ABSTRACT
Bacillus thuringiensis (Bt) was discovered in a diseased silk moth population at the beginning of this century by Ishiwata. The bacterium, then recorded as Bacillus soto, was lost but subsequently re-isolated in 1909 by Berliner from a diseased Mediterranean flour moth population (Berliner, 1915). Berliner described the presence of an inclusion body or crystal, now considered as the phenotypic characteristic discriminating Bacillus thuringiensis from Bacillus cereus. Already at the time of discovery Berliner suggested that pathogenic organisms could be used to control insect populations. In the late 1920s the bacterium was mass produced and used to control European corn borer in southeast Europe. The first commercial formulation, Sporéine, was produced in France in 1938. After the Second World War there was a rejuvenated interest in the use of Bt for the control of agricultural insect pests. In the 1950s it became clear that the insecticidal activity of the bacterium was predominantly determined by its crystal which is produced during sporulation (Hannay, 1953) and which contains proteins (Hannay and Fitz-James, 1955). Bt strains produce other products which have biocidal activities such as phospholipases, immunosuppresive agents, exotoxins (Lüthy, 1980) and vegetative insecticidal proteins (Chapter 7). The increasing number of Bt strains isolated from different sources by different people created a need to characterize Bt strains. One of the methods still in use today involves the immunological identification of flagella on the surface of the vegetative cells (de Barjac and Bonnefois, 1962), and so far some 45 serotypes have been described (Lecadet et al., 1994). A major step forward in the commercial success of Bt was the isolation of HD-1, a strain which proved far more potent than any other strain isolated so far and which still forms the basis of some of today’s commercial formulations (Dulmage, 1970). Until 1977 Bt was considered to be exclusively toxic to lepidopteran larvae. But then, from a pond in the Negev desert, Goldberg and Margalit (1977) isolated a Bt strain, Bacillus thuringiensis subsp. israelensis, which was specifically toxic to dipteran larvae such as mosquito and black fly larvae. Bt strains active against
dipteran larvae are now extensively used to control insect disease vectors for malaria and river blindness. In 1981 the first gene encoding a crystal protein was cloned (Schnepf and Whiteley, 1981). In 1983, Krieg et al. found a Bt strain, Bacillus thuringiensis subsp. tenebrionis, which proved highly toxic to some coleopteran larvae, including the Colorado potato beetle. This was the trigger for a massive isolation and screening effort for strains and crystal proteins with improved and new activities. Bt proved to be a ubiquitous organism and strains have been isolated from a wide range of sources such as forest soil, grain dust, bat dung, sea water, desert sand, and dead and diseased insects (Martin and Travers, 1989; Meadows, 1993). An interesting observation was that Bt strains are quite abundant on the phylloplane suggesting that they could help plants to protect themselves from insect feeding damage (Smith and Couche, 1991). Although Bt is not an obligate insect pathogen-it can easily be grown on standard microbiological media-it seems that multiplication in soil is rather limited (West et al., 1985). Spores can survive for several years and may occasionally germinate when in contact with nutrients derived from decaying grass or other organic material (West, 1985). The screening effort, mainly conducted at private companies, has led to the collection of perhaps as many as 50 000 isolates which represent several thousand unique strains. Only a fraction of these strains have been analyzed in full detail and one can expect many more interesting discoveries.
