In acute otitis media, which organism is the third most common after Streptococcus pneumoniae and Moraxella catarrhalis?

• A. Moraxella catarrhalis
• B. Streptococcus pneumoniae
• C. Haemophilus influenzae
• D. Pseudomonas aeruginosa

Answer: (C)

In acute otitis media, the lineup of common bacterial culprits follows a pattern: Streptococcus pneumoniae is the most frequent, Moraxella catarrhalis is next, and the third most common organism is Haemophilus influenzae. Non-typeable Haemophilus influenzae, which lacks a capsule, is a frequent middle-ear pathogen after a viral upper respiratory infection that disrupts normal Eustachian tube function and mucociliary clearance. This helps explain why H. influenzae often comes up as the third leading cause.

Pseudomonas aeruginosa is not a typical cause of acute otitis media; it more commonly appears with otitis externa or in certain chronic ear conditions or immunocompromised individuals. Understanding this distribution guides empiric therapy to cover these typical organisms, including beta-lactamase–producing strains of Haemophilus influenzae and Moraxella catarrhalis, which is why regimens like high-dose amoxicillin-clavulanate or certain cephalosporins are considered.