Midgut microbes influence the biological activity and binding of Cry toxins with BBMV in the midgut of Helicoverpa armigera

Helicoverpa armigera is one of the most important crop pests worldwide. Bacillus thuringiensis (Bt) toxin proteins have been deployed in transgenic plants for controlling insect pests. However, there is a possibility of development on insect resistance to Bt toxins deployed in transgenic plants. Therefore, we studied the biological activity of Bt toxins in  H. armigera larvae reared on diets with and without antibiotics. The H. armigera larvae reared on diets with antibiotics are resistant to Cry toxins as compared to the larvae reared on diets without antibiotics. There was a significant increase in food consumption, approximate digestibility, and efficiency of conversion of digested food in body matter in H. armigera larvae fed on diets with antibiotics. Trypsin activated Cry toxin showed greater binding affinity with the brush border membrane vesicles (BBMV) in the midgut than the chymotrypsin activated toxin. Decreased activity of aminopeptidase and alkaline phosphatase was observed in the larvae reared on diets with a cocktail of antibiotics + Cry1Ac or Cry1Ab as compared to the larvae reared on diets without antibiotics. Binding and pore forming activity of Cry1Ac and Cry1Ab to BBMVs was also studied by using FITC-labelled Cry toxins and voltage-sensitive dye 3, 3’-dipropylthiodicarbocyanine iodide, respectively. In BBMVs of H. armigera prepared from larvae reared on diets with antibiotics, there was reduced binding of Cry1Ac and Cry1Ab to the BBMV than the BBMV prepared from the larvae reared on artificial diet without anibiotics, suggesting that midgut microbes play a significant role in biological activity of Bt toxins, which has implications for development of insect resistance to Bt transgenic crops in future.