Giardia: An Anaerobic Parasite Without Mitochondria or Respiratory System

Giardia lamblia (also G. intestinalis, G. duodenalis) is a **flagellated protozoan parasite** with unique biology:

• No mitochondria: Possesses **mitosomes** (reduced organelles for iron‑sulfur cluster assembly).
• No oxidative phosphorylation: Strictly **anaerobic** – generates ATP via substrate‑level phosphorylation (glycolysis, arginine dihydrolase pathway).
• No respiratory chain: Does not consume O₂; lacks cytochromes.
• Microaerophilic tolerance: Survives low O₂ in proximal small intestine.

Metabolic Implications

• Dependent on host luminal nutrients (glucose, arginine).
• Produces acetate, ethanol, CO₂, H₂ as end products.
• Sensitive to oxygen – cultured anaerobically.

Clinical Relevance

• Infects duodenum/jejunum → malabsorption, diarrhea, steatorrhea, weight loss.
• Transmission: cysts in contaminated water/food, fecal‑oral.
• Treatment: Metronidazole, tinidazole, nitazoxanide (target anaerobic metabolism).

Giardia’s anaerobic metabolism is a therapeutic target and explains its niche in the upper small intestine.