Identification of chitin as a structural component of Giardia cysts

The intestinal parasite Giardia lamblia is a significant cause of diarrheal disease, which is perpetuated by the infective cyst form of the parasite. Although a rational approach to the control of giardiasis would be to inhibit cyst formation, nothing is known of the chemical composition of the cyst wall or of its biosynthesis. In these studies, we have shown that chitin is a major structural component of G. lamblia and G. muris cyst walls. This conclusion is based on the finding that chitinase specifically destroys the cyst wall, as revealed by electron microscopy. The presence of chitin was also shown directly by lectin binding studies. Of 12 lectins with diverse carbohydrate recognition specificity, only the N-acetylglucosamine-specific lectins wheat germ agglutinin, succinylated wheat germ agglutinin, and tomato lectin bound to cyst walls, as shown by fluorescence microscopy and cytochemistry. Wheat germ agglutinin binding was completely abolished by treatment of the cysts with purified chitinase. This effect was specific since it could be prevented by incubating the enzyme with chitin before treatment of the cysts. Treatment of cysts with N-acetyl-beta-glucosaminidase partially inhibited wheat germ agglutinin binding, whereas other glycosidases and proteases had no effect. These findings indicate that chitin is a major structural component of Giardia cyst walls and raise the possibility that inhibitors of chitin synthesis may be of use in preventing encystation and thus controlling spread of the disease.