29. The causative agents of dysentery (Shigella spp.). Classification. Morphology, biology, biochemical production. Antigenic composition. Pathogenesis and immunity. Microbiological diagnosis. Helicobacter pylori. Morphology, biology, biochemical production. Disease. Microbiological diagnosis. Therapy. Clostridium difficile. General characteristics.

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. Entry via ingestion → low infective dose (~100 organisms).
2. Invasion of M cells of Peyer’s patches in ileum.
3. Escape phagosome → replicate in cytoplasm → spread to adjacent epithelial cells.
4. Induce apoptosis in macrophages → release of IL-1 → inflammation.
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.