question as examined, in Hebrew)
- א. ביגואנידים – BIGUANIDES
- ב. אמינוגליקוזידים- AMINOGLYCOSIDES
- ג. אימידאזולים IMIDAZOLES
- ד. טריאזולים – TRIAZOLES
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Sources
- AAO Preferred Practice Pattern: Bacterial Keratitis, 2018 — Diagnosis and treatment
- Steroids for Corneal Ulcers Trial (SCUT) and Mycotic Ulcer Treatment Trial (MUTT I), 2012 / 2013 — Results
- AAO Basic and Clinical Science Course, Section 8: External Disease and Cornea, 2024-2025 — Infectious keratitis
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.