The synergism between Mtx1 from Bacillus sphaericus and Cyt1Aa from Bacillus thuringiensis to Culex quinquefasciatus

Abstract:

Mosquitocidal toxin 1 (Mtx1) was synthesized during vegetative phase of Bacillus sphaericus and it had been proved to have higher activity to Aedes spp. larvae and Binary toxin (Bin) resistance Culex larvae. The truncated 97kDa Mtx1 with a deletion of the signal peptide and the Cyt1Aa crystal protein, a 27.3kDa δ-endotoxin from Bacillus thuringiensis subsp. israelensis (Bti), were purified from Escherichia coli and B. thuringiensis recombinant strains respectively. Both purified toxins had high toxicity against Culex quinquefasciatus larvae. Bioassay result revealed the purified Mtx1 toxin had high toxicity against the target mosquito larvae, with LC₅₀ of 45.2ng/mL. However, the mixture of Mtx1 and Cyt1Aa exhibited higher toxicity against the mosquito larvae, with a lowest LC₅₀ value of 20.19ng/mL at the ratio of 1∶3. (Mtx1:Cyt1Aa). The calculated synergistic factor of different mixtures suggested a strong synergistic effect between Cyt1Aa toxin and Mtx1. Furthermore, the presence of Cyt1Aa in the mixture could induce early larval mortality, enhancing the activity of Mtx1 to the target mosquito larvae. The synergistic effect of Cyt1Aa on mortality of Mtx1 to mosquito larvae might be caused by the damage of the larval midgut-hemocoel barrier induced by the activated Cyt1Aa toxin, which enhanced the specific pathogenesis of Mtx1 on mosquito larvae. It is suggested that the co-application of Mtx1 and Cyt1Aa in future will be integrated for mosquito management.

Key words:Bacillus sphaericus; Bacillus thuringiensis; Mosquitocidal toxin; synergism; Culex quinquefasciatus; Mtx1; Cyt1Aa