Research on natural strains of Bacillus thuringiensis in different biotopes throughout the world

A screening programme to isolate new strains of the entomopathogenic bacteria Bacillus thuringiensis was undertaken on 4887 samples of various sources from 101 countries over the world: 1260 strains of the bacillus were isolated. Dust from mills and silos, as well as insects from nature, were more successful sources than soil samples, which emphasizes the diversity of biotopes where the bacillus is encountered. Electrophoretic characterization reveals the genetic variability of the species. An analysis of insecticidal properties of the isolated strains was performed on four insect species: Plutella xylostella (Lepidoptera: Plutellidae), Spodoptera littoralis (Lepidoptera: Noctuidae), Phaedon cochleariae (Coleoptera: Chrysomelidae), and Locusta migratoria or Schistocerca gregaria (Orthoptera: Acrididae). The most frequent strains (54%) were producing crystals constituted of proteins with molecular masses of 130-140 or 66 and 130-140 kDa and were toxic to Lepidoptera larvae. A significant number of strains (31) were larvicidal to Coleoptera while only one, H14 serotype, was active on Diptera. Numerous strains synthesize crystals made up of proteins with size differing from the already known toxins. Most of these strains were nonactive against the four insect species tested. One strain showing a protein band at 73 kDa had no insecticidal activity against P. cochleariae while it was toxic to P. xylostella.