Characterization of a mosquitocidal Bacillus thuringiensis serovar sotto strain isolated from Okinawa, Japan

Comparative Genomics of Bacillus thuringiensis Reveals a Path to Specialized Exploitation of Multiple Invertebrate Hosts

Understanding the genetic basis of host shifts is a key genomic question for pathogen and parasite biology. The Bacillus cereus group, which encompasses Bacillus thuringiensis and Bacillus anthracis, contains pathogens that can infect insects, nematodes, and vertebrates. Since the target range of the essential virulence factors (Cry toxins) and many isolates is well known, this group presents a powerful system for investigating how pathogens can diversify and adapt to phylogenetically distant hosts. Specialization to exploit insects occurs at the level of the major clade and is associated with substantial changes in the core genome, and host switching between insect orders has occurred repeatedly within subclades. The transfer of plasmids with linked cry genes may account for much of the adaptation to particular insect orders, and network analysis implies that host specialization has produced strong associations between key toxin genes with similar targets. Analysis of the distribution of plasmid minireplicons shows that plasmids with orf156 and orf157, which carry genes encoding toxins against Lepidoptera or Diptera, were contained only by B. thuringiensis in the specialized insect clade (clade 2), indicating that tight genome/plasmid associations have been important in adaptation to invertebrate hosts. Moreover, the accumulation of multiple virulence factors on transposable elements suggests that cotransfer of diverse virulence factors is advantageous in terms of expanding the insecticidal spectrum, overcoming insect resistance, or through gains in pathogenicity via synergistic interactions between toxins.IMPORTANCE Population genomics have provided many new insights into the formation, evolution, and dynamics of bacterial pathogens of humans and other higher animals, but these pathogens usually have very narrow host ranges. As a pathogen of insects and nematodes, Bacillus thuringiensis, which produces toxins showing toxicity to many orders of insects and other invertebrates, can be used as a model to study the evolution of pathogens with wide host ranges. Phylogenomic analysis revealed that host specialization and switching occur at the level of the major clade and subclade, respectively. A toxin gene co-occurrence network indicates that multiple toxins with similar targets were accumulated by the same cell in the whole species. This accumulation may be one of the strategies that B. thuringiensis has used to fight against host resistance. This kind of formation and evolution of pathogens represents a different path used against multiple invertebrate hosts from that used against higher animals.

Occurrence of parasporin-producing Bacillus thuringiensis in Vietnam

A total of 63 Bacillus thuringiensis isolates were recovered from urban soils of Hanoi, Vietnam. Of these, 34 were identified to 12 H serogroups. None of the isolates showed larvicidal activities against three lepidopterous insects. Three isolates belonging to the two serovars, colmeri (H21) and konkukian (H34), were highly toxic to larvae of the mosquito Aedes aegypti. Parasporal inclusion proteins of four isolates exhibited cytocidal activities against HeLa cells. Immunologically, proteins of four isolates were closely related to parasporin-1 (Cry31Aa), a parasporal protein that preferentially kills human cancer cells. Haemolytic activities were associated with parasporal proteins of the three mosquitocidal isolates but not with those of the four cancer-cell-killing isolates. PCR experiments and nucleotide sequence analysis revealed that the genes of four anti-cancer isolates are closely related to the gene parasporin-1 (cry31Aa) but are dissimilar to those of the three other existing parasporins. Our results suggest that the soil of northern Vietnam is a good reservoir of parasporin-producing B. thuringiensis.

Cloning and Characterization of Two Novel Genes, cry24B and s1orf2, from a Mosquitocidal Strain of Bacillus thuringiensis serovar sotto

Two new crystal protein genes, cry24B and s1orf2, were cloned from a mosquitocidal Bacillus thuringiensis serovar sotto strain. The cry24B and s1orf2 genes encoded a 76-kDa and 62-kDa protein, respectively. The Cry24B protein retained five conserved regions commonly found in the existing Cry proteins. The amino acid sequence of the S1ORF2 had a high homology to that of the ORF2 protein of B. thuringiensis serovar jegathesan. Southern hybridization experiments with a cry24B gene-specific probe revealed that these genes are located on two large plasmids of > 100 kb. When the two genes, cry24B and s1orf2, were expressed in an acrystalliferous B. thuringiensis host, the proteins were synthesized and accumulated as inclusions. These inclusions exhibited no larvicidal activities against three mosquito species: Aedes aegypti, Anopheles stephensi, and Culex pipiens molestus. Likewise, the inclusions contained no cytocidal activity against HeLa cells.

A new insertion variant, IS231I, isolated from a mosquito-specific strain of Bacillus thuringiensis

A new insertion variant belonging to the family IS231, designated IS231I, was isolated from a mosquito larvicidal strain of the Bacillus thuringiensis serovar sotto (H4ab). IS231I was 1653 bp long and delimited by two 20 bp inverted repeats with one mismatch, flanked by two perfect 11 bp direct repeats. The element contained a single open reading frame (ORF) encoding 478 amino acids and five conserved domains: N1, N2, N3, C1, and C2. The 5' noncoding region upstream of the ORF, presumed to form a stable stem-and-loop structure, was highly conserved in IS231I. The secondary structure conformation had a deduced free energy (DeltaG=25 degrees C) of -17.2 kcal/mol. Comparison of the IS231I amino acid sequence with those of the 10 existing IS variants revealed that the new variant shares 89% identity with IS231A and IS231B, 65-66% with IS231M and IS231N, and 38% with IS231W.