Lysinibacillus sphaericus is a Gram-positive, mesophilic, rod-shaped bacterium commonly found on soil. It can form resistant endospores that are tolerant to high temperatures, chemicals and ultraviolet light and can remain viable for long periods of time. It is of particular interest to the World Health Organization due to the larvicide effect of some strains against two mosquito genera, more effective than Bacillus thuringiensis, frequently used as a biological pest control. L. sphaericuscells in a vegetative state are also effective against Aedes aegypti larvae, an important vector of yellow fever and dengue viruses. L. sphaericus has five homology groups, with group II further dividing into subgroups IIA and IIB. Due to the low levels of homology between groups, it has been suggested that each might represent a distinct species, but owing to a lack of research on this topic, all remain designated as L. sphaericus.
Classification
The reclassification from Bacillus sphaericus to Lysinibacillus sphaericus is based on the fact that the Lysinibacillusgenus, in contrast to the type species of the genus Bacillus, contains peptidoglycan with lysine, aspartic acid, alanine and glutamic acide.
The entomopathogenic strains are found in the homology subgroup IIA, nonetheless, this group contains also non pathogenic isolates The insecticidal activity of some strains of L. sphaericus was first discovered in 1965 and further studies have shown mosquitoes to be the major target of this bacterium. There are reports of activity against other organisms such as the nematodeTrichostrongylus colubriformis to which it has lethal effects on the eggs. It is of important use in mosquito control programs worldwide and has high specificity against mosquito larvae in addition to being safe for mammals, fish, birds and nondipterean insects. The high toxicity strains produce during sporulation a binary toxincomposed ofBinA and BinB proteins, which is the major insecticidal component. The proteinBinB acts by binding to a receptor in the epithelial midgut cells, facilitating the entrance of BinA which causes cellular lysis. After being ingested by larvae, these proteins are solubilized in the gut and undergo proteolysis to active lower molecular weight derivatives. The vegetative cells of both high- and low-toxicity strains produce Mtx1, Mtx2 and Mtx3 toxins, but Mtx1 and Mtx2 are degraded by proteases during the stationary phase, consequently making them undetectable in sporulated cultures. In addition, the presence of binary-toxin genes and proteins has been determined in 18 pathogenic strains. Strains OT4b.2, OT4b.20, OT4b.25, OT4b.26 and OT4b.58 were found as toxic as the spores of the reference WHO strain 2362, against C. quinquefasciatus larvae.
