Corynebacterium diphtheriae (Diphtheria)

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Corynebacterium diphtheriae (Diphtheria)



Corynebacterium diphtheriae (Diphtheria)


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USMLE® Step 1 questions

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Corynebacterium diphtheriae (Diphtheria)

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USMLE® Step 1 style questions USMLE

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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?  

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Corynebacterium diphtheriae p. , 137

culture requirements for p. 124

exotoxin production p. 130

unvaccinated children p. 183


Corynebacterium diphtheriae p. , 130, 137

Myocarditis p. 481

Corynebacterium diphtheriae p. , 137


Corynebacterium diphtheriae p. , 137

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Corynebacterium diphtheriae or just C. diphtheriae takes its name from the Greek;. “Coryne” that means club, and “diphtheriae,” which means leather.

So to sum this up, C. diphtheriae is a club-looking bacteria that causes diphtheria, an infection with a characteristic tough leathery membrane that forms in the pharynx.

C diphtheria has four main subspecies; C. diphtheriae mitis, C. diphtheriae Intermedius, C. diphtheriae Gravis, and C. diphtheriae Belfanti.

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.


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