A 52-year-old male farmer from rural Maharashtra presents with a 10-day history of left eye pain, photophobia, and purulent discharge. He recalls a thorn prick injury while working in the field 2 weeks ago. On examination, visual acuity is 6/60. Slit-lamp examination reveals a central corneal ulcer with raised edges, stromal infiltration, and a hypopyon. Gram stain of the corneal scraping shows gram-positive cocci in clusters. Culture on blood agar grows golden-yellow colonies. What is the most likely causative organism?

Explanation

## Diagnosis: Staphylococcus aureus Corneal Ulcer ### Clinical Presentation **Key Point:** The combination of traumatic corneal injury (thorn prick), gram-positive cocci in clusters on Gram stain, and golden-yellow colonies on blood agar is pathognomonic for *Staphylococcus aureus*. ### Microbiological Features | Feature | S. aureus | P. aeruginosa | S. pneumoniae | M. catarrhalis | |---------|-----------|---------------|---------------|----------------| | Gram stain | Gram-positive cocci in clusters | Gram-negative rod | Gram-positive diplococci | Gram-negative diplococcus | | Culture appearance | Golden-yellow colonies | Green/blue pigment | Alpha-hemolytic | Oxidase-positive | | Catalase | Positive | Positive | Negative | Positive | | Coagulase | Positive | Negative | Negative | Negative | ### Clinical Pearl **High-Yield:** *Staphylococcus aureus* is the most common cause of bacterial corneal ulcers following **traumatic injury**, especially in agricultural settings. It produces a suppurative response with hypopyon formation. ### Risk Factors for S. aureus Keratitis - Corneal trauma (especially vegetative matter) - Contact lens wear - Ocular surface disease - Immunocompromised state ### Management Approach ```mermaid flowchart TD A[Bacterial corneal ulcer suspected]:::outcome --> B{Gram stain result}:::decision B -->|Gram-positive cocci in clusters| C[S. aureus likely]:::outcome C --> D[Start empirical therapy]:::action D --> E[Fortified cephalosporin + aminoglycoside]:::action E --> F[Hourly topical antibiotics]:::action F --> G[Culture confirmation]:::outcome G --> H[De-escalate if needed]:::action ``` ### Treatment **Key Point:** Empirical therapy for suspected *S. aureus* keratitis includes: 1. **Fortified cephalosporin** (cefazolin 5%) — excellent corneal penetration 2. **Fortified aminoglycoside** (tobramycin 1.3% or gentamicin 1.4%) — synergistic 3. Hourly instillation during waking hours 4. Cycloplegic agent (homatropine 5%) for pain relief **Warning:** Do NOT delay empirical therapy awaiting culture results — corneal perforation can occur within 48–72 hours. ### Prognosis With prompt treatment, *S. aureus* keratitis has a favorable prognosis, though scarring and visual loss depend on ulcer depth and location.