The combinatory effect of Cyt1Aa flexibility and specificity against dipteran larvae improves the toxicity of Bacillus thuringensis kurstaki toxins

Recombinant Beauveria bassiana expressing Bacillus thuringiensis toxin Cyt1Aa: a promising approach for enhancing Aedes mosquito control

The entomopathogenic fungus Beauveria bassiana provides an eco-friendly substitute to chemical insecticides for mosquito control. Nevertheless, its widespread application has been hindered by its comparatively slow efficacy in eliminating mosquitoes. To augment the potency of B. bassiana against Aedes mosquitoes, a novel recombinant strain, Bb-Cyt1Aa, was developed by incorporating the Bacillus thuringiensis toxin gene Cyt1Aa into B. bassiana. The virulence of Bb-Cyt1Aa was evaluated against Aedes aegypti and Aedes albopictus using insect bioassays. Compared to the wild-type (WT) strain, the median lethal time (LT50) for A. aegypti larvae infected with Bb-Cyt1Aa decreased by 33.3% at a concentration of 1 × 108 conidia/mL and by 22.2% at 1 × 107 conidia/mL. The LT50 for A. aegypti adults infected with Bb-Cyt1Aa through conidia ingestion was reduced by 37.5% at 1 × 108 conidia/mL and by 33.3% at 1 × 107 conidia/mL. Likewise, the LT50 for A. aegypti adults infected with Bb-Cyt1Aa through cuticle contact decreased by 33.3% and 30.8% at the same concentrations, respectively. Furthermore, the Bb-Cyt1Aa strain also demonstrated increased toxicity against both larval and adult A. albopictus, when compared to the WT strain. In conclusion, our study demonstrated that the expression of B. thuringiensis toxin Cyt1Aa in B. bassiana enhanced its virulence against Aedes mosquitoes. This suggests that B. bassiana expressing Cyt1Aa has potential value for use in mosquito control.

IMPORTANCE

Beauveria bassiana is a naturally occurring fungus that can be utilized as a bioinsecticide against mosquitoes. Cyt1Aa is a delta-endotoxin protein produced by Bacillus thuringiensis that exhibits specific and potent insecticidal activity against mosquitoes. In our study, the expression of this toxin Cyt1Aa in B. bassiana enhances the virulence of B. bassiana against Aedes aegypti and Aedes albopictus, thereby increasing their effectiveness in killing mosquitoes. This novel strain can be used alongside chemical insecticides to reduce dependence on harmful chemicals, thereby minimizing negative impacts on the environment and human health. Additionally, the potential resistance of B. bassiana against mosquitoes in the future could be overcome by acquiring novel combinations of exogenous toxin genes. The presence of B. bassiana that expresses Cyt1Aa is of significant importance in mosquito control as it enhances genetic diversity, creates novel virulent strains, and contributes to the development of safer and more sustainable methods of mosquito control.

Cyt1Aa toxin gene frequency in Bacillus thuringiensis isolates and its relation with pathogenicity for vector mosquitoes

Bacillus thuringiensis produces several virulence factors, the main ones being the Cry and Cyt toxins, present in the parasporal body produced during sporulation. The Cyt toxins have mechanisms specific for mosquitoes and Cyt1Aa, the most studied cytolytic toxin, is effective for mosquito control by acting in synergism with Cry toxins. The goal of the present work was to study the frequency of the codifying gene for Cyt1Aa in B. thuringiensis native isolates acquired from samples of soil, insect and water, as well as to verify any possible genetic polymorphism. 1,448 B. thuringiensis strains were used for DNA extraction and PCR technique, all with the use of a primer that amplifies a fragment of 300 pairs of the cyt1Aa gene. The strains that showed amplification in the PCR reaction were sequenced and compared to each other and to the sequences available at Genbank. 32 (2.3%) strains of B. thuringiensis showed positive amplification for the cyt1Aa gene. The highest frequency of isolates with cyt1Aa gene was acquired from samples coming from the Cerrado biome, both isolates from soil and from insects, equally with 3.4%. The cyt1Aa gene sequencing highlighted that, for that 300 bp region, the gene is conserved and there is no single-base polymorphism.