Giardia

The intestinal protozoan parasite Giardia duodenalis (syn. Giardia lamblia , Giardia intestinalis ) causes diarrhoea in humans and animals worldwide. The life cycle of G. duodenalis consists of two stages, the fl agellated trophozoite proliferating in the upper part of the small intestine and the non proliferative cyst representing the infectious stage of the parasite. Both stages can be handled in vitro and in vivo. Trophozoites are pear-shaped, motile cells exhibiting a convex dorsal and a concave ventral side. The cell body is formed by a microtubule cytoskeleton. The whole genome contained in two diploid nuclei per trophozoite has been sequenced and characterised. It has some prokaryote-like features such as short promoter sequences. Moreover, some key enzymes of energy and intermediate metabolisms share common features with prokaryotic enzymes and may have been acquired by lateral transfer. Giardia does not contain mitochondria and peroxisomes, but mitosomes, most likely an evolutionarily reduced version of a mitochondrion. The energy metabolism is chemoheterotrophic and works under anaerobic or semiaerobic conditions with glucose as main energy and carbon source and arginine as another important energy source. The present book chapter selectively reviews current knowledge in Giardia research highlighting its basic genetic, physiological and, to a lower extent, its immunological properties. Furthermore, this chapter also shows that G. duodenalis is a suitable cellular model system for the investigation of fundamental biological principles