Useful bacteria for agriculture – Bacillus thuringiensis and Bacillus sphaericus
Bacillus thuringiensis is a gram-positive bacterium that, during sporulation, synthesizes proteinaceous, parasporal crystals, which are toxic against a wide variety of target pests: many different species among the orders Lepidoptera, Diptera, Coleoptera as well as other insect orders, nematodes, mites and protozoa. B. thuringiensis can be recovered from many different samples and surfaces, and it does not need an insect for multiplication. However, many B.thuringiensis strains produce crystals that are not insecticidal.
Different toxin genes can be found in B.thuringiensis which receive different names: ICP, cry, kurhd1, Bta, etc. The proteins from the cry genes are protoxins which solubilize in the alkaline insect midgut and are proteolytically converted into a toxic core fragment. The regions that determine insecticidal specificity present a greater variation. Tipically, cry1 gene products kill Lepidopteran larvae, although some genes encode other proteins that are toxic to other species such as Coleoptera and Diptera. Cry2 gene products are toxic against Lepidoptera and in some cases Diptera. Some strains (HD-1) contain and express both cry1 and cry2 genes, exhibiting a dual insecticidal activity. And, for example: cry3, cry7 and cry8 genes are toxic against Coleoptera.
The use of Bt genes is also interesting in the genetic engineering of plants. When the Bt cry gene is transferred to a plant, such as corn, the genetically modified plant is able to produce the Bt cry protein, so it produces endotoxins that are able to kill different pests.
Bacillus sphaericus is also a gram-positive bacterium found in soil. It is interesting due to its insecticidal activity towards different organisms. However, its toxicity is specifically important against two mosquito genera: Culex and Anopheles. Some strains of B.sphaericus also produce crystal toxins during sporulation: the damage produced when these proteins are released in the larva’s midgut leads to the mosquitoes’ death.