Shigella genus and shigellosis

Question 1

General features and habitat of Shigella Genus

Answer 1

Gram – (Rod shaped)

Obligate pathogen (Enterobacteriaceae)

Medically Important Shigella Species:
S. dysenteriae, S. boydii, S. flexneri (many serotypes) and S. sonnei (only 1 serotype)

Habitat- only human colon (not in animal’s colon)

Immotile

Question 2

Biochemical Properties of Shigella Genus

Answer 2

Facultative intracellular

Acid resistant- only few number of organisms are required to cause infection

Relatively inactive- Lactose non-fermenter; H2S and Urease negative

Question 3

Pathogenesis of Shigella Genus

Answer 3

Transmitted by fecal-oral spread, human-to-human contact (common in day-cares)

1. Bacteria are ingested and proceed to intestine 

2. Phagocytosed by M cells, but escapes before degrade in phagolysosome. Then it uses
   M cell’s actin filaments to propel itself to enterocytes found laterally
3. This allows Shigella to avoid the immune system and produce shallow ulcers that causes bloody diarrhea combined with Inflammation in colon and terminal ileum
   Shigella has few days (~1-4) of incubation before symptoms are seen

Shiga Toxin (enterotoxin, AB type)- mainly by S. dysenteriae (serotype 1)
The toxin is internalized, then inhibiting translation by cleaving part of 60S ribosomal subunit

Question 4

Clinical Features Shigellosis most severe form- dysentery

Answer 4

First symptom- watery diarrhea and inflammation

Later contain blood, mucus and pus (severe cases can contain tissue debris)

Fever, tenesmus (constant urge to pass stool but unproductive)

In children (<10y) may increase the risk for Hemolytic-Uremic Syndrome (HUS):

Causes glomerular damage

This activate platelet aggregation causing decrease in platelet count- thrombocytopenia

Ultimately lysis RBC causing bleeding (hemolysis)

In severe cases symptoms include: Hyperpyrexia, convulsion, meningeal signs and shock

Question 5

Diagnosis of Shigella Genus

Answer 5

Culture from- native stool specimen (blood, mucus, pus)

Deoxycholate Citrate Agar (DCA) medium: colorless colonies (E. coli shows red)

Salmonella-Shigella (Hektoen) medium: green colonies (to distinguish from salmonella)

Eosin-Methylene Blue (EMB) medium: pink colonies (lactase negative)

Toxic detection by Serény test- inoculation of shigella in rabbit shows keratoconjunctivitis

Question 6

Treatment of Shigella Genus

Answer 6

Trimethoprim and Sulfamethoxazole (TMP-SMX), Fluoroquinolones, 3rd gen. Cephalosporin

Question 7

Prevention of Shigella Genus

Answer 7

Proper sanitation and food hygiene