Which organism is associated with atypical pneumonia and is commonly treated with macrolides or doxycycline?

• A. Streptococcus pneumoniae
• B. Haemophilus influenzae
• C. Mycoplasma pneumoniae
• D. Staphylococcus aureus

Answer: (C)

Atypical pneumonia is classically caused by Mycoplasma pneumoniae. This organism lacks a cell wall, so antibiotics that target cell wall synthesis, like penicillins and other beta-lactams, are ineffective. Because of this, treatments that inhibit protein synthesis or translation—macrolides (such as azithromycin or erythromycin) or doxycycline—are preferred. Clinically, it often presents as a walking pneumonia in younger people, with a gradual onset of dry cough and low-grade fever, and chest imaging showing diffuse interstitial or patchy involvement rather than a single lobar consolidation.

Streptococcus pneumoniae causes typical pneumonia with abrupt fever, productive cough, and lobar consolidation, usually treated with beta-lactams. Haemophilus influenzae is another respiratory pathogen more common in COPD and can cause pneumonia but is not the classic atypical pattern. Staphylococcus aureus can cause severe, often post-viral pneumonia and is not the archetype of atypical pneumonia.