Which of the following is the most common organism isolated in acute bacterial sinusitis?

Explanation

## Microbiology of Acute Bacterial Sinusitis **Key Point:** Streptococcus pneumoniae is the most common pathogen in acute bacterial sinusitis, followed by Haemophilus influenzae and Moraxella catarrhalis. ### Causative Organisms | Organism | Frequency | Clinical Notes | | --- | --- | --- | | *Streptococcus pneumoniae* | ~30–40% | Most common; gram-positive coccus | | *Haemophilus influenzae* | ~20–30% | Second most common; gram-negative rod | | *Moraxella catarrhalis* | ~10–15% | Gram-negative diplococcus | | *Pseudomonas aeruginosa* | <5% | Immunocompromised or chronic cases | | Anaerobes | ~5–10% | Odontogenic or chronic sinusitis | **High-Yield:** In acute sinusitis, the "big three" pathogens (S. pneumoniae, H. influenzae, M. catarrhalis) account for >80% of bacterial infections. These are the organisms targeted by empiric antibiotic therapy. **Clinical Pearl:** Pseudomonas and anaerobes are more common in chronic sinusitis, immunocompromised patients, or those with recent antibiotic exposure. In acute uncomplicated sinusitis, empiric coverage should target the top three organisms. ### Treatment Implications - **First-line empiric therapy:** Amoxicillin-clavulanate or fluoroquinolone (covers all three common pathogens) - **In penicillin allergy:** Fluoroquinolone or macrolide - **Culture indications:** Immunocompromised, severe disease, or treatment failure