Correlation between serovars of Bacillus thuringiensis and type I beta-exotoxin production

Evaluation of the Toxicity of Supernatant Cultures and Spore-Crystal Mixtures of Bacillus thuringiensis Strains Isolated from Algeria

Bacillus thuringiensis (Bt) is the most used technology for biological control of insect pathogens worldwide. In order to select new Bt candidates challenging the emergence of insect's resistance, a mass bioassay and molecular screening was performed on an autochthonous collection. Toxicity assays against neonate larvae of three lepidopteran species (Mamestra brassicae, Grapholita molesta, and Spodoptera exigua) were conducted using spore-crystal mixtures and supernatant cultures of 49 Bt isolates harboring at least one gene coding for a lepidopteran-specific insecticidal protein. A threshold of 30% of "functional mortality" was used to discriminate between "nontoxic" and "toxic" isolates. The toxicity of many Bt isolates competed with that of Btk-HD1. However, only three of them (Bl4NA, Bl5NA, and Bl9NA) showed high toxicity in both spore-crystal mixtures and supernatant cultures against the three lepidopteran species. The Bt isolates Bl4NA and Bl9NA express a protein of 130 kDa whereas the Bt isolate Bl5NA expresses a protein of 65-70 kDa. The LC-MS/MS results indicate that the major peptides in the 130 kDa band of Bl9NA were Cry1Da, Cry1Ca, Cry1Ab, and Cry1Aa, and those in the 70 kDa band of Bl5NA were Cry1Aa and Cry1Ca. The evaluation of the protein content of the supernatants by comparison to Btk-HD1 indicates the overproduction of Vip3 proteins in these strains (most likely Vip3Aa in Bl4NA and Bl9NA and Vip3Ca in Bl5NA). In addition, these three Bt strains do not produce β-exotoxins. Based on our results, the three selected strains could be considered promising candidates to be used in insect pest control.

Repertoire of the Bacillus thuringiensis Virulence Factors Unrelated to Major Classes of Protein Toxins and Its Role in Specificity of Host-Pathogen Interactions

Bacillus thuringiensis (Bt) is a Gram-positive soil bacteria that infects invertebrates, predominantly of Arthropoda phylum. Due to its immense host range Bt has become a leading producer of biopesticides applied both in biotechnology and agriculture. Cytotoxic effect of Bt, as well as its host specificity, are commonly attributed either to proteinaceous crystal parasporal toxins (Cry and Cyt) produced by bacteria in a stationary phase or to soluble toxins of Vip and Sip families secreted by vegetative cells. At the same time, numerous non-toxin virulence factors of Bt have been discovered, including metalloproteases, chitinases, aminopolyol antibiotics and nucleotide-mimicking moieties. These agents act at each stage of the B. thuringiensis invasion and contribute to cytotoxic properties of Bt strains enhancing toxin activity, ensuring host immune response evasion and participating in extracellular matrix degeneration. In this review we attempt to classify Bt virulence factors unrelated to major groups of protein toxins and discuss their putative role in the establishment of Bt specificity to various groups of insects.

Bacillus thuringiensis toxins: an overview of their biocidal activity

Bacillus thuringiensis (Bt) is a Gram positive, spore-forming bacterium that synthesizes parasporal crystalline inclusions containing Cry and Cyt proteins, some of which are toxic against a wide range of insect orders, nematodes and human-cancer cells. These toxins have been successfully used as bioinsecticides against caterpillars, beetles, and flies, including mosquitoes and blackflies. Bt also synthesizes insecticidal proteins during the vegetative growth phase, which are subsequently secreted into the growth medium. These proteins are commonly known as vegetative insecticidal proteins (Vips) and hold insecticidal activity against lepidopteran, coleopteran and some homopteran pests. A less well characterized secretory protein with no amino acid similarity to Vip proteins has shown insecticidal activity against coleopteran pests and is termed Sip (secreted insecticidal protein). Bin-like and ETX_MTX2-family proteins (Pfam PF03318), which share amino acid similarities with mosquitocidal binary (Bin) and Mtx2 toxins, respectively, from Lysinibacillus sphaericus, are also produced by some Bt strains. In addition, vast numbers of Bt isolates naturally present in the soil and the phylloplane also synthesize crystal proteins whose biological activity is still unknown. In this review, we provide an updated overview of the known active Bt toxins to date and discuss their activities.

Identification of distinct Bacillus thuringiensis 4A4 nematicidal factors using the model nematodes Pristionchus pacificus and Caenorhabditis elegans

Bacillus thuringiensis has been extensively used for the biological control of insect pests. Nematicidal B. thuringiensis strains have also been identified; however, virulence factors of such strains are poorly investigated. Here, we describe virulence factors of the nematicidal B. thuringiensis 4A4 strain, using the model nematodes Pristionchus pacificus and Caenorhabditis elegans. We show that B. thuringiensis 4A4 kills both nematodes via intestinal damage. Whole genome sequencing of B. thuringiensis 4A4 identified Cry21Ha, Cry1Ba, Vip1/Vip2 and β-exotoxin as potential nematicidal factors. Only Cry21Ha showed toxicity to C. elegans, while neither Cry nor Vip toxins were active against P. pacificus, when expressed in E. coli. Purified crystals also failed to intoxicate P. pacificus, while autoclaved spore-crystal mixture of B. thuringiensis 4A4 retained toxicity, suggesting that primary β-exotoxin is responsible for P. pacificus killing. In support of this, we found that a β-exotoxin-deficient variant of B. thuringiensis 4A4, generated by plasmid curing lost virulence to the nematodes. Thus, using two model nematodes we revealed virulence factors of the nematicidal strain B. thuringiensis 4A4 and showed the multifactorial nature of its virulence.

Toxicity intraperitoneal and intragastric route of Bacillus thuringiensis and Melia azedarach in mice

The aim of this investigation was the assessment of toxicity of two new isolates of Bacillus thuringiensis, and the aqueous extract of Melia azedarach through in vivo assays in CF1 mice. Bt 1958-2, Bt 2014-2 and the BTh Thuricide 63 standard isolates were grown in liquid usual glicosed medium, and Cry proteins were purified by centrifugation on a sucrose gradient. The supernatant was autoclaved at 121º C, 15min. to maintain the exotoxins. Dehydrated leaves of M. azedarach were used to prepare a 10% aqueous extract. Mice were treated either orally or intraperitoneally with a whole bacterial suspension (1.10(10) UFC/mL), a culture supernatant or purified crystal protein (50 µg/mL), and with the plant extract (50 µg/mL). The stomachs of the mice were collected and observed in stereomicroscopy, and the stomach contents were analyzed in 10% SDS-PAGE. Results showed that none of the oral treatments were toxic to mice, but intraperitoneal bacterial suspensions were lethal to the animals 6 - 24 hours after injection. In conclusion, the Cry proteins of the new B. thuringiensis isolates must be evaluated for their use as tools in the biotechnology field, since they do not show toxicity against mammals, intragastrically or peritoneally, just like the M. azedarach aqueous extract (10%), with those being indicated for the biological control of pest insects.

The Bacillus cereus group: novel aspects of population structure and genome dynamics

AIMS

To provide new insights into the population and genomic structure of the Bacillus cereus group of bacteria.

METHODS AND RESULTS

The genetic relatedness among B. cereus group strains was assessed by multilocus sequence typing (MLST) using an optimized scheme based on seven chromosomal housekeeping genes. A set of 48 strains from different clinical sources was included, and six clonal complexes containing several genetically similar isolates from unrelated patients were identified. Interestingly, several clonal groups contained strains that were isolated from similar human sources. Furthermore, comparative whole genome sequence analysis of 16 strains led to the discovery of novel ubiquitous genome features of the B. cereus group, such as atypical group II introns, IStrons, and hitherto uncharacterized repeated elements.

CONCLUSIONS

The B. cereus group constitutes a coherent population unified by the presence of ubiquitous and specific genetic elements which do not show any pattern, either in their sequences or genomic locations, which allows to differentiate between the member species of the group. Nevertheless, the population is very dynamic, as particular lineages of clinical origin can evolve to form clonal complexes. At the genome level, the dynamic behaviour is indicated by the presence of numerous mobile and repeated elements.

SIGNIFICANCE AND IMPACT OF THE STUDY

The B. cereus group of bacteria comprises species that are of medical and economic importance. The MLST data, along with the primers and protocols used, will be available in a public, web-accessible database (http://mlstoslo.uio.no).