A 52-year-old male farmer from rural Maharashtra presents with a 5-day history of right eye pain, photophobia, and purulent discharge. He reports a thorn prick injury to the cornea while working in the field 2 weeks ago. On examination, visual acuity is 6/60, corneal sensation is reduced, and there is a 4 mm central corneal ulcer with raised edges, stromal infiltration, and a hypopyon. Gram stain of corneal scrapings shows gram-positive cocci in clusters. Culture on blood agar shows golden-yellow colonies. What is the most likely causative organism?

Explanation

## Organism Identification **Key Point:** The clinical presentation of a traumatic corneal ulcer with gram-positive cocci in clusters and golden-yellow colonies on blood agar is pathognomonic for *Staphylococcus aureus*. ## Clinical Features Supporting S. aureus | Feature | S. aureus | P. aeruginosa | Aspergillus | Acanthamoeba | |---------|-----------|---------------|-------------|---------------| | **Gram stain** | Gram-positive cocci in clusters | Gram-negative rod | Fungal filaments | Trophozoites/cysts | | **Culture appearance** | Golden-yellow colonies | Green pigment, fruity odor | Septate hyphae | Difficult to culture | | **Corneal sensation** | May be preserved | Usually preserved | Often reduced | Markedly reduced | | **Hypopyon** | Common | Common | Less common | Rare | | **Stromal infiltration** | Moderate | Severe, rapid | Moderate | Severe, chronic | | **Trauma association** | Very common | Contact lens/trauma | Organic material trauma | Contact lens (contaminated water) | **High-Yield:** S. aureus is the most common bacterial cause of post-traumatic corneal ulcers in India, especially following agricultural injuries with organic material (thorns, soil). ## Pathophysiology of S. aureus Keratitis 1. Inoculation via traumatic injury (thorn, soil contamination) 2. Rapid epithelial invasion and stromal inflammation 3. Production of alpha-toxin and coagulase → tissue necrosis and abscess formation 4. Hypopyon formation from polymorphonuclear cell migration 5. Stromal infiltration with risk of perforation if untreated **Clinical Pearl:** The reduced corneal sensation in this case does not exclude bacterial keratitis—it reflects the degree of stromal inflammation and epithelial damage, not the organism type. Acanthamoeba keratitis typically presents with severe pain disproportionate to clinical findings and marked corneal hypoesthesia. ## Management Implications **Key Point:** S. aureus is sensitive to: - Topical fluoroquinolones (moxifloxacin, gatifloxacin) - Topical cephalosporins (cefazolin) - Topical aminoglycosides (tobramycin, gentamicin) Initial empirical therapy should cover both gram-positive and gram-negative organisms until culture results are available, using fortified antibiotics (cefazolin 5% + tobramycin 1.3%) or fluoroquinolone monotherapy (moxifloxacin 0.5% hourly). **Warning:** Do NOT delay treatment awaiting culture results in a patient with corneal ulcer and hypopyon—this is a sight-threatening emergency.