USMLE® Step 1 style questions USMLE
A sample is taken of an ulcerating papule in the inguinal region of a 26-year-old man who has had unprotected sex with multiple female partners in the past few months. Intracytoplasmic inclusions are seen in Giemsa staining of the sample taken from one of the swollen lymph nodes. Infection with which of the following is the most likely cause?
Chlamydia trachomatis exam links
Contributors:Evode Iradufasha, MD, Evan Debevec-McKenney
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.
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!
But the term binary fission is used to describe division of prokaryotic cells, which don’t have a nucleus, and therefore some steps in replication are different from mitosis.
Now, eventually, binary fission results in a huge number of reticulate bodies, which then start transitioning back to elementary bodies.
The cell eventually becomes too full, bursting open, and letting out a lot of elementary bodies in the surrounding fluids, where they attach to other cells to repeat the cycle over and over.
Now the resulting disease depends on the infecting serotype.
So serotypes A through C cause chlamydia conjunctivitis, or trachoma, in adults. Untreated, trachoma can progress to keratoconjunctivitis, a condition in which both the conjunctiva and the cornea are infected, which can result in total blindness if the cornea gets destroyed.
Serotypes D through K cause chlamydia - which is the most common sexually transmitted infection in both men and women.
In men, Serotypes D through K commonly infect the urethral mucosa, causing inflammation known as urethritis.
Sometimes the infection can spread to the prostate resulting in prostatitis.
In women, there can also be urethritis, but the infection is infamous for affecting the lower genital tract, causing vulvovaginitis when the vulva and the vagina are affected, and cervicitis when the cervix is involved.
From the cervix, the infection can extend up to the uterus, the fallopian tubes and the ovaries, causing pelvic inflammatory disease or PID.
Chlamydia trachomatis is a gram-negative, non-motile bacterium and an intracellular pathogen known to cause sexually transmitted chlamydia infection and associated complications, including infertility and pelvic inflammatory disease (PID). It is passed from one person to another through unprotected sexual intercourse, but it can also be spread from a mother to her baby during childbirth.