USMLE® Step 1 style questions USMLE
A 42-year-oldman presents to the emergency department with a fever and a cough productive of blood-tinged sputum. He is currently undomiciled. Temperature is 39 °C (102.2 °F), pulse is 114/min, respirations are 23/min, and blood pressure is 100/72 mmHg. On presentation, the patient is visibly intoxicated with alcohol and cannot answer any questions. On physical examination, the oropharynx is dry, and breath sounds are decreased in the right lower lobe on pulmonary auscultation. Which pathogens are most likely responsible for this patient’s condition?
Klebsiella pneumoniae exam links
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Kaylee Neff, Alexandru Duhaniuc, MD
Klebsiella pneumoniae is a Gram-negative rod-shaped bacteria, which belongs to a family of bacteria called the Enterobacteriaceae.
Klebsiella pneumoniae can normally colonize the oropharynx and the gastrointestinal tract.
It causes various hospital-acquired infections, such as pneumonia (hence the name) and is the third most common cause of urinary tract infections.
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.
Some Enterobacteriaceae like Klebsiella, Enterobacter, and Escherichia coli, can ferment lactose.
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.
Others like Salmonella and Shigella can’t ferment lactose, so their colonies will be colorless.
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.
This means that it protects the bacteria against phagocytosis by macrophages and neutrophils, allowing Klebsiella to escape destruction.
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.
This can combine with ammonium to form struvite stones that often form large staghorn renal calculi, or kidney stones.
Finally, this leads to urinary stasis, which starts a vicious circle, promoting bacterial multiplication, urinary alkalinization, and the deposition of new layers of struvite.
Klebsiella pneumoniae is a gram-negative, non-spore-forming, rod-shaped bacterium that is known to cause diseases in humans such as pneumonia, meningitis, septicemia, and urinary tract infections. Klebsiella pneumoniae is a facultative anaerobe, and lactose-fermenter, and is also able to produce an enzyme known as beta-lactamase, which makes it resistant to several antibiotics.