Strategies for the identification of Bacillus thuringiensis virulence factors = Estrategias para la identificación de factores de virulencia de Bacillus thuringiensis

Abstract

Bacillus thuringiensis is a gram-positive bacterium which synthesizes insecticidal virulence factors. The most important are the Cry proteins, which are produced during sporulation as crystal parasporal inclusions. These proteins along with the spores constitute the active ingredients of the most widely used biological pesticide to control agricultural insect pests and vectors of human and animal diseases. Some strains are capable of also expressing other insecticidal proteins called Vip or Sip during the vegetative growing phase. All these virulence factors have in common that are very specific, exhibiting a narrow host range of species within an insect order. In contrast, in some B. thuringiensis strains there are other secreted virulence factors such as β- exotoxins, chitinases, haemolysins, and enterotoxins, unlike Cry, Vip and Sip proteins, that are unspecific. In our laboratory, different strategies based on phenotypic and genotypic criteria have been successfully used to identify B. thuringiensis virulence factors: toxicity bioassays, competition binding assays performed on brush border membrane vesicles of insects, PCR-based detection of insecticidal toxin genes and comparative genomics. The knowledge of the virulence factors present in a particular strain is a key in understanding pathogenesis and essential for the development of new bioinsecticides or Bt plants.