Giardia

Giardia ( or ) is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada that colonise and reproduce in the small intestines of several vertebrates, causing the disease giardiasis. Their life cycle alternates between a binucleated motile trophozoite and an infective, metabolically inert, environmentally resistant tetranucleate cyst. Cysts are transmitted between hosts through the fecal–oral route, contaminated water and/or food. Giardia were first seen by the Dutch microscopist Antonie van Leeuwenhoek in 1681 under the light microscope. The genus is named after French zoologist Alfred Mathieu Giard.

== Characteristics == Giardia trophozoites are 12–15 μm long and 5–9 μm wide and have a shape of a pear bisected lengthwise. Like other diplomonads, Giardia has two transcriptionally operational nuclei that contain an equal number of well-defined chromosomes and replicate synchronously with cell division. The cytoskeleton of Giarida consists of a median body, 4 pairs of flagella (anterior, ventral, posterior and caudal) and an adhesive disc. Giardia lacks canonical mitochondria and Golgi complexes; instead, it possesses an endomembrane-vesicle system as well as reduced mitochondria, called mitosomes. These mitosomes are involved in the maturation of iron-sulfur proteins and do not participate in ATP synthesis. The adhesive disc, located on the ventral side, allows the parasite to attach itself to the host's intestinal epithelium. Trophozoites multiply via binary fission in the small intestine and encyst during the passage towards the large intestine.