Detection of Bacillus cereus virulence factors in commercial products of Bacillus thuringiensis and expression of diarrheal enterotoxins in a target insect

We examined isolates from 4 commercial bioinsecticides based on different strains of Bacillus thuringiensis subspecies (kurstaki, israelensis, aizawai, and tenebrionis) for the presence of genes encoding proteins with known enterotoxigenicity (nhe, hbl, cytk, ces) and various other putative virulence genes (piplc, sph, bceT, entFM, entS, entT). The piplc and bceT sequences were present in all the isolates; sph was found in aizawai and israelensis; entFM only in israelensis; and entS in kurstaki, israelensis, and tenebrionis. Our results corroborate previous findings that isolates used in commercial products contain all nhe and hbl component genes but not the ces gene. We ascertained that the cytK gene present in the kurstaki-, israelensis-, and aizawai-based products belongs to the cytK-2 type and not the more toxigenic cytK-1 variant originally isolated from enterotoxic Bacillus cereus. We provide the first evidence that hemolytic (hblA) and nonhemolytic (nheA, nheB, nheC) enterotoxin genes are expressed during septicemia in a target insect. This opens the door for their possible participation in pathogenesis in target insects. If enterotoxins do not contribute to bacterial pathogenesis in target insects, their genes could be deleted from commercial production strains to pre-empt perceptions of public health risks.