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.