An Overview on the Crystal Toxins from Bacillus thuringiensis

Abstract

Strains of Bacillus\rthuringiensis (Bt) are\rknown to produce crystalline proteins (δ-endotoxins) concomitantly with\rsporulation during their stationary phase of growth, which are demonstrated as lethal to\rlepidopeterous, coleopeterous and dipterous insects in addition to mites,\rnematodes, protozoa and flukes. Upon ingestion, the δ-nascent endotoxin\ris an inactive protoxin complex of (Cry alone or Cry and Cyt toxins together) high molecular mass, which is cleaved upon\ringestion into the active component proteins at the high alkaline environments\rin the digestive tract of these agricultural pests. Conventionally, Bt-crystals\rare being produced employing submerged or liquid fermentation techniques in commercial\rmedia, but recently many workers have used solid-state\rfermentation strategy for the enhanced production of Bt-toxin at low\rcost. Apart from δ-endotoxin, some isolates of Bt produce another\rclass of insecticidal small molecules called β-exotoxin (thuringiensin),\rwhich may be harmful to humans. Moreover, resistance to Bt developed in various target\rpest is yet another concern for Bt-industry.\rFollowing a brief introduction, this review addresses various toxins produced\rby various strains of Bt, Bt production media and media formulations with emphasis to solid-state fermentation,\rgeneral structure of Cry toxin, its mode of action, target pests, bioassay, resistance to Bt toxins and\rresistance management. Briefly, this review would provide the readers an\roverview on the general aspects of Bt toxin, its general structure and mechanism of action.