Pathogenicity is caused by the production of one or both of two unrelated protein toxins. One toxin of 100 kDa is produced by vegetative cells and bacteria producing this toxin alone are of low toxicity. The second toxin is produced at the time of sporulation and it accumulates in the sporangium as a parasporal body in much the same way as that found in B. thuringiensis. This is a binary toxin consisting of two distinct proteins of 51.4 and 41.9 kDa. Strains producing this toxin are of high toxicity.
Some strains produce both toxins and are of high toxicity.
Despite its attractive features as a biological control agent, efforts are being made to modify this bacterium to improve its toxicity and manufacturing characteristics. Initially, these genetic modifications are being done by inserting genes on plasmid vectors that also carry antibiotic resistance genes. The delivery of very large numbers of the spores of these bacteria into the environment might be undesirable if the spores are capable of germinating and if the resulting vegetative cells can transfer the antibiotic resistance genes to environmental microflora. We believe that if this can occur, the most likely site is in the mosquito larval cadaver where spores, the form delivered with toxin, may germinate to produce vegetative cells. The vegetative cells resulting from spore germination would be in close proximity to other bacteria in a nutrient rich environment, the larval cadaver, where conjugation might be possible.