Effect of a nematicidal Bacillus thuringiensis strain on free-living nematodes .3. Characterization of the intoxication process

The toxicity of Bacillus thuringiensis is temperature sensitive. Incubation of Caenorhabditis elegans with nematicidal B. thuringiensis strains at 16, 20, and 25 degrees C shows that toxicity decreases as temperature declines. At 16 degrees C, toxicity is completely lost, while it is maximal at 25 degrees C. Toxicity is pH sensitive and is significantly reduced when nematodes are incubated with the weak bases NH4Cl, chloroquine, acridine orange, methyl red, and neutral red. Based on these results, we proposed the hypothesis that the nematicidal factor is effectively internalized into the intestinal cells, a sharp deviation from the insecticidal B. thuringiensis toxins acting at the level of the brush border membrane. Although the absence of purified toxin prevents a more definitive elucidation of the mode of action, the results of this third and final part of this series of publications convincingly indicate that nematicidal B. thuringiensis do not hold the same promise as a biological control agent as the insecticidal B. thuringiensis strains.