A 28-year-old man presents with acute onset corneal ulcer following a minor corneal abrasion 48 hours ago. Gram stain of corneal scrapings shows gram-positive cocci in clusters. Culture on blood agar grows golden-yellow colonies. What is the most common bacterial organism causing corneal ulcers in the community setting?

Question

Accepted Answer

B. Staphylococcus aureus. ## Bacterial Corneal Ulcers — Most Common Organism

**Key Point:** Staphylococcus aureus is the most common cause of bacterial corneal ulcers in the community setting, accounting for 30–40% of all bacterial keratitis cases globally and in India. It is part of normal skin flora and causes infection through corneal trauma.

### Epidemiology of Bacterial Keratitis
| Organism | Prevalence | Setting | Risk Factor |
|----------|-----------|---------|-------------|
| **Staphylococcus aureus** | 30–40% | Community, trauma | Minor abrasion, skin contact |
| **Pseudomonas aeruginosa** | 25–35% | Contact lens, hospital | Lens wear, ocular surface disease |
| **Streptococcus pneumoniae** | 10–15% | Community | Systemic infection, immunocompromised |
| **Moraxella catarrhalis** | 5–10% | Community, elderly | Debilitated patients |

**High-Yield:** Staphylococcus aureus causes **indolent, slowly progressive** ulcers with **minimal suppuration** and **good response to antibiotics**, whereas Pseudomonas causes **aggressive, rapidly progressive** ulcers with **copious purulent discharge** and **poor prognosis**.

### Clinical Features of S. aureus Keratitis
1. **Onset:** Subacute (24–72 hours post-trauma)
2. **Ulcer morphology:** Central or paracentral, round to oval, well-demarcated
3. **Infiltration:** Mild to moderate stromal infiltration
4. **Hypopyon:** Present but usually mild
5. **Discharge:** Mucopurulent, not copious
6. **Vascularization:** Moderate neovascularization
7. **Prognosis:** Generally good with appropriate antibiotics

### Microbiological Identification
- **Gram stain:** Gram-positive cocci in **clusters** (grape-like arrangement)
- **Culture:** Blood agar → **golden-yellow colonies** (pigment production)
- **Catalase test:** Positive (distinguishes from Streptococcus)
- **Coagulase test:** Positive (confirms S. aureus)

**Clinical Pearl:** A history of **minor corneal trauma** (scratch, foreign body) with **subacute onset** (48–72 hours), **gram-positive cocci in clusters**, and **golden-yellow colonies** on culture is virtually diagnostic of S. aureus keratitis.

**Mnemonic:** **SAC** — **S**taphylococcus **A**ureus is the **C**ommonest cause (in community setting).

### Why S. aureus Dominates in Community Settings
1. **Ubiquitous:** Normal flora on skin and anterior ocular surface
2. **Trauma-associated:** Easily inoculated through minor corneal abrasions
3. **Lower virulence than Pseudomonas:** Slower progression allows time for diagnosis and treatment
4. **Antibiotic-sensitive:** Responds well to first-line agents (fluoroquinolones, cephalosporins)

[cite:Khurana HV Ophthalmology Ch 4; Sharma S et al. Epidemiology of bacterial keratitis Br J Ophthalmol 2010]

Answer

A. Streptococcus pneumoniae

Answer

C. Pseudomonas aeruginosa

Answer

D. Moraxella catarrhalis

Explanation

Bacterial Corneal Ulcers — Most Common Organism

Epidemiology of Bacterial Keratitis

Clinical Features of S. aureus Keratitis

Microbiological Identification

Why S. aureus Dominates in Community Settings

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Organism	Prevalence	Setting	Risk Factor
Staphylococcus aureus	30–40%	Community, trauma	Minor abrasion, skin contact
Pseudomonas aeruginosa	25–35%	Contact lens, hospital	Lens wear, ocular surface disease
Streptococcus pneumoniae	10–15%	Community	Systemic infection, immunocompromised
Moraxella catarrhalis	5–10%	Community, elderly	Debilitated patients