Prevalence, attachment ability and strength of Bacillus cereus and Bacillus thuringiensis on tomato

Bacillus thuringiensis, a spore-forming bacteria, is one of the most often used microbial biological control agents on plants in the world due to its production of insecticidal crystals. In the EU, fruit vegetables (e.g. tomato and bell pepper) are all often sprayed with B. thuringiensis biopesticides either in field or in greenhouse. In this study, from September 2020 to April 2021, the prevalence of presumptive B. cereus on tomatoes obtained from 6 supermarkets in Flanders region, Belgium was investigated. Isolates were subjected to microscopic identification to assess typical parasporal crystals for B. thuringiensis. In total 44 out 79 samples (56%) of vine tomatoes were detected presumptive B. cereus and numbers ranged from 27 to 1.26×105 cfu/g (median 1.3×103; 1 samples (1.3%) exceeding 105 cfu/g). In addition, the attachment ability and strength on tomato was shown to be much higher for the spores than what was the case for the vegetative cells of these B. cereus and B. thuringiensis strains. Interestingly, the commercial powder formulation of B. thuringiensis spores showed a lower attachment ability and strength (and thus would be easier washed off, or be reduced by rainfall or irrigation) than lab-harvested spores from these B. thuringiensis and B. cereus isolate