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?

Explanation

## 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]