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Shigella Dysenteriae (Dysentery Bacillus)
The genus Shigellaare exclusively parasites of human intestine and other primates. Shigella dysenteriae is the causative agent of bacillary dysentery or shigellosis in humans. It is a diarrheal illness which is characterized by frequent passage of bloodstained mucopurulent stools. The four important species of the genus Shigella are: Shigella dysenteriae, Shigella flexneri, Shigella sonnei and Shigella boydii.
Shigella are short, Gram negative rods (0.5µm× 1–3 µm in size). They are non – motile, non – sporing and non – capsulated (Figure 7.12).
• They are aerobes and facultative anaerobes. Optimum temperature is 37°C and optimum pH – 7.4.
• They can be grown on the following media and show the characteristic colony morphology (Table 7.10 & Figure 7.13).
Shigella dysenteriae produces toxins, which is of 3 types, namely, endotoxin, exotoxin and verocytotoxin. The mode of action of these toxins is illustrated in the Table 7.11
The pathogenic mechanism of Shigella dysenteriaeis discussed below in flowchart 7.5.
Source of Infection – Patient or carriers
Route of entry – faecal – oral route
Site of infection – Large intestine Incubation Period – Less than 48 hours (1–7 days)
Mode of transmission – Food, finger, faeces and flies
• Frequent passage of loose, scanty faeces containing blood and mucus.
• Abdominal cramps and tenesmus (straining to defecate).
• Fever and vomiting.
• Hemolytic uremic syndrome (It is a condition caused by the abnormal destruction of red blood cells).
Specimens: Fresh stool is collected.
Direct Microscopy: Saline and Lugol’s iodine preparation of faeces show large number of pus cells, and erythrocytes.
Culture: For inoculation, it is best to use mucus flakes (if present in the specimen) on MacConkey agar and SS agar. After overnight incubation at 37°C, the plates are observed for characteristic colonies, which is confirmed by Grams staining and biochemical reactions.
• Uncomplicated shigellosis is a self – limiting condition that usually recovers spontaneously.
• In acute cases, oral rehydration therapy (ORT) is done.
• In all severe cases, the choice of antibiotic should be based on the sensitivity of prevailing strain.
• Many strains are sensitive to Nalidixic acid and Norfloxacin.Improving personal and environmental sanitation.
• The detection and treatment of patients and carriers.
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