Correspondence of high levels of beta-exotoxin I and the presence of cry1B in Bacillus thuringiensis

Examination of 640 natural isolates of Bacillus thuringiensis showed that the 58 strains (9%) whose supernatants were toxic to Anthonomus grandis (Coleoptera: Curculionidae) produced between 10 and 175 mug of beta-exotoxin I per ml. We also found that 55 (46%) of a sample of 118 strains whose culture supernatants were not toxic to A. grandis nevertheless produced between 2 and 5 mug/ml. However, these amounts of beta-exotoxin I were below the threshold for detectable toxicity against this insect species. Secretion of large amounts of beta-exotoxin I was strongly associated with the presence of cry1B and vip2 genes in the 640 natural B. thuringiensis isolates studied. We concluded that strains carrying cry1B and vip2 genes also possess, on the same plasmid, genetic determinants necessary to promote high levels of production of beta-exotoxin I.