## Acute Bacterial Conjunctivitis — Causative Organisms **Key Point:** Staphylococcus aureus is the most common cause of acute bacterial conjunctivitis worldwide, including in India, accounting for approximately 50–70% of cases. ### Organism Frequency Table | Organism | Frequency | Clinical Features | | --- | --- | --- | | Staphylococcus aureus | 50–70% | Purulent discharge, lid crusting, mild systemic symptoms | | Streptococcus pneumoniae | 10–15% | Mucopurulent discharge, may cause keratitis | | Haemophilus influenzae | 10–15% | Acute onset, chemosis, preauricular lymphadenopathy | | Neisseria gonorrhoeae | 1–3% | Hyperacute, copious purulent discharge, risk of corneal ulceration | | Chlamydia trachomatis | Chronic/neonatal | Chronic follicular conjunctivitis, inclusion bodies | | Pseudomonas aeruginosa | <1% (non-contact lens) | Associated with contact lens wear, risk of corneal involvement | **High-Yield:** In acute bacterial conjunctivitis, Staph. aureus is the most frequent isolate; Neisseria gonorrhoeae causes hyperacute disease (copious pus within 24–48 hours) and is a medical emergency due to corneal perforation risk. **Clinical Pearl:** Neisseria gonorrhoeae and Chlamydia trachomatis are sexually transmitted and require systemic antibiotics; topical therapy alone is insufficient. ### Why Staphylococcus aureus is the Answer Staphylococcus aureus is a gram-positive coccus that colonizes the skin and eyelid margins. It is the leading cause of community-acquired acute bacterial conjunctivitis and responds well to topical fluoroquinolones or aminoglycosides. 
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