Complete Genome Sequence of the Larvicidal Bacterium Lysinibacillus sphaericus Strain OT4b.25

Bacterial Toxins Active against Mosquitoes: Mode of Action and Resistance

Larvicides based on the bacteria Bacillus thuringiensis svar. israelensis (Bti) and Lysinibacillus sphaericus are effective and environmentally safe compounds for the control of dipteran insects of medical importance. They produce crystals that display specific and potent insecticidal activity against larvae. Bti crystals are composed of multiple protoxins: three from the three-domain Cry type family, which bind to different cell receptors in the midgut, and one cytolytic (Cyt1Aa) protoxin that can insert itself into the cell membrane and act as surrogate receptor of the Cry toxins. Together, those toxins display a complex mode of action that shows a low risk of resistance selection. L. sphaericus crystals contain one major binary toxin that display an outstanding persistence in field conditions, which is superior to Bti. However, the action of the Bin toxin based on its interaction with a single receptor is vulnerable for resistance selection in insects. In this review we present the most recent data on the mode of action and synergism of these toxins, resistance issues, and examples of their use worldwide. Data reported in recent years improved our understanding of the mechanism of action of these toxins, showed that their combined use can enhance their activity and counteract resistance, and reinforced their relevance for mosquito control programs in the future years.

Horizontal transfer of large plasmid with type IV secretion system and mosquitocidal genomic island with excision and integration capabilities in Lysinibacillus sphaericus

We identified a ~30-kb genomic island (named GI8) carrying the binary toxin gene operon binA/binB on both the chromosome and large pBsph plasmid in the mosquitocidal Lysinibacillus sphaericus C3-41 strain. We found that GI8 is related to the occurrence of binA/binB within L. sphaericus and displays excision and integration capability by recognizing the attB region, which consists of a 2-nt target site (AT) flanked by an 11-nt imperfect inverted repeat. pBsph and two pBsph-like plasmids (p2362 and p1593) were found to carry a type IV secretion system (T4SS) and displayed transmissibility within a narrow host range specific to L. sphaericus. GI8 can be co-transferred with pBsph as a composite element by integration into its attB site, then excised from pBsph and re-integrated into the chromosomal attB site in the new host. The potential hosts of GI8, regardless of whether they are toxic or non-toxic to mosquito larvae, share good collinearity at the chromosomal level. Data indicated that the appearance of the mosquitocidal L. sphaericus lineage was driven by horizontal transfer of the T4SS-type conjugative plasmid and GI8 with excision and specific integration capability.

Efficacy of the vegetative cells of Lysinibacillus sphaericus for biological control of insecticide-resistant Aedes aegypti

Background

The control of Aedes aegypti is usually based on chemical insecticides, but the overuse of these compounds has led to increased resistance. The binary toxin produced by Lysinibacillus sphaericus in the final stages of sporulation is used for mosquito control due to its specificity against the culicid larvae; however, it has been proved that Ae. aegypti is refractory for this toxin. Currently, there is no evidence of the use of L. sphaericus vegetative cells for mosquito biocontrol. Therefore, in this study, the vegetative cells of three L. sphaericus strains were assessed against a field-collected Ae. aegypti, resistant to temephos, and the reference Rockefeller strain.

Results

Vegetative cells of L. sphaericus 2362, III(3)7 and OT4b.25 produced between 90% and 100% of larvae mortality in the reference Rockefeller strain. Effective concentrations of each L. sphaericus strain for the four larval stages ranged from 1.4 to 2 × 10⁷ CFU/ml. Likewise, a consortium of L. sphaericus assessed against a field-collected Ae. aegypti resistant to temephos and the Rockefeller strain caused 90% of larvae mortality. Concentrations of L. sphaericus consortium that resulted in larvae mortality of field-collected and Rockefeller Ae. aegypti ranged from 1.7 to 2.5 × 10⁷ CFU/ml. The vegetative cells of L. sphaericus have no effect on the Ae. aegypti eggs and pupae.

Conclusions

The vegetative cells of L. sphaericus are effective against Ae. aegypti larvae, meaning that it could be used in the biological control of these mosquito species. Since the L. sphaericus consortium was effective against temephos-resistant Ae. aegypti, vegetative cells could be an alternative to overcome insecticide-resistant populations. Further studies, should be conducted to reveal the mode of action and the toxic principle of L. sphaericus vegetative cells.

Comparative genomics reveals Lysinibacillus sphaericus group comprises a novel species

Background

Early in the 1990s, it was recognized that Lysinibacillus sphaericus, one of the most popular and effective entomopathogenic bacteria, was a highly heterogeneous group. Many authors have even proposed it comprises more than one species, but the lack of phenotypic traits that guarantee an accurate differentiation has not allowed this issue to be clarified. Now that genomic technologies are rapidly advancing, it is possible to address the problem from a whole genome perspective, getting insights into the phylogeny, evolutive history and biology itself.

Results

The genome of the Colombian strain L. sphaericus OT4b.49 was sequenced, assembled and annotated, obtaining 3 chromosomal contigs and no evidence of plasmids. Using these sequences and the 13 other L. sphaericus genomes available on the NCBI database, we carried out comparative genomic analyses that included whole genome alignments, searching for mobile elements, phylogenomic metrics (TETRA, ANI and in-silico DDH) and pan-genome assessments. The results support the hypothesis about this species as a very heterogeneous group. The entomopathogenic lineage is actually a single and independent species with 3728 core genes and 2153 accessory genes, whereas each non-toxic strain seems to be a separate species, though without a clear circumscription. Toxin-encoding genes, binA, B and mtx1, 2, 3 could be acquired via horizontal gene transfer in a single evolutionary event. The non-toxic strain OT4b.31 is the most related with the type strain KCTC 3346.

Conclusions

The current L. sphaericus is actually a sensu lato due to a sub-estimation of diversity accrued using traditional non-genomics based classification strategies. The toxic lineage is the most studied with regards to its larvicidal activity, which is a greatly conserved trait among these strains and thus, their differentiating feature. Further studies are needed in order to establish a univocal classification of the non-toxic strains that, according to our results, seem to be a paraphyletic group.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3056-9) contains supplementary material, which is available to authorized users.

Complete genome sequencing and comparative genomic analysis of functionally diverse Lysinibacillus sphaericus III(3)7

Lysinibacillus sphaericus III(3)7 is a native Colombian strain, the first one isolated from soil samples. This strain has shown high levels of pathogenic activity against Culex quinquefaciatus larvae in laboratory assays compared to other members of the same species. Using Pacific Biosciences sequencing technology we sequenced, annotated (de novo) and described the genome of strain III(3)7, achieving a complete genome sequence status. We then performed a comparative analysis between the newly sequenced genome and the ones previously reported for Colombian isolates L. sphaericus OT4b.31, CBAM5 and OT4b.25, with the inclusion of L. sphaericus C3-41 that has been used as a reference genome for most of previous genome sequencing projects. We concluded that L. sphaericus III(3)7 is highly similar with strain OT4b.25 and shares high levels of synteny with isolates CBAM5 and C3-41.