USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 64-year-old man is brought to the emergency department by the ambulance following a seizure at home. He appears acutely unwell on admission, and is complaining of headaches and bright lights. His temperature is 39.2°C, blood pressure is 98/55 mmHg, and pulse is 110/min. His past medical history is significant for diffuse large B-cell lymphoma, for which he is on chemotherapy. A CT head shows no acute abnormalities, and a lumbar puncture confirms the suspected diagnosis of bacterial meningitis. Gram-positive bacilli are isolated on blood culture. Which of the following is the most appropriate pharmacotherapy for treating this patient's meningitis?
Listeria was discovered by a Scottish doctor, J.H.H. Pirie, who named it in honor of of British surgeon Joseph Lister,.
It’s also called “monocytogenes” because when it was inoculated in rabbits, their monocyte levels increased.
Ok now, L. monocytogenes is a rod-shaped bacteria with a thick peptidoglycan cell wall.
So when gram-stained, it takes in purple dye, making it a gram-positive bacteria.
It is catalase and oxidase positive - which means it produces both these enzymes.
L. monocytogenes is a facultative intracellular pathogen, meaning it can live both outside or inside of its host’s cells.
It doesn’t form spores and it is facultatively anaerobic, meaning that it can survive in both aerobic and anaerobic environments.
Now, when L. monocytogenes is cultivated on blood agar medium, its colonies cause beta-hemolysis, also called complete hemolysis.
That’s because it produces toxins called beta hemolysins, which hydrolyze the hemoglobin within red blood cells to transparent yellow color byproducts.
L. monocytogenes is a motile bacteria, with a very interesting way to move, that depends on both its location, and the temperature. In an extracellular environment, this bacteria moves by beating its flagella creating a characteristic tumbling motility.
But this is only possible at 37 degrees Celsius and below.
That’s because FlaA, the gene that codes for flagellin, which is the structural protein that makes up the flagella becomes downregulated as the temperature rises up to 37 degree Celsius.
So above 37 degrees, there’s reduced production of flagellin proteins, meaning no flagella being made, rendering L. monocytogenes non-motile.
In an intracellular environment, L. monocytogenes moves by an actin-based motility.
It starts with this bacteria producing a protein called Actin assembly-inducing protein or just ACTA, which recruits small actin filaments at one end.
As more and more actin filaments get recruited and polymerized behind the bacteria’s end pole, that propels the bacteria forward, like a rocket.
L. monocytogenes is a foodborne bacteria, meaning people get infected when they eat contaminated food - especially unpasteurized dairy products and cold deli meats, and this bacteria is known to survive even at very low temperatures in the fridge.
Once this bacteria gets into the gut, it uses its attachment proteins called internalins to attach to the host’s receptor proteins, such as E cadherin located on goblet cells of the intestinal mucosa.
After attaching to the host’s cells, this bacteria is 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 what’s referred to a internalization vacuole.
Inside the vacuole, Listeria releases listeriolysin O, and bacterial phospholipases, which are enzymes that degrade the vacuolar membrane.
This releases the bacteria in the host’s cell cytoplasm, where it starts using the host’s cell resources and food, and multiply into many identical bacterial cells, by the process of binary fission, which means every bacterial cell splits in two identical copies.
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.