Lectin binding by Giardia lamblia

Surface carbohydrates of Giardia lamblia were examined using six plant lectins chosen because of their specificity for major carbohydrates moieties. The binding to axenically grown G. lamblia trophozoites was assessed in both a quantitative microagglutination assay and a fluorescence assay. Of the six lectins tested, wheat germ agglutinin (WGA) agglutinated the highest percentage (22.9 +/- 3.7%) of live trophozoites, and fluorescein-labeled WGA (100 micrograms/ml) bound to 98 +/- 5% of them. Since the carbohydrate specificity of WGA includes both N-acetyl-D-glucosamine (GlcNAc) and sialic acid, inhibition experiments were performed. GlcNAc inhibited he binding of WGA to G. lamblia in both assays to a greater extent than did sialic acid. Binding of WGA was not altered by prior treatment of trophozoites with neuraminidase, suggesting that WGA was binding to GlcNAc moieties on G. lamblia and not to sialic acid. The remaining five lectins either bound nonspecifically or exhibited low percentages of binding. The apparent presence of GlcNAc but not sialic acid or other exposed surface carbohydrates may be important in the interaction of G. lamblia with its human host.