1. Dr Sabrina Moyo Department of Microbiology and Immunology
Shigella
Dr Sabrina Moyo
Department of Microbiology and Immunology

2. Definition An enterobacteriaceae Gram negative bacilli.
Readily growth O2 + An O2.
Metabolically active, fermenting a variety of substrates.
Mostly non-motile, non sporing, non acid fast, 2-4um x um rounded ends.

3. Morphology & Physiology
  Small Gram-negative, facultatively anaerobic, coliform bacillus
  Non-motile (no H antigen)
  Possess capsule (K antigen) and O antigen
  K antigen not useful in serologic typing, but can interfere with O antigen determination
  O antigens: A, B, C, D correspond respectively to the four species
  Non-lactose fermenting
  Bile salts resistant: trait useful for selective media
ferment glucose
reduce nitrates (NO3 to NO2 or N2)
are oxidase negative

4. Taxonomy Family Enterobacteriaceae
Shigella dysenteriae: most serious form of bacillary dysentery
Shigella flexneri: shigellosis in underdeveloped countries
Shigella sonnei: shigellosis in developed countries
Shigella boydii

5. Taxonomy
Classification: 4 groups or species on the based on difference in O antigen and some biochemical reactions
Group A - S. dysenteriae 1-10
group B - S. flexneri 1-6
group C - S. boydii 1-15
group D - S. sonnei 1

6. Clinical Syndromes (Shigellosis)
  Ranges from asymptomatic infection to severe bacillary dysentery
  Two-stage disease: watery diarrhea changing to dysentery with frequent small stools with blood and mucus, tenesmus, cramps, fever
Early stage:
  Watery diarrhea attributed to the enterotoxic activity of Shiga toxin
  Fever attributed to neurotoxic activity of toxin  

7. Clinical Syndromes Process involves: 1. Ingestion
2. Non-invasive colonization and cell multiplication
3. Production of the enterotoxin by the pathogenic bacteria in the small intestine;
 Second stage:
  Adherence to and tissue invasion of large intestine
  Typical symptoms of dysentery
  Cytotoxic activity of Shiga toxin increases severity

8. Epidemiology
  Shigellosis is a major cause of diarrheal disease (developing nations)
  Major cause of bacillary dysentery (severe second stage form of shigellosis)
   Leading cause of infant diarrhea and mortality (death) in developing countries

9. Epidemiology Shigella occurs naturally in higher primates
Spread from human to human via the fecal-oral route
 Less frequently, transmission by ingestion of contaminated food or water
  Outbreaks usually occur in close communities;
  Secondary transmission occurs frequently

10. Epidemiology
 Low infectious dose ( CFU) with 1-3 day incubation period
Carriage of the organism persists for approximately one month following convalescence

11. Pathogenesis & Immunity
Invasiveness involves attachment (adherence) and internalization
Controlled by a multi-gene virulence plasmid
Organisms penetrate through colonic mucosa
invade and multiply in the colonic epithelium
Not beyond the epithelium into the lamina propria

12. Pathogenesis & Immunity
  Bacterial cells preferentially attach to and invade into M cells in Peyer's patches of small intestine
M cells typically transport foreign antigens from the intestine to underlying macrophages,
Shigella can lyse the phagocytic vacuole (phagosome) and replicate in the cytoplasm

13. Pathogenesis & Immunity
Exotoxin (Shiga toxin) is neurotoxic, cytotoxic, and enterotoxic, encoded by chromosomal genes,
Enterotoxic effect: Shiga toxin adheres to small intestine receptors
Blocks absorption (uptake) of electrolytes, glucose, and amino acids from the intestinal lumen

14. Pathogenesis & Immunity
Cytotoxic effect: B subunit of Shiga toxin binds host cell glycolipid in large intestine,
Inactivate the 60S ribosomal subunit,
Inhibit protein synthesis, causing cell death, microvasculature damage to the intestine, and hemorrhage (blood and fecal leukocytes in stool)
  Neurotoxic effect: Fever, abdominal cramping are considered signs of neurotoxicity

15. Laboratory Identification:
Closely related to Escherichia
Species (serogrouping and biochemical analysis
Stool specimens and rectal swabs should be cultured soon after collection or placed in appropriate transport medium (Cary-Blair medium)
Readily isolated on selective/differential agar media (XLD, SS, and brilliant green agar
Lactose nonfermenter

16. Treatment, Prevention & Control:
Dehydration is problem to attend
Treat carriers, major source of organisms; Cirpflxacin , Erythromycin
Antibiotic resistance is a major problem
Proper sewage disposal and water chlorination
Oral vaccines of Shigella: E. coli hybrids or Shigella mutants offers immunity for six months to one year

17. THANK YOU!

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