Immunological localization of Bacillus thuringiensis serovar israelensis toxins in midgut cells of intoxicated Anopheles gambiae larvae (Diptera: Culicidae).

Fourth instar larvae of Anopheles gambiae were intoxicated with doses of purified crystals from Bacillus thuringiensis serovar israelensis (Bti) corresponding to 50-fold the LC50 after 24 h. Midguts were dissected after various contact times, then processed for immuno-light and -electron microscopy. Immunodetection on thin sections was performed using affinity-purified rabbit IgG against Bti crystal CryIVD or CytA polypeptides, in combination with anti-rabbit IgG/peroxidase. Both polypeptides were detected by optical and electron microscopy after 15 min of contact with Bti crystals on the apical brush border of midgut cells, but only in the gastric caeca and posterior stomach. No specific signal was detected in the other parts of the midgut, i.e. the cardia cells and the anterior stomach. These results confirm that mosquito midgut cells are the primary target for the toxins and that binding to specific receptors on the apical microvilli membrane is the initial step of delta-endotoxin action.