Genetic exchange between Bacillus thuringensis subsp. kurstaki and the indigenous microbiota in soils of Sardinia

Sine 1989, sprays of commercial preparations of Bacillus thuringiensis Berliner subsp. kurstaki (Btk) (usually a mixture of cells, spores, and parasporal crystals) have been used to defend the cork oak forests in Sardinia (Italy) against the defoliation by gypsy moth (Lymantria dispar L.). Up to now, the efficacy of the treatment rather than the possible effect on the environment has been evaluated. In a previous work we have demonstrated that Btk and its toxin introduced into soils in sprays can persist for long periods (at least 88 months for Btk and at least 28 months for its toxin), and there was no correlation between numbers of Btk and toxin detected in the Calangianus soil. In order to investigate the potential for genetic exchange of Btk with autochthonous bacteria, we have done the screening of 36 soil samples for each area treated with Bacillus thuringiensis subsp. kurstaki (Btk) in 1993, 1997, and 1998. Colonies of autochthonous bacteria, identified as B. mycoides, have been isolated from the soil treated five years after spraying, and the molecular analysis has indicated that they have acquired part of the cry1Ab gene from the released Btk. These results suggest that the gene transfer between introduced Btk and native bacteria can occur under natural field conditions and that the persistence of the Btk favours this genetic exchange.