Shigella: Disease, Properties, Pathogenesis, Lab Diagnosis

By Acharya Tankeshwar •  Updated: 05/03/22 •  4 min read
Shigella

Shigellae are Gram-negative, non-motile, non-spore-forming, rod-shaped highly infectious bacteria of the Enterobacteriaceae family.

Shigella species include; Shigella sonnei, S. boydii, S. flexneri, and S. dysenteriae.

Classification

There are 4 major subgroups and 43 recognized serotypes of Shigella.

Subgroup Serotypes and Subtypes
Group A: Shigella dysenteriae 15 serotypes
Group B: Shigella flexneri8 serotypes and 9 subtypes
Group C: Shigella boydii19 serotypes
Group D: Shigella sonnei1 serotype

*S. dysentriae is the most virulent but S. sonnei is the most common isolated species worldwide.

Diseases

Shigellosis (also called bacillary dysentery).

Routes of transmission

Humans are the only host of Shigella. Fecal-oral route is the primary means of human-to-human transmission. The five “Fs” of disease transmission are:

  1. Food- ingestion ( water, food, drinks)
  2. Flies
  3. Fomites-(taps, toilet seats, etc)
  4. Fingers
  5. Feces

Pathogenesis

Some strains of Shigella produce enterotoxins and Shiga toxin. Shigella species are tolerant to low pH and are able to transit the harsh environment of the stomach. Pathogenic mechanism of shigellosis is complex and involves:

  1. Shigellas are taken up by M cells and transported beneath the epithelium. Macrophage takes up shigellas, dies, and releases the bacteria.
  2. The bacteria enter the inferior and lateral aspects of the epithelial cells by inducing endocytosis. The endosomes are quickly lysed leaving shigellas free in the cytoplasm.
  3. Actin filaments quickly form a tail pushing the shigellas into next cell.
  4. Shigella multiply in the cytoplasm and infection spread to next cell.
  5. Infected cells die and slough off, intense response of acute inflammatory cells (neutrophils), bleeding and abscess formation.

Infectious dose (ID): As few as 10 to 200 cells can cause disease, depending on the age and condition of the host.

Laboratory diagnosis

Freshly passed stool is the preferred sample. The sample must reach the laboratory within 2 hours of collection. If a delay is anticipated transport media should be used to retain the viability of the organism.

Macroscopic examination: scanty fecal matter, bright red in color, and presence of mucus

Microscopic examination: Plenty of pus cells, RBCs & a dead background (no motile bacteria)

Transport Media for Shigella

  1. Buffered glycerol water,
  2. Cary-Blair transport media,
    Transported in the cool box or refrigerated if a delay is likely.

The sample can be plated directly in either MacConkey agar or deoxycholate citrate agar (DCA) or xylose lysine desoxycholate (XLD) agar, Salmonella-Shigella agar, or after enrichment in selenite F broth (incubated at 37o C)

Colony characteristics:

  1. Shigellae form non-lactose fermenting pale-colored or colorless colonies on MacConkey’s agar or EMB agar or DCA.  S. sonnei forms pink colonies.
  2. Xylose Lysine Desoxycholate (XLD) Agar: Shigellae produce red-pink colonies without black centers.
  3. Salmonella-Shigella (SS) agar: Despite its name, this medium is not suitable for isolating shigellae as it is inhibitory to most strains.
  4. On the Triple Sugar Iron (TSI) agar test, they cause an alkaline slant and an acid butt, with no gas and no hydrogen sulphide. Confirmation of the organism as Shigella and determination of its group is done by slide agglutination test.

Biochemical properties of Shigellae

  1. Catalase +ve (Shigella dysenteriae type 1 is always catalase -ve)
  2. Lactose negative (S. sonnei is a late lactose and sucrose fermenter)
  3. Hydrogen sulphide production: Negative
  4. Urease test: Negative
  5. Oxidase test: Negative
  6. Citrate utilization test: negative
  7. Lysine decarboxylase (LDC) test: Negative
  8. Ornithine decarboxylase (ODC) test: Negative except S. sonnei which is ODC positive.
  9. Beta-galactosidase (ONPG) test: Negative
    S. sonnei and upto 15% Shigella dysenteriae type 1 strains and minority of S. boydii strains are ONPG positive.
  10. Acid produced from a number of carbohydrates.
    • Glucose is fermented with acid only (gas is produced by a few strains only)
    • Mannitol is fermented by all except S.dysenteriae
    • Lactose & sucrose are not fermented except by S.sonnei

Serology of Shigella species

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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