A 38-year-old woman who wears monthly contact lenses presents with acute onset left eye pain, redness, and purulent discharge for 2 days. On slit-lamp examination, there is a central corneal ulcer with sharply demarcated borders, dense stromal infiltration, and a large hypopyon. Corneal scraping shows Gram-negative rods that are oxidase-positive and mucoid in culture. The ulcer is unresponsive to topical fluoroquinolone monotherapy. What is the most appropriate next step in management?

Explanation

## Management of Aggressive Bacterial Corneal Ulcer **Key Point:** A Gram-negative, oxidase-positive, mucoid organism in a contact lens wearer with acute, aggressive keratitis unresponsive to monotherapy is *Pseudomonas aeruginosa*. This organism requires aggressive combination topical therapy with a fluoroquinolone PLUS a cephalosporin PLUS an aminoglycoside. ### Organism Identification: *Pseudomonas aeruginosa* | Characteristic | Finding | |---|---| | **Gram stain** | Gram-negative rod | | **Oxidase test** | Positive (key differentiator) | | **Culture morphology** | Mucoid, greenish pigment | | **Risk factor** | Contact lens wear, corneal abrasion | | **Virulence** | High — produces exotoxins and elastase | | **Antibiotic resistance** | Intrinsic resistance to many β-lactams | **High-Yield:** *Pseudomonas aeruginosa* is the most common cause of contact lens–associated bacterial keratitis and the most aggressive bacterial corneal pathogen. Monotherapy (even with fluoroquinolones) frequently fails; combination therapy is the standard of care. ### Treatment Algorithm for *Pseudomonas* Keratitis ```mermaid flowchart TD A[Gram-negative rod, oxidase-positive<br/>Pseudomonas aeruginosa]:::outcome --> B[Aggressive keratitis?]:::decision B -->|Yes| C[Combination topical therapy]:::action C --> D[Fluoroquinolone<br/>e.g., moxifloxacin 0.5%]:::action C --> E[Cephalosporin<br/>e.g., cefazolin 5%]:::action C --> F[Aminoglycoside<br/>e.g., tobramycin 1.3%]:::action D --> G[Hourly instillation<br/>day 1-3]:::action E --> G F --> G G --> H[Monitor daily]:::action H --> I{Response?}:::decision I -->|Good| J[Taper frequency]:::action I -->|Poor| K[Consider oral fluoroquinolone<br/>+ systemic anti-inflammatory]:::action ``` **Clinical Pearl:** The oxidase test is the quickest way to identify *Pseudomonas* at the bedside. A positive oxidase test in a Gram-negative rod from a corneal ulcer in a contact lens wearer is diagnostic until proven otherwise. ### Why Monotherapy Fails 1. **Intrinsic resistance:** *Pseudomonas* has chromosomal resistance to many β-lactams and aminoglycosides due to efflux pumps and low outer membrane permeability. 2. **Synergy:** Combination therapy (fluoroquinolone + cephalosporin + aminoglycoside) achieves better corneal penetration and bactericidal activity than any single agent. 3. **Rapid progression:** Pseudomonal keratitis can perforate within 24–48 hours if undertreated. **Mnemonic for Pseudomonas Keratitis: **CONTACT** - **C**ontact lens wear (primary risk) - **O**xidase positive (identification) - **N**eed combination therapy (not monotherapy) - **T**opical + systemic antibiotics - **A**ggressive, rapidly progressive - **C**ephalosporin + aminoglycoside + fluoroquinolone - **T**obramycin (aminoglycoside of choice) **Tip:** Instill topical antibiotics hourly in the first 3 days, then taper based on clinical response. Systemic fluoroquinolones (e.g., ciprofloxacin 750 mg BD) provide additional corneal penetration and are often added in severe cases. [cite:Khurana Comprehensive Ophthalmology Ch 4; Agarwal Textbook of Ophthalmology Ch 5]