Binding of the Bacillus sphaericus (Eubacteriales: Bacillaceae) Toxin to Midgut Cells of Mosquito (Diptera: Culicidae) Larvae: Relationship to Host Range

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Abstract
Fluorescent-labelled Bacillus sphaericus (Neide) toxin was found to bind to sharply delineated regions of the proximal lobes of the gastric caecum and the posterior midgut of susceptible Culex quinquefasciatus Say larvae but did not bind to midgut cells of resistant Aedes aegypti (L.) and A. triseriatus (Say) larvae. Failure of this toxin to bind to midgut cells of certain mosquito species may be responsible for the restricted host range of B. sphaericus . The toxin appeared to enter the cells of C. quinquefasciatus larvae by receptor-mediated endocytosis, and it accumulated in the cells in bright fluorescent endocytic vesicles. Binding of fluorescent toxin to C. quinquefasciatus midgut cells and activity of the toxin to these larvae in bioassays was reduced by a lectin, wheat germ agglutinin. C. quinquefasciatus larvae ingested the toxin from clean water, and toxin passed through a column of kaolin in the larval gut to bind to midgut cells and intoxicate the larvae. A. aegypti larvae similarly ingested fluorescent-labelled toxin and other proteins from clean water. These results indicate that these larvae drink the surrounding medium.

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