The Lack of Tools to Track Bacillus thuringiensis (Bt) Based Insecticide Isolates from Farm to Fork

Introduction : Bacillus thuringiensis (Bt) is a widespread, sporeforming bacteria with a complex life cycle. Due to its ability to produce parasporal crystalline inclusions that show insecticidal properties, it has become the main microorganism used for pest control in organic farming. Today, it is successfully used as a bioinsecticide against caterpillars, beetles, and flies, including mosquitoes and blackflies. Yet from a farm to fork prospective, the major issue relies on the lack of tools to distinguish Bt-based bioinsecticide isolates from other closely related strains of the Bacillus cereus group that may be involved in food poisoning or food spoilage.

Purpose: The aim of this study was to collect isolates from Bt-based commercial products and develop a PFGE method to track B. cereus contaminants.

Methods: Commercial Bt-based products were used to isolate Bt strains that were further characterized by the presence of toxin encoding genes, Guinebretiere et al. phylogenetic classification, as well as the observation of parasporal crystalline inclusions using a phase contrast microscope. The Pulsed-Field Gel Electrophoresis (PFGE) subtyping protocol was adapted from Liu, et al. (1997) and Zhong, et al. (2006).

Results: For most prevalent Bt strains used in Europe, all tested isolates showed the production of huge parasporal crystals, belonging to group IV phylogenetic classification, during the sporulation phase, . PFGE is the gold standard used by the CDC and European public health laboratory network for foodborne disease surveillance. Molecular fingerprints generated clearly enabled the clustering of strains belonging to the subspecies Bt aizawai, Bt kurstaki and Bt israelensis.

Significance:   The promising results obtained in this study will be extended to Bt strains used in the composition of Bt-based products used outside Europe and, also, to a selection of strains to be representative of B. cereus diversity. When validated, this tool could be applied to track Bt contamination from farm to fork.