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In this article we will discuss about Shigella:- 1. Morphology of Shigella 2. Cultural Characteristics of Shigella 3. Pathogenicity 4. Colicin Typing 5. Treatment 6. Prevention and Control.
Contents:
- Morphology of Shigella
- Cultural Characteristics of Shigella
- Pathogenicity of Shigella
- Colicin Typing of Shigella
- Treatment of Shigella
- Prevention and Control of Shigella
1. Morphology of Shigella:
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They are short Gram-negative bacilli, 0.5 p x 1-3 p in size, fimbriate, non-motile, non-sporing and non-capsulated.
2. Cultural Characteristics of Shigella:
They are aerobes and facultative anaerobes, grow best at 37°, pH 7.4 on ordinary media. Colonies are small, about 2 mm in diameter, circular, convex, smooth and transparent; on MacConkey’s agar medium they are pale or colourless except Sh. sonnei which is late lactose fermenter.
DCA and Salmonella and Shigella (SS) agar medium are useful selective media, but their growth is inhibited on Wilson and Blair medium. On XLD, Shigella produce pink colonies without black centres.
3. Pathogenicity of Shigella:
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Infection is by ingestion of a much lower dose of shigellae than that of salmonellae (except S. typhi). After reaching the large intestine, shigellae multiply locally with resultant inflammation of the mucosa which, in severe cases, may progress to ulcer formation and sloughing of larger areas of mucosa; bacteriaemia is very rare as their invasion is usually restricted to mucosa membrane.
Laboratory diagnosis can be done by immediate isolation of the bacillus from faeces obtained by rectal swab or fresh stool. The specimens should be transported in glycerol saline (Fig. 5.2(a)) never in highly alkaline transport Cary Blair medium used for vibrio.
Mucus flakes are preferred and can be inoculated on MacConkey agar and DCA media. After incubating overnight at 37°C, pale colonies are tested for motility and biochemical reactions. The identification is confirmed by slide agglutination with polyvalent or monovalent sera. Identification of shigellae in faeces can also be done by fluorescent antibody technique. Serodiagnosis is of no value.
4. Colicin Typing of Shigella:
Bacteriophage typing of Shigella is cumbersome, therefore colicin typing is adopted in many centres. Sh. sonnei and Sh. boydii produce Colicin bacteriocins which have wide range of activity against enteric bacilli and, on this basis, Sh. sonnei strains (Group D) are subdivide into 17 Colicin types against 15 indicator strains and each type is characterised by the production of specific colicin.
5. Treatment of Shigella:
Oral rehydration therapy (ORT) is adequate in all uncomplicated shigellosis cases. Routine antibacterial treatment is not indicated in dysentery because of the multiple drug resistance of shigellae. However, antibodies are reserved for the severe toxic cases.
6. Prevention and Control of Shigella:
Man is the main source of infection (bacillary dysentery). He may transmit dysentery bacilli by touching door handle of latrine with the contaminated fingers. The disease may also be transmitted by contaminated food through faeces and flies from person to person. The infection can be controlled by satisfactory sanitation along with the detection and treatment of patients and carriers.
