Transfer of plasmid pBC16 between Bacillus thuringiensis strains in non-susceptible larvae

Plasmid transfer was investigated in larvae of insects of the orders Coleoptera, Diptera, and Lepidoptera. The effects of introducing Bacillus thuringiensis strains in live non-susceptible larvae, and in the presence of added insecticidal toxins to kill the larvae, were examined. Plasmid transfer was not detected as the strains passed through non-susceptible live larvae, but was detected when the larvae were toxin-killed. The results indicate that growth of B. thuringiensis and plasmid transfer between strains while simply passing through an insect gut system is an infrequent event. In toxin-killed larvae, a more complex picture was recorded. B. thuringiensis subsp. kurstaki transferred pBC16 at a lower rate in killed Phaedon cochleriae larvae compared to previous work studying transfer with this strain in susceptible Lacanobia oleracea larvae. Similarly, B. thuringiensis subsp. tenebrionis transferred pBC16 in killed L. oleracea larvae, while no transfer in susceptible P. cochleriae larvae was detected. The results indicate that gene transfer was more frequent in killed L. oleracea larvae. When both B. thuringiensis strains were studied in Aedes aegypti, transfer of pBC16 was detected in toxin-killed larvae. This was surprising since in similar studies with strain B. thuringiensis subsp. israelensis that kills mosquitoes, transfer of pBC16 was not detected in mosquito cadavers. The improved transfer frequency of B. thuringiensis subsp. kurstaki and subsp. tenebrionis compared to B. thuringiensis subsp. israelensis in laboratory broth culture could account for this difference in detection of transfer within killed insects.