Toxicity of Bacillus thuringiensis β-exotoxins and δ-endotoxins to Drosophila melanogaster, Ephestia kuhniella and human erythrocytes
A total of 73 Bacillus thuringiensis (Bt) strains were screened for the presence of non-hemolytic insecticidal β-exotoxin-free δ-endotoxins. Out of them, 45 Bt strains produced δ-endotoxins with specific insecticidal activity against Drosophila melanogaster and/or Ephestia kuhniella larvae. The thermostable β-exotoxin was observed only in 15 Bt strains and appeared to exhibit dual non-specific insecticidal activity against both D. melanogaster and E. kuhniella larvae and showed in vitro hemolysis for human erythrocytes. It was found that β-exotoxin was produced by Bt strains belonging to five serovars (israelensis, kenyae, kurstaki, pakistani, and tohokuensis) and two non-serotypable strains. This result suggests that β-exotoxin production is a strain-specific property rather than a serovar-specific property. To our knowledge, this is the first study that demonstrates β-exotoxins production association with Bt strains belonging to serovars israelensis, pakistani, and tohokuensis. The plasmid DNA profiles of some β-exotoxin producing Bt strains shared large plasmid patterns which may have the common β-exotoxin regulatory gene(s). It was found that 16 local Bt strains, 15 of which belonged to five serovars (aizawai, israelensis, kurstaki, morrisoni, and pakistani) and one was autoagglutinated strain, produced non-hemolytic insecticidal β-exotoxin-free δ-endotoxins. Based on random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR), the genotypic relatedness among these 16 Bt strains was investigated. The strains were grouped into two clusters. Bt strains within serovars israelensis were grouped in two subclusters, Bt strains within serovars aizawai were genomically homogeneous and clustered together, while the other serovars were grouped together in one subcluster. The autoagglutinated strain was clustered within serovar israelensis. Thus, these δ-endotoxins can be developed for the use in Bt-based insecticidal preparations.
Keywords: Endotoxin, exotoxin, hemolytic, serovar, thuringiensis