Centre for Research in Medical Entomology (ICMR), 4, Sarojini Street, Chinna Chokkikulam, Madurai-625 002, Tamilnadu (India).
Culex quinquefasciatus Say larvae were administered with Bacillus sphaericus (Bs) and Bacillus thuringiensis var. israelensis (Bti) spore toxin to understand ultrastructural changes in the midgut epithelial cells. Toxin exposed larvae exhibited a progressive poisoning, and finally mortality occurred by 24 and 48 h for Bti and Bs. Before toxin treatment, midgut epithelial cells were observed ultrastructurally, a well developed microvilli encomposed with quite large number of parallel microvillus and microfibrillar core at the basement region. Also, epithelial brush border membrane and cytoplasmic cell organelles were clearly distinguished below microvilli. These structural integrity of midgut epithelial cells were disturbed by Bs toxic effect and caused slow but steady gross ultrastructural changes in the microvilli. Formation of innumerable vacuoles in the cytoplasm, appearance of cytoplasmic filaments, mitochondrial swelling, discernible cisterna and progressive number of lysosomes by a rapid multiplication process were also observed. In the advanced stage (36 and 48 h), a total loss of cytoplasmic network was noticed in the cell and the cell organelles became empty by phagocytosis. Whereas, the structural integrity of cell was disturbed more rapidly whithin a short period (S to 10 h) of larval exposure to Bti toxin. A scanty number of microvilli were evident immediately after this toxin exposure and they become bubbled. A total collapse in cytoplasm was also evident by leaving behind loss of cytoplasmic network communication and cell debris. Lysosome proliferation and appearance of secondary lysosomes were not clear in the larvae as was seen in Bs treated larvae, while examining ultra thin serial sections. This suggested that both the bacterial strains, viz., B. sphaericus and B. thuringiensis var. israelensis were toxic to mosquitoes, but have separate entity in respect of mode of action to cause severe pathological lesions in the midgut epithelial cells.
