Mosquitocidal bacterial toxins (Bacillus sphaericus and B. thuringiensis serovar israelensis): Mode of action, cytopathological effects and mechanism of resistance

Subbiah Poopathi* and S. Abidha

Unit of Microbiology and Immunology, Vector Control Research Centre (Indian Council of Medical Research), Medical complex, Indira Nagar, Puducherry – 60 5006, India.

*Corresponding author. E-mail: Subbiahpoopathi@rediffmail.com. Tel: 91-9443957479. Fax: 91-413-2272041.

Accepted 4 September, 2010

Abstract

Bacillus sphaericus Neide (Bs) and Bacillus thuringiensis serovar israelensis deBarjac (Bti) provide effective alternatives to broad spectrum larvicides in many situations with little or no environmental impact. Taking into account environmental benefits including safety for humans and other non-target organisms, reduction of pesticide residues in the aquatic environment, increased activity of most other natural enemies and increased biodiversity in aquatic ecosystems, their advantages are numerous. In addition to recombinant bacteria used as larvicides, research is also underway to develop transgenic algae and cyanobacteria using larvicidal endotoxins of Bti and Bs. The advent of recombinant DNA technology is now having an enormous impact on agriculture and medicine and it is appropriate that the ability to manipulate and recombine genes with this technology be applied to improving larvicides for vector control. These new recombinant bacteria are as potent as many synthetic chemical insecticides yet are much less prone to resistance, as they typically contain a mixture of endotoxins with different modes of action. The existing recombinants also have what can be considered disadvantageous in that they do not show significantly improved activity against aedine and anopheline mosquitoes in comparison to Bti. But it may be possible to overcome this limitation using some of the newly discovered mosquitocidal proteins such as the Mtx proteins and peptides such as the trypsin-modulating oostatic factor which could be easily engineered for high expression in recombinant bacteria. While other microbial technologies such as recombinant algae and other bacteria are being evaluated, it has yet to be shown that these are as efficacious and environmentally friendly as Bti and Bs. By combining the genes from a variety of organisms, it should ultimately be possible to design `smart' bacteria that will seek out and kill larvae of specific vector mosquitoes. Thus, recombinant bacteria show excellent promise for development and use in operational vector control programs, hopefully within the next few years.

Key words: Bacillus sphaericus, Bacillus thuringiensis serovar israelensis, bacterial toxins, Culex quinquefasciatus, Anopheles stephensi, Aedes aegypti, mode of action, resistance, management of resistance.