## Microbiology of CSOM Discharge ### Most Common Organisms in CSOM **Key Point:** Pseudomonas aeruginosa is the most frequently isolated organism from chronic suppurative otitis media discharge, accounting for 40–50% of aerobic bacterial isolates in Indian studies. ### Organism Frequency in CSOM | Organism | Frequency (%) | Clinical Notes | |---|---|---| | Pseudomonas aeruginosa | 40–50 | Most common; gram-negative rod; foul-smelling discharge | | Staphylococcus aureus | 20–30 | Second most common; can be MRSA | | Proteus mirabilis | 10–15 | Gram-negative rod; alkaline urine odor | | Streptococcus pneumoniae | 5–10 | More common in acute OM; rare in CSOM | | Escherichia coli | 5–10 | Gram-negative rod | | Anaerobes | 20–30 | Often polymicrobial; Bacteroides, Peptostreptococcus | ### Why Pseudomonas Dominates CSOM 1. **Environmental organism** — thrives in moist, warm ear canal 2. **Biofilm formation** — resistant to antibiotics and host immunity 3. **Chronic drainage** — creates ideal anaerobic/microaerophilic conditions 4. **Virulence factors** — exotoxins, proteases, lipopolysaccharide **High-Yield:** Pseudomonas aeruginosa is the pathognomonic organism of CSOM; its presence suggests chronicity and biofilm-mediated disease. ### Clinical Pearl Foul-smelling, purulent discharge in CSOM is classically associated with Pseudomonas aeruginosa. Fluoroquinolones (ciprofloxacin, ofloxacin) are the drugs of choice for Pseudomonas-associated CSOM. **Mnemonic:** **PACS** = Pseudomonas Aeruginosa in Chronic Suppurative otitis media. [cite:Hazarika 5e Ch 9] 
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