USMLE® Step 1 style questions USMLE
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A 38-year-old man comes to the emergency department for evaluation of fever and cough that have been going on for the past four days. The cough is productive and yields purulent, foul-smelling sputum. Temperature is 38.9°C (102°F), pulse is 102/min, and blood pressure is 121/65 mmHg. The patient’s pulse oximetry shows 91% on room air. Crackles are heard bilaterally. Chest radiograph reveals infiltrations in the basilar region of both lungs. Sputum Gram-stain is notable for neutrophils and Gram-negative organisms. Subsequent sputum culture yields growth of Klebsiella and Bacteroides species. Which of the following is most likely to be found upon further interviewing this patient?
Content Reviewers:Rishi Desai, MD, MPH
Klebsiella pneumoniae is a Gram-negative rod-shaped bacteria, which belongs to a family of bacteria called the Enterobacteriaceae.
Now, Klebsiella pneumoniae has a thin peptidoglycan layer, so like other Gram-negative bacteria, it stains pink.
And since it’s a bacillus, it looks like a little pink rod under the microscope.
Klebsiella pneumoniae is non-motile, non-spore forming, and facultative anaerobe.
This means it can live even without oxygen, although it grows better in an aerobic environment.
So, it prefers places like lungs,throat, or respiratory airways, as well as ventilators in the ICU where there is an unlimited flow of oxygen.
Alright, now Klebsiella pneumoniae is urease positive, which means it can produce an enzyme called urease that dissociates urea into carbon dioxide and ammonia.
This can be tested by transferring a pure sample of bacteria from the culture to a sterile tube containing a mixture of “urea agar” broth and phenol red.
Then, the mixture is incubated.
So, with Klebsiella, urease makes urea dissociate into carbon dioxide and ammonia.
Ammonia then makes the mixture change color from orange-yellow to bright pink.
Finally, Klebsiella pneumoniae grows well on MacConkey agar, which is a medium that contains a pH sensitive dye and lactose.
This medium helps identify whether Gram-negative bacteria are lactose fermenters or not.
This results in the production of the acid that makes the pH sensitive dye turn pink - so their colonies will be pink.
Klebsiella has an abundant polysaccharide capsule which leads to the formation of very mucoid and viscous pink colonies.
Pathogenesis and virulence factors
Now, Klebsiella pneumoniae has a number of virulence factors that are like assault weaponry that help it attack and destroy the host cells and evade the immune system.
First, Klebsiella pneumoniae is encapsulated, which means it’s covered by a polysaccharide layer called a capsule.
This capsule is a major virulence factor because of its antiphagocytic ability.
On the capsule, there are pili, which are hair-like extensions that help the bacteria attach to host cells.
Underneath the capsule, there’s an outer membrane, which consist of lipopolysaccharides, or LPS.
Now, LPS has the ability to avoid complement-mediated killing by inhibiting the formation of the membrane attack complex and preventing membrane damage and bacterial cell death.
Finally, it needs iron to thrive and replicate, so it produces a siderophore, which is a term used for a group of small, high-affinity, iron chelating compounds that snatch iron from host cells.
In the urinary tract, Klebsiella can also use urease to convert the urea that’s normally present in urine to ammonia and carbon dioxide.
Ammonia can then combine with hydrogen to form ammonium, which increases urine PH - so the urine becomes more alkaline.
Alkaline urine promotes the precipitation of phosphate, calcium, and magnesium.
Finally, this leads to urinary stasis, which starts a vicious circle, promoting bacterial multiplication, urinary alkalinization, and the deposition of new layers of struvite.