Levels of biopesticide Bacillus thuringiensis strains on tomatoes, leaf lettuce and head lettuce from retailers and producers in Germany

In order to reduce the use of chemical pesticides, bioinsecticides based on Bacillus thuringiensis (Bt) are increasingly applied in agriculture. However, food safety aspects of these bioinsecticides are controversially discussed. One issue that hampers this discussion is a lack of knowledge concerning the presence of biopesticide Bt strains on foodstuffs. In our study, we analysed Bacillus cereus (sensu lato) isolates obtained within different monitoring programs in 2016, 2021 and 2022 in Germany. Here, the competent food control authorities tested samples of tomatoes (n = 426), leaf lettuce (n = 428) and head lettuce (n = 605). From representative numbers of samples, B. cereus (s.l.) isolates were submitted to the Federal Institute for Risk Assessment (BfR), where parasporal crystal producing isolates underwent whole genome sequencing and single nucleotide polymorphism (SNP) analysis to determine the genetic similarity to the seven biopesticide Bt strains approved for use for lepidopteran larvae control in the EU. Results of tomato isolates were initially published in 2020, but were reassessed in the current report. Samples from which isolates were submitted to the BfR contained Bt with ratios of 99 % for tomatoes (94 of 95 samples), 22 % for leaf lettuce (37 of 165 samples) and 29 % for head lettuce (52 of 177 samples). Overall, 227 of the 235 analysed Bt isolates (97 %) differed by ≤ 14 core genome SNPs from a biopesticide Bt strain, indicating a biopesticide origin. The respective strains were rarely detected at the level of ≥ 105 cfu/g. The applied methodology to distinguish the biopesticide strains may help to monitor these strains and to identify sources of B. cereus (s.l.) contaminations in the food production.