Genome stability of Bacillus thuringiensis subsp israelensis isolates

Swedish soil isolates biochemically classified as Bacillus thuringiensis su bsp. israelensis were further examined for genetic diversity by multilocus enzyme electrophoresis (MLEE), random amplified polymorphic DNA analysis (R APD), pulse field gel electrophoresis (PFGE), and Southern blotting, and we re compared with reference strains. All the tested strains belonging to the Br. israelensis serotype H14 were found to be identical, as judged from th e RAPD analysis. MLEE analysis gave a similar result; only one H14 strain w as found to differ from the remaining H14 strains by one null allele. PFGE analysis confirmed a very close relationship between the H14 strains but re vealed an SfiI restriction fragment of variable size. Southern blot analyse s were carried out with probes for the chromosomally encoded flagellin gene (s) and the plasmid-encoded mosquitocidal toxins. All probes gave similar h ybridization patterns in the H14 strains. The mosquito toxin probes hybridi zed only to the H14 strains, except for one probe hybridizing to strain 6:3 , which was originally isolated from the same soil sample as strains 6:11 a nd 6:12. Because the RAPD, MLEE, and PFGE analyses showed that strain 6:3 a ppears to be unrelated to strains 6:11 and 6.12, the presence of a mosquito toxin sequence in strain 6:3 may suggest that gene transfer has occurred.