Submitted to: PLoS One
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 24, 2011
Publication Date: March 25, 2011
Citation: Blackburn, M.B., Martin, P.A., Kuhar, D.J., Farrar, R.R., Gundersen, D.E. 2011. The occurrence of Photorhabdus-like toxin complexes in Bacillus thuringiensis. PLoS One. 6(3):e18122.
Interpretive Summary: Bacillus thuringiensis (Bt) is an important bacteria used in organic gardening and farming to control moths and some beetles that are agricultural pests. One drawback of Bt is that a given strain of the bacteria usually has a very narrow range of activity against moths or beetles, but not both. We have discovered that some types of Bt produce an unusual toxin normally produced by another insect-killing bacterium that lives in the gut of certain small worms called nematodes. These toxins affect a wider range of insects than the toxins normally produced by Bt. We show that the Bts that produce these unusual toxins have a wider range of activity affecting both moths and beetles. We describe ways that Bts producing these toxins can be identified more easily. This information will be of interest to scientists prospecting for new strains of Bt with enhanced activity against a range of pests or interested in novel biocontrol strains.
Recently, genomic sequencing of a Bacillus thuringiensis (Bt) isolate from our collection revealed the presence of an apparent operon encoding an insecticidal toxin complex (Tca) similar to that first described from the entomopathogen Photorhabdus luminescens. To determine whether these genes are widespread among Bt strains we screened isolates from the collection for the presence of tccC, one of the genes needed for the expression of fully functional toxin complexes. Among 80 isolates chosen to represent relatively abundant biochemical phenotypes, 17 were found to encode a TccC. Phylogenetic analysis of the 80 isolates by multilocus sequence typing revealed that all the isolates possessing a tccC gene were restricted to sequence type (ST) 23 and ST 240, which are most closely related to Bt varieties israelensis, morrisoni, tenebrionis and toumanoffi. Sequencing of the entire ~17 kb tca operon from two isolates representing each of the two sequence types revealed 99% sequence identity. Optical mapping of DNA from Bt isolates of ST 23 (strain IBL 122) and ST 240 (strain IBL 200) revealed nearly identical plasmids of ca. 333 and 338 kb, respectively. Selected isolates were found to be toxic to gypsy moth larvae, but were not as effective as a commercial strain of Bt kurstaki. Some isolates tested, particularly those belonging to ST 23, were found to inhibit growth of Colorado potato beetle. Relative quantitative real-time PCR-based assays for Tc-encoding Bt revealed both tcaA and tcaB genes were expressed within infected gypsy moth larvae.