Variation in binding of Bacillus sphaericus toxin and wheat germ agglutinin to larval midgut cells of six species of mosquitoes.

Bacillus sphaericus toxin labeled with fluorescein isothiocyanate was readily ingested by Culex pipiens, Aedes aegypti, Anopheles stephensi, Anopheles gambiae, Anopheles quadrimaculatus, and Anopheles albimanus larvae. Fluorescent toxin bound to the luminal cell surface in discrete regions of the posterior midgut and gastric caecum in C. pipiens. In Anopheles spp., toxin bound in a variable pattern to these structures and central and anterior midgut as well. The toxin did not bind to midgut cells of A. aegypti. The toxin was internalized in bright fluorescent vesicles in C. pipiens, but was not internalized in Anopheles spp. and appeared to be weakly bound in these larvae, leaking rapidly from the gut surface. The lectin, wheat germ agglutinin, which interferes with binding of the B. sphaericus toxin, bound to the posterior midgut and gastric caecum of all species, but was not internalized. These results suggest that the sugar moiety of the receptor is not solely responsible for specificity of this toxin, and that binding to Culex spp. midgut cells may be highly specific and of high affinity, whereas binding to Anopheles spp. cells may be nonspecific and/or of low affinity.