Deletion by in vivo recombination shows that the 28-kilodalton cytolytic polypeptide from Bacillus thuringiensis subsp. israelensis is not essential for mosquitocidal activity

Cys183 and Cys258 in Cry49Aa toxin from Lysinibacillus sphaericus are essential for toxicity to Culex quinquefasciatus larvae

The two-component Cry48Aa/Cry49Aa toxin produced by Lysinibacillus sphaericus shows specifically toxic to Culex quinquefasciatus mosquito larvae. Cry49Aa C-terminal domain is responsible for specific binding to the larval gut cell membrane, while its N-terminal domain is required for interaction with Cry48Aa. To investigate functional role of cysteine in Cry49Aa, four cysteine residues at positions 70, 91, 183, and 258 were substituted by alanine. All mutants showed similar crystalline morphology and comparable yield to that of the wild type except that the yield of the C91A mutant was low. Four cysteine residues did not involve in disulfide bond formation within or between Cry49Aa molecules. Cys91, Cys183, and Cys258 are essential for larvicidal activity against C. quinquefasciatus larvae, while Cys70 is not. Substitution at C91, C183, and C258 caused weaker Cry48Aa- Cry49Aa interaction, while mutations at C183 and C258 reduced the binding capacities to the larval gut cell membrane. Thus, Cysteine residues at position 91, 183, and 258 in Cry49Aa are required for full toxicity of Cry48Aa/Cry49Aa toxin.

Identification of Cyt2Ba from a New Strain of Bacillus thuringiensis and Its Toxicity in Bradysia difformis

Bradysia difformis is one of the most damaging pests in mushroom production in China. In this study, eight Bacillus thuringiensis strains were analyzed for insecticidal activity in B. difformis. The strain JW-1 showed the highest insecticidal activity against B. difformis larvae, but did not inhibit the mycelial growth of Pleurotus ostreatus and P. geesteranus. The 16S rRNA gene (1397 bp) and cyt2 gene (792 bp) were obtained from strain JW-1. The phylogenetic tree based on 16S rRNA gene and Cyt2 toxin showed that strain JW-1 was a member of B. thuringiensis and Cyt2 toxin belonged to Cyt2Ba toxin cluster. The Cyt2Ba toxin from strain JW-1 was overexpressed in E. coli as a fusion protein and the fusion protein (70 kDa) was purified by Ni-IDA affinity chromatography. The purified Cyt2Ba fusion protein was toxic to B. difformis larvae (LC₅₀ was 2.25 ng/mL). The identification of Cyt2Ba from strain JW-1 and confirmation of the insecticidal activity of Cyt2Ba in B. difformis provided a new means of biological control of the important pest in mushroom production.

Interaction of Lysinibacillus sphaericus Cry48Aa/Cry49Aa toxin with midgut brush-border membrane fractions from Culex quinquefasciatus larvae

The Cry48Aa/Cry49Aa mosquitocidal toxin from Lysinibacillus sphaericus was uniquely composed of a three-domain (Cry) toxin and binary (Bin) toxin-like protein, with high toxicity against Culex spp. However, its mode of action against the target mosquitoes is still unknown. In this study, Cry48Aa, Cry49Aa and its N- and C-terminal truncated proteins were expressed and purified, and the binding affinities of the purified proteins with midgut brush-border membrane fractions (BBMFs) from Culex quin-quefasciatus larvae were performed. The results showed that both Cry48Aa and Cry49Aa have specific and high binding affinity to BBMFs, with dissociation constants of 9.5 ± 1.8 and 25.4 ± 3.8 nM, respectively. Competition assays demonstrated that Cry49Aa C-terminal derivatives were able to bind to the BBMFs, whereas Far-Western dot blot analysis revealed that its N-terminal constructs interacted with Cry48Aa. Nevertheless, larvicidal activity was almost lost when Cry49Aa truncated proteins, either individually or in pairs, combined with Cry48Aa. It is concluded that Cry49Aa is responsible for receptor binding and interaction with Cry48Aa and plays an important role in the mechanism of action of these two-component toxins.

Mtx toxins from Lysinibacillus sphaericus enhance mosquitocidal cry-toxin activity and suppress cry-resistance in Culex quinquefasciatus

The interaction of Mtx toxins from Lysinibacillus sphaericus (formerly Bacillus sphaericus) with Bacillus thuringiensis subsp. israelensis Cry toxins and the influence of such interactions on Cry-resistance were evaluated in susceptible and Cry-resistant Culex quinquefasciatus larvae. Mtx-1 and Mtx-2 were observed to be active against both susceptible and resistant mosquitoes; however varying levels of cross-resistance toward Mtx toxins were observed in the resistant mosquitoes. A 1:1 mixture of either Mtx-1 or Mtx-2 with different Cry toxins generally showed moderate synergism, but some combinations were highly toxic to resistant larvae and suppressed resistance. Toxin synergy has been demonstrated to be a powerful tool for enhancing activity and managing Cry-resistance in mosquitoes, thus Mtx toxins may be useful as components of engineered bacterial larvicides.

The utility of camptothecin as a synergist of Bacillus thuringiensis var. kurstaki and nucleopolyhedroviruses against Trichoplusia ni and Spodoptera exigua

We studied the effect of combining microbial pesticides with camptothecin (CPT) on the mortality of two lepidopteran insects: Trichoplusia ni (Hübner) and Spodoptera exigua (Hübner). CPT is an alkaloid that is often used as an anticancer agent. Here, CPT was evaluated as a microbial pesticide synergist of Bacillus thuringiensis (Bt) and insect baculovirus. The toxicity of CPT and its synergistic effects on two microbial pesticides were studied using the diet overlay method. Bioassay results showed that CPT significantly enhances the toxicity of Bt variety kurstaki to S. exigua and T ni. In addition, CPT strongly enhanced the infectivity of Autographa californica (Speyer) multinucleocapsid nucleopolyhedrovirus (AcMNPV) and S. exigua nucleopolyhedrovirus (SeMNPV). Using light microscopy, we found that CPT disrupts the peritrophic membrane of T. ni larvae and severely affects the structure of the midgut, resulting in an abnormal gut lumen morphology. We speculate that CPT increases toxicity by affecting the permeability of the peritrophic membrane.

Inheritance, stability, and dominance of cry resistance in Culex quinquefasciatus (Diptera: Culicidae) selected with the three cry toxins of Bacillus thuringiensis subsp. israelensis

Mendelian crosses were used to study the mode of inheritance of Cry toxin resistance in a Culex quinquefasciatus Say (Diptera: Culicidae) colony (CqAB11A) that evolved insecticide resistance under laboratory selection with a deletion mutant of Bacillus thuringiensis subsp. israelensis de Barjac lacking the Cyt1Aa toxin component but containing its three major Cry toxins, Cry4Aa, Cry4Ba, and Cry11Aa. High levels of resistance were observed to Cry toxins. F1 offspring of reciprocal crosses to a sensitive colony showed intermediate levels of resistance with no maternal effect, indicating autosomal inheritance. Dose-response data for backcross offspring deviated significantly from the monofactorial model when tested with Cry4Aa + Cry4Ba + Cry11Aa, Cry4Aa + Cry4Ba, or Cry11Aa. However, tests with Cry11Ba from B. thuringiensis subsp. jegathesan (Seleena, Lee, Lecadet) fit the monofactorial model. Dominance of F1 offspring was calculated at different concentrations of Cry-toxin suspensions and, as reported for other Cry-resistant Culex, generally decreased as concentration increased. A subset of colony CqAB11A was reared without selection pressure for 18 generations with little change in susceptibility, indicating a highly homozygous population. Consistent with reports for other Cry-resistant Culex, the data show these mosquitoes evolved resistance to B. thuringiensis Cry toxins at multiple loci in response to selection pressure and that cross-resistance to Cry11Ba was conferred by one of those loci.

Expression of cryIVA and cryIVB Genes, Independently or in Combination, in a Crystal-Negative Strain of Bacillus thuringiensis subsp. israelensis

The cryIVA and cryIVB genes, encoding the 125- and 135-kDa proteins, respectively, of Bacillus thuringiensis subsp. israelensis, were cloned either alone or together into a shuttle vector and expressed in a nontoxic strain of B. thuringiensis subsp. israelensis. The CryIVB protein was produced at a high level during sporulation and accumulated as inclusions; in contrast, the CryIVA polypeptide did not form such structures unless it was cloned on a higher-copy-number plasmid. Transcriptional fusions between the cryIVA or cryIVB gene promoter and the lacZ gene were constructed. The poor synthesis of CryIVA was not due to a poor efficiency of transcription from the cryIVA gene promoter. Mosquitocidal assays performed with purified inclusions showed that CryIVA was toxic for larvae of the species Aedes aegypti, Anopheles stephensi, and Culex pipiens, whereas CryIVB displayed activity only toward Aedes aegypti and Anopheles stephensi. The activity of inclusions containing both polypeptides was higher than that of single-peptide inclusions but was not as high as that of the native crystals, which contain at least four polypeptides.

Influence of Exposure to Single versus Multiple Toxins of Bacillus thuringiensis subsp. israelensis on Development of Resistance in the Mosquito Culex quinquefasciatus (Diptera: Culicidae)

The impending widespread use of transgenic crop plants encoding a single insecticidal toxin protein of Bacillus thuringiensis has focused attention on the perceived risk of rapid selection of resistance in target insects. We have used Bacillus thuringiensis subsp. israelensis toxins as a model system and determined the speed and magnitude of evolution of resistance in colonies of the mosquito Culex quinquefasciatus during selection for 28 consecutive generations with single or multiple toxins. The parental strain was synthesized by combining approximately 500 larvae from each of 19 field collections obtained from the states of California, Oregon, Louisiana, and Tennessee. At least 10,000 larvae were selected in each generation of each line at an average mortality level of 84%. The susceptibilities of the parental and selected lines were compared in parallel tests in every third generation by using fresh suspensions of toxin powders. The normal toxin complement of B. thuringiensis subsp. israelensis consists of four toxins, CryIVA, CryIVB, CryIVD, and CytA. Resistance became evident first in the line that was selected with a single toxin (CryIVD), attaining the highest level (resistance ratio [RR], >913 at 95% lethal concentration) by generation F(inf28) when the study was completed. Resistance evolved more slowly and to a lower level (RR, >122 by F(inf25)) in the line selected with two toxins (CryIVA+CryIVB) and lower still (RR, 91 by F(inf28)) in the line selected with three toxins (CryIVA+CryIVB+ CryIVD). Resistance was remarkably low (RR, 3.2) in the line selected with all four toxins. The results reveal the importance of the full complement of toxins found in natural populations of B. thuringiensis subsp. israelensis as an effective approach to resistance management.

Evolution of resistance to the Bacillus sphaericus Bin toxin is phenotypically masked by combination with the mosquitocidal proteins of Bacillus thuringiensis subspecies israelensis

Two insecticidal bacteria are used as larvicides to control larvae of nuisance and vector mosquitoes in many countries, Bacillus thuringiensis ssp. israelensis and B. sphaericus. Field studies show both are effective, but serious resistance, as high as 50 000-fold, has evolved where B. sphaericus is used against Culex mosquitoes. To improve efficacy and deal with even greater potential problems of resistance, we previously developed several recombinant larvicidal bacteria that combine the best mosquitocidal proteins of these bacteria. In the present study, we report laboratory selection studies using our best recombinant strain against larvae of Culex quinquefasciatus. This recombinant, Bti/BsBin, is a strain of B. thuringiensis ssp. israelensis engineered to produce a large amount of the B. sphaericus binary (Bin) toxin, which makes it more than 10-fold as mosquitocidal as the its parental strains. Here we show that larvae exposed to Bti/BsBin failed to develop significant resistance after 30 successive generations of heavy selection pressure. The highest level of resistance obtained at the LC(95) level was 5.2-fold, but declined to less than two-fold at the 35th generation. Testing the selected populations against B. sphaericus alone showed resistance to Bin evolved, but was masked by combination with B. thuringiensis ssp. israelensis. These results suggest that recombinant bacterial strains have improved mosquito and vector management properties compared with the wild-type strains used in current commercial formulations, and should prove useful in controlling important human diseases such as malaria and filariasis on a long-term basis, even when used intensively under field conditions.

Assessment of protoxin composition of Bacillus thuringiensis strains by use of polyacrylamide gel block and mass spectrometry

Assessment of protoxin composition in Bacillus thuringiensis parasporal crystals is principally hampered by the fact that protoxins in a single strain usually possess high sequence homology. Therefore, new strategies towards the identification of protoxins have been developed. Here, we established a powerful method through embedding solubilized protoxins in a polyacrylamide gel block coupled to liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of in-gel-generated peptides for protoxin identification. Our model study revealed that four protoxins (Cry1Aa, Cry1Ab, Cry1Ac and Cry2Aa) and six protoxins (Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa, Cyt1Aa, and Cyt2Ba) could be rapidly identified from B. thuringiensis subsp. kurstaki HD1 and subsp. israelensis 4Q2-72, respectively. The experimental results indicated that our method is a straightforward tool for analyzing protoxin expression profile in B. thuringiensis strains. Given its technical simplicity and sensitivity, our method might facilitate the present screening program for B. thuringiensis strains with new insecticidal properties.

A nonribosomal peptide synthetase gene tzw1 is involved in zwittermicin A biosynthesis in Bacillus thuringiensis G03

A 4.20-kb SspI fragment from Bacillus thuringiensis G03 was cloned and sequenced. Sequencing analysis revealed two complete open reading frames (ORF; tzw1 and tzw2), and one incomplete ORF (tzw3) (GenBank accession no. EU293887). Tzw1 encodes a putative nonribosomal peptide synthetase with thiolation and condensation domains localized at the C-termini, whereas tzw2 and tzw3 encode acyl carrier protein and Acyl-CoA dehydrogenase, respectively. To investigate the function of tzw1 in zwittermicin A (ZA) biosynthesis, an in-frame deletion of 1,461 bp within tzw1 was constructed. The mutant abolished ZA production. Complementation of the mutant with cloned tzw1 restored ZA productivity. These results revealed that tzw1 is required for ZA biosynthesis in B. thuringiensis G03.

Diversity of Colombian strains of Bacillus thuringiensis with insecticidal activity against dipteran and lepidopteran insects

AIM

To evaluate the genetic and molecular diversity and insecticidal activity of Bacillus thuringiensis isolates from all the natural regions of Colombia.

METHODS AND RESULTS

A total of 445 isolates from a collection of B. thuringiensis were characterized. The parasporal crystal morphology that was most abundant was bipyramidal (60%). Almost 10% of the isolates were toxic to Spodoptera frugiperda and 5.6% against Culex quinquefasciatus larvae. cry gene content determined by PCR indicated that 10.6% of the isolates contained cry1 genes and 1.1% contained cry2, cry4 or cry11 genes. Protein content of the parasporal crystal was determined by SDS-PAGE; 25 and 18 different protein profiles were found in isolates active against S. frugiperda and C. quinquefasciatus, respectively.

CONCLUSIONS

Bacillus thuringiensis presents great genetic and molecular diversity even in isolates from the same soil sample. Moreover, the diversity and activity of the isolates might have a relationship with the geographical origin of the samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results obtained here indicate that some of the B. thuringiensis isolates characterized in this study are potential control agents that could be used in programmes against mosquitoes and S. frugiperda.

Cloning and characterization of a novel crystal protein from a native Bacillus thuringiensis isolate highly active against Aedes aegypti

We characterized a novel Bacillus thuringiensis isolate native to Argentina (FCC 41) that exhibits a mosquitocidal activity higher than the reference B. thuringiensis subsp. israelensis. This isolate shows a rounded crystal harboring two major proteins of about 70-80 kDa. Moreover, we cloned and sequenced the encoding gene of one of the crystal proteins (Cry) consisting of an open reading frame of 2061 pb that encodes a protein of 687 amino acid residues. The deduced amino acid sequence has a predicted relative molecular mass of 78 kDa and is 52% and 45% identical to those of the reported Cry24Aa and Cry24Ba sequences, respectively. The novel Cry protein was designated as Cry24Ca, which also exhibited larvicidal activity against Aedes aegypti when its encoding gene was expressed in an Escherichia coli host strain.

Conjugal transfer of a toxin-coding megaplasmid from Bacillus thuringiensis subsp. israelensis to mosquitocidal strains of Bacillus sphaericus

Both Bacillus sphaericus and Bacillus thuringiensis subsp. israelensis produce mosquitocidal toxins during sporulation and are extensively used in the field for control of mosquito populations. All the known toxins of the latter organism are known to be encoded on a large plasmid, pBtoxis. In an attempt to combine the best properties of the two bacteria, an erythromycin resistance-marked pBtoxis plasmid was transferred to B. sphaericus by a mating technique. The resulting transconjugant bacteria were significantly more toxic to Aedes aegypti mosquitoes and were able to overcome resistance to B. sphaericus in a resistant colony of Culex quinquefasciatus, apparently due to the production of Cry11A but not Cry4A or Cry4B. The stability of the plasmid in the B. sphaericus host was moderate during vegetative growth, but segregational instability was observed, which led to substantial rates of plasmid loss during sporulation.

New strategy for identification of novel Cry-type genes from Bacillus thuringiensis strains

We designed five degenerate primers for detection of novel cry genes from Bacillus thuringiensis strains. An efficient strategy was developed based on a two-step PCR approach with these primers in five pair combinations. In the first step, only one of the primer pairs is used in the PCR, which allows amplification of DNA fragments encoding protein regions that include consensus domains of representative proteins belonging to different Cry groups. A second PCR is performed by using the first-step amplification products as DNA templates and the set of five primer combinations. Cloning and sequencing of the last-step amplicons allow both the identification of known cry genes encoding Cry proteins covering a wide phylogenetic distance and the detection and characterization of cry-related sequences from novel B. thuringiensis isolates.

Response of larval Chironomus tepperi (Diptera: Chironomidae) to individual Bacillus thuringiensis var. israelensis toxins and toxin mixtures

The biopesticide Bacillus thuringiensis israelensis (B.t.i.) is highly toxic to the larvae of Chironomus tepperi, an important pest of aerially sown rice in southern Australia. In this study, all of the known Cry genes and the Cyt1A gene from B.t.i. were expressed and tested for individual toxicity against fourth instar C. tepperi larvae. Possible synergism between toxins in two component mixtures involving all toxins except Cry10A was also evaluated. Of the Cry toxins, only Cry11A and Cry4B displayed substantial toxicity; however, both were 10- to 20-fold less toxic than the parental B.t.i. strain. The only detected synergy was between the mildly toxic Cry4A and Cyt1A toxins. In direct contrast to previous studies with mosquitoes, mixtures of Cry11A/Cry4B and Cry11A/Cyt1A were mildly antagonistic. The activity of Cry11A and Cry4B is sufficient to justify investigation as to whether their expression in transgenic rice plants could provide control of C. tepperi larvae.

Synergy between toxins of Bacillus thuringiensis subsp. israelensis and Bacillus sphaericus

Synergistic interactions among the multiple endotoxins of Bacillus thuringiensis subsp. israelensis de Barjac play an important role in its high toxicity to mosquito larvae and the absence of insecticide resistance in populations treated with this bacterium. A lack of toxin complexity and synergism are the apparent causes of resistance to Bacillus sphaericus Neide in particular Culex field populations. To identify endotoxin combinations of the two Bacillus species that might improve insecticidal activity and manage mosquito resistance to B. sphaericus, we tested their toxins alone and in combination. Most combinations of B. sphaericus and B. t. subsp. israelensis toxins were synergistic and enhanced toxicity relative to B. sphaericus, particularly against Culex quinquefasciatus Say larvae resistant to B. sphaericus and Aedes aegypti (L.), a species poorly susceptible to B. sphaericus. Toxicity also improved against susceptible Cx. quinquefasciatus. For example, when the CytlAa toxin from B. t. subsp. israelensis was added to Bin and Cry toxins, or when native B. t. subsp. israelensis was combined with B. sphaericus, synergism values as high as 883-fold were observed and combinations were 4-59,000-fold more active than B. sphaericus. These data, and previous studies using cytolytic toxins, validate proposed strategies for improving bacterial larvicides by combining B. sphaericus with B. t. subsp. israelensis or by engineering recombinant bacteria that express endotoxins from both strains. These combinations increase both endotoxin complexity and synergistic interactions and thereby enhance activity and help avoid insecticide resistance.

Laboratory selection for resistance to Bacillus thuringiensis subsp. jegathesan or a component toxin, Cry11B, in Culex quinquefasciatus (Diptera: Culicidae)

The bacteria Bacillus thuringiensis subsp. israelensis and Bacillus sphaericus produce insecticidal toxins used to control mosquito larvae throughout the world. Unfortunately, there are few alternative insecticides with similar activity and environmental safety, which may limit the long-term success of these insecticides. Bacillus thuringiensis subsp. jegathesan is another bacterium with toxins that are active against mosquitoes and has potential for development as a commercial product. B. t. subsp. jegathesan would be ineffective if cross-resistance was detected or if treated mosquito populations evolved resistance. B. t. subsp. jegathesan was evaluated for its potential for selecting insecticide resistance in Culex quinquefasciatus Say. Susceptibility changes in mosquitoes selected with the wild-type strain were compared with susceptibility changes in mosquitoes selected with Cry11B, a component toxin of B. t. subsp. jegathesan. Resistance was detected in generation 18 in the Cry11B-selected colony, reached a maximum of 38-fold, and was present through generation 40. The B. t. subsp. jegathesan-selected colony evolved 13-fold resistance in generation 22, but resistance declined to 2.3-fold in generation 26 and remained low throughout the study. Cry11B-selected mosquitoes showed no significant resistance to the wild-type bacterium, whereas B. t. subsp. jegathesan-selected mosquitoes expressed significant resistance to Cry11B. Both colonies displayed cross-resistance to component toxins of B. t. subsp. israelensis, but they lacked cross-resistance to that wild-type strain. The patterns of resistance and cross-resistance in this study are consistent with the patterns previously observed in mosquitoes selected with B. t. subsp. israelensis and suggest that B. t. subsp. jegathesan might also be at low risk for resistance.

Toxicity and synergism in transgenic Escherichia coli expressing four genes from Bacillus thuringiensis subsp. israelensis

The genes cyt1Aa and p20, encoding, respectively, cytolytic and accessory proteins of Bacillus thuringiensis subsp. israelensis, were introduced into previously constructed clones expressing cry4Aa and cry11Aa in Escherichia coli (Ben-Dov et al., 1995). Fifteen clones with all possible combinations of the four genes were obtained and found to express the genes included. Two new combinations, pVE4-ADRC and pVE4-ARC, expressing cyt1Aa, p20 and cry4Aa, with or without cry11Aa, respectively, were more toxic than their counterparts without cyt1Aa. They displayed the highest toxicity against Aedes aegypti larvae ever reached in transgenic bacteria. Five out of the six clones (except pVE4-DC) containing cry4Aa or cry11Aa (with or without p20) displayed varying levels of synergism with cyt1Aa: they are 1.5-to 34-fold more toxic than the respective clones without cyt1Aa against exposed larvae. Their lethal times also decreased (they kill larvae quicker), more so at higher cell concentrations. These clones are anticipated to dramatically reduce the likelihood of resistant development in the target organisms (Wirth et al., 1997).

Distribution of S-layers on the surface of Bacillus cereus strains: phylogenetic origin and ecological pressure

Bacillus anthracis, Bacillus cereus and Bacillus thuringiensis have been described as members of the Bacillus cereus group but are, in fact, one species. B. anthracis is a mammal pathogen, B. thuringiensis an entomopathogen and B. cereus a ubiquitous soil bacterium and an occasional human pathogen. In two clinical isolates of B. cereus, in some B. thuringiensis strains and in B. anthracis, an S-layer has been described. We investigated how the S-layer is distributed in B. cereus, and whether phylogeny or ecology could explain its presence on the surface of some but not all strains. We first developed a simple biochemical assay to test for the presence of the S-layer. We then used the assay with 51 strains of known genetic relationship: 26 genetically diverse B. cereus and 25 non-B. anthracis of the B. anthracis cluster. When present, the genetic organization of the S-layer locus was analysed further. It was identical in B. cereus and B. anthracis. Nineteen strains harboured an S-layer, 16 of which belonged to the B. anthracis cluster. All 19 were B. cereus clinical isolates or B. thuringiensis, except for one soil and one dairy strain. These findings suggest a common phylogenetic origin for the S-layer at the surface of B. cereus strains and, presumably, ecological pressure on its maintenance.

A general strategy for identification of S-layer genes in the Bacillus cereus group: molecular characterization of such a gene in Bacillus thuringiensis subsp. galleriae NRRL 4045

Despite its possible role in virulence, there has been little molecular characterization of members of the S-layer protein family of the Bacillus cereus group. It is hypothesized that the components of the S-layers are likely to display similar anchoring structures in Bacillus thuringiensis and Bacillus anthracis. Based on inferred sequence similarities, a DNA fragment encoding the cell-wall-anchoring domain of an S-layer component of BAC: thuringiensis subsp. galleriae NRRL 4045 was isolated. The complete gene was identified and sequenced. An ORF of 2463 nt was identified, which was predicted to encode a protein of 821 aa, SlpA. The mature SlpA protein (792 residues) carries three S-layer homology (SLH) motifs towards its amino terminus, each about 50 aa long. These motifs were sufficient to bind Bac. thuringiensis and Bac. anthracis cell walls in vitro by interacting with peptidoglycan-associated polymers, confirming a common wall-anchoring mechanism. The slpA null-allele mutant was constructed and shown to possess no other abundant surface protein. It is proposed that the method described in this paper could be applied to the identification and deletion of any Bac. cereus S-layer gene and is of great value for the molecular and functional characterization of these genes.

The Bacillus thuringiensis cyt genes for hemolytic endotoxins constitute a gene family

In the same way that cry genes, coding for larvicidal delta endotoxins, constitute a large and diverse gene family, the cyt genes for hemolytic toxins seem to compose another set of highly related genes in Bacillus thuringiensis. Although the occurrence of Cyt hemolytic factors in B. thuringiensis has been typically associated with mosquitocidal strains, we have recently shown that cyt genes are also present in strains with different pathotypes; this is the case for the morrisoni subspecies, which includes strains biologically active against dipteran, lepidopteran, and coleopteran larvae. In addition, while one Cyt type of protein has been described in all of the mosquitocidal strains studied so far, the present study confirms that at least two Cyt toxins coexist in the more toxic antidipteran strains, such as B. thuringiensis subsp. israelensis and subsp. morrisoni PG14, and that this could also be the case for many others. In fact, PCR screening and Western blot analysis of 50 B. thuringiensis strains revealed that cyt2-related genes are present in all strains with known antidipteran activity, as well as in some others with different or unknown host ranges. Partial DNA sequences for several of these genes were determined, and protein sequence alignments revealed a high degree of conservation of the structural domains. These findings point to an important biological role for Cyt toxins in the final in vivo toxic activity of many B. thuringiensis strains.

Characterization of the operon encoding the alternative sigma(B) factor from Bacillus anthracis and its role in virulence

The operon encoding the general stress transcription factor sigma(B) and two proteins of its regulatory network, RsbV and RsbW, was cloned from the gram-positive bacterium Bacillus anthracis by PCR amplification of chromosomal DNA with degenerate primers, by inverse PCR, and by direct cloning. The gene cluster was very similar to the Bacillus subtilis sigB operon both in the primary sequences of the gene products and in the order of its three genes. However, the deduced products of sequences upstream and downstream from this operon showed no similarity to other proteins encoded by the B. subtilis sigB operon. Therefore, the B. anthracis sigB operon contains three genes rather than eight as in B. subtilis. The B. anthracis operon is preceded by a sigma(B)-like promoter sequence, the expression of which depends on an intact sigma(B) transcription factor in B. subtilis. It is followed by another open reading frame that is also preceded by a promoter sequence similarly dependent on B. subtilis sigma(B). We found that in B. anthracis, both these promoters were induced during the stationary phase and induction required an intact sigB gene. The sigB operon was induced by heat shock. Mutants from which sigB was deleted were constructed in a toxinogenic and a plasmidless strain. These mutants differed from the parental strains in terms of morphology. The toxinogenic sigB mutant strain was also less virulent than the parental strain in the mouse model. B. anthracis sigma(B) may therefore be a minor virulence factor.

Laboratory selection for resistance to Bacillus sphaericus in Culex quinquefasciatus (Diptera: Culicidae) from California, USA

A previously untreated field population of Culex quinquefasciatus Say, collected near Bakersfield, CA, was subjected to intensive laboratory selection with the bacterial insecticide Bacillus sphaericus Neide (strain 2362) at a level producing 95% mortality. Resistance rapidly appeared and resistance levels increased such that fourth instars of generation 12 were able to survive a concentration of B. sphaericus that was 7,000 times higher than the median lethal concentration (LC50) of the susceptible reference colony. Similar resistance levels were detected in first instars. Cross-resistance in the selected colony was detected toward B. sphaericus strains 1593 and 2297, but little or no cross-resistance was observed toward B. sphaericus strains IAB59 or ISPC5 (= WHO 2173). Cross-resistance also was not detected toward the bacterial insecticide Bacillus thuringiensis subsp. israelensis, toward a recombinant strain expressing both B. thuringiensis subsp. israelensis and B. sphaericus (strain 1593) toxins, toward individual or multiple toxins from B. thuringiensis subsp. israelensis, or toward conventional synthetic insecticides. Genetic analysis revealed that B. sphaericus resistance was inherited as a recessive trait and controlled by a single major locus. These data are discussed in relation to cases of field resistance toward this biopesticide in the Cx. pipiens (L.) complex.

Production of Cry11A and Cry11Ba toxins in Bacillus sphaericus confers toxicity towards Aedes aegypti and resistant Culex populations

Cry11A from Bacillus thuringiensis subsp. israelensis and Cry11Ba from Bacillus thuringiensis subsp. jegathesan were introduced, separately and in combination, into the chromosome of Bacillus sphaericus 2297 by in vivo recombination. Two loci on the B. sphaericus chromosome were chosen as target sites for recombination: the binary toxin locus and the gene encoding the 36-kDa protease that may be responsible for the cleavage of the Mtx protein. Disruption of the protease gene did not increase the larvicidal activity of the recombinant strain against Aedes aegypti and Culex pipiens. Synthesis of the Cry11A and Cry11Ba toxins made the recombinant strains toxic to A. aegypti larvae to which the parental strain was not toxic. The strain containing Cry11Ba was more toxic than strains containing the added Cry11A or both Cry11A and Cry11Ba. The production of the two toxins together with the binary toxin did not significantly increase the toxicity of the recombinant strain to susceptible C. pipiens larvae. However, the production of Cry11A and/or Cry11Ba partially overcame the resistance of C. pipiens SPHAE and Culex quinquefasciatus GeoR to B. sphaericus strain 2297.

Two novel delta-endotoxin gene families cry26 and cry28 from Bacillus thuringiensis ssp. finitimus

Genes cry26Aal and cry28Aal were cloned from Bacillus thuringiensis ssp. finitimus strain B-1166 VKPM. This strain forms insecticidal crystal bodies either outside or inside the exosporium. The deduced amino acid sequence of the cry26Aal gene product included seven residues determined to be an N-terminal part of a chymotrypsin-treated delta-endotoxin isolated from the same strain. Earlier this protein was detected in both free and spore-associated types of crystals [Revina et al., Biokhimia (1999) in press]. Neither BtI nor BtII promoter sequences were found upstream of the open reading frames in both genes. Southern hybridization has shown that the surroundings of both genes at least 3 kb upstream and downstream of the open reading frames are unique. We suggest that the protein Cry26Aal in both types of crystal bodies is synthesized under the control of one and the same genomic locus.

Characterization of Bacillus thuringiensis serovar bolivia (serotype H63), a novel serovar isolated from the Bolivian high valleys

The type strain Bacillus thuringiensis var. bolivia (serotype H63), isolated from the Bolivian high valleys, has been characterized at different levels. Its parasporal crystal has an unusual shape and it is composed of a protein of 155 kDa which shows two bands of 75 and 80 kDa after activation. Analysis by PCR shows the presence of cry1 genes, and amplification with specific primers gave products for cry1 E, cry1 D, cry4 A and cry4 B with sizes different to those expected. Immunoblotting tests showed positive reaction for Cry1 E, Cry3 A, Cry4 A and Cry11 A crystal proteins. The plasmid pattern revealed two large and two small plasmids. Toxicity tests were performed against 14 insects and a slight toxicity was found against Plutella xylotella and Trichoplusia ni.

Variable cross-resistance to Cry11B from Bacillus thuringiensis subsp. jegathesan in Culex quinquefasciatus (Diptera: Culicidae) resistant to single or multiple toxins of Bacillus thuringiensis subsp. israelensis

A novel mosquitocidal bacterium, Bacillus thuringiensis subsp. jegathesan, and one of its toxins, Cry11B, in a recombinant B. thuringiensis strain were evaluated for cross-resistance with strains of the mosquito Culex quinquefasciatus that are resistant to single and multiple toxins of Bacillus thuringiensis subsp. israelensis. The levels of cross-resistance (resistance ratios [RR]) at concentrations which caused 95% mortality (LC95) between B. thuringiensis subsp. jegathesan and the different B. thuringiensis subsp. israelensis-resistant mosquito strains were low, ranging from 2.3 to 5.1. However, the levels of cross-resistance to Cry11B were much higher and were directly related to the complexity of the B. thuringiensis subsp. israelensis Cry toxin mixtures used to select the resistant mosquito strains. The LC95 RR obtained with the mosquito strains were as follows: 53.1 against Cq4D, which was resistant to Cry11A; 80.7 against Cq4AB, which was resistant to Cry4A plus Cry4B; and 347 against Cq4ABD, which was resistant to Cry4A plus Cry4B plus Cry11A. Combining Cyt1A with Cry11B at a 1:3 ratio had little effect on suppressing Cry11A resistance in Cq4D but resulted in synergism factors of 4.8 and 11.2 against strains Cq4AB and Cq4ABD, respectively; this procedure eliminated cross-resistance in the former mosquito strain and reduced it markedly in the latter strain. The high levels of activity of B. thuringiensis subsp. jegathesan and B. thuringiensis subsp. israelensis, both of which contain a complex mixture of Cry and Cyt proteins, against Cry4- and Cry11-resistant mosquitoes suggest that novel bacterial strains with multiple Cry and Cyt proteins may be useful in managing resistance to bacterial insecticides in mosquito populations.

Bacillus thuringiensis and its pesticidal crystal proteins

During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism's pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.

Improvement of Bacillus sphaericus toxicity against dipteran larvae by integration, via homologous recombination, of the Cry11A toxin gene from Bacillus thuringiensis subsp. israelensis

Integrative plasmids were constructed to enable integration of foreign DNA into the chromosome of Bacillus sphaericus 2297 by in vivo recombination. Integration of the aphA3 kanamycin resistance gene by a two-step procedure demonstrated that this strategy was applicable with antibiotic resistance selection. Hybridization experiments evidenced two copies of the operon encoding the binary toxin from B. sphaericus in the recipient strain. The Bacillus thuringiensis subsp. israelensis cry11Aal gene (referred to as cry11A), encoding a delta-endotoxin with toxicity against Culex, Aedes, and Anopheles larvae, was integrated either by a single crossover event [strain 2297 (::pHT5601), harboring the entire recombinant plasmid] or by two successive crossover events [strain 2297 (::cry11A)]. The level of the Cry11A production in B. sphaericus was high; two crystalline inclusions were produced in strain 2297 (::pHT5601). Synthesis of the Cry11A toxin conferred toxicity to the recombinant strains against Aedes aegypti larvae, for which the parental strain was not toxic. Interestingly, the level of larvicidal activity of strain 2297 (::pHT5601) against Anopheles stephensi was as high as that of B. thuringiensis subsp. israelensis and suggested synergy between the B. thuringiensis and B. sphaericus toxins. The toxicities of parental and recombinant B. sphaericus strains against Culex quinquefasciatus were similar, but the recombinant strains killed the larvae more rapidly. The production of the Cry11A toxin in B. sphaericus also partially restored toxicity for C. quinquefasciatus larvae from a population resistant to B. sphaericus 1593. In vivo recombination therefore appears to be a promising approach to the creation of new B. sphaericus strains for vector control.

CytA enables CryIV endotoxins of Bacillus thuringiensis to overcome high levels of CryIV resistance in the mosquito, Culex quinquefasciatus

Cry proteins produced by Bacillus thuringiensis are selective biodegradable insecticides used increasingly in bacterial insecticides and transgenic plants as alternatives to synthetic chemical insecticides. However, the potential for development of resistance and cross-resistance in target insect populations to Cry proteins used alone or in combination threatens the more widespread use of this novel pest control technology. Here we show that high levels of resistance to CryIV proteins in larvae of the mosquito, Culex quinquefasciatus, can be suppressed or reduced markedly by combining these proteins with sublethal quantities of CytA, a cytolytic endotoxin of B. thuringiensis. Resistance at the LC95 level of 127-fold for a combination of three CryIV toxins (CryIVA, B, and D), resulting from 60 generations of continuous selection, was completely suppressed by combining sporulated powders of CytA in a 1:3 ratio with sporulated powders of a CryIVA, CryIVB, and CryIVD strain. Combining the CytA strain with a CryIVA and CryIVB strain also completely suppressed mosquito resistance of 217-fold to the latter toxins at the LC95 level, whereas combination of CytA with CryIVD reduced resistance in a CryIVD-selected mosquito strain from greater than 1,000-fold to less than 8-fold. The CytA/CryIV model provides a potential molecular genetic strategy for engineering resistance management for Cry proteins directly into bacterial insecticides and transgenic plants.

PCR-based approach for detection of novel Bacillus thuringiensis cry genes

A two-step strategy, named exclusive PCR or E-PCR, has been developed to overcome the main limitation of PCR, which is the detection of already-known sequences only. This strategy allows the ability to detect and further clone and sequence genes for which no specific primers are available and in which a variable region exists between two conserved regions. This approach has been applied to Bacillus thuringiensis cryI genes by the use of mixtures of degenerate and specific primers recognizing well-known sequences. The first step allows the accurate identification of already-characterized cryI genes by the use of three primers. During the second step, the same sets of primers are used to exclude known sequences and to positively detect cryI genes unrecognized by any specific primer. The method, as well as its application to detect, clone, and sequence a novel cryIB gene, is described in this article.

Cloning and characterization of a cytolytic and mosquitocidal delta-endotoxin from Bacillus thuringiensis subsp. jegathesan

A cytolytic toxin gene encoding a 30.1-kDa Cyt2Bb1 toxin protein from B. thuringiensis subsp. jegathasan was cloned employing a limited-growth PCR screening method with forward and reverse oligonucleotide primers designed from N-terminal amino acid sequences of native and trypsin-cleaved protein, respectively. The expressed protein showed little cross-reactivity to the antibody raised against the Cyt1Aa protein. Unlike Cyt1Aa and Cyt2Aa expression, there was little or no visible crystal inclusion formation under microscopic observation. The amino acid sequence alignment indicated 31 and 66% identity to Cyt1Aa and Cyt2Aa, respectively. The sequence alignment for five known cytolytic proteins indicated three highly conserved regions, two in the loop regions between alpha-helices and beta-sheets and one in the loop region between beta-sheets 5 and 6. beta-Blocks 4 to 7 are also conserved, not only structurally but also among the amino acids in the hydrophobic faces. Mosquitocidal activity assays indicated that the Cyt2Bb toxin had less toxicity than Cyt1Aa and had about 600-times-lower toxicity than the wild-type whole toxin crystal. However, both the Cyt2Bb and the Cyt1Aa toxin showed comparable levels of hemolytic activity.

Identification of a gene for Cyt1A-like hemolysin from Bacillus thuringiensis subsp. medellin and expression in a crystal-negative B. thuringiensis strain

A gene designated cyt1Ab1, encoding a 27,490-Da protein, was isolated from Bacillus thuringiensis subsp. medellin (H30 serotype) by using an oligonucleotide probe corresponding to the cyt1Aa1 gene. The sequence of the Cyt1Ab1 protein, as deduced from the sequence of the cyt1Ab1 gene, was 86% identical to that of the Cyt1Aa1 protein and 32% identical to that of the Cyt2Aa1 protein from B. thuringiensis subsp. kyushuensis. The cyt1Ab1 gene was flanked upstream by a p21 gene, in the same orientation, encoding a 21,370-Da protein that showed 84% similarity to the putative chaperone P20 protein from B. thuringiensis subsp. israelensis and downstream, on the opposite strand, by a sequence showing 85% identity to the IS240A insertion sequence. The cyt1Ab1 gene was expressed at a high level in a nontoxic strain of B. thuringiensis subsp. israelensis in which large inclusions of the Cyt1Ab1 protein were produced. Purified Cyt1Ab1 crystals were as hemolytic as those of the Cyt1Aa1 protein and were twice as hemolytic as those from the wild-type strain. Mosquitocidal activity toward Aedes aegypti, Anopheles stephensi, and Culex pipiens larvae was assayed. The toxicity of the Cyt1Ab1 protein was slightly lower than that of the Cyt1Aa1 protein for all three mosquito species, and Cyt1Ab1 was 150, 300, and 800 times less active toward Culex, Anopheles, and Aedes larvae, respectively, than were the native crystals from B. thuringiensis subsp. medellin.

Cloning and expression of the first anaerobic toxin gene from Clostridium bifermentans subsp. malaysia, encoding a new mosquitocidal protein with homologies to Bacillus thuringiensis delta-endotoxins

A gene (cbm71) encoding a 71,128-Da mosquitocidal protein (Cbm71) was obtained by screening a size-fractionated XbaI digest of total genomic DNA from Clostridium bifermentans subsp. malaysia CH18 with two gene-specific oligonucleotide probes. The sequence of the Cbm71 protein, as deduced from the sequence of cbm71, corresponds to that of the 66-kDa protein previously described as one of the mosquitocidal components of C. bifermentans subsp. malaysia. Cbm71 shows limited similarities with Bacillus thuringiensis delta-endotoxins, especially in the four first conserved blocks. However, Cbm71 was not immunologically related to any of the Cry toxins and thus belongs to a novel class of mosquitocidal protein. The cbm71 gene was expressed in a nontoxic strain of B. thuringiensis, and Cbm71 was produced during sporulation and secreted to the supernatant of culture. Trichloroacetic-precipitated supernatant preparations were toxic for mosquito larvae of the species Aedes aegypti, Culex pipiens, and Anopheles stephensi.

Biochemical, immunological and toxicological characteristics of the crystal proteins of Bacillus thuringiensis subsp. medellin

Characterization of the insecticidal and hemolytic activity of solubilized crystal proteins of Bacillus thuringiensis (Bt) subsp. medellin (Btmed) was performed and compared to solubilized crystal proteins of isolates 1884 of B. thuringiensis subsp. israelensis (Bti) and isolate PG-14 of B. thuringiensis subsp. morrisoni (Btm). In general, at acid pH values solubilization of the Bt crystalline parasporal inclusions (CPI) was lower than at alkaline pH. The larvicidal activity demonstrated by the CPI of Btmed indicated that optimal solubilization of CPI takes place at a pH value of 11.3, in Bti at pH values from 5.03 to 11.3 and in Btm at pH values from 9.05 to 11.3 Hemolytic activity against sheep red blood cells was mainly found following extraction at pH 11.3 in all Bt strains tested. Polyacrylamide gel electrophoresis under denaturing conditions revealed that optimal solubilization of the CPI in all Bt strains takes place at the alkaline pH values from 9.05 to 11.3. An enriched preparation of Btmed crystals was obtained, solubilized and crystal proteins were separated on a size exclusion column (Sephacryl S-200). Three main protein peaks were observed on the chromatogram. The first peak had two main proteins that migrate between 90 to 100 kDa. These proteins are apparently not common to other Bt strains isolated to date. The second and third peaks obtained from the size exclusion column yielded polypeptides of 68 and 28-30 kDa, respectively. Each peak independently, showed toxicity against 1st instar Culex quinquefasciatus larvae. Interestingly, combinations of the fractions corresponding to the 68 and 30 kDa protein showed an increased toxicity. These results suggest that the 94 kDa protein is an important component of the Btmed toxins with the highest potency to kill mosquito larvae. When crystal proteins of Bti were probed with antisera raised independently against the three main protein fractions of Btmed, the only crystal protein that showed cross reaction was the 28 kDa protein. These data suggest that Btmed could be an alternative bacterium for mosquito control programs in case mosquito larval resistance emerges to Bti toxic proteins.

Cloning and expression of a novel toxin gene from Bacillus thuringiensis subsp. jegathesan encoding a highly mosquitocidal protein

A gene, designated cry11B, encoding a 81,293-Da crystal protein of Bacillus thuringiensis subsp. jegathesan was cloned by using a gene-specific oligonucleotide probe. The sequence of the Cry11B protein, as deduced from the sequence of the cry11B gene, contains large regions of similarity with the Cry11A toxin (previously CryIVD) from B. thuringiensis subsp. israelensis. The Cry11B protein was immunologically related to both Cry11A and Cry4A proteins. The cry11B gene was expressed in a nontoxic strain of B. thuringiensis, in which Cry11B was produced in large amounts during sporulation and accumulated as inclusions. Purified Cry11B inclusions were highly toxic for mosquito larvae of the species Aedes aegypti, Culex pipiens, and Anopheles stephensi. The activity of Cry11B toxin was higher than that of Cry11A and similar to that of the native crystals from B. thuringiensis subsp. jegathesan, which contain at least seven polypeptides.