General Overview
Shigella caused bloodstained mucopurulent stool, often it called as bacillary dysentery or shigellosis.
Furthermore, the dysentery caused by Shigella species called as enterocolitis. Some species produce enterotoxins.
They are typical members of Enterobacteriaceae and closely they are related to Escherichia. Shigella are divided into four sub groups as,
- Shigella dysenteriae – serogroup A
- Shigella flexneri – serogroup B
- Shigella boydii – serogroup C
- Shigella sonnei – serogroup D
Shigella have O antigens in the cell wall. They are the ones useful for the dividing of genus Shigella into groups A, B, C and D.
According to the estimations annually 164.7 million episodes of shigellosis occurred all over the world.
Mainly they occur in developing countries and 61% of deaths involving children under 5 years.
This infection occurs only in humans and transmission mainly occur through faeco-oral route, in the places where there is poor sanitation, overcrowding and unhygienic conditions.
Less number of organisms is enough to make the disease.
House flies are the main source of transporters which carry Shigella from faeces to food.There are no any carrier stages found in Shigella as in Salmonella.
However, Shigella are rapidly killed by drying.
Patients with Shigella dysentery show symptoms such as fever, abdominal pain and cramps and diarrhea.
Microscopy
Shigella are Gram negative bacilli. They are non-motile, non-spore forming and non-capsulated.
Methylene Blue stained fecal specimens give a clue for the diagnosis. In Shigellosis neutrophils can be seen. Â Â
Culture
Shigella are non-lactose fermenters and do not produce H2S. They are also do not produce gas from fermenting glucose.
Shigella are aerobes and facultative anaerobes. They grow temperatures between 10-45°C and their optimal growth temperature is 37°C.
A selective medium is used for the isolation of Shigella. Specimens should be cultured without delay as Shigella destroyed by drying. Faeces is the preferable specimen than a rectal swab for culture.
XLD agar
Red pink colour colonies are given without black centers by Shigella on XLD agar. Colonies are 2-4 mm in diameter.
DCA and MacConkey agar
Non lactose fermenting pale colour colonies are produced. Colonies are 1-2 mm in diameter.
Though they have given the name Salmonella-Shigella (SS) agar, it is not suitable for the isolation of Shigella as most Shigella strains are inhibited by this medium.
- On KIA medium Shigella produce pink (alkaline) slope and yellow (acid) butt. It indicates fermentation of glucose only.
- Lactose negative
- H2S negative
- Urease negative
- Oxidase negative
- Lysine decarboxylase (LDC) negative
- Ornithine decarboxylase (ODC) negative except sonnei which is ODC positive
- Beta-galactosidase (ONPG) negative
- Shigella can be serogrouped by their O antigens, using polyvalent group antisera and monovalent antisera.
PCR (polymerase chain reaction) test can be used for the identification of Shigella plasmid antigen H.
Tissue culture can be done for the identification of Shiga toxins (Vero and HeLa cell tests).
Treatment
Antibiotics are usually used for the treatment of severe Shigellosis. In minor conditions Fluid and Electrolyte replacement is done (hydration).
For the treatment of dysentery in children and severe infection ciprofloxacin, ceftriaxone and pivmecillinam are used.
As many organisms show multiple drug resistance, it is necessary to perform formal antibiotic sensitivity tests.
Trimethoprimsulfamethoxazole is also can be used as an alternative drug.
Prevention
There is no vaccine available for the prevention of Shigellosis.
Interruption of the transmission of organisms via fecal- oral route may helpful for the prevention.
Proper disposal of waste materials, proper chlorination of water, handwashing when handling foods and personal hygiene may helpful to prevent the disease.
There are no any recommended prophylactic antibiotics for the prevention of the disease.
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