Synergy between Toxins ofBacillus thuringiensissubsp.israelensisandBacillus sphaericus

Abstract

Synergistic interactions among the multiple endotoxins of Bacillus thuringiensis subsp. israelensis de Barjac play an important role in its high toxicity to mosquito larvae and the absence of insecticide resistance in populations treated with this bacterium. A lack of toxin complexity and synergism are the apparent causes of resistance to Bacillus sphaericus Neide in particular Culex field populations. To identify endotoxin combinations of the two Bacillus species that might improve insecticidal activity and manage mosquito resistance to B. sphaericus, we tested their toxins alone and in combination. Most combinations of B. sphaericus and B. t. subsp. israelensis toxins were synergistic and enhanced toxicity relative to B. sphaericus, particularly against Culex quinquefasciatus Say larvae resistant to B. sphaericus and Aedes aegypti (L.), a species poorly susceptible to B. sphaericus. Toxicity also improved against susceptible Cx. quinquefasciatus. For example, when the CytlAa toxin from B. t. subsp. israelensis was added to Bin and Cry toxins, or when native B. t. subsp. israelensis was combined with B. sphaericus, synergism values as high as 883-fold were observed and combinations were 4-59,000-fold more active than B. sphaericus. These data, and previous studies using cytolytic toxins, validate proposed strategies for improving bacterial larvicides by combining B. sphaericus with B. t. subsp. israelensis or by engineering recombinant bacteria that express endotoxins from both strains. These combinations increase both endotoxin complexity and synergistic interactions and thereby enhance activity and help avoid insecticide resistance.

Evolution of Resistance inCulex quinquefasciatus(Say) Selected With a RecombinantBacillus thuringiensisStrain-Producing Cyt1Aa and Cry11Ba, and the Binary Toxin, Bin, FromLysinibacillus sphaericus

“…Our post hoc measurements of synergy for binary interactions generally agreed with earlier reports. We observed a high level of synergy between Bin and Cry11Ba that was similar to those of Bin with other Cry toxins (Wirth et al 2004b). However, the interaction of Cry11Ba and Bin against our Bin-resistant colony, although synergistic, was considerably lower than reported for other Cry toxins (Wirth et al 2004b).…”

“…We observed a high level of synergy between Bin and Cry11Ba that was similar to those of Bin with other Cry toxins (Wirth et al 2004b). However, the interaction of Cry11Ba and Bin against our Bin-resistant colony, although synergistic, was considerably lower than reported for other Cry toxins (Wirth et al 2004b). Consequently, Cry11Ba may be poorly effective at suppressing high levels of Bin resistance, and this may be a contributing factor to the evolution of resistance to the recombinant.…”

Cytopathological Effects of Bacillus sphaericus Cry48Aa/Cry49Aa Toxin on Binary Toxin-Susceptible and -Resistant Culex quinquefasciatus Larvae

“…A similar effect was seen when the mixture of Bin toxin from B. sphaericus and Cry11Aa from B. thuringiensis serovar israelensis was used to overcome Bin toxin resistance in CqRL1/2362 larvae, which was comparable to the effect caused by Cry48Aa/Cry49Aa. With the Bin/Cry11Aa toxin combination, the major effect observed was mitochondrial (22,29). The effects of the Cry48Aa/Cry49Aa mixture on Bin-toxinresistant larvae were similar to those on the Bin-toxin-susceptible (CqSF) larvae and indicated that the action of the mixture does not rely on the presence of the Cqm1 receptor on the midgut cells.…”

Detection of an Allele Conferring Resistance to Bacillus sphaericus Binary Toxin in Culex quinquefasciatus Populations by Molecular Screening

“…Selection of resistance can be largely minimized through the coordinated use of Bin toxin with other insecticidal proteins, in mixtures or in rotation, in the scope of integrated control programs (22,39,40). Monitoring of population susceptibility, particularly focusing on the frequency of resistance alleles, is essential, and this study demonstrates the effectiveness of molecular tools to determine such frequency in Culex natural populations.…”