In this section we like to present you the main results of the BeeT project. BeeT is engineered to produce a toxin specific for Varroa destructor, produce the toxin on the right time and incapable of escaping the hive alive. To accomplish this, we performed multiple experiments and created different models. The outcome is in short shown on this page.

In order to improve on existing methods, BeeT should effect Varroa mites only. To accomplish this we decided to make use of Cry toxins. These toxins are naturally produced by Bacillus thuringiensis and because of this also known as BT toxins. A functional Cry toxin is only effective when specific binding occurs to the gut membrane of the target organism. Hereafter, the Cry toxins will form pores into the cell membrane, which results in cell death. As cell death occurs, the gut membrane becomes porous. Consequently, the organism dies. ¹ To find a Cry toxin active against V. destructor we engineered our own toxins and we searched in nature for one as well.

Due to the parasitic nature of Varroa mites, testing Cry toxins proved to be very problematic. To overcome this problem we developed an in vitro test for Cry toxins. Out of the membranes of the target organism, brush border membrane vesicles (BBMVs) were made and incorporated with 6-carboxyfluorescein. A functional Cry toxins will create pores into the BBMVs, which then results in the leaking of fluorophores out of the BBMVs. Due to self-quenching behaviour of 6-carboxyfluorescein, this can be measured as an increase in fluorescence. As a proof of principle, BBMVs from the gut of Tenebrio molitor were made and loaded with 6-carboxyfluorescein to test the pore formation ability of Cry3Aa, which is known to be toxic to T. molitor larvae ³. Figure 1a shows how the fluorescence increases of BBMVs incorporated with fluorophores in the presence and absence of Cry3Aa. A kinetic value could be coupled to this process. These values for multiple measurements for BBMVs in the presence and absence of Cry3Aa are shown in Figure 1b. From this can be concluded that the presence of a functional Cry protein results can be measured.

To engineer a Cry toxin active against V. destructor we took the Cry3Aa toxin as a starting point. This Cry toxin consists of three domains, from which one is responsible for the binding. In 1996, Rajamohan et al. demonstrated that mutations in the binding sites of Cry toxins can both decrease and enhance the specificity of a toxin towards its target organism². Three putative binding sites have been identified after analysing the 3D structure of the binding domain of Cry3Aa. The putative sites were changed with random mutagenesis and the adapted proteins were cloned into E. coli. 144 Cry proteins were produced and tested for activity on BBMVs from V. destructor as described previously. From these measurements 24 candidates were selected to test further. The results are shown in Figure 2. From these results, it can be concluded that the third binding site (amino acids 410-416) seems to be a good candidate for future engineering and specificity adaptation of this particular Cry toxin. Due to the relatively high deviation in reaction speed for the toxins 3.3.3 and 3.3.7, these should not be taken into account, as they are rather inconclusive. This leaves us with one proper candidate – the toxin mutant Cry3.3.5.

In order to find the right specific binding motif, phage display was performed. Phages with a binding motif on their exterior were exposed to the gut membrane of V. destructor. Hereafter, the bound phages were isolated and analysed. The filamentous bacteriophage M13 was used with a 12-mer library (The Ph.D.™-12 Phage Display Peptide Library). The phages were fed to Varroa mites and exposed to BBMVs originating from Varroa mites. The recovered phages were isolated and sequenced. The consensus sequences of the binding motif of the 12-mer both in vivo and in vitro are shown in Figure 3.

Alongside creating a Cry toxin ourselves, we searched in nature for one as well. We gathered 800 death Varroa mites and looked for B. thuringiensis or related species inside these mites that might have been the cause the death. Figure 4 shows the morphology of B. thuringiensis and two found strains. Five out of 106 isolates were identified as Bacillus-like species. One strain, not B. thuringiensis, showed the presence of a large overexpressed protein and was sent for sequencing. We are waiting with excitement for the results.