Patterns of plasmid DNA in crystalliferous and acrystalliferous strains of Bacillus thuringiensis

Elucidating the genomic history of commercially used Bacillus thuringiensis subsp. tenebrionis strain NB176

Bacillus thuringiensis subsp. tenebrionis (Btt) produces a coleopteran-specific crystal protoxin protein (Cry3Aa δ-endotoxin). After its discovery in 1982, the strain NB125 (DSM 5526) was eventually registered in 1990 to control the Colorado potato beetle (Leptinotarsa decemlineata). Gamma-irradiation of NB125 resulted in strain NB176-1 (DSM 5480) that exhibited higher cry3Aa production and became the active ingredient of the plant protection product Novodor^® FC. Here, we report a comparative genome analysis of the parental strain NB125, its derivative NB176-1 and the current commercial production strain NB176. The entire genome sequences of the parental and derivative strains were deciphered by a hybrid de novo approach using short (Illumina) and long (Nanopore) read sequencing techniques. Genome assembly revealed a chromosome of 5.4 to 5.6 Mbp and six plasmids with a size range from 14.9 to 250.5 kbp for each strain. The major differences among the original NB125 and the derivative strains NB176-1 and NB176 were an additional copy of the cry3Aa gene, which translocated to another plasmid as well as a chromosomal deletion (~ 178 kbp) in NB176. The assembled genome sequences were further analyzed in silico for the presence of virulence and antimicrobial resistance (AMR) genes.

Molecular Characterization of Native Bacillus thuringiensis Strains from Root Nodules with Toxicity Against the Fall Armyworm (FAW, Spodoptera frugiperda) and Brinjal Ash Weevil (Myllocerus subfasciatus)

The fall armyworm is an exotic pest which destroys a wide variety of crops Querywhereas the brinjal ash weevil is a serious pest of eggplant and other solanaceous vegetables. The goal of this research is to find a sustainable and ecologically friendly bio-control agent for managing FAW and brinjal ash weevils. Twelve natural Bacillus thuringiensis strains were isolated from cowpea root nodules, and the Gram-positive cells with characteristic Bt crystal structures were discovered using phase contrast and scanning electron microscopy. There were bipyramidal, cuboidal, rhombus, and spherical crystals. The Bt cry gene content was characterized by PCR analysis, which revealed the presence of cry1, cry1I, cry3, cry7, cry7,8, cry14, cry26, and cry55 genes. The identity of Bt was confirmed by cloning and sequencing the cry genes. In the nucleotide sequences, no pseudo genes or indels were found in cry sequences. SDS-PAGE examination indicated the presence of bands ranging in size from 13 to 130 kDa, with 50-60 kDa being the most common. When compared to the control, the new native Bt strains were lethal, with pathogenicity ranging from 93 to 100% against S. frugiperda larvae and M. subfasciatus adults. The studies revealed that the native strains with conserved regions of 16S rRNA genes were compared to NCBI database sequences and classified as native Bt strains with 99-100% similarity to known Bt strains. In conclusion, native Bt strains from cowpea root nodules were shown to have bio-insecticidal activity against fall armyworm larvae and brinjal ash weevil adults.

Role of plasmid plasticity and mobile genetic elements in the entomopathogen Bacillus thuringiensis serovar israelensis

Bacillus thuringiensis is a well-known biopesticide that has been used for more than 80 years. This spore-forming bacterium belongs to the group of Bacillus cereus that also includes, among others, emetic and diarrheic pathotypes of B. cereus, the animal pathogen Bacillus anthracis and the psychrotolerant Bacillus weihenstephanensis. Bacillus thuringiensis is rather unique since it has adapted its lifestyle as an efficient pathogen of specific insect larvae. One of the peculiarities of B. thuringiensis strains is the extent of their extrachromosomal pool, with strains harbouring more than 10 distinct plasmid molecules. Among the numerous serovars of B. thuringiensis, 'israelensis' is certainly emblematic since its host spectrum is apparently restricted to dipteran insects like mosquitoes and black flies, vectors of human and animal diseases such as malaria, yellow fever, or river blindness. In this review, the putative role of the mobile gene pool of B. thuringiensis serovar israelensis in its pathogenicity and dedicated lifestyle is reviewed, with specific emphasis on the nature, diversity, and potential mobility of its constituents. Variations among the few related strains of B. thuringiensis serovar israelensis will also be reported and discussed in the scope of this specialised insect pathogen, whose lifestyle in the environment remains largely unknown.

Experimental evolution in silico: a custom-designed mathematical model for virulence evolution of Bacillus thuringiensis

Most mathematical models on the evolution of virulence are based on epidemiological models that assume parasite transmission follows the mass action principle. In experimental evolution, however, mass action is often violated due to controlled infection protocols. This "theory-experiment mismatch" raises the question whether there is a need for new mathematical models to accommodate the particular characteristics of experimental evolution. Here, we explore the experimental evolution model system of Bacillus thuringiensis as a parasite and Caenorhabditis elegans as a host. Recent experimental studies with strict control of parasite transmission revealed that one-sided adaptation of B. thuringiensis with non-evolving hosts selects for intermediate or no virulence, sometimes coupled with parasite extinction. In contrast, host-parasite coevolution selects for high virulence and for hosts with strong resistance against B. thuringiensis. In order to explain the empirical results, we propose a new mathematical model that mimics the basic experimental set-up. The key assumptions are: (i) controlled parasite transmission (no mass action), (ii) discrete host generations, and (iii) context-dependent cost of toxin production. Our model analysis revealed the same basic trends as found in the experiments. Especially, we could show that resistant hosts select for highly virulent bacterial strains. Moreover, we found (i) that the evolved level of virulence is independent of the initial level of virulence, and (ii) that the average amount of bacteria ingested significantly affects the evolution of virulence with fewer bacteria ingested selecting for highly virulent strains. These predictions can be tested in future experiments. This study highlights the usefulness of custom-designed mathematical models in the analysis and interpretation of empirical results from experimental evolution.

pXO16 from Bacillus thuringiensis serovar israelensis: Almost 350 kb of terra incognita

Bacillus thuringiensis strains usually harbor large sets of plasmids, some of which carrying the entomopathogenic δ-endotoxins. B. thuringiensis serovar israelensis, active on Dipteran larvae, carries the very large conjugative plasmid pXO16 (350 kb). pXO16 displays a macroscopic aggregation phenotype when liquid cultures of conjugative partners are mixed. Its conjugative apparatus is able of transferring itself and other non-conjugative and non-mobilizable plasmids in a fast and very efficient manner. Even though its conjugative kinetics and capabilities have been extensively studied, the genetic bases for this unique transfer system remain largely unknown. In this work, the sequence of pXO16 has been identified in the existing sequenced genome of B. thuringiensis sv. israelensis HD-789 as corresponding to the p01 plasmid. Despite pXO16 sequence being highly coding, few CDS possess homologs in the databases. However, potential regions responsible for the aggregation phenotype and the plasmid replication have been highlighted. The common orientation of all CDS and the presence of a high number of potential paralogs suggested a phage-like nature. Concerning conjugative functions, no significant type IV secretion system homologs have been found, indicating that pXO16 encodes an unforeseen conjugative system.

The dynamics of cooperative bacterial virulence in the field

Laboratory experiments have shown that the fitness of microorganisms can depend on cooperation between cells. Although this insight has revolutionized our understanding of microbial life, results from artificial microcosms have not been validated in complex natural populations. We investigated the sociality of essential virulence factors (crystal toxins) in the pathogen Bacillus thuringiensis using diamondback moth larvae (Plutella xylostella) as hosts. We show that toxin production is cooperative, and in a manipulative field experiment, we observed persistent high relatedness and frequency- and density-dependent selection, which favor stable cooperation. Conditions favoring social virulence can therefore persist in the face of natural population processes, and social interactions (rapid cheat invasion) may account for the rarity of natural disease outbreaks caused by B. thuringiensis.

Molecular cloning and characterization of the insecticidal crystal protein gene of Bacillus thuringiensis var. tenebrionis

The insecticidal crystal protein gene of the coleopteran-toxic Bacillus thuringiensis var. tenebrionis has been isolated, and the nucleotide sequence has been determined. A total DNA library from var. tenebrionis was made in the plasmid vector pUC12. By using a synthetic 27-base oligonucleotide corresponding to a stretch of nine N-terminal amino acids of a tryptic fragment of purified crystal protein of var. tenebrionis as a probe, recombinant colonies were screened by in situ hybridization for the presence of the crystal protein gene. Positive clones obtained from this screening were further tested for toxicity. One recombinant, NSBP544 (which contained a 5.9-kilobase BamHI insert), was toxic to larvae of Colorado potato beetle. Immunoblot analysis revealed that this clone produces two crystal-specific antigens of 65 and 73 kDa as do sporulating var. tenebrionis cells. However, purified crystal inclusions from var. tenebrionis contain a primary peptide component of 65 kDa. A 1932-base-pair open reading frame with a coding capacity of 73,119 Da has been identified by nucleotide sequencing analysis of the cloned crystal protein. In addition, mung bean nuclease mapping indicates that transcription of the crystal protein of var. tenebrionis initiates 130 base pairs upstream from the translational start site. Southern blot analysis using an internal 0.7-kilobase EcoRI fragment of pNSBP544 as a probe revealed that the crystal protein gene is located on a 90-MDa plasmid.

Environmental factors determining the epidemiology and population genetic structure of the Bacillus cereus group in the field

Bacillus thuringiensis (Bt) and its insecticidal toxins are widely exploited in microbial biopesticides and genetically modified crops. Its population biology is, however, poorly understood. Important issues for the safe, sustainable exploitation of Bt include understanding how selection maintains expression of insecticidal toxins in nature, whether entomopathogenic Bt is ecologically distinct from related human pathogens in the Bacillus cereus group, and how the use of microbial pesticides alters natural bacterial populations. We addressed these questions with a MLST scheme applied to a field experiment in which we excluded/added insect hosts and microbial pesticides in a factorial design. The presence of insects increased the density of Bt/B. cereus in the soil and the proportion of strains expressing insecticidal toxins. We found a near-epidemic population structure dominated by a single entomopathogenic genotype (ST8) in sprayed and unsprayed enclosures. Biopesticidal ST8 proliferated in hosts after spraying but was also found naturally associated with leaves more than any other genotype. In an independent experiment several ST8 isolates proved better than a range of non-pathogenic STs at endophytic and epiphytic colonization of seedlings from soil. This is the first experimental demonstration of Bt behaving as a specialized insect pathogen in the field. These data provide a basis for understanding both Bt ecology and the influence of anthropogenic factors on Bt populations. This natural population of Bt showed habitat associations and a population structure that differed markedly from previous MLST studies of less ecologically coherent B. cereus sample collections. The host-specific adaptations of ST8, its close association with its toxin plasmid and its high prevalence within its clade are analogous to the biology of Bacillus anthracis. This prevalence also suggests that selection for resistance to the insecticidal toxins of ST8 will have been stronger than for other toxin classes.

Control of insects by bacteria

All bacteria in microbial insecticides are species ofBacillusand form spores since they have to survive in the environment and on the shelf. They can be formulated as wettable powders, suspensions and dusts for application with conventional pest control machinery. All are safe to man and virtually all non-target organisms. Development costs are relatively low, but host specificity greatly restricts markets, the largest beingca. 2000 tons per annum in the West forB. thuringiensis. All act only after ingestion, a disadvantage because there is no contact action and usually only larvae are attacked. Three main groups have special features that determine their commercial success.

TheB. popilliaegroup is produced onlyin vivowhich limits production by three small firms. The Japanese beetle has been controlled in grassland in the warm parts of the USA by single applications of spores in heaps, spaced 2 m each way. The bacterium spreads slowly to untreated areas, is very persistent and kills only by infection.

TheB. thuringiensisgroup kills larvae of Lepidoptera, mosquitoes and blackflies, mainly by gut poisoning with a protein crystal toxin. It rapidly paralyses mouthparts and gut, stopping crop damage. It is readily produced by deep liquid fermentation, but does not persist and needs repeated application during the pest season. Products containing no beta exotoxin can be applied at unlimited dosage to food crops up to harvest. Only one application is needed for stored grain. After 20 years' use of strains against Lepidoptera, a different strain is now used commercially against mosquitoes and blackflies (only 5 years after its discovery), although improvements in formulation for aquatic application are needed. A recent new product based on the beta exotoxin is used in Finland only against flies in pig houses because it has some vertebrate toxicity.

TheB. sphaericusgroup is similar toB. thuringiensis, except that its proteinaceous toxin is different, is situated in the spore wall in strain 1593, and attacks only mosquitoes. Now at the pilot production stage, its commercial future depends on whether it is more potent thanB. thuringiensisagainst certain species and whether it can recycle to give effective extended mosquito control in some environments.

Intensive selection from natural isolates has improved potency 100 to 600 fold. This selective effort must be maintained and improved by genetic manipulation, which can be used to develop greater potential, particularly since DNA coding for the crystal toxin is carried on plasmids. This also gives speculative hope that the toxin may be incorporated into natural aquatic bacteria for mosquito control and into plants for protection against lepidopterous larvae. A great advantage is that these bacteria do not harm beneficial fauna to cause pest resurgence. At present, the main use lies in integrated pest control systems, although bacteria are not likely to supplant chemical insecticides on a large scale in the near future.

Bacterial Insecticidal Toxins

Over the years it has been important for humans to control the populations of harmful insects and insecticides have been used for this purpose in agricultural and horticultural sectors. Synthetic insecticides, owing to their various side effects, have been widely replaced by biological insecticides. In this review we attempt to describe three bacterial species that are known to produce insecticidal toxins of tremendous biotechnological, agricultural, and economic importance. Bacillus thuringiensis (BT) accounts for 90% of the bioinsecticide market and it produces insecticidal toxins referred to as delta endotoxins. The other two bacteria belong to the genera Photorhabdus and Xenorhabdus, which are symbiotically associated with entomopathogenic nematodes of the families Heterorhabditidae and Steinernematidae respectively. Whereas, Xenorhabdus and Photorhabdus exist in a mutualistic association with the entomopathogenic nematodes, BT act alone. BT formulations are widely used in the field against insects; however, over the years there has been a gradual development of insect resistance against BT toxins. No resistance against Xenorhabdus or Photorhabdus has been reported to date. More recently BT transgenic crops have been prepared; however, there are growing concerns about the safety of these genetically modified crops. Nematodal formulations are also used in the field to curb harmful insect populations. Resistance development to entomopathogenic nematodes is unlikely due to the physical macroscopic nature of infection. Xenorhabdus and Photorhabdus transgenes have not yet been prepared; but are predicted to be available in the near future. In this review we start with an overview of the synthetic insecticides and then discuss Bacillus thuringiensis, Xenorhabdus nematophilus, and Photorhabdus luminescens in greater detail.

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.

Molecular characterization of a DNA fragment harboring the replicon of pBMB165 from Bacillus thuringiensis subsp. tenebrionis

BACKGROUND

Bacillus thuringiensis belongs to the Bacillus cereus sensu lato group of Gram-positive and spore-forming bacteria. Most isolates of B. thuringiensis can bear many endogenous plasmids, and the number and size of these plasmids can vary widely among strains or subspecies. As far as we know, the replicon of the plasmid pBMB165 is the first instance of a plasmid replicon being isolated from subsp. tenebrionis and characterized.

RESULTS

A 20 kb DNA fragment containing a plasmid replicon was isolated from B. thuringiensis subsp. tenebrionis YBT-1765 and characterized. By Southern blot analysis, this replicon region was determined to be located on pBMB165, the largest detected plasmid (about 82 kb) of strain YBT-1765. Deletion analysis revealed that a replication initiation protein (Rep165), an origin of replication (ori165) and an iteron region were required for replication. In addition, two overlapping ORFs (orf6 and orf10) were found to be involved in stability control of plasmid. Sequence comparison showed that the replicon of pBMB165 was homologous to the pAMbeta1 family replicons, indicating that the pBMB165 replicon belongs to this family. The presence of five transposable elements or remnants thereof in close proximity to and within the replicon control region led us to speculate that genetic exchange and recombination are potentially responsible for the divergence among the replicons of this plasmid family.

CONCLUSION

The replication and stability features of the pBMB165 from B. thuringiensis subsp. tenebrionis YBT-1765 were identified. Of particular interest is the homology and divergence shared between the pBMB165 replicon and other pAMbeta1 family replicons.

Minireplicon from pBtoxis of Bacillus thuringiensis subsp. israelensis

A 2.2-kb fragment containing a replicon from pBtoxis, the large plasmid that encodes the insecticidal endotoxins of Bacillus thuringiensis subsp. israelensis, was identified, cloned, and sequenced. This fragment contains cis elements, including iterons, found in replication origins of other large plasmids and suggests that pBtoxis replicates by a type A theta mechanism. Two genes, pBt156 and pBt157, encoding proteins of 54.4 kDa and 11.8 kDa, respectively, were present in an operon within this minireplicon, and each was shown by deletion analysis to be essential for replication. The deduced amino acid sequences of the 54.4-kDa and 11.8-kDa proteins showed no substantial homology with known replication (Rep) proteins. However, the 54.4-kDa protein contained a conserved FtsZ domain, and the 11.8 kDa protein contained a helix-turn-helix motif. As FtsZ proteins have known functions in bacterial cell division and the helix-turn-helix motif is present in Rep proteins, it is likely that these proteins function in plasmid replication and partitioning. The minireplicon had a copy number of two or three per chromosome equivalent in B. thuringiensis subsp. israelensis but did not replicate in B. cereus, B. megaterium, or B. subtilis. A plasmid constructed to synthesize large quantities of the Cry11A and Cyt1A endotoxins demonstrated that this minireplicon can be used to engineer vectors for cry and cyt gene expression.

Complete nucleotide sequence of pBMB67, a 67-kb plasmid from Bacillus thuringiensis strain YBT-1520

The complete nucleotide sequence of a large (67kb) cryptic plasmid pBMB67 from Bacillus thuringiensis strain YBT-1520 was determined. Of the 74 predicted open reading frames (ORFs), 25 (34%) were assigned putative functions, 18 (24%) encoded conserved hypothetical proteins, and 31 (42%) had no homology to any genes present in the current open databases. The ORFs with similar functions were organized in a modular structure; thus, the DNA sequence of pBMB67 could be functionally divided into three modules, including a 39kb transfer region encoding homologs of the Agrobacterium tumefaciens VirB/D4 system components VirB1, VirB4, VirB11, and VirD4, as well as homologs of Gram-positive conjugation proteins. We also found a potential operon that was analogous to the Rap-Phr cassettes from Bacillus subtilis, which are involved in cell-cell communication and transcriptional regulation. Thus, we suggest that pBMB67 is likely to be implicated in cell-cell signaling and plays a role in the regulation of several cellular processes, with the production of exoprotease being one of the candidates.

The plasmid pBMBt1 from Bacillus thuringiensis subsp. darmstadiensis (INTA Mo14-4) replicates by the rolling-circle mechanism and encodes a novel insecticidal crystal protein-like gene

This work describes a novel rolling-circle replicating (RCR) plasmid pBMBt1 from Bacillus thuringiensis subsp. darmstadiensis (INTA Mo14-4) encoding an insecticidal crystal protein-like gene. pBMBt1 (6700 bp) contains three ORFs and their putative transcription initiation sites and Shine-Dalgarno sequences were localized. ORF1 encodes a 34.6 kDa protein which showed identity with the protein CryC53 from B. thuringiensis subsp. cameroun (24.6%), the Cry15Aa insecticidal crystal protein from B. thuringiensis subsp. thompsoni (21.9%) and the Mtx3 protein from Bacillus sphaericus (27.8%). The ORF2 (52.3 kDa) showed a 74% identity with the Mob protein coded by pUIBI-1 from B. thuringiensis subsp. entomocidus and 64% identity with the Mob protein of pBMY1 from Bacillus mycoides; both Mob proteins belong to the pMV158 superfamily. To evaluate the Mob protein, the plasmid pHTMob14-4 was constructed. This plasmid shows transfer frequencies of 9.1x10(-6) in B. thuringiensis subsp. israelensis (4Q7Gm(R)). The ORF3 (23.6 kDa) gene product is homologous to the Rep protein from the plasmid pBMYdx of B. mycoides (37.6%). A putative double-strand origin with significant homology to that of B. thuringiensis plasmids, and an ssoA-type single-strand origin were also identified. Detection of single-stranded pBMBt1 DNA replicating intermediaries suggests that replication occurs via the rolling-circle mechanism.

The Bacillus thuringiensis linear double-stranded DNA phage Bam35, which is highly similar to the Bacillus cereus linear plasmid pBClin15, has a prophage state

Bam35, a 15-kbp double-stranded DNA phage, infects Bacillus thuringiensis. Recently, sequencing of the related Bacillus cereus revealed a 15.1-kbp linear plasmid, pBClin15. We show that pBClin15 closely resembles Bam35 and demonstrate conversion of Bam35 to a prophage. This state is common, as several B. thuringiensis strains release Bam35-related viruses.

Expression of a Bacillus thuringiensisdelta-endotoxin cry1Ab gene in Bacillus subtilis and Bacillus licheniformis strains that naturally colonize the phylloplane of tomato plants (Lycopersicon esculentum, Mills)

AIMS

To introduce a cry gene into microorganisms that naturally colonize the phylloplane of tomato plants to improve the persistence of the Cry proteins for controlling a South American tomato moth (Tuta absoluta, Meyrick, 1917).

METHODS AND RESULTS

A cry1Ab gene isolated from a native Bacillus thuringiensis strain (LM-466), showing a relevant activity against T. absoluta larvae, was cloned into the shuttle vector pHT315 (Arantes and Lereclus 1991). The construct was introduced by electroporation into native Bacillus subtilis and Bacillus licheniformis strains, both natural inhabitants of the tomato phylloplane. Western analysis and toxicity assays against the target larvae proved that the successful expression of the gene was accomplished in host bacteria. Recombinant toxin displayed a similar LC50 value in comparison to native donor strain LM-466. Both transformed Bacillus survived for at least 45 days on the tomato leaf surface.

CONCLUSIONS

Plant-associated microorganisms that naturally colonize the phylloplane could be useful as recombinant microbial delivery systems of toxin genes of B. thuringiensis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Modified microorganisms capable of surviving on leaf surfaces for several weeks with insecticidal activity should allow for a reduction in pesticide application.

Composition and ecological distribution of cry proteins and their genotypes of Bacillus thuringiensis isolates from warehouses in China

The composition and distribution of insecticidal crystal proteins (Cry proteins) and their genotypes of Bacillus thuringiensis isolates from warehouses were evaluated through SDS-PAGE and PCR techniques. The results showed that the electrophoretic patterns of delta-endotoxin crystal preparations were divided into five types. The isolates containing approximately 135 kDa with a 65-kDa protein or only a approximately 135-kDa protein, which amounted to 55.74 and 35.25% of all isolates respectively, were the two major profiles of Cry protein isolated. The distribution of cry genes of B. thuringiensis from warehouses was highly variable. Cry protein genotypes detected in B. thuringiensis isolates included cry1Aa5, cry1Ab9, cry1Ac5, cry1Ba, cry1Ca1, cry1Da1, cry1Ea3, cry2, and cry3 genes, but not cry1Fa2. Among them, cry2, cry1Ac5, and cry1Ab9 genes were the most common in our B. thuringiensis isolates. Most B. thuringiensis isolates contained several cry genes in a total of 18 profiles. Among them, cry1Ac5 with cry1Ea3; cry1Aa5, cry1Ab9, cry1Ac5 with cry1Ea3; and cry1Aa5, cry1Ab9 with cry1Ac5 were the three principal profiles. The distribution of the Cry proteins and cry genes in isolates depended on geography and type of warehouses. Gene profiles may be used as markers for insecticidal activity of B. thuringiensis strains, but they did not directly reflect the toxic level of B. thuringiensis strains. The serotype of B. thuringiensis strains did not directly reflect the specific cry gene profiles in the strains, but certain relationships can be established between the serotype and cry genotype.

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

Two new Bacillus thuringiensis strains have been classified by the H antigen of the cells and differentiated by their morphological, biochemical and molecular characteristics. The flagellar agglutination showed that both strains bore specific H antigens which allowed their classification as the new serotypes H57 and H59. The serovar names proposed for the type strains characterized in this work are B. thuringiensis ser. pirenaica, for the H serotype 57, and B. thuringiensis ser. iberica, for the H serotype 59. Further characterization of these strains, by means of SDS-PAGE, Western inmunodetection, plasmid profile and cry-gene identification by polymerase chain reaction, confirmed the originality of the two novel serotypes. Toxicity tests carried out against several insect species, belonging to the orders Lepidoptera, Diptera and Coleoptera, showed no detectable insecticidal activity for either of the B. thuringiensis strains.

Replication mechanism and sequence analysis of the replicon of pAW63, a conjugative plasmid from Bacillus thuringiensis

A 5.8-kb fragment of the large conjugative plasmid pAW63 from Bacillus thuringiensis subsp. kurstaki HD73 containing all the information for autonomous replication was cloned and sequenced. By deletion analysis, the pAW63 replicon was reduced to a 4.1-kb fragment harboring four open reading frames (ORFs). Rep63A (513 amino acids [aa]), encoded by the largest ORF, displayed strong similarity (40% identity) to the replication proteins from plasmids pAMbeta1, pIP501, and pSM19035, indicating that the pAW63 replicon belongs to the pAMbeta1 family of gram-positive theta-replicating plasmids. This was confirmed by the facts that no single-stranded DNA replication intermediates could be detected and that replication was found to be dependent on host-gene-encoded DNA polymerase I. An 85-bp region downstream of Rep63A was also shown to have strong similarity to the origins of replication of pAMbeta1 and pIP501, and it is suggested that this region contains the bona fide pAW63 ori. The protein encoded by the second large ORF, Rep63B (308 aa), was shown to display similarity to RepB (34% identity over 281 aa) and PrgP (32% identity over 310 aa), involved in copy control of the Enterococcus faecalis plasmids pAD1 and pCF10, respectively. No significant similarity to known proteins or DNA sequences could be detected for the two smallest ORFs. However, the location, size, hydrophilicity, and orientation of ORF6 (107 codons) were analogous to those features of the putative genes repC and prgO, which encode stability functions on plasmids pAD1 and pCF10, respectively. The cloned replicon of plasmid pAW63 was stably maintained in Bacillus subtilis and B. thuringiensis and displayed incompatibility with the native pAW63. Hybridization experiments using the cloned replicon as a probe showed that pAW63 has similarity to large plasmids from other B. thuringiensis subsp. kurstaki strains and to a strain of B. thuringiensis subsp. alesti.

Identification of novel cry-type genes from Bacillus thuringiensis strains on the basis of restriction fragment length polymorphism of the PCR-amplified DNA

Two pairs of universal oligonucleotide primers were designed to probe the most conserved regions of all known cryI-type gene sequences so that the amplified PCR fragments of the DNA template from Bacillus thuringiensis strains may contain all possible cryI-type gene sequences. The restriction fragment length polymorphism (RFLP) patterns of the PCR-amplified fragments revealed that 14 distinct cry-type genes have been identified from 20 B. thuringiensis strains. Those cry-type genes included cryIA(a), cryIA(a), cryIA(b), cryIA(b), cryIA(c), cryIB, cryIC, cryIC, cryIC(b), cryID, cryIE, cryIF, cryIF, and cryIII (a dagger at the end of a gene designation indicates a novel cry-type gene determined by restriction mapping or DNA sequences). Among them, the sequences of cryIA(a), cryIA(b), cryIB, cryIC, cryIF, and cryIII were found to be different from the corresponding published cry gene sequences. Interestingly, five cry-type genes [cryIA(a)-, cryIB-, cryIC-, cryIC(b)-, and cryIF-type genes] and seven cry-type genes [cryIA(a)-, cryIA(b)-, cryIB-, cryIC-, cryIC(b)-, cryIF-, and cryIII-type genes] have been detected from B. thuringiensis subsp. morrisoni HD-12 and B. thuringiensis subsp. wuhanensis, respectively. Therefore, the PCR-RFLP typing system is a facile method to detect both known and novel cry genes existing in B. thuringiensis strains.

Enhanced production of insecticidal proteins in Bacillus thuringiensis strains carrying an additional crystal protein gene in their chromosomes

A two-step procedure was used to place a cryIC crystal protein gene from Bacillus thuringiensis subsp. aizawai into the chromosomes of two B. thuringiensis subsp. kurstaki strains containing multiple crystal protein genes. The B. thuringiensis aizawai cryIC gene, which encodes an insecticidal protein highly specific to Spodoptera exigua (beet armyworm), has not been found in any B. thuringiensis subsp. kurstaki strains. The cryIC gene was cloned into an integration vector which contained a B. thuringiensis chromosomal fragment encoding a phosphatidylinositol-specific phospholipase C, allowing the B. thuringiensis subsp. aizawai cryIC to be targeted to the homologous region of the B. thuringiensis subsp. kurstaki chromosome. First, to minimize the possibility of homologous recombination between cryIC and the resident crystal protein genes, B. thuringiensis subsp. kurstaki HD73, which contained only one crystal gene, was chosen as a recipient and transformed by electroporation. Second, a generalized transducing bacteriophage, CP-51, was used to transfer the integrated cryIC gene from HD73 to two other B. thuringiensis subsp. kurstaki stains. The integrated cryIC gene was expressed at a significant level in all three host strains, and the expression of cryIC did not appear to reduce the expression of the endogenous crystal protein genes. Because of the newly acquired ability to produce the CryIC protein, the recombinant strains showed a higher level of activity against S. exigua than did the parent strains. This two-step procedure should therefore be generally useful for the introduction of an additional crystal protein gene into B. thuringiensis strains which have multiple crystal protein genes and which show a low level of transformation efficiency.

Determination and Distribution of cry -Type Genes of Bacillus thuringiensis Isolates from Taiwan

Using PCR with a set of specific oligonucleotide primers to detect cryI -type genes, we were able to screen the cry -type genes of 225 Bacillus thuringiensis soil isolates from Taiwan without much cost in time or labor. Some combinations of cry genes (the cry -type profile) in a single isolate were unique. We identified five distinct profiles of crystal genes from the B. thuringiensis soil isolates from Taiwan. The cry genes included cryIA ( a ), cryIA ( b ), cryIA ( c ), cryIC, cryID , and cryIV . Interestingly, 501 B. thuringiensis isolates (93.5% of the total number that we identified) were isolated from areas at high altitudes. The profiles of cry -type genes were distinct in all isolation areas. The distribution of cry -type genes of our isolates therefore depended on geography. Using PCR footprinting to detect cryIC -type genes, we identified two distinct cryIC footprints from some of our isolates, indicating that these isolates may contain novel cryIC -type genes. B. thuringiensis isolates containing cryIA ( a )-, cryIA ( b )-, and cryIA ( c )-type genes exhibited much greater activity against Plutella xylostella than did other isolates, indicating that multiple cry -type genes may be used as markers for the prediction of insecticidal activities.

A complete physical map of a Bacillus thuringiensis chromosome

Bacillus thuringiensis is the source of the most widely used biological pesticide, through its production of insecticidal toxins. The toxin genes are often localized on plasmids. We have constructed a physical map of a Bacillus thuringiensis chromosome by aligning 16 fragments obtained by digestion with the restriction enzyme NotI. The fragments ranged from 15 to 1,350 kb. The size of the chromosome was 5.4 Mb. The NotI DNA fingerprint patterns of 12 different B. thuringiensis strains showed marked variation. The cryIA-type toxin gene was present on the chromosome in four strains, was extrachromosomal in four strains, and was both chromosomal and extrachromosomal in two strains. A Tn4430 transposon probe hybridized to 5 of the 10 cryIA-positive chromosomal fragments, while cryIA and the transposon often hybridized to different extrachromosomal bands. Ten of the strains were hemolytic when grown on agar plates containing human erythrocytes. Nine of the strains were positive when assayed for the presence of Bacillus cereus enterotoxin. We conclude that B. thuringiensis is very closely related to B. cereus and that the distinction between B. cereus and B. thuringiensis should be reconsidered.

Arbitrary primer polymerase chain reaction, a powerful method to identify Bacillus thuringiensis serovars and strains

Arbitrary primer polymerase chain reaction technology has been applied to the identification of commercial strains of Bacillus thuringiensis by using total DNAs extracted from single bacterial colonies as templates. Characteristic DNA banding patterns can be readily and reproducibly obtained by agarose gel electrophoresis. This method has been used to distinguish commercial products containing B. thuringiensis serovar kurstaki (3a3b). When a single primer was used this method was capable of producing discriminating DNA fingerprints for 33 known serovars. Differentiation from the closely related species Bacillus cereus is also readily achieved. This technique should prove to be a powerful tool for identification and discrimination of individual B. thuringiensis strains.

The construction of Bacillus thuringiensis strains expressing novel entomocidal delta-endotoxin combinations

Using our recently reported method of electroporation to transform Bacillus thuringiensis [Bone & Ellar (1989) FEMS Microbiol. Lett. 58, 171-178], cloned B. thuringiensis entomocidal delta-endotoxin genes have been introduced into several native B. thuringiensis strains. In many cases the resulting transformants expressed both their native toxins and the cloned toxin, producing strains with broader toxicity spectra. The introduction of the var. tenebrionis toxin gene into B. thuringiensis var. israelensis resulted in a strain with activity against Pieris brassicae (cabbage white butterfly), an activity which neither parent strain possesses. We discuss further the possibility of synergism and also the problems associated with introducing cloned DNA by this method.

Transformation of Bacillus cereus vegetative cells by electroporation

Transformation of untreated vegetative cells of Bacillus cereus 569 with plasmid pC194 (1.8 megadaltons) by high-voltage electroporation resulted in a maximum of 2 x 10(-5) transformants per viable cell. Transformation of a 130-megadalton plasmid occurred at a comparable frequency. The method was simple, rapid, and yielded transformant colonies in 14 to 24 h. Transformation was obtained with unpurified total plasmid DNA.

The biotechnology of Bacillus thuringiensis

One of the challenges in the application of biotechnology to pest control is the identification of agents found in nature which can be used effectively. Biotechnology offers the potential of developing pesticides based on such agents which will provide environmentally sound and economically feasible insect control alternatives. Such an agent, the insect pathogen Bacillus thuringiensis, is the subject of intense investigations in several laboratories. Insecticides which use the entomocidal properties of B. thuringiensis are currently produced and sold worldwide; new products are currently in the development stage. Herein, the biology and genetics of B. thuringiensis and the problems associated with current products are critically reviewed with respect to biotechnology. Moreover, the economic and regulatory implications of technologically advanced products are evaluated.

Bacillus thuringiensis and related insect pathogens

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Two different parasporal inclusions are produced by Bacillus thuringiensis subsp. finitimus

Bacillus thuringiensis subsp. finitimus produced at least two parasporal inclusions. One inclusion was formed within the exosporium and remained with the spore after mother cell lysis. A second inclusion formed somewhat later exterior to the exosporium. Each inclusion contained a major polypeptide of about 135,000 daltons with unique antigenic determinants. This subspecies contained only two plasmids, of 98 and 77 megadaltons (MDa). Strains cured of these plasmids produced only the free inclusion. Since the plasmid-cured strains did not contain DNA sequences homologous to plasmid DNA, the gene for the free-inclusion protein must be encoded in the chromosome. In contrast, the enclosed parasporal inclusion was produced only when the plasmid of 98 MDa was present. In addition, transfer of the 98-MDa plasmid to Bacillus cereus resulted in transcipients that produced small inclusions enclosed within the exosporium, and the protein extracted from these inclusions reacted with antibody specific for enclosed inclusion protein of B. thuringiensis subsp. finitimus. Genes in both the chromosome and a plasmid function in the synthesis of distinct parasporal proteins in this subspecies.

Cloning and expression of the lepidopteran toxin produced by Bacillus thuringiensis var. thuringiensis in Escherichia coli

The Bacillus thuringiensis var. thuringiensis strain 3A produces a proteinaceous parasporal crystal toxic to larvae of a variety of lepidopteran pests including Spodoptera littoralis (Egyptian cotton leaf worm), Heliothis zeae, H. virescens and Boarmia selenaria. By cloning of individual plasmids of B. thuringiensis in Escherichia coli, we localized a gene coding for the delta-endotoxin on the B. thuringiensis plasmid of about 17 kb designated pTN4. Following partial digestion of the B. thuringiensis plasmid pTN4 and cloning into the E. coli pACYC184 plasmid three clones were isolated in which toxin production was detected. One of these hybrid plasmids pTNG43 carried a 1.7-kb insert that hybridized to the 14-kb BamHI DNA fragments of B. thuringiensis var. thuringiensis strains 3A and berliner 1715. This BamHI DNA fragment of strain berliner 1715 has been shown to contain the gene that codes for the toxic protein of the crystal (Klier et al., 1982). No homologous sequences have been found between pTNG33 and the DNA of B. thuringiensis var. entomocidus strain 24, which exhibited insecticidal activity against S. littoralis similar to that of strain 3A.

Recent aspects of genetic manipulation in Bacillus thuringiensis

The conjugative plasmid pAM beta 1 was transferred from Streptococcus faecalis to several strains of Bacillus thuringiensis by a filter-mating process. From a transconjugant clone of B. thuringiensis a hybrid plasmid resulting from an in vivo insertion into pAM beta 1 of a 3 Md DNA sequence was isolated. This 3 Md DNA molecule (Th sequence) is related to several host plasmids found in different serotypes of B. thuringiensis. A reciprocal conjugation-like process involving the transfer of pAM beta 1 from B. thuringiensis to S. faecalis was also demonstrated. The comparison of the restriction maps of the crystal genes from plasmid and chromosomal origins of different serotypes, six of which having been cloned in E. coli, revealed the existence of two classes of genes which are very similar in the map corresponding to the N-terminal part of the protein, and which differ essentially in the 3' region. The presence of the transposon-like Th sequence was found in several cases associated with the crystal gene in the same host plasmid, and a model for their structural organization is proposed.

Molecular cloning of the delta-endotoxin gene of Bacillus thuringiensis var. israelensis

A transformant of Bacillus megaterium, VB131, was isolated which carries a 6.3-kb XbaI segment of the crystal toxin gene of Bacillus thuringiensis var. israelensis (BTI) cloned in a vector plasmid pBC16 to yield pVB131. The chimeric plasmid DNA from VB131 was introduced into a transformable Bacillus subtilis strain by competence transformation. Both the B. megaterium VB131 strain and the B. subtilis strain harboring the chimeric plasmid produced irregular, parasporal, phase-refractile, crystalline inclusions (Cry+) during sporulation. The sporulated cells as well as the isolated crystal inclusions of the pVB131-containing B. megaterium and B. subtilis strains were highly toxic to the larvae of Aedes aegypti. Also, the solubilized crystal protein preparation from VB131[pVB131] showed clear immuno cross-reaction with antiserum to the BTI crystal toxin. 32P-labeled pVB131 plasmid DNA showed specific hybridization with a 112-kb plasmid DNA of Cry+ strains of BTI, and no hybridization with other plasmid or chromosomal DNA of either Cry+ or Cry- variants. These results are in agreement with our previous findings (González and Carlton, 1984) that the 112-kb plasmid of BTI is associated with the production of the crystal toxin.