Legionella vs Mycoplasma and Chlamydia pneumoniae
How to tell Legionella and Mycoplasma and Chlamydia pneumoniae apart when a board question makes them look alike.
Legionella is severe water-linked pneumonia with GI/CNS signs and hyponatremia; Mycoplasma is walking pneumonia with cold agglutinins; Chlamydia pneumoniae is mild hoarse bronchitis-pneumonia.
Side by side
Legionella causes atypical pneumonia with high fever, diarrhea, confusion, hyponatremia, water-source exposure, and severe disease in older adults, smokers, or immunocompromised patients.
Mycoplasma causes walking pneumonia with dry cough and cold agglutinins; Chlamydia pneumoniae causes mild atypical pneumonia with pharyngitis, hoarseness, or bronchitis.
Legionella is an intracellular gram-negative rod that grows in macrophages and contaminated water systems; it requires special culture media and is often detected by urine antigen.
Mycoplasma lacks a cell wall and injures respiratory epithelium through adherence; Chlamydia pneumoniae is an obligate intracellular organism with elementary and reticulate bodies.
Urine antigen, silver stain, buffered charcoal yeast extract culture, hyponatremia, transaminitis, and watery diarrhea support Legionella.
Cold agglutinin hemolysis supports Mycoplasma; intracellular inclusions or serology/PCR with hoarseness and bronchitis support Chlamydia pneumoniae.
The trap
All three can cause atypical pneumonia with dry cough, so severity, exposure, extrapulmonary clues, and organism biology separate them.
Watch it explained
Test it nowThe repair move
For atypical pneumonia, do not stop at dry cough; add water exposure, GI/CNS signs, cold agglutinins, or hoarseness.
Try a free question
CoreAn older smoker develops high fever, dry cough, watery diarrhea, confusion, hyponatremia, and pneumonia after a hotel conference. Which organism is most likely?
Show answer and explanation
Answer: A. Legionella pneumophila. Water exposure with severe atypical pneumonia, diarrhea, confusion, and hyponatremia supports Legionella.
Why the other choices are tempting
- B. Mycoplasma causes walking pneumonia and can cause cold agglutinin hemolysis.
- C. Chlamydia pneumoniae more often causes mild pneumonia with hoarseness or bronchitis.
- D. Streptococcus pneumoniae causes typical lobar pneumonia rather than this water-linked systemic pattern.
- E. Bordetella pertussis causes paroxysmal cough with lymphocytosis.
Flashcards
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What exposure pattern supports Legionella pneumonia?
Contaminated water systems such as hotels, air conditioning, or plumbing support Legionella.
What lab clue supports Legionella over other atypical pneumonias?
Hyponatremia with positive urine antigen or special charcoal yeast extract culture supports Legionella.
Quick answers
What is the fastest way to tell Legionella from Mycoplasma and Chlamydia pneumoniae?
Legionella is severe water-linked pneumonia with GI/CNS signs and hyponatremia; Mycoplasma is walking pneumonia with cold agglutinins; Chlamydia pneumoniae is mild hoarse bronchitis-pneumonia.
What is the key clue for Legionella?
Legionella causes atypical pneumonia with high fever, diarrhea, confusion, hyponatremia, water-source exposure, and severe disease in older adults, smokers, or immunocompromised patients.
What is the key clue for Mycoplasma and Chlamydia pneumoniae?
Mycoplasma causes walking pneumonia with dry cough and cold agglutinins; Chlamydia pneumoniae causes mild atypical pneumonia with pharyngitis, hoarseness, or bronchitis.
What is the common trap with Legionella vs Mycoplasma and Chlamydia pneumoniae?
All three can cause atypical pneumonia with dry cough, so severity, exposure, extrapulmonary clues, and organism biology separate them.