AssessmentsCorynebacterium diphtheriae (Diphtheria)
Corynebacterium diphtheriae (Diphtheria)
USMLE® Step 1 style questions USMLE
A 6-year-old boy who recently immigrated to the United States from Asia is admitted to the hospital with dyspnea. Vital signs are within normal limits other than an elevated respiratory rate. Physical examination shows a gray pseudomembrane in the oropharynx along with cervical lymphadenopathy. Which of the following describes the organism responsible for this patient's disease?
OK, now, C. diphtheriae has a thick peptidoglycan cell wall that takes in purple dye when Gram-stained - so it’s a gram-positive bacteria.
It is aerobic, which means it requires oxygen to grow, and it doesn’t form spores.
Now, when stained with Albert’s stain, these bacteria demonstrate some unique features.
They look like green, club-shaped bacteria with metachromatic granules, which are these dark blue dots made of phosphate, located at the bacterial poles.
When many, and clustered together, these bacteria seem to be arranged in a characteristic pattern that resembles Chinese letters.
Finally, C. diphtheriae is a fastidious bacteria.
This means it can only grow on special nutrients-enriched media.
The medium commonly used to grow this bug is cysteine-tellurite blood agar on which C. diphtheriae grow into black colonies.
Alright, any of the C. diphtheriae subspecies can be either toxigenic or not, depending on whether or not they produce the diphtheria toxin, or DT for short.
DT is a cytotoxic protein, where cytotoxic means it causes damage to host cells.
In fact, all the C. diphtheriae subspecies start out as non-toxigenic, but they become toxigenic after they’re infected by a beta-bacteriophage.
This is a kind of virus that attaches to bacteria and merges its own genome with the bacteria’s.
The beta-bacteriophage genome contains tox-genes, which code for diphtheria toxin production.
Now, DT has two main subunits, A and B, joined by a disulfide bond, with each of the subunits playing a specific role in the invasion and destruction to the host's cells.
The B subunit, which is the Bigger portion of DT complex, helps Binding to the host’s cell membrane.
After attaching to the host’s cells, the whole DT complex gets slowly engulfed by the cell membrane, which invaginates to form a sac on its inner side.
The sac then separates from the actual cell membrane forming a vesicle called an endosome.
Within the host’s cell cytoplasm, the medium inside the endosome becomes more acidic, and as a consequence, the disulfide bond holding the two subunits together becomes weak and eventually break, separating the subunits.
The A subunit then diffuses through the endosome membrane into the cytoplasm, where it goes straight to the ribosomes.
Here, it interferes with cell protein synthesis.
This happens because the A subunit has an ADP-ribose group, which attaches to the elongation factor - EF2, an important ribosomal protein that joins amino acids together during protein synthesis.
This process is called EF2 ADP-ribosylation, and it results in complete deactivation of the EF2, which stops protein synthesis, leading to cell death.
Most often, the bacteria can be transmitted from one person to another mainly by respiratory droplets, following coughing or sneezing, in which case it causes pharyngeal diphtheria, but they can also enter the body through open lesions on the skin, causing cutaneous diphtheria.
Following inhalation of infected respiratory droplets, C. diphtheriae attaches to the pharyngeal epithelial cells, where they release DT toxin.
This causes local inflammation that leads to necrosis of pharyngeal tissue, and neck swelling.