The safety of Bacillus thuringiensis to mammals investigated by oral and subcutaneous dosage

Six strains of Bacillus thuringiensis were tested with two commercially available kits for their ability to produce Bacillus cereus-type enterotoxin and by dipteran bioassay for the production of beta-exotoxin. All of the strains were positive for enterotoxin production including three which have been used world-wide for many years to control pest insects. Rats given oral doses totalling 1 x 10(12) spores ( +crystals), over three weeks, or a single subcutaneous dose of 1 x 10(6) spores ( +crystals) showed no ill-effects in terms of their condition or in the pathology of their internal organs: this was in spite of the strain of B. thuringiensis used (13B) being an active producer of both beta-exotoxin and enterotoxin. A commercial insecticide containing B. thuringiensis was sprayed onto spinach leaves. After normal food preparation regimes some leaves retained residual spore loads sufficient for a strongly enterotoxic strain to cause food poisoning in humans. These findings suggest that the agricultural use of some, previously unvalidated, strains of B. thuringiensis could give rise to cases of food poisoning and that rodents are unsuitable for testing the safety to humans of oral exposure to this organism.