## Microbiology of Acute Suppurative Otitis Media **Key Point:** Streptococcus pneumoniae remains the most common bacterial pathogen in acute suppurative otitis media (ASOM) in children, followed closely by non-typeable Haemophilus influenzae and Moraxella catarrhalis. ### Causative Organisms — Frequency Ranking | Organism | Frequency | Notes | | --- | --- | --- | | *Streptococcus pneumoniae* | ~30–40% | Most common; serotypes 19F, 23F, 6B most prevalent | | Non-typeable *Haemophilus influenzae* | ~25–30% | Second most common; type b now rare due to Hib vaccine | | *Moraxella catarrhalis* | ~10–15% | Third most common; more common in younger children | | Group A Streptococcus | <5% | Rare; usually follows viral URI | | Viruses alone | ~20–25% | RSV, influenza, parainfluenza; often precede bacterial superinfection | **High-Yield:** The "Big Three" pathogens account for >85% of bacterial ASOM cases. *S. pneumoniae* is the single most common agent and the most likely to cause severe disease and complications. **Clinical Pearl:** In neonates and very young infants (<6 weeks), gram-negative organisms (E. coli, Klebsiella) and Staphylococcus aureus are more common and must be considered in empiric therapy. ### Pathogenesis Sequence 1. Viral URI → mucosal inflammation and Eustachian tube dysfunction 2. Bacterial colonization of nasopharynx 3. Ascent through Eustachian tube → middle ear infection 4. Pus accumulation → pressure increase → potential perforation **Mnemonic:** **SPHiM** — *S. pneumoniae*, *Pseudomonas* (in chronic/resistant cases), *H. influenzae*, *M. catarrhalis*. (Note: Pseudomonas is rare in acute ASOM but common in chronic suppurative otitis media.) 
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