Mycoplasma pneumoniae

Quick Profile

• Classification: Atypical bacterium — no cell wall, not gram-stainable
• Key labs: Cold agglutinins (IgM against I antigen on RBCs), elevated cold agglutinin titer; PCR of respiratory specimen for confirmation
• Colonization: Respiratory mucosa via attachment protein P1
• Clinical style: Insidious-onset, mild lower respiratory illness that does not warrant hospitalization

Identification And Classification

• Smallest self-replicating organism; lacks peptidoglycan cell wall entirely
• Not visible on Gram stain
• Grows very slowly on special Eaton agar (not used clinically)
• Distinguished from Chlamydia by ability to grow on cell-free media

Reservoir And Transmission

• Exclusive human reservoir
• Respiratory droplet transmission
• High-yield populations: adolescents, young adults, college students, military recruits
• Endemic year-round with epidemic cycles every few years

Virulence Factors And Pathogenesis

• P1 surface protein attaches to respiratory epithelium and prevents mucociliary clearance
• Hydrogen peroxide and superoxide production injure the epithelium
• Immune-mediated injury contributes to extrapulmonary manifestations

High-Yield Disease Patterns

Walking Pneumonia (Atypical Pneumonia)

• Gradual onset over 1–2 weeks; productive cough, low-grade fever, malaise
• Patient feels ill but continues daily activities (hence “walking” pneumonia)
• Chest X-ray shows bilateral interstitial (diffuse, patchy) infiltrates disproportionate to mild symptoms
• Hallmark clue: CXR looks worse than the patient appears

Extrapulmonary Manifestations

• Bullous myringitis: hemorrhagic blisters on the tympanic membrane
• Cold agglutinin hemolytic anemia: IgM anti-I autoantibodies agglutinate RBCs in the cold
• Erythema multiforme: targetoid skin lesions
• Stevens-Johnson syndrome in severe cases
• Myocarditis, encephalitis, Guillain-Barré syndrome (rare)

Diagnosis

• Clinical diagnosis in young adults with walking pneumonia presentation
• Cold agglutinin titer above 1:64 (non-specific but useful clue)
• Respiratory PCR is the definitive test of choice
• Serology (IgM/IgG) useful for epidemiology but less helpful acutely

Treatment And Resistance

• First-line: Azithromycin or doxycycline (macrolides or tetracyclines)
• Alternative: Fluoroquinolones (levofloxacin, moxifloxacin) for adults
• Beta-lactams and vancomycin are completely ineffective because there is no cell wall target
• Duration: 5 days for azithromycin; 5–14 days for doxycycline

For antibiotic mechanisms, see Pharmacology: Antimicrobials.

Exam Traps

• Beta-lactams do not work against Mycoplasma; there is no cell wall to inhibit
• Cold agglutinins are IgM antibodies that can cause hemolysis in the cold extremities
• Chest X-ray appearance is typically worse than the patient’s clinical condition
• Bullous myringitis on the ear exam is a classic board clue pointing directly to Mycoplasma