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RESEARCH PRODUCT

Distribution, frequency and diversity of Bacillus thuringiensis in olive tree environments in Spain

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Summary Bacillus thuringiensis was isolated from samples collected from olive tree related habitats (olive groves, olive storage facilities and oil mills) of different olive producing regions in Spain. This bacterium was found in a high percentage (92%) of samples. From 72 samples analysed, 2244 sporulating colonies were selected and observed under phase contrast microscopy. From these, 414 colonies (18%) were classified as B. thuringiensis based on the production of parasporal crystals. A great variability of spore, crystal size and morphology was observed. The results indicate that olive groves are as rich as soil olive mills and olive storage facilities regarding the B. thuringiensis population. Analysis by SDS-PAGE of the proteins in the parasporal crystal showed a considerable heterogeneity between B. thuringiensis strains. This was especially relevant in samples obtained from oil mills and storage centres, indicating the high polymorphism in this species. Strains isolated from the same sample showing the same SDS-PAGE profile were not considered different, and just one of them was selected for further tests. A total of 188 out of the 414 B. thuringiensis isolates were shown to be different strains and tested for their toxicity against adult animals of the olive fly Bactrocera oleae. Approximately 40% of the strains caused similar or higher mortality than the dipteran active strain B. thuringiensis israelensis (45% mortality). Most strains (71%) producing a toxicity higher than 65% came from olive mills and olive storage facilities. Three of these strains exhibited a mortality higher than 70% in our test conditions.