diplomonads

Diplomonads are a group of flagellated, anaerobic protists that belong to the Excavata, commonly placed in the phylum Metamonada and the order Diplomonadida. The group includes several genera, most notably Giardia, with Giardia lamblia (also known as Giardia intestinalis or Giardia duodenalis) as the best known species. Diplomonads are primarily parasitic or commensal in the gastrointestinal tracts of vertebrates and some invertebrates.

Cell biology and morphology are distinctive. Diplomonads are typically binucleate, containing two nuclei, and possess multiple flagella. They have a simplified cytoskeleton and a ventral adhesive disc used to attach to the intestinal epithelium. They lack conventional mitochondria and instead retain reduced organelles called mitosomes, which perform limited mitochondrial-related functions such as iron-sulfur cluster assembly.

Life cycle and metabolism are adapted to an intestinal lifestyle. Diplomonads alternate between a motile trophozoite form in the host and environmentally resistant cysts excreted in feces. Trophozoites attach to the intestinal lining and reproduce by binary fission. Cysts are ingested in contaminated water or food, and excystation releases trophozoites in the host. They are strictly anaerobic and derive energy mainly from glycolysis and fermentation, with mitosomes reflecting their reduced energy metabolism.

Medical and ecological significance is centered on Giardia species, which cause giardiasis in humans and other animals. Transmission occurs through cyst-contaminated water or food and during person-to-person contact. Diagnosis uses stool antigen tests or microscopy, and treatment typically involves nitroimidazole drugs such as metronidazole or tinidazole, sometimes nitazoxanide. Control emphasizes safe drinking water and improved sanitation.