1. IDENTIFYING CHARACTERISTICS
The above Picture is of Giardia lamblia trophozoites, as they appear with the scanning electron microscope. (Original image by Arturo Gonzalez, CINVESTAV, Mexico. Used with permission of Javier Ambrosio, UNAM, Mexico.)
2. TAXONOMIC DESCRIPTIONGiardia is regarded as the most common flagellate in the human digestive tract and is highly contagious, especially in its resting cyst stage. This protozoa has a characteristic tear-drop shape and measures 10-15 micrometers in shape. It has twin nuclei that give it the appearance of having "eyes". Giardia also has an adhesive disk which is a rigid structure reinforced by supelicular microtubules. There are two median bodies that are important in differentiating between Giardia species. Also, Giardia has 4 pairs of flagella, one anterior pair, two posterior pairs, and a caudal pair. Giardia have no mitochondria, endoplasmic reticulum, golgi, or lysosomes. They have two distinct life stages. As trophozoites, they are motile and swim free and rapidly in a spiral motion in the intestinal lumen of animals. They also attach, via their adhesive disc, to the intestinal epithelium. As a cyst, the Giardia resting stage, they can withstand harsh environments and stay dormant for weeks to months. Giardia reproduce by binary fission and attachment to a surface, such as the intestinal epithelium, is required for this to occur. Their main food source is glucose. It is obtained by a process of diffusion or by pinocytosis. They are aerotolerant anaerobes and require a reducing environment. Food reserves are stored as glycogen. Glucose catabolism via the glycolytic pathway results in production of ethanol, acetate, and carbon dioxide.
The above picture is another example of a Giardia lamblia trophozoite.
The two nuclei are easy to see in this image. (Original
image from a Japanese language site tentatively titled "Internet Atlas of
Human Parasitology.")
Selective media are available and, on isolation, Giardia colonies are nearly colorless. However, off white, and light pink pigmentation have also been observed. There are many ways to isolate Giardia. A few methods for isolating the trophozoite stage is the isolation and purification by nylon fiber columns, and isolation by vibration and sedimentation. A few methods for isolating the cyst stage are the Rendtorff method, and sucrose density gradient centrifugation
Widely distributed in nature, Giardia species are pathogenic for humans
and animals.
These parasites are located in the duodenum, jejunum, and upper
ileum. Giardia is transmitted to humans primarily through
contaminated water. The cyst is the form that the disease is usually
transmitted in. Cysts germinate in the gastrointestinal tract and bring
about the symptoms of giardiasis.
The above is a representative painting of Giardia by Russell Kightley
Erlandson, Stanley L. and Ernest A. Meyer. 1984. Giardia and Giardiasis: biology, pathogenesis, and epidemiology. Plenum Press. New York City.
Meyer, E.A. 1990. Giardiasis. Solemun Press. Amsterdam; New York; Elsevier.
Salfeder, Karlhanns. 1992. Atlas of Parasitic Pathology. Kluwer Academic Publishers. Boston; Dordrecht.
Thompson, R.C.A., J.A. Reynoldson and A.J. Lymbrey. 1994. Giardia: from molecules to disease. Oxon Press. CAB International
Check out the Bad Bug Book at http://vm.cfsan.fda.gov/~mow/chap22.html This site contains The U.S. Food and Drug Administration's guidelines and thoughts in Giardia lambilia.
Another good site is http://www.geocities.com/CollegePark/Lab/4551/
. This site discusses the evolutionary aspects of Giardia, and
recent discoveries regarding these amazing protozoa.
| MICROBES | SOILS | CYCLES | APPLICATIONS |