USMLE® Step 1 style questions USMLE
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A 25-year-old man comes to the emergency department with a fever, headache, and watery diarrhea. His abdomen has been hurting since last night. He noticed a small amount of blood in his most recent bowel movement, 2 hours ago. A stool sample is obtained and is positive for a lactose-negative, hydrogen sulfide-producing bacteria. Which of the following mechanisms is most likely responsible for causing the symptoms associated with the causative bacteria?
Content Reviewers:Viviana Popa, MD
Salmonella is a bacterium belonging to the family Enterobacteriaceae.
There are two main species: Salmonella bongori and Salmonella enterica, and the latter has six subspecies.
One of the subspecies is enterica, which has over 2500 serotypes that can be divided into two main groups based on the clinical symptoms they cause- so typhoidal or non-typhoidal Salmonella.
The non-typhoidal group, can infect humans and animals and cause a variety of disease states.
OK, but generally, Salmonella are encapsulated gram-negative, rod bacteria – meaning, they have a polysaccharide layer outside the cell envelope and look like little red or pink sticks on a gram stain.
They’re facultative intracellular pathogens, meaning they can live both outside or inside of its host’s cells.
And have flagella, making them motile, but don’t form spores.
They’re also facultative anaerobes, so they can undergo respiratory and fermentative metabolism; and they can ferment glucose but not lactose; are oxidase negative, and produce hydrogen sulfide gas.
And while a variety of media can be used to selectively identify Salmonella, among them is Triple Sugar Iron agar which produces a black precipitate when hydrogen sulfide is produced.
Now, once Salmonella is ingested and reaches the distal ileum of the small intestine, it tends to target the epithelial layer of the mucosal lining where it uses surface appendages to adhere to microfold cells, or M-cells.
And these M-cells eat, or phagocytose, the bacteria from the intestinal lumen and spit it out into the underlying Peyer’s patches - a type of mucosal immune tissue that extends into the submucosa.
Responding immune cells can also damage the mucosa as they travel to the site of infection.
Now, usually, the infection is uncomplicated and limited to the mucosa, where it’s often destroyed by the local immune cells.
But, in some cases, the infection can become invasive, gaining access to nearby blood vessels, causing bacteremia.
From the bloodstream, the bacteria can spread to other organs like the brain, bones, liver, or spleen.
Now, reservoirs for non-typhoidal Salmonella include infected humans and animals, particularly birds, reptiles, mammals, and amphibians.
So, transmission can be fecal-to-oral and foodborne through consumption of contaminated raw or undercooked animal products; especially poultry, meat, and eggs, or unpasteurized milk or milk-products.