Shigella: antigenic structure, cultural characteristics and biochemical tests

Genus: Shigella

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.

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.

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.

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

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.

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