Shigella are not normal inhabitants of the human gut unlike other members of the Enterobacteriaceae. Shigella are responsible for diarrheal diseases, which may be severe; Salmonella typhi is also invasive and gives rise to systemic infection.
Contains four species of importance to man as causes of bacillary dysentery:
- Sh.flexneri and
Characteristics: Gram-negative rods. Non-motile. Non-capsulate. Capable of aerobic and anaerobic respiration.
Transmission: Human pathogens spread by fecal-oral route, especially in crowded conditions. Small infective dose.
Diseases: Bacillary dysentery, very rarely invasive.
Non-fastidious, bile-tolerant, Lactose non-fermenters. Full identification requires use of biochemistry and serologic tests for O antigens.
Sample Collection: Faeces for culture
Transport medium: Sach’ buffered glycerol saline
Methods for Identification of Organism:
- Microscopy: Shigella is Gram negative, non-motile & non capsulated rod.
- Culture: Nutrient agar, MacConkey agar, D.C.A (Deoxycholate Citrate Agar) & S.S. (salmonella- shigellae) agar
- MacConkey agar: Colonies are colorless except shigella sonnei which is late lactose fermenter.
- D.C.A agar and S.S. agar (Selective Medium): Colonies are colorless.
- Biochemical Reactions:
- IMVIC= -+- –
- Glucose is fermented with the production of acid without gas.
- Only Sh. sonnei ferment lactose and sucrose late on further incubation.
- Fermentation of mannitol is of importance in classification.
- TSI: Slant/Butt – Pink/Yellow
Identification is confirmed by polyvalent and monovalent sera.
4. Antibiotic sensitivity test.