Cloning and Expression of the Binary Toxin Gene from Bacillus sphaericus IAB872 in a Crystal-Minus Bacillus thuringiensis subsp. israelensis

Stability, oviposition attraction, and larvicidal activity of binary toxin from Bacillus sphaericus expressed in Escherichia coli

Bacillus sphaericus produces a two-chain binary toxin composed of BinA (42 kDa) and BinB (51 kDa), which are deposited as parasporal crystals during sporulation. The toxin is highly active against Culex larvae and Aedes and Anopheles mosquitoes, which are the principal vectors for the transmission of malaria, yellow fever, encephalitis, and dengue. The use of B. sphaericus and Bacillus thuringiensis in mosquito control programs is limited by their sedimentation in still water. In this study, the binA and binB genes were cloned and the recombinant BinAB protein was expressed in three strains of Escherichia coli. These recombinant strains were used in a toxicity assay against Culex quinquefasciatus larvae. The highest expression level was achieved when both proteins were expressed in a single operon construct. The BinAB protein expressed in the E. coli Arctic strain showed higher larvicidal activity than either of the recombinant proteins from the E. coli Ril or pLysS strains. Furthermore, it had the highest oviposition attraction (49.1%, P < 0.05). These data suggest that biologically active recombinant BinA and BinB toxins might be useful in mosquito control programs, delivered by inactivated bacterial cells or in traps.

Lack of cross-resistance to Mtx1 from Bacillus sphaericus in B. sphaericus-resistant Culex quinquefasciatus (Diptera: Culicidae)

The toxicities of Mtx1 toxin against dipteran and lepidopteran species have been evaluated in this study. It was shown that Mtx1 has little or no toxicity to the tested lepidopteran species, but has moderate-level toxicity to Aedes albopictus Skuse (Diptera: Culicidae) and high-level toxicity to both susceptible and binary toxin-resistant Culex quinquefasciatus Say (Diptera: Culicidae). The LC(50) values of Mtx1 against a susceptible C. quinquefasciatus colony SLCq and two resistant colonies RLCq1/C3-41 and RLCq2/IAB59 selected in the laboratory with Bacillus sphaericus (Mayer & Neide) strains C3-41 and IAB59 respectively were 0.508, 0.854 and 0.675 mg L(-1) respectively. The data indicate that Mtx1 has a different mode of action from the binary toxin, and that it could be an alternative toxin to delay or overcome resistance development to binary toxin in C. quinquefasciatus.

Mosquitocidal toxin from Bacillus sphaericus induces stronger delayed effects than binary toxin on Culex quinquefasciatus (Diptera: Culicidae)

We studied the toxicity and delayed effects of a mosquitocidal toxin (Mtx1) and a binary toxin (Bin) produced in Escherchia coli E-TH21 and Bacillus thuringiensis B-CW1, respectively, on Culex quinquefasciatus (Diptera: Culicidae). Bioassay results showed that both E-TH21 powder and B-CW1 sporulated culture were highly toxic against susceptible Cx. quinquefasciatus, with LC50 values of 0.65 and 1.70 mg/liter against third and fourth instars at 48 h, respectively. After initial 48-h exposure of larvae to different concentrations of Mtx1 and Bin, significant continued mortality could be observed in larval, pupal, and emergence stages of Cx. quinquefasciatus. Importantly, the Mtx1 could induce higher cumulative larval and preadult mortalities than Bin toxin on the target mosquito. This finding is important for understanding the mode of action of Mtx1 and Bin toxins and for developing a new bioassay procedure for evaluation of toxicity of Bacillus sphaericus Neide, some strains of which produce Mtx1 and Bin, in the laboratory and field.

The synergistic activity between Cry1Aa and Cry1c from Bacillus thuringiensis against Spodoptera exigua and Helicoverpa armigera

AIMS

To investigate the interaction between two crystal proteins, Cry1Aa and Cry1C, for future development of biopesticides based on Bacillus thuringiensis, toxicities of the two individual proteins and in combinations have been determined against Spodoptera exigua and Helicoverpa armigera larvae, and synergism between the proteins has been evaluated using synergistic factor.

METHODS AND RESULTS

SDS-PAGE showed that Cry1Aa and Cry1C proteins could be expressed in acrystalliferous B. thuringiensis 4Q7 strain, with molecular weights of 135 and 130 kDa respectively. The bioassay results indicated a synergistic activity between Cry1Aa and Cry1C against S. exigua and H. armigera, and the highest toxicities could be observed in the combination of Cry1Aa and Cry1C at a ratio of 1 : 1.

CONCLUSION

The two toxins, Cry1Aa and Cry1C, interact synergistically to exhibit higher toxicity against S. exigua and H. armigera.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the investigation on the synergistic activity between two B. thuringiensis Cry1 toxins. It can be applied to the rational design of new generations of B. thuringiensis biopesticides and to strategies for management of resistant insects.

Tightly bound binary toxin in the cell wall of Bacillus sphaericus

We have shown that urea-extracted cell wall of entomopathogenic Bacillus sphaericus 2297 and some other strains is a potent larvicide against Culex pipiens mosquitoes, with 50% lethal concentrations comparable to that of the well-known B. sphaericus binary toxin, with which it acts synergistically. The wall toxicity develops in B. sphaericus 2297 cultures during the late logarithmic stage, earlier than the appearance of the binary toxin crystal. It disappears with sporulation when the binary toxin activity reaches its peak. Disruption of the gene for the 42-kDa protein (P42) of the binary toxin abolishes both cell wall toxicity and crystal formation. However, the cell wall of B. sphaericus 2297, lacking P42, kills C. pipiens larvae when mixed with Escherichia coli cells expressing P42. Thus, the cell wall toxicity in strongly toxic B. sphaericus strains must be attributed to the presence in the cell wall of tightly bound 51-kDa (P51) and P42 binary toxin proteins. The synergism between binary toxin crystals and urea-treated cell wall preparations reflects suboptimal distribution of binary toxin subunits in both compartments. Binary toxin crystal is slightly deficient in P51, while cell wall is lacking in P42.