Chlamydia trachomatis or just C. trachomatis, is a gram-negative bacteria that strictly infects humans, and it’s divided into 15 serotypes, also known as serovars.
A serovar groups together bacteria with similar surface antigens, and so are likely to cause the same type of infection.
Serotypes A through C cause chlamydia conjunctivitis in adults, which also called trachoma.
Serotypes D through K cause a genital infection called chlamydia.
Lastly, serotypes L1, L2 and L3 - and there’s no particular reason that these are numbered - infect the lymph nodes, causing a disease called lymphogranuloma venereum, or LVG.
No matter the serotype, C. trachomatis is a gram-negative bacteria, meaning it cannot retain the crystal violet stain used during gram staining.
However, unlike other common gram-negative bacteria which have a thin layer of murein, also known as peptidoglycan in their cell wall, the cell wall of C. trachomatis has no any murein at all - so it can’t retain pink safranin dye used during Gram staining, either.
So, C. trachomatis is best stained with Giemsa stain, which colors them pinkish-blue.
What is more, unlike most bacteria, chlamydia requires vial cells or embryonated hen's egg for culture, which is technically difficult and expensive, so cultures are only done for research purposes.
Chlamydia trachomatis is also non-motile, and an obligate aerobe, meaning it absolutely depends on oxygen to survive.
It’s also an obligate intracellular pathogen, because it’s unable to make its own ATP for energy, so it needs to use another cell’s resources.
Ok now, when C. trachomatis 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.