Effect of nematicidal Bacillus thuringiensis strains on free-living nematodes .1. Light microscopic observations, species and biological stage specificity and identification of resistant mutants of Caenorhabditis elegans

Light-microscopic observations of the toxic action of Bacillus thuringiensis spore/crystals reveals that, in Caenorhabditis elegans, the intestine is destroyed in two stages over a period of 24 h. The anterior ring of four cells is the first and foremost target. Observations indicate that the intestine is the only tissue being destroyed. Screening of fourteen additional rhabditid nematode species against three nematicidal B. thuringiensis strains active against C. elegans, resulted in only one additional sensitive nematode species, and indicates a high species specificity of the nematicidal factor. However, in contrast to insect-specific B. thuringiensis toxins, the nematicidal toxin exhibits low developmental stage specificity against C. elegans; all developmental stages, including adult nematodes are sensitive. Moreover, sensitivity increases as development proceeds. Using ethyl methyl sulfonate induced mutagenesis two mutants of C. elegans have been recovered, exhibiting reduced sensitivity of up to 50 % against one of the nematicidal strains. Moreover, one of the mutants exhibited cross-resistance to a second nematicidal B. thuringiensis strain against which it was not screened. Preliminary data indicate that the reduced sensitivity in the mutants is not due to reduced pharyngeal pumping activity.