Occurrence of Bacillus thuringiensis on cured tobacco leaves

A worldwide survey was conducted to evaluate the frequency and distribution of Bacillus thuringiensis populations on cured tobacco leaves during post-harvest storage. In total, 133 tobacco samples of different types and origins were analyzed. Nine percent of the samples showed the presence of B. thuringiensis, and 24 B. thuringiensis strains were isolated and characterized. The majority of the isolates produced bipyramidal crystals, and three fourths of them showed a second type of crystal protein (cuboidal or heterogeneous crystals). Only three isolates showed the rhomboidal crystal morphology characteristic of the anti-coleopteran B. thuringiensis subsp. tenebrionis. PCR analysis with primers specific for cry1 and cry3 genes revealed eight distinct cry gene profiles. The results of this study indicate that B. thuringiensis is naturally present at low frequency on the phylloplane of cured tobacco leaves and that its distribution is worldwide.