Yersinia enterocolitica

Definitions & Key takeaways

Yersinia enterocolitica is a Gram-negative bacterium belonging to the Enterobacteriaceae family that causes a diarrheal illness called yersiniosis. It secretes bacterial proteins called Yops, which block the secretion of proinflammatory cytokines and inhibit macrophage activation. Yersinia enterocolitica is transmitted through pet feces, contaminated milk, or pork and causes a gastrointestinal infection that can mimic appendicitis. It can spread systemically and cause septicemia, which can be rapidly fatal if left untreated. Even after the infection, there may be associated sequelae such as reactive arthritis and erythema nodosum.

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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.
Now, some animals such as cattle, deer, pigs and birds are natural hosts for Yersinia enterocolitica. So, it’s transmitted through pet feces, usually from puppies, contaminated milk or pork and it causes a gastrointestinal infection called yersiniosis.
So, it can affect various segments of the gastrointestinal tract such as the ileum causing terminal ileitis, the right colon causing enterocolitis or the abdominal mesenteric lymph nodes causing mesenteric lymphadenitis which can lead to a disease that can mimic appendicitis, called pseudoappendicitis.
In people with an immunosuppressive state or with iron overload conditions such as sickle-cell disease or beta-thalassemia, the bacteria can spread systemically and cause septicemia which can be rapidly fatal if left untreated.
Finally, even after the infection, meaning after the bacteria has been eliminated from the body, there may be still associated sequelae, and this seems to happen more often who have the HLA-B27 antigen, which is a protein found on the surface of leukocytes.
One example is reactive arthritis which usually develops 1 to 3 weeks after the infection. The mechanism is not well known but it’s considered to be caused by bacterial antigens which are somehow deposited in the joints, and cause a delayed inflammatory response.
Another example is erythema nodosum which is an inflammatory condition characterized by inflammation of the fat cells under the skin.
This happens because of a delayed hypersensitivity reaction to bacterial antigens which may have gotten inside the fat cells.
Now, symptoms of enterocolitis include fever, diarrhea with blood and mucus, abdominal pain. With pseudoappendicitis, there’s fever, abdominal pain, tenderness in the right lower quadrant of the abdomen.
Fever is also present with septicemia, along with chills, hypotension and tachycardia. As for the post-infectious sequelae, with reactive arthritis, there’s joint pain and swelling, and with erythema nodosum there may red or purple, painful lesions, mainly on the legs.
Now, diagnosis is made by identifying the bacteria in cultures from a stool sample. Reactive arthritis can be diagnosed by identifying inflammatory cells or bacteria in a synovial fluid sample.
Infection can also be confirmed by demonstrating increased titers of antibodies in the serum using serological tests such as tube agglutination or ELISA but this is not usually done, because it’s easier to culture the bacteria.
Finally, an ultrasound or CT scan is needed to exclude appendicitis in cases with pseudoappendicitis. Yersinia enterocolitica infections can be treated usually with third-generation cephalosporins and it’s also susceptible to trimethoprim-sulfamethoxazole, aminoglycosides, fluoroquinolones and tetracyclines.
Alright, as a quick recap, Yersinia enterocolitica is a Gram-negative bacillus. It can be motile or non-motile, it’s non-spore forming, facultative anaerobic, facultative intracellular, non-lactose fermenter and oxidase negative.
It grows well on MacConkey agar into colourless colonies and on CIN agar into white, sharp-bordered colonies with a deep-red center.
Its virulence factors include adhesins, which are involved in the colonization of intestinal mucosa, and also Yops, which inhibit phagocytosis.
It’s transmitted through pet feces, contaminated milk and pork and can cause enterocolitis, terminal ileitis, mesenteric lymphadenitis as well as septicemia.
Post-infectious sequelae include reactive arthritis and erythema nodosum. Diagnosis relies on identifying the bacteria in stool cultures.
Treatment can be done with third-generation cephalosporins, trimethoprim-sulfamethoxazole, aminoglycosides, fluoroquinolones and tetracyclines.