Shigella is a gram-negative, rod-shaped bacterium that belongs to the tribe Escherichieae within the family Enterobacteriaceae. These bacteria are closely related to the genus Escherichia which is best known for the E. coli species. Unlike E. coli, none of the Shigella species are part of the normal human gastrointestinal flora. The presence of these bacteria almost always causes a form of dysentery which can range from mild to severe, depending on the infecting species.
Shigella was first isolated in 1896 by Japanese microbiologist Kiyoshi Shiga. Today there are four different species/groups present within the genus Shigella. The groups were formed based on the properties of the O-antigen. The O-antigen is present within the lipopolysaccharide found in the outer membranes of gram-negative bacteria. It is one of the antigens responsible for causing an immune response in the body against invading bacteria. The groups/species are as follow:
Shigella dysenteriae (Group A)
Shigella flexneri (Group B)
Shigella boydii (Group C)
Shigella sonnei (Group D)
Each of the species has a multitude of serotypes with the exception of Group D, which only contains one. This division allows laboratory workers to use serological grouping to determine which Shigella species have been isolated. In some cases, certain species can have a K-antigen which is also known as a capsule. The presence of a capsule can interfere with serological grouping and must be removed with laboratory procedures.
In general, all members of the genus Shigella are non-motile, do not hydrolyze urea, do not decarboxylate (remove carboxyl groups) lysine, do not produce hydrogen sulfide (H2S), do not utilize mucate or acetate as a carbon source, do not produce gas with the fermentation of glucose, and do not ferment lactose. There are a few exceptions within each species:
S. flexneri can produce gas with the fermentation of glucose.
S. sonnei ferments lactose slowly on MacConkey agar.
S. sonnei can decarboxylate ornithine.
S. sonnei is ONPG test positive which is used to detect beta-galactosidase activity. Other species have negative or variable results.
As with all bacteria, positive or negative tests simply means that 90% or more of the strains isolated for each species show those results. It is possible for laboratory technicians to see atypical results. There are also several rapid screening tests available, and a multitude of other biochemical tests are available to distinguish between the species. Shigella bacteria are considered to be fragile organisms, and must be plated for culture in a lab as soon as possible. They are susceptible to chemical and physical agents such high concentrations of bile and acids, as well as disinfectants.
This is important because humans are the only known reservoir for Shigella. Transmission most often occurs by the fecal-oral route. They can be transmitted by fingers, infected food and water, and by flies. People most at risk for contraction of the bacteria are those with poor hygiene or in very crowded situations, young children, and those that participate in oral-anal sexual situations. Shigella infections are considered to be highly communicable because less than 200 bacilli are necessary for the disease to be seen in a previously healthy individual.
Many cases of mild gastrointestinal upset caused by Shigella organisms are self-limiting. They are characterized by mild fever, stomach cramps, and watery diarrhea. Shigella has an incubation period ranging from 1-7 days but signs and symptoms can be seen as early as two or three days after infection. The infection can last up to a week. According to the Center for Disease Control, the most commonly isolated species in the United States is S. sonnei followed by S. flexneri. These numbers could be under reported simply because many people may choose to just allow the infection to run its course without a culture being completed.
Shigella is most known for the severe bacillary dysentery it can cause. Epidemics of these cases are seen in developing countries and are caused by S. dysenteriae. These infections cause severe diarrhea, vomiting, high fever, and bloody stools. The mortality rate is 5-10% and mostly affects children under the age of 10. Children under the age of one are most at risk for fatal infections. Shigella can be successfully treated with a range of antibiotics, including Ciprofloxacin, Azithromycin, and Fluoroquinolones. A vaccine is still being researched because there are about 160 million yearly cases of Shigella related dysentery worldwide resulting in about 1.5 million deaths a year.
Hemolytic Uremic Syndrome is a possible complication for people affected by S. dysenteriae because of the Shiga toxin produced by this species. Persons infected with S. flexneri have a small chance of developing a complication known as Reiter’s syndrome. This syndrome can last for months or years and consists of painful urination, irritated eyes, and joint pains. It can lead to chronic arthritis.
The best way for people to avoid any Shigella bacteria is to practice proper hygiene, especially hand washing. Basic food preparation safety should be followed at all times. Child care facilities or any place where diaper changing takes place should have those areas regularly disinfected after every changing, with proper disposal of waste. Anyone recently infected with Shigella should not prepare meals for anyone else until they have had no signs or symptoms for at least two days. Travelers should only drink boiled or treated water to avoid contamination.
Mahon, C. R., Lehman, D. C., & Manuselis, G. (2007). Textbook of diagnostic microbiology (3rd ed.). St. Louis, Mo.: Saunders Elsevier.