USMLE® Step 2 style questions USMLE
A 12-year-old boy comes to the emergency department with severe right lower quadrant abdominal pain. Parents note the patient has felt hot for 3 days. His temperature is 38.9°C (102.0°F). Laboratory studies show leukocytosis. The patient is brought to the operating room for an appendectomy, and the surgeon finds visible inflammation around the appendix, terminal ileum, and mesenteric lymph nodes, but the appendix itself is normal. A week later the patient begins complaining of joint pain, dysuria, headache, and malaise. He has had bloody diarrhea for the past week. What is the most likely source of the pathogen causing his illness?
Yersinia enterocolitica is a Gram-negative bacillus that belongs to a family of bacteria called the Enterobacteriaceae.
The Yersinia genus got its name from Alexandre Yersin, who discovered it, and enterocolitica refers to intestine and colon, so Yersinia enterocolitica causes a diarrheal illness, called yersiniosis.
Now, a little bit of microbe anatomy and physiology.
First, Yersinia enterocolitica has a thin peptidoglycan layer, so it doesn’t retain the crystal violet dye during Gram staining.
Instead, like any other Gram-negative bacteria, it stains pink with safranin dye.
And since it’s a Gram-negative bacillus, it looks like a little pink rod under the microscope.
Interestingly, Yersinia enterocolitica can be both motile and non-motile depending on the temperature.
So, at 25 degrees Celsius it’s motile and at 37 degrees Celsius it’s non-motile.
It’s facultative anaerobe, so it can survive in both aerobic and anaerobic environments, and also facultative intracellular which means it can survive both outside and inside the cells.
Finally, it’s non-spore forming, so it doesn’t make spores, and oxidase negative, so it doesn’t produce this enzyme.
Yersinia enterocolitica 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, Escherichia coli and Enterobacter, can ferment lactose, which results in the production of acid, and this makes the pH sensitive dye turn pink - so their colonies will be pink.
Yersinia enterocolitica, however, is a non-lactose fermenter, so it forms colorless colonies on MacConkey agar.
Yersinia enterocolitica also grows on Cefsulodin-Irgasan-Novobiocin agar, or CIN agar for short.
After 24 hours of incubation, it forms white, sharp-bordered colonies with a deep-red center, that looks like bull’s eyes colonies.
Finally, the triple sugar iron test, or TSI for short can be done to assess hydrogen sulfide production.
This medium contains three sugars - lactose, glucose and sucrose, as well as iron and a pH sensitive dye.
If the bacteria produces hydrogen sulfide, that reacts with the iron, and a black precipitate forms in the test tube.
Yersinia enterocolitica doesn’t, so no precipitate forms.
Now, Yersinia enterocolitica 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, Yersinia enterocolitica can use adhesins such as YadA and Ail to attach to gut epithelial cells.
Once attached, it uses another virulence factor, called type III secretion system, or T3SS, to secrete bacterial proteins called Yersinia outer proteins, or Yops for short.
Yops block secretion of proinflammatory cytokines like TNF-alpha and IL-8, and seem to also inhibit macrophage activation.
The end result is that Yops suppress the host inflammatory response, and help Yersinia avoid phagocytosis.
Now, the bacteria can replicate inside Peyer’s patches, which are patches of lymphoid tissue located in the gut wall, and from there it can spread to more distant lymphoid tissues such as the mesenteric lymph nodes.
Also it needs iron to survive, so to get iron, it uses a siderophore, which is a term used for a group of small, high-affinity, iron chelating compounds that snatch iron from host cells.
Finally, Yersinia also produces an enterotoxin called Yst, which is considered to be involved in the diarrheal disease but its role is not well defined because diarrheal symptoms have been found in the absence of this enterotoxin.