Bacterial larvicides for vector control: mode of action of toxins and implications for resistance

Abstract

Vector control can be an effective strategy to interrupt disease transmission and biolarvicides based on the entomopathogenic bacteria Bacillus sphaericus, and Bacillus thuringiensis serovar israelensis (Bti) have been successfully used to control species of public health relevance from the genera Aedes, Culex, Anopheles and Simulium. The most important feature of these agents is their ability to produce insecticidal proteins with selective action on the larval midgut. These protoxins are produced as crystals that, once ingested by larvae, are processed into active toxins, interact with receptors in the midgut epithelium and trigger cytopathological effects leading to larval death. B. sphaericus and Bti toxins share the initial steps of the mode of action; however, they interact with different midgut molecules. B. sphaericus presents a single larvicidal factor, the binary (Bin) toxin, whose action relies on the binding to one class of midgut receptors, while Bti crystals contain four protoxins (Cry4Aa, Cry4Ba, Cry11Aa and Cyt1Aa), which display interactions with multiple midgut receptors. The mode of action of B. sphaericus displays a greater potential for resistance selection, compared to Bti, and, to date, there is no record of insect resistance to the latter, contrarily to B. sphaericus. The set of mosquitocidal toxins and their interaction with midgut target sites are described in this review, as well as the implications for the potential to select resistance amongst exposed populations. These biolarvicides have specific mode of action that rely on unique interactions and make them the most selective agents to control Diptera insects actually available.

Keywords:

• biolarvides
• vectors
• Bacillus sphaericus
• Bti
• mode of action
• resistance

Acknowledgements

We thank Henry R. Rupp, MA, for kindly reviewing this manuscript, Dr. Cláudia Maria Fontes de Oliveira, Dr. Sinval Brandão Filho and Dr. Zulma Medeiros for supporting its preparation. This work was supported by Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco-FACEPE (APQ 0427-2.13/08), Conselho Nacional de Pesquisa-CNPq (472491/2012-1) and Fundação Osvaldo Cruz-FIOCRUZ.