Shigella dysenteriae

Dysentery is a clinical condition of multiple origin. It could be bacillary or amoebic in nature. Dysentery is characterised by frequent discharge of blood stained, mucopurulent stools. Infection with this organism often leads to ulceration of the intestinal epithelium.

Shigella  - gram-negative, facultatively anaerobic, rod-shaped, non motile, non-sporing, non capsulated - extremely pathogenic and causes severe dysentery.   Fimbriae may be present.

Shigella produce an exotoxin (Shiga toxin) which disrupts protein synthesis and produces endothelial damage. Infection with this organism often leads to ulceration of the intestinal epithelium. Shigella spread via fecal-oral and person-to-person transmission

Kiyoshi Shiga - Japanese physician and bacteriologist- In 1896, Shiga discovered and identified Shigella dysenteriae which caused dysentery in Japan, and the Shiga toxin which is produced by the bacteria. He conducted research on other diseases such as tuberculosis and trypanosomiasis.

Most individuals are infected with Shigellae when they ingest food or water contaminated with human fecal material. This results in Bacillary dysentery.

Shigella can survive upto 30 days in milk, eggs and cheese.

Bacillary dysentery is characterized by severe abdominal cramps and the frequent painful passage of low volume stools containing blood and pus

MORPHOLOGY

Shigella are short Gram -ve rods-non-sporing, non-motile-non-capsulated. Fimbriae are present only in S. flexneri

CULTURAL CHARACTERISTICS 

Aerobic and facultative anaerobes. Optimum temperature 37°C. 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.

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)

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)

XLD - best selective media for Shigella - 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°C. 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.

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 (Sub groupA):

1)These are mannitol non-fermenting, consists of 12 serotypes.

2)Shigella dysenteriae type-1 forms exotoxin – Shiga toxin

3)3 types of toxic activity have been demonstrated in Shigella culture filtrates.( Neurotoxicity, enterotoxicity, and cytotoxicity/verotoxin)

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 flexneri - belongs to group B. 

S. flexneri infections can usually be treated with antibiotics, although some strains have become resistant.

Shigella boydii (Subgroup C):

Boyd first described this strain from India (1931).

S. boydii isolates -18 serotypes have been identified.

S. boydii is the most genetically divergent species of the genus Shigella

Shigella sonnei (Subgroup D):

Isolated by Danish bacteriologist Carl Olaf Sonne (1915) in Germany.

Ferment lactose and sucrose late, indole negative.

Causes mildest form of bacillary dysentery.

Shigella sonnei together with Shigella flexneri, is responsible for 90% of shigellosis cases. 

Resistance

1) Shigella are killed at 56°C in one hour and by 1% phenol in 30 minutes.

2) In ice they last for 1-6 months.

3) They remain viable in moist environment.

4) In faeces they die within few hours due acidity produced by growth of coliforms.

Biochemical tests of Shigella

Carbohydrate utilization:

1)Most strains utilize sugar to produce acid but not gas though some strain S. flexneri and S. boydii form gas.

2)Glucose is fermented by almost all strains.

3)Lactose is not fermented within 24 hrs.

4) S. sonnei and some strains of S. dysenteriae produce acid from lactose after prolonged incubation.

5)Mannitol fermentation - differentiates Group A strain (which do not ferment mannitol) from group B, C and D, most strains of which ferment it.

6) Sucrose is not fermented except S. sonnei and some strains of S. 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

Epidemiology

• Human beings are the only natural hosts

• Transmission through

1. Contaminated water & food
2. Contaminated fingers, flies, food, fomites (door handles, water taps, lavatory seats)
3. In young male homosexuals, through gay bowel syndrome