Which organism is the most common cause of acute bacterial conjunctivitis in adults?

Explanation

## Bacterial Conjunctivitis — Causative Organisms **Key Point:** Staphylococcus aureus is the most common cause of acute bacterial conjunctivitis in adults, accounting for approximately 30–50% of cases. ### Epidemiology by Organism | Organism | Prevalence in Adults | Clinical Features | Notes | |----------|----------------------|-------------------|-------| | **Staphylococcus aureus** | 30–50% (most common) | Purulent discharge, lid crusting, may cause preseptal cellulitis | Often associated with poor hygiene, contact lens wear | | Streptococcus pneumoniae | 5–15% | Mucopurulent discharge, chemosis | Less common than S. aureus in modern practice | | Haemophilus influenzae | 10–20% | Mucopurulent discharge, preauricular lymphadenopathy | More common in children; non-typeable strains predominate | | Neisseria gonorrhoeae | <1% in adults | Hyperacute purulent discharge, severe chemosis, risk of corneal ulceration | Sexually transmitted; requires systemic antibiotics | **High-Yield:** In children, Haemophilus influenzae (non-typeable) and Streptococcus pneumoniae are more common; in adults, Staphylococcus aureus dominates. **Clinical Pearl:** Gram-positive cocci (especially S. aureus) are responsible for the majority of acute bacterial conjunctivitis cases. Fluoroquinolone eye drops (e.g., ofloxacin, moxifloxacin) provide broad coverage and are first-line empiric therapy [cite:Kanski's Clinical Ophthalmology 9e Ch 3]. ### Why S. aureus Predominates - Normal skin flora with high prevalence in community - Easy transmission via contaminated hands or contact lenses - Ability to cause both acute and chronic infection - Often associated with staphylococcal blepharoconjunctivitis (lid margin involvement)