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Plasmid transfer between Bacillus thuringiensis subsp. israelensis strains in laboratory culture, river water, and
D J Thomas1, J A Morgan, J M Whipps
1Department of Plant Pathology and Microbiology, Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, United Kingdom.
Abstract:
Plasmid transfer between strains of Bacillus thuringiensis subsp. israelensis was studied under a range of environmentally relevant laboratory conditions 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 water conditions, and the maximum transfer ratio was 10(-3) transconjugant per recipient under optimal conditions. Transfer of conjugative plasmid pXO16::Tn5401 was also detected under this range of conditions. However, a maximum 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 this plasmid and the formation of spores by the introduced donor and recipient strains. In contrast, transfer of plasmid pXO16::Tn5401 was detected in water, 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 and 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 reduced transfer rate in insects compared to broth cultures may be accounted for by competition from the background bacterial population present in the mosquito gut and diet or by the maintenance of a large population of B. thuringiensis spores in the insects.
Insights
Plasmid transfer in Bacillus thuringiensis subsp. israelensis was observed in vitro and in mosquito larvae, with conjugative plasmids showing higher transfer rates than mobilizable ones, especially in aquatic environments.
Area of Science:
- Microbiology
- Genetics
- Environmental Science
Background:
- Plasmid transfer is a key mechanism for genetic exchange in bacteria.
- Bacillus thuringiensis subsp. israelensis (Bti) is an important entomopathogen used for mosquito control.
Purpose of the Study:
- To investigate plasmid transfer dynamics of Bti under various environmental conditions.
- To compare the transfer efficiency of different plasmid types (mobilizable vs. conjugative).
Main Methods:
- In vitro studies under varying temperatures, pH, and water availability.
- Experiments conducted in natural river water.
- Plasmid transfer assays in killed mosquito larvae.
Main Results:
- Mobilizable plasmid pBC16 showed low transfer in vitro (10^-3) and no transfer in river water or larvae.
- Conjugative plasmid pXO16::Tn5401 exhibited high transfer in vitro (1.0) and detectable transfer in river water (10^-2) and larvae (10^-3).
- Plasmid transfer in river water was not linked to bacterial growth, suggesting new transfer events.
Conclusions:
- Conjugative plasmids are more likely to transfer between Bti strains in diverse environmental conditions compared to mobilizable plasmids.
- Environmental factors significantly influence plasmid transfer efficiency.
- Understanding plasmid transfer is crucial for assessing the ecological impact and evolution of Bti strains.