Chlamydia pneumoniae

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.

The reticulate body can use the host cell resources to divide, and it does that by binary fission - which means every reticulate body splits in two identical copies of reticular bodies.

On a side note, if this sounds similar to mitosis… well, it is!