A 28-year-old woman presents with a 2-week history of persistent dry cough, low-grade fever (38.2°C), and malaise. She reports recent exposure to a colleague with similar symptoms. Physical examination reveals scattered crackles on auscultation. Chest X-ray shows bilateral interstitial infiltrates. Blood culture and routine bacterial culture remain sterile. PCR for respiratory viruses is negative. Which of the following is the most likely causative organism?

Explanation

## Clinical Presentation Analysis **Key Point:** Mycoplasma pneumoniae causes "atypical pneumonia" or "walking pneumonia" characterized by a prolonged dry cough, constitutional symptoms, and interstitial infiltrates on imaging. ### Diagnostic Features Supporting M. pneumoniae | Feature | M. pneumoniae | Typical Bacterial Pneumonia | |---------|---------------|----------------------------| | **Cough character** | Dry, persistent (2–4 weeks) | Productive, early | | **Fever pattern** | Low-grade, prolonged | High-grade, acute onset | | **Culture results** | Negative on routine media | Positive on blood/sputum culture | | **CXR findings** | Bilateral interstitial infiltrates | Lobar consolidation, air bronchograms | | **Epidemiology** | Person-to-person, clusters | Sporadic | | **Viral PCR** | Negative | Not applicable | **High-Yield:** M. pneumoniae lacks a cell wall (contains only a cell membrane with sterols), so it does NOT grow on standard bacterial culture media — this explains the negative blood and routine bacterial cultures in this case. ### Pathophysiology 1. Organism attaches to respiratory epithelium via P1 adhesin protein 2. Produces hydrogen peroxide and superoxide, causing local inflammation 3. Triggers immune-mediated damage → prolonged cough and interstitial pneumonia 4. Incubation period: 2–3 weeks; symptoms persist 4–6 weeks even with treatment **Clinical Pearl:** The combination of **negative routine cultures + interstitial infiltrates + dry cough + recent exposure cluster** is pathognomonic for atypical pneumonia, especially M. pneumoniae. ### Confirmatory Tests - **PCR for M. pneumoniae** (most sensitive and specific; gold standard) - **Cold agglutinins** (nonspecific IgM antibodies; positive in ~50% of cases) - **Serology** (IgM/IgG antibodies; useful retrospectively) - **Culture on PPLO or Eaton medium** (slow, 2–4 weeks; rarely used clinically) [cite:Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases 9e Ch 175] **Mnemonic: CAMP** — **C**old agglutinins, **A**typical pneumonia, **M**ycoplasma, **P**ersistent dry cough.