Using combinatorics to screen Bacillus thuringiensis isolates for toxicity against Manduca sexta and Plutella xylostella

Screening Bacillus thuringiensis (Bt) isolates or strains for toxicity has traditionally been performed with one bacterial isolate at a time versus a specific insect. By testing Bt strains in groups, we more rapidly identified 28 of 147 Bt isolates as toxic to either diamondback moth, Plutella xylostella (L.), or tobacco hornworm, Manduca sexta (L.). The use of freeze-dried diet and directed pooling of isolates for toxicity testing decreased the number of bioassays required to identify toxic strains by as much as 60% for a given group of isolates. Three of the B. thuringiensis isolates were more toxic to diamondback moth than a standard commercial strain. This method parallels the concept of combinatorics used for screening compounds in the pharmaceutical industry by the use of bacterial strains rather than chemicals.