Microbiology · MU Sofia

56. Genus Shigella.

Covers the general characteristics of Shigella as Gram-negative, non-motile rods and how they are told apart from Salmonella, followed by the four species grouped into subgroups A to D in a table. It then takes virulence factors, including Shiga toxin, through pathogenesis, epidemiology and the clinical forms of shigellosis, bacillary dysentery and haemolytic uremic syndrome. Diagnosis, treatment with antibiotic options and prevention close the note.

Inside: 10 sections · 6 labelled pictures · 3 tables · 16 flashcards · 3 min

Topic 56 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 10 sections, about 323 words (3 min of reading), with 16 flashcards.

In this note

  1. Genus Shigella
  2. General Characteristics
  3. Classification
  4. Virulence Factors
  5. Pathogenesis
  6. Epidemiology
  7. Diseases
  8. Diagnosis
  9. Treatment
  10. Prevention

General Characteristics

  • Gram-negative rods (G–ve), non-spore forming.
  • Non-motile (key distinguishing point from Salmonella).
  • Do not form capsules.
  • Facultative anaerobes.
  • Belong to the family Enterobacteriaceae.
  • Oxidase (-), Lactose non-fermenters, H₂S (-) — helps differentiate from Salmonella.
  • Grow well on selective and differential media like MacConkey agar (lactose – = pale colonies) and DCA (do not produce H₂S → no black center).
  • Do not ferment mannitol (except S. sonnei which ferments it slowly).

Classification

  • 4 main species (grouped into subgroups A–D):

Pathogenesis

  1. 1. Entry via ingestion → low infective dose (~100 organisms).
  2. 2. Invasion of M cells of Peyer’s patches in ileum.
  3. 3. Escape phagosome → replicate in cytoplasm → spread to adjacent epithelial cells.
  4. 4. Induce apoptosis in macrophages → release of IL-1 → inflammation.
  5. 5. Shiga toxin (if produced) → damages intestinal epithelium and endothelial cells (kidneys, CNS) → possible Hemolytic Uremic Syndrome (HUS).

Epidemiology

  • Reservoir: Humans only (no animal reservoir unlike Salmonella).
  • Spread via faecal-oral route — contaminated food/water, direct contact, poor hygiene.
  • Highly contagious, especially in crowded places like schools, day-cares, refugee camps.
  • More common in children under 5 years.

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