Elimination of Gut Bacteria from Helicoverpa armigera (Lepidoptera: Noctuidae) Using Antibiotics Reduces the Binding and Pore-Forming Activity of Cry Toxins

Abstract

Bacillus thuringiensis (Bt) has been used as a biopesticide for controlling lepidopteran pests, including the cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae). To eliminatethe gut bacteria, the larvae were reared on diet containing an antibiotic cocktail composed of seven antibiotics (gentamycin, neomycin, chloramphenicol, ampicillin, streptomycin, rifampicin, and penicillin). In presence of Cry1Ac/Ab, the diet absorption studies documented an increase in relative consumption rate (RCR), approximate digestibility (AD), efficiency of conversion of digested food (ECD), and relative growth rate (RGR) in larvae reared on antibiotics compared to larvae reared on an artificial diet. Activities of aminopeptidase-N (APN) and alkaline phosphatase (ALP) significantly increased in larvae maintained on an artificial diet treated with Cry1Ac/Ab toxins. However, in presence of Cry1Ac/Ab, the APN and ALP activities were reduced in larvae reared antibiotics. In vitro binding and pore-forming activity of the Cry toxins with the brush border membrane vesicles (BBMVs) was significantly reduced in larvae reared on antibiotics compared to the larvae reared on an artificial diet alone. We infer that compared to the H. armigera larvae having gut bacteria, the toxicity of Cry proteins was minimized in larvae sans bacteria, which, in turn, leads to reduced susceptibility of larvae toward Bt toxins.