Evaluation of the Toxicity of Supernatant Cultures and Spore-Crystal Mixtures of Bacillus thuringiensis Strains Isolated from Algeria.

Abstract

Bacillus thuringiensis (Bt) is the most used technology for biological control of insect pathogens worldwide. In order to select new Bt candidates challenging the emergence of insect's resistance, a mass bioassay and molecular screening was performed on an autochthonous collection. Toxicity assays against neonate larvae of three lepidopteran species (Mamestra brassicae, Grapholita molesta, and Spodoptera exigua) were conducted using spore-crystal mixtures and supernatant cultures of 49 Bt isolates harboring at least one gene coding for a lepidopteran-specific insecticidal protein. A threshold of 30% of "functional mortality" was used to discriminate between "nontoxic" and "toxic" isolates. The toxicity of many Bt isolates competed with that of Btk-HD1. However, only three of them (Bl4NA, Bl5NA, and Bl9NA) showed high toxicity in both spore-crystal mixtures and supernatant cultures against the three lepidopteran species. The Bt isolates Bl4NA and Bl9NA express a protein of 130kDa whereas the Bt isolate Bl5NA expresses a protein of 65-70kDa. The LC-MS/MS results indicate that the major peptides in the 130kDa band of Bl9NA were Cry1Da, Cry1Ca, Cry1Ab, and Cry1Aa, and those in the 70kDa band of Bl5NA were Cry1Aa and Cry1Ca. The evaluation of the protein content of the supernatants by comparison to Btk-HD1 indicates the overproduction of Vip3 proteins in these strains (most likely Vip3Aa in Bl4NA and Bl9NA and Vip3Ca in Bl5NA). In addition, these three Bt strains do not produce β-exotoxins. Based on our results, the three selected strains could be considered promising candidates to be used in insect pest control.