Application of Bacillus thuringiensis strains with conjugal and mobilizing capability drives gene transmissibility within Bacillus cereus group populations in confined habitats.

Background: Bacillus thuringiensis bacteria share similar genetic, physiological, and biochemical characteristics with other members of the Bacillus cereus group. Their diversity and entomopathogenic origin are related to their mobile genetic elements. However, the effects of wide-spread application of B. thuringiensis-based pesticides on genetically related B. cereus group populations present in the environment remain poorly understood.

Results: We first identified pBMB76 from B. thuringiensis tenebrionis as a new conjugative plasmid. Mixed mating experiments suggested that pBMB76 may compete with pHT73, another known conjugative plasmid. Applications of single (tenebrionis 4AA1 and kurstaki HD73 carrying pBMB76 and pHT73, respectively) and mixed (4AA1 + HD73) B. thuringiensis strains were performed in confined plot habitats (soil and leaf) over two planting seasons. In total, 684 B. cereus group isolates were randomly selected from different treatment sets, and the transmissibility and occurrence rate of potential conjugative plasmids were surveyed. Results showed that the percentage of isolates with plasmid mobility was markedly enhanced in the B. thuringiensis-sprayed groups. Furthermore, we performed multi-locus sequence typing (MLST) for a subset of 291 isolates, which indicated that the dominant sequence types in the treated habitats were identical or related to the corresponding sprayed formulations.

Conclusions: The application of B. thuringiensis strains with conjugal and mobilizing capability drove gene transmissibility within the B. cereus group populations in confined habitats and potentially modified the population structure.