Bacillus thuringiensis toxin in C. elegans
By Sarah Adrianowycz
Bacillus thuringiensis as an organism
Bacillus thuringiensis is prevalent in the soil, and is a gram –positive, spore-forming member of the Bacillus ceres group. The Bacillus ceres group is a collection of seven genetically similar species, three of which are so alike that they are occasionally considered the same organism, except for their vastly different pathogenic outcomes (Kho et al. 2011). It is these different pathogens, or external expression of the organism’s phenotype, which are often used by researchers to determine which species are distinct. In the case of B. thuringiensis the organism is typified by the production of specific invertebrate related pathogens (Ceuppens et al., 2013). However, the issue with using phenotypic differences to differentiate among these organisms is that the differences frequently stem from virulence factors contained on plasmids, which are extremely variable and transient. These genetic elements separate from the primary genome can be exchanged through horizontal gene transmission, lost in the environment, or selected against during culturing in the laboratory. Despite the issues with distinguishing strains based upon their ability to cause disease, B. thuringiensis is often distinguished by the ability to form crystal proteins. These products are insecticidal in nature and referred to as cry genes for the genes that code their expression (Ceuppens et al.). In the wild, most strains of B. thuringiensis are capable of producing anywhere from 1-4 types of cry proteins (Kho et al., 2011).The ability to produce multiple cry genes may be explained because of the synergistic effect that some of these proteins have in taking over a host when working together, or because different potential hosts have different levels of susceptibility to specific cry proteins (E. Schnephf, 1998).
It is the ability to produce these crystal proteins that gives B. thuringiensis value as an insecticide and has made it especially interesting to humans who are looking into biological applications of the cry proteins. These toxins have been utilized both in direct application on to crops and as a source of transgenic elements that provide resistance against the orders Lepidoptera, Diptera, and Coleoptera (E. Schnepf, 1998) when incorporated into the genomes of genetically modified organisms. The method of action within this subset of proteins makes them especially effective against insects but hasn’t produced evidence of negative effects against either humans or other higher trophic level organisms, as is typically the case for insecticides. Because of these perceived benefits and the lack of obvious repercussions, the utilization of plants with cry proteins has become common place since 1996, when genetically modified potatoes, corn, and potatoes with versions of cry genes were made commercially available (Schnepf).