Bacillus thuringiensis, which produces several proteins that are specifically toxic to plant pests, is used widely as an agricultural biocontrol agent. However, concern about the widespread use of B. thuringiensis on food crops is growing because B. thuringiensis is closely related to B. cereus, a known food contaminant that can cause diarrhea in humans due to the expression of enterotoxin genes. Most commercial B. thuringiensis strains also contain and express enterotoxin genes.

The major difference between B. thuringiensis and B. cereus appears to be the presence of plasmids in B. thuringiensis that encode an insecticidal crystal toxin. Therefore, constructing enterotoxin-deficient mutants of existing commercial B. thuringiensis strains should enhance the safety of B. thuringiensis for food crops while maintaining its effectiveness as a biocontrol agent.

UW–Madison researchers previously developed methods for making Bacillus strains in which a component of the HBL enterotoxin is disrupted. However, while these strains demonstrated reduced enterotoxin activity, they continued to exhibit low levels of enterotoxin.

UW–Madison researchers have now created improved mutants of B. thuringiensis for use as bioinsecticides on food crops. In the modified strains, four distinct operons, each comprising three genes that encode unique enterotoxins that have been implicated in food poisoning, have been replaced with copies containing deletions, rendering the enterotoxins non-functional. The quadruple enterotoxin-deficient strains do not produce the enterotoxin products that are associated with human toxicity, yet perform as well as the wild-type B. thuringiensis strain.

File Number: P08212US02