USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 32-year-old man comes to the clinic with inguinal ulcers that he noticed two days ago. He is sexually active and began a relationship with a new partner one month ago. They use condoms inconsistently. He immigrated from Papua New Guinea two months ago. Temperature is 37.0°C (98.6°F), pulse is 90/min, respirations are 20/min, and blood pressure is 120/75 mmHg. The abdomen is soft and nontender. Shallow, nontender ulcers are noted along the shaft and base of the penis with associated erythema. Painful, bilateral inguinal lymphadenopathy is present. An additional flesh-colored papule is noted at the base of the penis. Which of the following is most likely the causative organism responsible for this patient’s symptoms?
A serovar groups together bacteria with similar surface antigens, and so are likely to cause the same type of infection.
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.
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.
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.