• Dysentery is a clinical condition of multiple etiology characterized by frequent passage of blood-stained mucopurulent stool.
• The causative agent of bacillary dysentery (Disease characterized by severe abdominal cramps and the frequent painful passage of low volume stools containing blood and pus) belong to the genus Shigella.
•  Shigella is named after ‘Shiga’ who in (1896) isolated the first member of this genus from epidemic dysentery in Japan.
• Other Bacilli such as entero-invasive E. coli, Vibrio parahemolyticus, Campylobacter can also cause the clinical feature of dysentery.
• Enterobacteria (fermentative, facultative anaerobes, oxidase -ve, gram -ve rods).
• Non lactose fermenting, non-motile, mostly anaerogenic urease -ve, non-citrate utilizing and KCN sensitive.
• Shigella are killed at 56^oC in 1 hour and by 1% phenol in 30 minutes.
• In faeces they die wthin a few hours due to acidity produced by faecal coliform.
• S. sonnei are more resistant than other Shigella they can tolerate low temperature if adequate moisture is present.
• They can survive over 6 months in water at room temperature.

Morphology of Shigella:

• Shigella are short gram -ve rods.
• (2-4) x 0.6 ml, non-sporing, non-motile
• Non-capsulated
• Fimbriae are present only in S. flexneri

Antigenic structure of Shigella:

• Shigella are differentiated by their ‘O’ antigens into serotypes.
• These are classified into 4 structures or subgroups based on a combination of biochemical and serological characteristics.
• Shigella dysenteriae (Subgroup A):
  • These are mannitol non-fermenting, consists of 10 serotypes.
  • Shigella dysenteriae type-1 forms a toxin.
  • 3 types of toxic activity have been demonstrated in Shigella culture filtrates.
    ( Neurotoxicity, enterotoxicity, and cytotoxicity)
• Shigella flexneri (Subgroup B):
  • Named after Flexner, who first time described first of the mannitol fermenting Shigella from Phillipines (1900).
  • Based on type specific and group specific antigen, they have been classified into six serotypes (1-6) and several subtypes.
• Shigella boydii (Subgroup C)
  • Consists of dysentery bacilli that resemble S. flexneri biochemically, but not antigenically.
  • After Boyd who first described this strain from India (1931).
  • S. boydii isolates least frequently.
  • 15 serotypes have been identified.
• Shigella sonnei (Subgroup D)
  • 1^st time isolated by Sonne (1915) in Germany.
  • Ferment lactose and sucrose late, indole negative.
  • Causes mildest form of bacillary dysentery.

Cultural characteristics of Shigella:

• Aerobic and facultative anaerobes.
• Optimum temperature 37^oC (Exception S. sonnei grow even at 10^oC and 45^oC).
• They grow on ordinary media however less readily than other Enterobacteria.
• Nutrient agar and Blood agar:
  • On Nutrient agar and Blood agar, Colony are smooth, circular convex greyish or colorless, translucent often 2-3 mm diameter.
  • Those of S. sonnei are slightly larger and opaque than others.
• MacConkey agar (MA):
  • On MA, colonies are pale and yellowish (non-lactose fermenting). Exception S. sonnei being late lactose fermenting become pink when incubation period is prolonged.
• Deoxycholate citrate agar (DCA):
  • On DCA, excellent selective medium for isolation of Shigella from faeces.
  • Colonies are pale and similar to though usually slightly smaller 1-1.5mm diameter and more translucent than those of Salmonella. They do not form black center.
• Xylose lysine deoxycholate agar (XLD):
  • On XLD, probably the best selective media for Shigella being less inhibitory to S. dysenteriae and S. flexneri than DCA.
  • Colonies are red and unlike those of most Salmonella without black centers.
• Peptone water and nutrient broth:
  • Good growth with uniform turbidity on incubation over night at 37^oC.
  • In some cases, especially fimbriated form a surface pellicle on longer incubation.
• Selenite F-broth:
  • Selenite F-broth enrich S. sonnei and S. flexneri but inhibitory to other Shigella.

Biochemical tests of Shigella:

• Carbohydrates utilization:
  • Most strains utilize sugar to produce acid but not gas though some strain S. flexneri and S. boydii form gas.
  • Glucose is fermented by almost all strains.
  • Lactose is not fermented within 24hrs.
  • However, S. sonnei and some strains of S. dysenteriae produce acid from lactose after prolonged incubation.
  • Mannitol fermentation is important characteristics. This differentiated Group A strain (which do not ferment mannitol) from group B, C and D, most strains of which ferment it.
  • Dulcitol is not fermented by most Shigella.
  • Sucrose is not fermented except S. sonnei andsomestrains of S. flexneri.
  • Adonitol and Inositol are also not fermented.
  • Xylose is not fermented except mannitol -ve biotype of Shigella flexneri.
• Methyl red test: +ve
• VP test: -ve
• Reduce nitrate to nitrite
• Catalase +ve
• Indole -ve,
• Citrate -ve
• H₂S -ve
• Urease -ve
• KCN growth (-ve).
• Gelatin not liquified.
• Decarboxylation test:
  • Group A, B and C fail to decarboxylate lysine and ornithine.
  • S. sonnei decarboxylate ornithine but not lysine

Pathogenesis of Bacillary dysentery caused by Shigella:

• Shigella causes bacillary dysentery.
• Infection occurs by ingestion.
• The minimum infective dose is low (10-100) bacilli.
• Being capable of initiating the disease probably because they survive gastric acidity better than other enterobacteria.
• Their pathogenic mechanism resembles those of Entero-invasive E. coli.
• The epithelial cell of the villi is the large intestine and multiply inside them spreading laterally to involve adjacent cell and penetrate into the lamina.
• Inflammatory reaction develops with capillary thrombosis resulting in necrosis of patches of epithelium which slough offs and leaves transverse superficial ulcers behind.
• Bacteremia may occur in sever infection, particularly in malnourished children and in AIDs patient.
• Has short incubation period (1-7 days usually 48 hrs).
• The onset and clinical course are variable and are largely determined by the virulence of infective strains.
• Shigellosis has high death rate especially in young children. Most death are caused by S. dysenteriae type 1.

Clinical manifestation of bacillary dysentery caused by Shigella:

• The clinical features of Shigella dysenteriae type 1 infection includes:
•  toximea, sometimes bacteremia and severe dysentery leading to marked dehydration and protein loss
•  Inflammation and ulceration of the large intestine
•  Hemorrhage, abdominal pain and high fever
• Death occur from circulatory collapse or kidney failure

Shigella: antigenic structure, cultural characteristics and biochemical tests

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