Shigella

SHIGELLA

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.





GENUS SHIGELLA:

Contains four species of importance to man as causes of bacillary dysentery:

  • Sh.dysentery,
  • Sh.boydii,
  • Sh.flexneri and
  • Sh.sonnei.

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.

Laboratory Identification:

Non-fastidious, bile-tolerant, Lactose non-fermenters. Full identification requires use of biochemistry and serologic tests for O antigens.




LABORATORY DIAGNOSIS:

Sample Collection: Faeces for culture
Transport medium: Sach’ buffered glycerol saline

 

Methods for Identification of Organism:

  1. Microscopy: Shigella is Gram negative, non-motile & non capsulated rod.
  2. 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.
  3. 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.