A 35-year-old man presents with nasal obstruction, purulent nasal discharge, and facial pain for 3 weeks. CT paranasal sinuses shows opacification of multiple sinuses. Which is the most common organism isolated in acute bacterial sinusitis?

Explanation

## Most Common Bacterial Pathogen in Acute Sinusitis **Key Point:** Streptococcus pneumoniae is the single most common organism in acute bacterial sinusitis, followed closely by Haemophilus influenzae and Moraxella catarrhalis. ### Epidemiology of Acute Sinusitis Pathogens | Organism | Frequency | Clinical Notes | |----------|-----------|----------------| | Streptococcus pneumoniae | 25–35% | Most common; gram-positive diplococcus | | Haemophilus influenzae | 20–30% | Second most common; gram-negative coccobacillus | | Moraxella catarrhalis | 10–15% | Third most common; gram-negative diplococcus | | Pseudomonas aeruginosa | < 5% | Seen in immunocompromised, cystic fibrosis | | Anaerobes | 5–10% | Odontogenic sinusitis, chronic disease | **High-Yield:** The "Big 3" organisms account for ~70% of acute bacterial sinusitis cases. S. pneumoniae leads because of its prevalence in upper respiratory flora and ability to invade mucosal barriers. ### Clinical Context **Clinical Pearl:** In acute sinusitis following a viral upper respiratory infection, the bacterial superinfection is typically caused by organisms that colonize the nasopharynx. S. pneumoniae is the most frequent colonizer and pathogen. **Warning:** Pseudomonas aeruginosa and other gram-negative organisms are NOT common in immunocompetent patients with acute sinusitis — they emerge in chronic disease, immunosuppression, or cystic fibrosis. [cite:Scott-Brown's Otolaryngology 8e Ch 12]