Definition, Classification, Morphology and cultural Characteristics of Shigella
Class: Gamma Proteobacteria
Type species: S. dysenteriae
Definition of Shigella:
They are fermentative, facultative anaerobic non spore forming gram negative bacilli, non motile, catalase positive (except Shigella dysentriae type 1), oxidase negative, do not produce gas form carbohydrates with few exceptions. None forms acetylmethyl carbinol or ferments adonitol, inositol or salicin, lysine decarboxylase, liquifies gelatin, deaminates phenylalanine, utilises malonate or produce H2S. Based on DNA- DNA hybridisation studies, this genus is closely related to Escherichia coli . G+C content of DNA varies from 48-53 mol%.
They are grouped into following 4 species on the basis of serological and some biochemical tests:
Group A (S dysenteriae, 12 serotypes)
Group B (S flexneri, 6 serotypes)
Group C (S boydii, 18 serotypes)
Group D (S sonnei, 1 serotype)
Groups A, B, and C are very similar physiologically while S. sonnei can be differentiated from the other serogroups by positive β-D-galactosidase and ornithine decarboxylase biochemical reactions.
Morphology of Shigella
They are short, gram negative rods, about 2-4 µm in length x 0.6µm in breadth. They are non spore forming, non capsulated, non motile bacilli. Type1 fimbria occurs only in Shigella flexnerri.
Cultural characteristics of Shigella
They are aerobes and facultative anaerobe with a growth temperature ranging from 10oC – 45oC (optimal temperature being 37oC). S. sonnei grow well even at 10oC and 45oC. They grow well on conventional ordinary media but none grows on a simple glucose ammonium salts medium unless supplemented with nicotinic acid and for some strains other growth
On nutrient agar after overnight incubation, colonies are small often 2-3 mm in diameter, circular, convex , greyish or colorless, smooth and translucent. Colonies of S. sonnei are slightly larger and more opaque than other Shigella.
- sonnei has two antigenic forms: Form 1is like the colonies of other Shigella while form 2 arise by irreversible variation from form 1 and are seen in early subcultures from primary isolates, are larger and less opaque and have irregular, matt surface and a crenated edge.
On Monkey agar, colonies are pale and yellowish due to the absence of lactose fermentation. An exception is S. sonnei, which is a late lactose fermenter and colonies become pink when incubation is prolonged beyond 24 hours. Deoxycholate citrate agar and Xylose lysine deoxycholate agar is a useful selective medium (Shigella donot have black centres on these medium as Salmonella). Growth is inhibited on Wilson and Blair’s bismuth sulphite medium.
Shigella sps can be killed by heating at 56oC in one hour and by 1% phenol in 30 minutes. They can survive in ice for 1-6 months. In faeces, they die within hours due to tha acidity produced by growth of coliforms. S. sonnei is the most resistant species.
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