What kind of bacteria is Shigella?

There are four different species of Shigella:

  • Shigella dysenteriae,
  • Shigella flexneri,
  • Shigella boydii, and
  • Shigella sonnei.

Shigellas are Gram – negative microorganisms. They do not form spores, and are able to survive the proteases and acids of the intestinal tract. Shigellas are facultative anaerobic and have no capsule. The four different types of Shigellas vary in their genomic structure. Shigella flexneri has 6 serotypes, Shigella boydii has 18 serotypes, Shigella sonnei has 1 serotype, while Shigella dysenteria has 12 serotypes. The best optimal temperature for Shigellas to grow and multiply is 37 degrees.

Shigella dysenterie includes 12 serotypes, doesn’t ferment the mulberry. Shigella flexneri, which consists of 6 serotypes, ferments the mulberry. Shigella boydii, which consists of 18 serotypes, ferment the mulberry and have very similar biochemical structure as Shigella flexneri, but very different antigenic structure. Shigella sonnei, which consists of 1 serotype, ferments the mulberry and lactose.

Shigellas do ferment the glucose, but don’t produce gas, except Shigella flexneri 6 and Shigella boydii 14. They do not hydrolyse urea, do not produce H2S, and are generally lactose negative, except of Shigella sonnei.

Shigella sonnei usually gives infections which are mild and the symptoms last shorter, when compared to other Shigella types. Shigella flexneri causes mild infection, which lasts longer than the one caused by Shigella sonnei. Bacterial infection caused by Shigella flexneri tends to cause chronic complications like reactive arthritis. This is more likely to occur in genetically predisposed persons.

Shigella pathogens use a mixed acid fermentation pathway to metabolize substrates. Products of this anaerobic pathway include ethanol, acetic acid, lactic acid, succinic acid, formic acid, and carbon dioxide.

Shigellas penetrate the mucous of the colon, which results in degradation of the epithelium and an acute inflammation in the lamina propria of the colon. This will cause inflammation of the colon, presence of mucus and blood into the intestinal lumen, which are secreted outside the human body and can be seen in the diarrheal feces.

The bacterial infection with Shigella, is more likely to occur in the period between March – April, and August – October. Shigella flexneri is usually found and spread through contaminated water, while Shigella sonnei is usually found and spread through contaminated food. Humans are the primary reservoir of Shigella species.

What is Shigellosis?

Shigellosis is an intestinal disease caused from one of the Shigella species. This infection is characterized by bloody diarrhea. Shigella can be passed through direct contact with the bacteria in the stool.Shigellosis is more likely to occur in children between the ages 2 and 4. The infection occurs when the shigella is accidently swollen like:

  • Touching the mouth – when hands are not washed after toilet use or change of the diaper.
  • Eat contaminated food
  • Swallow contaminated water.

Fecal oral contamination is the cause of the infection.

Signs and symptoms of Shigellosis

Signs and symptoms of Shigellosis usually begin after a day or two from the infection. There are two main types of the disease. One is the mild to moderate form of the gastrointestinal infection, while the other one is a more severe form, which can lead to serious complication. Signs and symptoms generally include: abdominal pain, abdominal cramps, nausea, vomiting, tenesmus, fever, but the most characteristic sign is the diarrhea, which contains mucus and blood. In the severe to moderate form, diarrhea is very severe, but mostly watery, while in serious cases it is less in quantity, but contains lots of blood and mucus. Some people may be only the carriers of Shigella, without being sick themselves, but still being contagious.

Usually Shigellosis clears up without complications. However, in some cases complications are possible. These complications include:

  • Dehydration – due to diarrhea and vomiting a big amount of fluids, ions, bicarbonate and electrolytes is lost. This will lead to low blood pressure, weakness, sunken eyes, lack of tears and dry diapers in babies. Severe dehydration can lead to circulatory shock and death.
  • Seizures – occur after a very high fever. It is not known for sure, if this seizures result from the high fever, or from the bacterial infection with Shigella.
  • Hemolytic uremic syndrome – it is a very rare complication of Shigellosis, which can lead to a low red blood cell count, low platelet count and acute kidney failure.
  • Reactive arthritis –develops in response to this bacterial infection. Signs and symptoms include swollen joints due to inflammation, red and warm in touch. Usually, the ankle, knee and hip joints are affected.
  • Toxic megacolon – very rare but serious and dangerous complication of Shigellosis, which could be even life threatening. Signs and symptoms of toxic megacolon include: severe abdominal pain, abdominal cramps, high fever, weakness, etc. This is all due to a paralysis of the colon, which becomes like a sack. This sack formed colon is filled with feces, eventually it will rupture, causing peritonitis. This is a life threatening condition, which requires immediate emergency surgery.
  • Rectal prolapse – straining during the bowel movements may cause the mucous membrane, or lining, of the rectum to move out through the anus.

How is Shigellosis diagnosed?

Diagnose of Shigellosis is confirmed through samples taken from the stool, which are tested in laboratories, for the presence of Shigella and their toxins.


How is Shigellosis treated?

Treatment of Shigellosis consists in using antibiotics. Drugs that treat diarrhea should be avoided, because they will make the situation worse. In case of severe dehydration, replacement of fluids, ions, electrolytes and bicarbonates are necessary. This usually requires treatment in the hospital, especially for young children or elderly adults with severe dehydration. Treatment should be started right away, and it is very important to prevent any possible complication of the disease, which might be even life threatening, like for example toxic megacolon.

Avoiding and preventing a bacterial infection with Shigella is very important. Good personal hygiene, washing the hands several times a day, especially after the use of toilet or diaper change, drinking supervised water, etc., are routine little things that will help prevent this disease. There is no vaccine for Shigella.