Bacillus thuringiensis (Bt) is a spore-forming bacterium used as a bio-pesticide due to its insect toxicity. Bt was used as a model organism in a risk evaluation project. Prevalence, survival and spreading, as well as genome stability, putative virulence genes, and gene transfer were examined. Field release of a marked Bt subsp. israelensis (Bti) strain resulted in a minor, transient increase of Bt-like bacteria.

The bacterial population structure returned to as before release after seven weeks (paper I). Physical chromosome maps were established for Bt subsp. gelechiae, alesti and kurstaki using pulsed-field gel electrophoresis (PFGE). Co-expressed virulence genes are scattered on the chromosome and are not located in operons or pathogenicity islands (paper II). Bt subsp. alesti and kurstaki have identical chromosome maps although their serotypes differ. Sequences homologous to Bacillus cereus enterotoxin genes were found on both chromosomes and on a plasmid in Bt subsp. alesti/kurstaki (paper III). A 5.2 kb region from Bt subsp. alesti was cloned and sequenced, and found to contain three flagellin genes constituting a fla-operon, a putative flagellar motor switch protein gene fliM, and a transglycosylase-like protein gene tlp.

Deletion mutants in the region are avirulent and do not express previously identified virulence factors (paper IV). Chromosomal genes were transferred by transduction to Bt soil isolate strains, using a bacteriophage originating from soil (paper V).