## Most Common Organism in CSOM **Key Point:** Pseudomonas aeruginosa is the most frequently isolated organism from chronic suppurative otitis media, particularly in cases with active drainage and central perforations. ### Organism Prevalence in CSOM | Organism | Frequency | Clinical Context | | --- | --- | --- | | *Pseudomonas aeruginosa* | 30–50% | Most common; associated with foul-smelling discharge, granulation tissue | | *Staphylococcus aureus* | 15–30% | Second most common; often with acute exacerbations | | *Streptococcus pneumoniae* | 5–10% | More common in acute OM; rare in chronic disease | | *Haemophilus influenzae* | 5–10% | Declining prevalence post-vaccination | | Anaerobes (mixed flora) | 10–20% | Foul odour suggests anaerobic involvement | ### Why Pseudomonas Dominates in CSOM 1. **Biofilm formation:** Pseudomonas forms resistant biofilms on chronically inflamed mucosa and bone. 2. **Aerobic environment:** The chronically draining ear maintains aerobic conditions favourable to Pseudomonas. 3. **Virulence factors:** Produces proteases and exotoxins that perpetuate inflammation and tissue destruction. 4. **Antibiotic resistance:** Often resistant to common oral antibiotics, explaining treatment failures and chronicity. **High-Yield:** In any CSOM case with foul-smelling discharge and central perforation, Pseudomonas should be the first suspected organism. Culture and sensitivity are essential because empiric therapy often fails. **Clinical Pearl:** The presence of granulation tissue, polyps, or bone erosion in CSOM is highly suggestive of Pseudomonas infection. Fluoroquinolone ear drops (ciprofloxacin, ofloxacin) are first-line topical therapy. **Tip:** If the question mentions "foul-smelling discharge" or "chronic drainage," think Pseudomonas. If it mentions "acute exacerbation," think Staph aureus.
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