Plasmid transfer between Bacillus thuringiensis subsp israelensis strains in laboratory culture, river water, and dipteran larvae

Plasmid transfer between strains of Bacillus thuringiensis subsp. israelens is was studied under a range of environmentally relevant laboratory conditi ons in vitro, in river water, and in mosquito larvae. Mobilization of pBC16 was detected in vitro at a range of temperatures, pH values, and available mater conditions, and the maximum transfer ratio was 10(-3) transconjugant per recipient under optimal conditions. Transfer of conjugative plasmid pX O16::Tn5401 was also detected under this range of conditions. However, a ma ximum transfer ratio of 1.0 transconjugant per recipient was attained, and every recipient became a transconjugant. In river water, transfer of pBC16 was not detected, probably as a result of the low transfer frequency for th is plasmid and the formation of spores by the introduced donor and recipien t strains. In contrast, transfer of plasmid pXO16::Tn5401 was detected in w ater, but at a lower transfer ratio (ca. 10(-2) transconjugant per donor). The number of transconjugants increased over the first 7 days, probably as a result of new transfer events between cells, since growth of both donor a nd recipient cells in water was not detected. Mobilization of pBC16 was not detected in killed mosquito larvae, but transfer of plasmid pXO16::Tn5401 was evident, with a maximum rate of 10-3 transconjugant per donor. The redu ced transfer rate in insects compared to broth cultures may be accounted fo r by competition from the background bacterial population present in the mo squito gut and diet or by the maintenance of a large population of B. thuri ngiensis spores in the insects.