Nematoda: susceptibility of the egg to Bacillus thuringiensis toxins

Crystalline toxins from Bacillus thuringiensis israelensis and B.t. kurstaki were lethal in vitro to eggs of the nematode Trichostrongylus colubriformis. The LD50 values for the two toxins were 0.38 ng and 37.5 micrograms total protein/ml, respectively. After 1 week at ambient temperature, the LD50 of B.t. kurstaki decreased to less than 4 micrograms/ml. Toxin from B.t. israelensis had no effect within 48 hr on survival of adult nematodes or on their feeding in vitro. Third-stage larvae of T. colubriformis were also unaffected by B.t. israelensis toxin. Exposure of third-stage larvae of Nippostrongylus brasiliensis to 1.1 micrograms total protein/ml of B.t. israelensis for 4 hr had no effect on their infectivity in mice, based on recovery of helminths at 7 days after infection. Similar exposure of 5-day-old N. brasiliensis and subsequent transfer into the intestine of mice gave recoveries that were similar to the untreated control. Thirty strains of B. thuringiensis caused mortality in nematode eggs, but over a 77,000-fold range of activity was found, based on the LD50 values. Toxin from B.t. israelensis was lethal to eggs of six zooparasitic and one free-living species of nematode, but the LD50 values varied 28-fold. Addition of B.t. israelensis to feces that contained eggs of T. colubriformis reduced subsequent recovery of larvae, with an LD50 of 260 ng/g of feces.