Overview of the Basic Biology of Bacillus thuringiensis with Emphasis on Genetic Engineering of Bacterial Larvicides for Mosquito Control

Abstract

The insecticidal bacterium, Bacillus thuringiensis, consists of a wide variety of subspecies, most of which are insecticidal for either lepidopteran, coleopteran, or dipteran insect larvae. Subspecies such as B. thuringiensis subsp. kurstaki have been used with remarkable safety for more than forty years to control lepidopteran pests in agriculture and forestry, and over the past thirty years, B. thruingeinsis subsp. israelensis, has proven to be a safe and effective larvicide for controlling mosquito and black fly larvae. Studies of the basic biology of B. thuringiensis have shown that it produces a variety of insecticidal proteins produced during vegetative growth and sporulation that determines its activity for insect species belonging to different orders, with the most important of these being the Cry proteins active against lepidopteran and coleopteran pests, and a combination of Cry and Cyt proteins for mosquitoes and blackflies. After intoxication by these proteins, spores typically germinate and invade larvae, contributing to insect mortality. Whereas strains of many wild type isolates have been commercialized and are now used worldwide, the use of recombinant DNA techniques, i.e., genetic engineering, has been used over the past decade to recombine the proteins of different B. thuringiensis strains with those of B. sphaericus to generate recombinant larvicides as much as ten-fold more toxic than the parental strains. In this chapter, we begin with a general overview of the basic biology of B. thuringiensis and B. sphaericus, then show how studies of its molecular genetics combined with recombinant DNA techniques have been used to generate highly improved bacterial larvicides for control of nuisance and vector mosquitoes.