Shigella

Shigella

Shigella is a genus of Gram –negative , non-spore forming rod-shaped bacteria closely related to Escherichia coli and salmonella ,the causative agent of human shigellosis,shigella causes disease in primates but not in other mammals , During infection, it typically causes bacillary dysentery. The genus is named after kiyoshi shiga who first discovered it in 1898

Classification of Shigella:

Shigella genus is classified into four pathogenic species on manifold fermentation (acid, no gas) and antigenic structures.

Species                                             Group                                                Mannitol

1. Shigella dysenteriae                   A                                                         -
2. Shigella flexneri                           B                                                         +
3. Shigella boydii                                                                           C                                                         +
4. Shigella  sonnei                                                                             D                                                         +

Morphology of Shigella:

Shape                         : Rod –shape

Size                            :2.5×0.5   µm,

Flagella                      :   Absent.

Spore                          :Absent.

Capsule                      :Absent.

Motility                    Non-motile.

Staining of Shigella:

They are Gram –negative.

Cultural characters of Shigella:

1. Aerobes or facultative anaerobes.
2. Optimum temperature :37°C
3. Grow on ordinary media and are capable of growing in presence of bile .
4. On Mac-Conkey agar and DCA: Grows as pale colonies.
5. On XLD agar : Produces 1 -2 mm red colonies.
6. On SS agar: Grows as pale colonies.

Biochemical characters of Shigella:

1. As member of the family: Enterobacteriaceae they invariably ferment glucose and reduce nitrate to nitrite.
2. They produce acid only and no gas during fermentation of sugars . Only exception is Shigella Flexneri  type 6 that produces acid and gas.
3. They are non-lactose fermenters except shigella sonnei which is late lactose fermented.
4. They are non-urea splitters.
5. Mannitol fermentation and in dole production are variable.