BACILLUS-SPHAERICUS TOXINS - MOLECULAR-BIOLOGY AND MODE OF ACTION

Bacillus sphaericus is a spore-forming aerobic bacterium, several stra ins of which are pathogenic for mosquito larvae. During sporulation, t he most active strains produce a crystal toxin with a high degree of l arvicidal activity. The toxin is composed of two proteins of 51.4 and 41.9 kDa, which are encoded by highly conserved chromosomal genes. Aft er B. sphaericus is ingested, these proteins are released in the larva 's midgut, and, in susceptible mosquito species, bind to a specific re ceptor present on midgut brush-border membranes. The resulting damages to the midgut cells leads to the mosquitoes' death. During vegetative growth, some B. sphaericus strains also synthesize mosquito larvicida l proteins of 100 and 30.8 kDa (Mtx toxins), the mode of action of whi ch is still unknown. The mechanism of acquisition of the recessive mos quito resistance to the crystal toxin varies with selection conditions .