Identification and characterization of the new bacillus thuringiensis serovars pirenaica (serotype H57) and iberica (serotype H59)

Insecticidal Activity of Bacteria from Larvae Breeding Site with Natural Larvae Mortality: Screening of Separated Supernatant and Pellet Fractions

Mosquitoes can transmit to humans devastating and deadly pathogens. As many chemical insecticides are banned due to environmental side effects or are of reduced efficacy due to resistance, biological control, including the use of bacterial strains with insecticidal activity, is of increasing interest and importance. The urgent actual need relies on the discovery of new compounds, preferably of a biological nature. Here, we explored the phenomenon of natural larvae mortality in larval breeding sites to identify potential novel compounds that may be used in biological control. From there, we isolated 14 bacterial strains of the phylum Firmicutes, most of the order Bacillales. Cultures were carried out under controlled conditions and were separated on supernatant and pellet fractions. The two fractions and a 1:1 mixture of the two fractions were tested on L3 and early L4 Aedes albopictus. Two concentrations were tested (2 and 6 mg/L). Larvae mortality was recorded at 24, 48 and 72 h and compared to that induced by the commercialized B. thuringiensis subsp. israelensis. Of the 14 strains isolated, 11 were active against the A. albopictus larvae: 10 of the supernatant fractions and one pellet fraction, and mortality increased with the concentration. For the insecticide activity prediction in three strains of the Bacillus cereus complex, PCR screening of the crystal (Cry) and cytolytic (Cyt) protein families characteristic to B. thuringiensis subsp. israelensis was performed. Most of the genes coding for these proteins' synthesis were not detected. We identified bacterial strains that exhibit higher insecticidal activity compared with a commercial product. Further studies are needed for the characterization of active compounds.

Diversity of Bacillus thuringiensis Strains From Qatar as Shown by Crystal Morphology, δ-Endotoxins and Cry Gene Content

Bacillus thuringiensis (Bt) based insecticidal formulations have been recognized as one of the most successful, environmentally safe and sustainable method of controlling insect pests. Research teams worldwide are in search of Bt diversity giving more choices of bio-insecticides and alternatives to address insect resistance. In fact, there are many unexplored ecologies that could harbor novel Bt strains. This study is the first initiative to explore Bt strain diversity in Qatar. A collection of 700 Bt isolates was constructed. Scanning electron microscopy of Bt crystals showed different crystal forms, with a high abundance of spherical crystals compared to the bipyramidal ones. Among the spherical crystals, four different morphologies were observed. The δ-endotoxin content of parasporal crystals from each Bt isolate revealed that there are 16 different protein profiles among the isolates of the collection. On the other hand, plasmid pattern analysis showed seven different plasmid profiles. Their insecticidal activity was predicted by exploring the δ-endotoxin coding genes and conducting qualitative insecticidal bioassays. 19 smooth spherical crystal producing isolates have been identified that could be possible candidates for endotoxin production targeting Dipteran insects. Another group of 259 isolates producing bipyramidal and cuboidal crystals could target Lepidopteran and Coleopteran insects. The remaining 422 isolates have novel profiles. In conclusion, Qatari soil ecology provides a good collection and diversity of Bt isolates. In addition to strains harboring genes encoding common endotoxins, the majority are different and very promising for the search of novel insecticidal endotoxins.

The Replacement of five Consecutive Amino Acids in the Cyt1A Protein of Bacillus thuringiensis Enhances its Cytotoxic Activity against Lung Epithelial Cancer Cells

Cyt1A protein is a cytolytic protein encoded by the cyt gene of Bacillus thuringiensis subsp. israelensis (Bti) as part of the parasporal crystal proteins produced during the sporulation. Cyt1A protein is unique compared to the other endotoxins present in these parasporal crystals. Unlike δ-endotoxins, Cyt1A protein does not require receptors to bind to the target cell and activate the toxicity. It has the ability to affect a broad range of cell types and organisms, due to this characteristic. Cyt1A has been recognized to not only target the insect cells directly, but also recruit other endotoxins by acting as receptors. Due to these mode of actions, Cyt1A has been studied for its cytolytic activity against human cancer cell lines, although not extensively. In this study, we report a novel Cyt1A protein produced by a Bti strain QBT229 isolated from Qatar. When tested for its cytotoxicity against lung cancer cells, this local strain showed considerably higher activity compared to that of the reference Bti and other strains tested. The possible reasons for such enhanced activity were explored at the gene and protein levels. It was evidenced that five consecutive amino acid replacements in the β8 sheet of the Cyt1A protein enhanced the cytotoxicity against the lung epithelial cancer cells. Such novel Cyt1A protein with high cytotoxicity against lung cancer cells has been characterized and reported through this study.

Molecular characterisation of Bacillus thuringiensis strain MEB4 highly toxic to the Mediterranean flour moth Ephestia kuehniella Zeller (Lepidoptera: Pyralidae)

BACKGROUND

Cry2 proteins play an essential role in current Bacillus thuringiensis (Bt) applications and in the prevention of insect resistance to Cry1A toxins. This paper reports on the screening and characterisation of novel Bt strains harbouring effective cry2A-type genes and higher insecticidal activity to Ephestia kuehniella.

RESULTS

A total of 29 native Bt strains were screened to search for the potent strain against E. kuehniella. The plasmid pattern of the selected strains showed interesting variability. PCR-RFLP analysis of two amplified regions showed high sequence identity within the selected cry2A-type genes. SDS-PAGE and western blot analysis revealed the presence of Cry2Aa toxin only in the MEB4 and BLB240 strains. The activation of Cry2Aa protoxins by larval midgut juice, trypsin or chymotrypsin enzymes revealed significant differences in terms of proteolysis profiles. Interestingly, a 49 kDa band was detected in the proteolysis pattern of BLB240, suggesting the presence of a chymotrypsin cleavage site that might have affected its insecticidal activity. Further, bioassays demonstrated that MEB4 (103.08 ± 36 µg g(-1)) was more active than BLB240 (153.77 ± 45.65 µg g(-1)) against E. kuehniella.

CONCLUSION

Based on its potent insecticidal activity, the MEB4 strain could be considered to be an effective alternative agent for the control of E. kuehniella.

Draft genome sequences of two Bacillus thuringiensis strains and characterization of a putative 41.9-kDa insecticidal toxin

In this work, we report the genome sequencing of two Bacillus thuringiensis strains using Illumina next-generation sequencing technology (NGS). Strain Hu4-2, toxic to many lepidopteran pest species and to some mosquitoes, encoded genes for two insecticidal crystal (Cry) proteins, cry1Ia and cry9Ea, and a vegetative insecticidal protein (Vip) gene, vip3Ca2. Strain Leapi01 contained genes coding for seven Cry proteins (cry1Aa, cry1Ca, cry1Da, cry2Ab, cry9Ea and two cry1Ia gene variants) and a vip3 gene (vip3Aa10). A putative novel insecticidal protein gene 1143 bp long was found in both strains, whose sequences exhibited 100% nucleotide identity. The predicted protein showed 57 and 100% pairwise identity to protein sequence 72 from a patented Bt strain (US8318900) and to a putative 41.9-kDa insecticidal toxin from Bacillus cereus, respectively. The 41.9-kDa protein, containing a C-terminal 6× HisTag fusion, was expressed in Escherichia coli and tested for the first time against four lepidopteran species (Mamestra brassicae, Ostrinia nubilalis, Spodoptera frugiperda and S. littoralis) and the green-peach aphid Myzus persicae at doses as high as 4.8 µg/cm² and 1.5 mg/mL, respectively. At these protein concentrations, the recombinant 41.9-kDa protein caused no mortality or symptoms of impaired growth against any of the insects tested, suggesting that these species are outside the protein’s target range or that the protein may not, in fact, be toxic. While the use of the polymerase chain reaction has allowed a significant increase in the number of Bt insecticidal genes characterized to date, novel NGS technologies promise a much faster, cheaper and efficient screening of Bt pesticidal proteins.

Characterization of Tunisian Bacillus thuringiensis strains with abundance of kurstaki subspecies harbouring insecticidal activities against the lepidopteran insect Ephestia kuehniella

The study of 257 crystal-producing Bacillus thuringiensis isolates from bioinsecticide free soil samples collected from different sites in Tunisia, was performed by PCR amplification, using six primer pairs specific for cry1, cry2, cry3, cry4, and vip3A genes, by the investigation of strain plasmid pattern, crystal morphology and delta-endotoxin content and by the assessment of insecticidal activities against the lepidopteran insect Ephestia kuehniella. Based on plasmid pattern study, 11 representative strains of the different classes were subjected to morphological and molecular analyses. The comparison of the PFGE fingerprints confirmed the heterogeneity of these strains. B. thuringiensis kurstaki strains, harbouring at the same time the genes cry1A, cry2, cry1Ia, and vip3A, were the most abundant (65.4%). 33.34% of the new isolates showed particular delta-endotoxin profiles but no PCR products with the used primer sets. B. thuringiensis israelensis was shown to be also very rare among the Tunisian B. thuringiensis isolates diversity. These findings could have considerable impacts for the set up of new pest control biological agents.

Plasmid patterns of Bacillus thuringiensis type strains

Practically all Bacillus thuringiensis strains contain a set of self-replicating, extrachromosomal DNA molecules or plasmids, which vary in number and size in the different strains. The plasmid patterns obtained from gel electrophoresis have previously been used as a tool to characterize strains, but comparison of the plasmid patterns has been limited in the number and diversity of strains analyzed. In this report, we were able to compare the plasmid patterns of 83 type strains (out of 84) and 47 additional strains from six serotypes. The information obtained from this comparison showed the importance of this tool as a strain characterization procedure and indicates the complexity and uniqueness of this feature. For example, with one exception, all type strains showed a unique plasmid pattern. All were unique in such a way that none showed even a single comigrating plasmid in the agarose gels, and therefore, cluster analysis was impossible, indicating that plasmid patterns are qualitative rather than quantitative features. Furthermore, comparison between strains belonging to the same serotype showed a great difference in variability. Some serotypes (e.g., israelensis) showed the same basic pattern among all its strains, while other serotypes (e.g., morrisoni) showed a great diversity of patterns. These results indicate that plasmid patterns are valuable tools to discriminate strains below the serotype level.

Association analysis between serotype, cry gene content, and toxicity to Helicoverpa armigera larvae among Bacillus thuringiensis isolates native to Spain

Serotyping, cry gene content, and toxicity to Helicoverpa armigera were determined for 178 isolates of Bacillus thuringiensis native to Spain. A total of 13 different cry1 and cry2 genes were detected when isolates were screened by PCR analysis. Results showed that cry2 and cry1Ia were the most frequent cry genes in the collection (74 and 57%, respectively); whereas cry1D, cry1Aa, cry1Ab, and cry1C were only moderately abundant (49, 48, 47, and 36%, respectively). The most uncommon cry genes were cry1Ac, cry1E, cry1B, cry1Ib, cry1Ad, cry1F, and cry1G, with frequencies of 24, 14, 13, 8, 5, 5, and 1%, respectively. The distribution of some cry genes was somewhat associated with particular serovars. For example, genes cry1C and cry1D were especially frequent in the serovar aizawai, while cry1B was very frequent in the serovar thuringiensis. Bioassays against H. armigera larvae showed a wide variation in the insecticidal potency, even among strains sharing the same set of cry genes and within the same serotype.

PCR-based identification of Bacillus thuringiensis pesticidal crystal genes

The polymerase chain reaction (PCR) is a molecular tool widely used to characterize the insecticidal bacterium Bacillus thuringiensis. This technique can be used to amplify specific DNA fragments and thus to determine the presence or absence of a target gene. The identification of B. thuringiensis toxin genes by PCR can partially predict the insecticidal activity of a given strain. PCR has proven to be a rapid and reliable method and it has largely substituted bioassays in preliminary classification of B. thuringiensis collections. In this work, we compare the largest B. thuringiensis PCR-based screenings, and we review the natural occurrence of cry genes among native strains. We also discuss the use of PCR for the identification of novel cry genes, as well as the potential of novel technologies for the characterization of B. thuringiensis strains.

Contents of cry genes and insecticidal toxicity of Bacillus thuringiensis strains from terrestrial and aquatic habitats

AIMS

Two Bacillus thuringiensis collections from terrestrial and aquatic habitats were compared in order to study the possible interrelationships between habitat and biological characteristics (serovar, cry genes content and toxicity).

METHODS AND RESULTS

Bacillus thuringiensis strains were characterized by serology, PCR, and one-dose treatment against the noctuids Helicoverpa armigera and Spodoptera exigua, and the dipteran Tipula oleracea. A total of 12 and 10 different serovars were identified within terrestrial and aquatic strains, respectively. The number of non-toxic strains was greater in aquatic (41.6%) than in terrestrial habitats (5.3%). The genes cry1C, cry1D and cry1E were significantly more frequent in the terrestrial habitat. The cry1B gene was very frequent within thuringiensis strains.

CONCLUSIONS

A high diversity was found in terms of serovars present and cry genes content in both collections. The relative frequency of individual cry genes was different in both collections, and a serovar-dependent distribution of the cry1B gene was found. Some strains sharing the same set of cry genes differed in their toxicity, suggesting important differences in gene expression.

SIGNIFICANCE AND IMPACT OF THE STUDY

The inter-relationships between serology, cry gene content and toxicity may allow a better understanding of B. thuringiensis ecology.