A 38-year-old man with a history of contact lens overwear presents with a 5-day history of corneal ulcer. Slit-lamp examination shows a central, well-demarcated ulcer with a greenish tinge and mucopurulent discharge. Gram stain of corneal scraping shows Gram-negative rods. Which additional investigation would be most useful to confirm the organism and guide antibiotic selection?

Explanation

## Organism Identification in Gram-Negative Corneal Ulcer **Key Point:** In a contact lens–associated corneal ulcer with Gram-negative rods, culture on blood agar and chocolate agar with oxidase testing is the most appropriate next investigation to identify *Pseudomonas aeruginosa* and guide fluoroquinolone therapy. ### Clinical Context The presentation is classic for **Pseudomonas aeruginosa** infection: - Contact lens overwear (major risk factor) - Greenish tinge (pyoverdine pigment) - Mucopurulent discharge - Gram-negative rod morphology - Rapid progression (5 days) **High-Yield:** *Pseudomonas* is the most common Gram-negative corneal pathogen in contact lens wearers and causes aggressive, sight-threatening ulcers. ### Diagnostic Algorithm for Gram-Negative Rods ```mermaid flowchart TD A[Gram-negative rod on corneal scraping]:::outcome --> B[Culture on blood/chocolate agar]:::action B --> C[Oxidase test]:::decision C -->|Oxidase positive| D[Pseudomonas aeruginosa]:::outcome C -->|Oxidase negative| E[Enterobacteriaceae<br/>Moraxella]:::outcome D --> F[Fluoroquinolone monotherapy]:::action E --> G[Cephalosporin ± aminoglycoside]:::action ``` ### Differential Identification of Gram-Negative Rods | Organism | Oxidase | Pigment | Odor | Culture Medium | Key Feature | |----------|---------|---------|------|-----------------|-------------| | **Pseudomonas aeruginosa** | **+** | Green/blue | Fruity | Blood, chocolate | Motile, mucoid colonies | | **Moraxella** | + | None | None | Blood, chocolate | Diplococcus, oxidase + | | **Enterobacteriaceae** | − | Variable | None | Blood, MacConkey | Lactose fermentation | | **Neisseria** | + | None | None | Chocolate (fastidious) | Gram-negative diplococcus | **Mnemonic for Oxidase-Positive Gram-Negative Rods:** **P-M-N** = *Pseudomonas*, *Moraxella*, *Neisseria* ### Why Oxidase Test Is Critical 1. **Pseudomonas** is oxidase-positive → requires **fluoroquinolone monotherapy** (moxifloxacin 0.5% or ciprofloxacin 0.3%) 2. **Moraxella** is oxidase-positive but less aggressive → also responds to fluoroquinolones 3. **Enterobacteriaceae** are oxidase-negative → require **cephalosporin ± aminoglycoside** **Clinical Pearl:** Oxidase-positive Gram-negative rods in a contact lens ulcer = *Pseudomonas* until proven otherwise. Start fluoroquinolone immediately; delay risks corneal perforation and vision loss. ### Why Other Investigations Are Inappropriate - **Sabouraud agar:** For fungal pathogens, not Gram-negative bacteria - **Confocal microscopy:** Adjunctive imaging, does not identify organism or guide therapy - **AS-OCT with polarization:** Structural assessment, not microbiological diagnosis [cite:Kanski Clinical Ophthalmology 9e Ch 6; Robbins Pathologic Basis of Disease 10e Ch 8]