Chlamydia pneumoniae


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Chlamydia pneumoniae


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Chlamydia pneumoniae


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USMLE® Step 1 questions

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High Yield Notes

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Chlamydia pneumoniae

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USMLE® Step 1 style questions USMLE

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A 31-year-old man presents to the clinic with one week of fever, lethargy, and a non-productive cough. Chest x-ray shows bilateral diffuse infiltrates and he is diagnosed with pneumonia. A sputum sample is collected; under light microscopy; it shows several respiratory epithelial cells with small circular inclusions. No bacteria are seen on Gram stain of the sputum sample. Which of the following is the most likely pathogen causing this patient’s pneumonia?  

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Chlamydophila pneumoniae p. , 146

pneumonia p. 176

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Content Reviewers


Alex Aranda

Salma Ladhani, MD

With Chlamydia pneumoniae, which also used to be called Chlamydophila pneumoniae in the past, Chlamydia comes from “cloak” and pneumoniae means, well, pneumonia.

So, Chlamydia pneumoniae are bacteria that hide inside host cells, particularly cells of the respiratory tract, and they cause pneumonia in humans.

Now, Chlamydia pneumoniae is classically considered Gram-negative, because it can't retain the crystal violet dye used during gram staining.

Additionally, it has an outer lipopolysaccharide membrane which all Gram-negative bacteria also have. However, other Gram-negative bacteria also have a thin peptidoglycan layer under the lipopolysaccharide membrane, whereas Chlamydia pneumoniae doesn’t.

Without it, Chlamydia can’t retain the pink safranin dye used during Gram staining, so they’re not technically Gram-negative either, which is why they’re more correctly called atypical bacteria.

To visualize them, a Giemsa stain is required, which makes them look blue or reddish blue.

Now, Chlamydia pneumoniae is non-motile and round-shaped, and is also an obligate aerobe, meaning that they depend on oxygen for survival.

Additionally, Chlamydia pneumoniae are obligate intracellular pathogens which means they are unable to generate ATP so they rely on other cells for energy.

When outside of a host cell, Chlamydia pneumoniae are metabolically inactive.

So, this means that they can’t grow on artificial mediums but rather, require a host cell for culture.

Ok now, when C. pneumoniae enters a host cell, it undergoes a life cycle that alternates between two distinct forms.

The first is the small spore-looking form called the elementary body, and it’s the infective form of this bacteria.

After the elementary body enters the host cell, it gets enclosed in a vacuole called an inclusion, where it transforms into a metabolically active, star-looking form, called the reticulate body.


Chlamydia pneumonia (also known as chlamydophila pneumoniae) is a round-shaped non-motile bacteria, and an obligate intracellular pathogen because they live inside host cells to survive. Chlamydia pneumoniae is known to cause pneumonia, as its name suggests, but it has also been associated with atherosclerosis, coronary artery disease, and asthma exacerbations. The symptoms of Chlamydia pneumoniae pneumonia are usually mild fever, sore throat, and dry cough.


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