A large transmissible plasmid is required for crystal toxin production in Bacillus thuringiensis variety israelensis

Plasmid - Chromosome interplay in natural and non-natural hosts: global transcription study of three Bacillus cereus group strains carrying pCER270 plasmid

The Bacillus cereus group comprises genetically related Gram-positive spore-forming bacteria that colonize a wide range of ecological niches and hosts. Despite their high degree of genome conservation, extrachromosomal genetic material diverges between these species. The discriminating properties of the B. cereus group strains are mainly due to plasmid-borne toxins, reflecting the importance of horizontal gene transfers in bacterial evolution and species definition. To investigate how a newly acquired megaplasmid can impact the transcriptome of its host, we transferred the pCER270 from the emetic B. cereus strains to phylogenetically distant B. cereus group strains. RNA-sequencing experiments allowed us to determine the transcriptional influence of the plasmid on host gene expression and the impact of the host genomic background on the pCER270 gene expression. Our results show a transcriptional cross-regulation between the megaplasmid and the host genome. pCER270 impacted carbohydrate metabolism and sporulation genes expression, with a higher effect in the natural host of the plasmid, suggesting a role of the plasmid in the adaptation of the carrying strain to its environment. In addition, the host genomes also modulated the expression of pCER270 genes. Altogether, these results provide an example of the involvement of megaplasmids in the emergence of new pathogenic strains.

Conjugation across Bacillus cereus and kin: A review

Horizontal gene transfer (HGT) is a major driving force in shaping bacterial communities. Key elements responsible for HGT are conjugation-like events and transmissible plasmids. Conjugative plasmids can promote their own transfer as well as that of co-resident plasmids. Bacillus cereus and relatives harbor a plethora of plasmids, including conjugative plasmids, which are at the heart of the group species differentiation and specification. Since the first report of a conjugation-like event between strains of B. cereus sensu lato (s.l.) 40 years ago, many have studied the potential of plasmid transfer across the group, especially for plasmids encoding major toxins. Over the years, more than 20 plasmids from B. cereus isolates have been reported as conjugative. However, with the increasing number of genomic data available, in silico analyses indicate that more plasmids from B. cereus s.l. genomes present self-transfer potential. B. cereus s.l. bacteria occupy diverse environmental niches, which were mimicked in laboratory conditions to study conjugation-related mechanisms. Laboratory mating conditions remain nonetheless simplistic compared to the complex interactions occurring in natural environments. Given the health, economic and ecological importance of strains of B. cereus s.l., it is of prime importance to consider the impact of conjugation within this bacterial group.

The Distribution of Several Genomic Virulence Determinants Does Not Corroborate the Established Serotyping Classification of Bacillus thuringiensis

Bacillus thuringiensis, commonly referred to as Bt, is an object of the lasting interest of microbiologists due to its highly effective insecticidal properties, which make Bt a prominent source of biologicals. To categorize the exuberance of Bt strains discovered, serotyping assays are utilized in which flagellin serves as a primary seroreactive molecule. Despite its convenience, this approach is not indicative of Bt strains' phenotypes, neither it reflects actual phylogenetic relationships within the species. In this respect, comparative genomic and proteomic techniques appear more informative, but their use in Bt strain classification remains limited. In the present work, we used a bottom-up proteomic approach based on fluorescent two-dimensional difference gel electrophoresis (2D-DIGE) coupled with liquid chromatography/tandem mass spectrometry(LC-MS/MS) protein identification to assess which stage of Bt culture, vegetative or spore, would be more informative for strain characterization. To this end, the proteomic differences for the israelensis-attributed strains were assessed to compare sporulating cultures of the virulent derivative to the avirulent one as well as to the vegetative stage virulent bacteria. Using the same approach, virulent spores of the israelensis strain were also compared to the spores of strains belonging to two other major Bt serovars, namely darmstadiensis and thuringiensis. The identified proteins were analyzed regarding the presence of the respective genes in the 104 Bt genome assemblies available at open access with serovar attributions specified. Of 21 proteins identified, 15 were found to be encoded in all the present assemblies at 67% identity threshold, including several virulence factors. Notable, individual phylogenies of these core genes conferred neither the serotyping nor the flagellin-based phylogeny but corroborated the reconstruction based on phylogenomics approaches in terms of tree topology similarity. In its turn, the distribution of accessory protein genes was not confined to the existing serovars. The obtained results indicate that neither gene presence nor the core gene sequence may serve as distinctive bases for the serovar attribution, undermining the notion that the serotyping system reflects strains' phenotypic or genetic similarity. We also provide a set of loci, which fit in with the phylogenomics data plausibly and thus may serve for draft phylogeny estimation of the novel strains.

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.

Comparative genomics of extrachromosomal elements in Bacillus thuringiensis subsp. israelensis

Bacillus thuringiensis subsp. israelensis is one of the most important microorganisms used against mosquitoes. It was intensively studied following its discovery and became a model bacterium of the B. thuringiensis species. Those studies focused on toxin genes, aggregation-associated conjugation, linear genome phages, etc. Recent announcements of genomic sequences of different strains have not been explicitly related to the biological properties studied. We report data on plasmid content analysis of four strains using ultra-high-throughput sequencing. The strains were commercial product isolates, with their putative ancestor and type B. thuringiensis subsp. israelensis strain sequenced earlier. The assembled contigs corresponding to published and novel data were assigned to plasmids described earlier in B. thuringiensis subsp. israelensis and other B. thuringiensis strains. A new 360 kb plasmid was identified, encoding multiple transporters, also found in most of the earlier sequenced strains. Our genomic data show the presence of two toxin-coding plasmids of 128 and 100 kb instead of the reported 225 kb plasmid, a co-integrate of the former two. In two of the sequenced strains, only a 100 kb plasmid was present. Some heterogeneity exists in the small plasmid content and structure between strains. These data support the perception of active plasmid exchange among B. thuringiensis subsp. israelensis strains in nature.

Detection of the cryptic prophage-like molecule pBtic235 in Bacillus thuringiensis subsp. israelensis

Bacillus thuringiensis has long been recognized to carry numerous extrachromosomal molecules. Of particular interest are the strains belonging to the B. thuringiensis subsp. israelensis lineage, as they can harbor at least seven extrachromosomal molecules. One of these elements seems to be a cryptic molecule that may have been disregarded in strains considered plasmid-less. Therefore, this work focused on this cryptic molecule, named pBtic235. Using different approaches that included transposition-tagging, large plasmid gel electrophoresis and Southern blotting, conjugation and phage-induction experiments, in combination with bioinformatics analyses, it was found that pBtic235 is a hybrid molecule of 235,425 bp whose genome displays potential plasmid- and phage-like modules. The sequence of pBtic235 has been identified in all sequenced genomes of B. thuringiensis subsp. israelensis strains. Here, the pBtic235 sequence was considered identical to that of plasmid pBTHD789-2 from strain HD-789. Despite the fact that the pBtic235 genome possesses 240 putative CDSs, many of them have no homologs in the databases. However, CDSs coding for potential proteins involved in replication, genome packaging and virion structure, cell lysis, regulation of lytic-lysogenic cycles, metabolite transporters, stress and metal resistance, were identified. The candidate plasmidial prophage pBtic235 exemplifies the notable diversity of the extrachromosomal realm found in B. thuringiensis.

Bacillus thuringiensis toxins: an overview of their biocidal activity

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

Influence of lysogeny of Tectiviruses GIL01 and GIL16 on Bacillus thuringiensis growth, biofilm formation, and swarming motility

Bacillus thuringiensis is an entomopathogenic bacterium that has been used as an efficient biopesticide worldwide. Despite the fact that this bacterium is usually described as an insect pathogen, its life cycle in the environment is still largely unknown. B. thuringiensis belongs to the Bacillus cereus group of bacteria, which has been associated with many mobile genetic elements, such as species-specific temperate or virulent bacteriophages (phages). Temperate (lysogenic) phages are able to establish a long-term relationship with their host, providing, in some cases, novel ecological traits to the bacterial lysogens. Therefore, this work focuses on evaluating the potential influence of temperate tectiviruses GIL01 and GIL16 on the development of different life traits of B. thuringiensis. For this purpose, a B. thuringiensis serovar israelensis plasmid-cured (nonlysogenic) strain was used to establish bacterial lysogens for phages GIL01 and GIL16, and, subsequently, the following life traits were compared among the strains: kinetics of growth, metabolic profiles, antibiotics susceptibility, biofilm formation, swarming motility, and sporulation. The results revealed that GIL01 and GIL16 lysogeny has a significant influence on the bacterial growth, sporulation rate, biofilm formation, and swarming motility of B. thuringiensis. No changes in metabolic profiles or antibiotic susceptibilities were detected. These findings provide evidence that tectiviruses have a putative role in the B. thuringiensis life cycle as adapters of life traits with ecological advantages.

Transgenic organisms expressing genes from Bacillus thuringiensis to combat insect pests

Various subspecies (ssp.) of Bacillus thuringiensis (Bt) are considered the best agents known so far to control insects, being highly specific and safe, easily mass produced and with long shelf life.1 The para-crystalline body that is produced during sporulation in the exosporium includes polypeptides named δ-endotoxins, each killing a specific set of insects. The different entomopathogenic toxins of various Bt ssp. can be manipulated genetically in an educated way to construct more efficient transgenic bacteria or plants that express combinations of toxin genes to control pests.2 Joint research projects in our respective laboratories during the last decade demonstrate what can be done by implementing certain ideas using molecular biology with Bt ssp. israelensis (Bti) as a model system. Here, we describe our progress achieved with Gram-negative bacterial species, including cyanobacteria, and some preliminary experiments to form transgenic plants, mainly to control mosquitoes (Diptera), but also a particular Lepidopteran and Coleopteran pest species. In addition, a system is described by which environment-damaging genes can be removed from the recombinants thus alleviating procedures for obtaining permits to release them in nature.

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.

Genetic basis for alkaline activation of germination in Bacillus thuringiensis subsp. israelensis

Differences in activation between spores from strains of Bacillus thuringiensis subsp. israelensis with and without the toxin-encoding plasmid pBtoxis are demonstrated. Following alkaline activation, the strain bearing pBtoxis shows a significantly greater germination rate. Expression of just three genes constituting a previously identified, putative ger operon from this plasmid is sufficient to produce the same phenotype and characterizes this operon as a genetic determinant of alkaline activation.

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.

Transfer and expression of the mosquitocidal plasmid pBtoxis in Bacillus cereus group strains

The toxicity of Bacillus thuringiensis subsp. israelensis to dipteran larvae (mosquitoes and black flies) depends on the presence of the pBtoxis plasmid. In this paper, two antibiotic resistance tagged pBtoxis were transferred by conjugation to other Bacillus cereus group strains. Among 15 potential recipients, only a lepidopteran active B. thuringiensis subspecies kurstaki and a B. cereus strain received the plasmid pBtoxis with a low transfer rate of about 10(-8) transconjugants/recipient. The resulting B. thuringiensis subspecies kurstaki transconjugant was active to both lepidopteran and dipteran targets and the B. cereus transconjugant was active against dipteran insects. Phase contrast microscopy showed that the B. cereus transconjugants could produce only round crystalline inclusion bodies while B. thuringiensis subspecies kurstaki transconjugant could produce both round and bipyramidal crystals during sporulation. SDS-PAGE revealed that all the major mosquitocidal proteins from pBtoxis could express in the two transconjugants, including Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa and Cyt1Aa. However, none of the experiment showed any indications of mobilising abilities of pBtoxis. The limited number of strains, which could receive and maintain pBtoxis using a conjugational helper plasmid, indicates a very narrow host range of the B. thuringiensis subsp. israelensis pBtoxis plasmid.

Comparative genome analysis ofBacillus cereusgroup genomes withBacillus subtilis

Genome features of the Bacillus cereus group genomes (representative strains of Bacillus cereus, Bacillus anthracis and Bacillus thuringiensis sub spp. israelensis) were analyzed and compared with the Bacillus subtilis genome. A core set of 1381 protein families among the four Bacillus genomes, with an additional set of 933 families common to the B. cereus group, was identified. Differences in signal transduction pathways, membrane transporters, cell surface structures, cell wall, and S-layer proteins suggesting differences in their phenotype were identified. The B. cereus group has signal transduction systems including a tyrosine kinase related to two-component system histidine kinases from B. subtilis. A model for regulation of the stress responsive sigma factor sigmaB in the B. cereus group different from the well studied regulation in B. subtilis has been proposed. Despite a high degree of chromosomal synteny among these genomes, significant differences in cell wall and spore coat proteins that contribute to the survival and adaptation in specific hosts has been identified.

Plasmid transfer in bacilli by a self-transmissible plasmid p19 from a Bacillus subtilis soil strain

The cryptic 95-kb plasmid p19 of the Bacillus subtilis 19 soil strain promotes the transfer of a small kanamycin resistance plasmid pUB110. To facilitate direct selection for p19 transfer, a plasmid derivative carrying the chloramphenicol resistance gene was constructed. The frequency of transfer of the large plasmid between cells of B. subtilis 19 approached 100% but was more than two orders of magnitude lower when the strain B. subtilis 168 was a recipient. However, when the restriction-deficient strain B. subtilis 168 was a recipient, the transfer efficiency was almost completely recovered. The effectiveness of pUB110 mobilization was virtually not altered in all these cases. pC194 was not mobilized by p19. The kinetics of p19 conjugative transfer is also presented.

pGIL01, a linear tectiviral plasmid prophage originating from Bacillus thuringiensis serovar israelensis

Bacillus thuringiensis serovar israelensis harbours, in addition to several circular plasmids, a small linear molecule of about 15 kb. Sequence analysis of this molecule, named pGIL01, showed the presence of at least 30 ORFs, five of which displayed similarity with proteins involved in phage systems: a B-type family DNA polymerase, a LexA-like repressor, two potential muramidases and a DNA-packaging protein (distantly related to the P9 protein of the tectiviral phage PRD1). Experimental evidence confirmed that pGIL01 indeed corresponds to the linear prophage of a temperate phage. This bacteriophage, named GIL01, produces small turbid plaques and is sensitive to organic solvents, which suggests the presence of lipid components in its capsid. Experiments using proteases and exonucleases also revealed that proteins are linked to the genomes of both pGIL01 prophage and GIL01 phage at their 5' extremities. Altogether, these features are reminiscent of those of phages found in the Tectiviridae family, and more specifically of those of PRD1, a broad-host-range phage of Gram-negative bacteria. Dot-blot hybridization, PFGE, PCR and RFLP analyses also showed the presence of pGIL01 variants in the Bacillus cereus group.

The hidden lifestyles of Bacillus cereus and relatives

Bacillus cereus sensu lato, the species group comprising Bacillus anthracis, Bacillus thuringiensis and B. cereus (sensu stricto), has previously been scrutinized regarding interspecies genetic correlation and pathogenic characteristics. So far, little attention has been paid to analysing the biological and ecological properties of the three species in their natural environments. In this review, we describe the B. cereus sensu lato living in a world on its own; all B. cereus sensu lato can grow saprophytically under nutrient-rich conditions, which are only occasionally found in the environment, except where nutrients are actively collected. As such, members of the B. cereus group have recently been discovered as common inhabitants of the invertebrate gut. We speculate that all members disclose symbiotic relationships with appropriate invertebrate hosts and only occasionally enter a pathogenic life cycle in which the individual species infects suitable hosts and multiplies almost unrestrained.

Conjugative plasmid transfer in gram-positive bacteria

Conjugative transfer of bacterial plasmids is the most efficient way of horizontal gene spread, and it is therefore considered one of the major reasons for the increase in the number of bacteria exhibiting multiple-antibiotic resistance. Thus, conjugation and spread of antibiotic resistance represents a severe problem in antibiotic treatment, especially of immunosuppressed patients and in intensive care units. While conjugation in gram-negative bacteria has been studied in great detail over the last decades, the transfer mechanisms of antibiotic resistance plasmids in gram-positive bacteria remained obscure. In the last few years, the entire nucleotide sequences of several large conjugative plasmids from gram-positive bacteria have been determined. Sequence analyses and data bank comparisons of their putative transfer (tra) regions have revealed significant similarities to tra regions of plasmids from gram-negative bacteria with regard to the respective DNA relaxases and their targets, the origins of transfer (oriT), and putative nucleoside triphosphatases NTP-ases with homologies to type IV secretion systems. In contrast, a single gene encoding a septal DNA translocator protein is involved in plasmid transfer between micelle-forming streptomycetes. Based on these clues, we propose the existence of two fundamentally different plasmid-mediated conjugative mechanisms in gram-positive microorganisms, namely, the mechanism taking place in unicellular gram-positive bacteria, which is functionally similar to that in gram-negative bacteria, and a second type that occurs in multicellular gram-positive bacteria, which seems to be characterized by double-stranded DNA transfer.

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

beta-Exotoxin is a thermostable metabolite produced by some strains of Bacillus thuringiensis. Because of vertebrate toxicity, most commercial preparations of B. thuringiensis are prepared from isolates that do not produce beta-exotoxin. The aim of the present study was to find out the possible relationship between serovars of B. thuringiensis and beta-exotoxin production. A specific HPLC assay for type I beta-exotoxin has been used to detect this exotoxin in supernatants from final whole cultures of 100 strains belonging to four serovars of B. thuringiensis: thuringiensis, kurstaki, aizawai, and morrisoni. For each serovar, 25 strains randomly chosen from two Spanish collections were analyzed. Frequency of beta-exotoxin production was higher in B. thuringiensis serovar thuringiensis, whereas only two strains from serovar kurstaki showed beta-exotoxin production. None of the 25 strains belonging to serovars aizawai and morrisoni was found to produce this compound. Along with data from other studies, serovars can be classified as "common," "seldom," or "rare" beta-exotoxin producers. The serovar-dependent beta-exotoxin production is discussed in relation to the evolutionary process of serovar differentiation, the plasmid compatibility and limited plasmid exchange between serovars, and with the serovar-dependent regulation of plasmid-encoded genes.

Bacillus thuringiensis in fecal samples from greenhouse workers after exposure to B. thuringiensis-based pesticides

In a study of occupational exposure to Bacillus thuringiensis, 20 exposed greenhouse workers were examined for Bacillus cereus-like bacteria in fecal samples and on biomonitoring filters. Bacteria with the following characteristics were isolated from eight individuals: intracellular crystalline inclusions characteristic of B. thuringiensis, genes for and production of B. cereus enterotoxins, and positivity for cry11 as determined by PCR. DNA fingerprints of the fecal isolates were identical to those of strains isolated from the commercial products used. Work processes (i.e., spraying) correlated with the presence of B. thuringiensis in the fecal samples (10(2) to 10(3) CFU/g of feces). However, no gastrointestinal symptoms correlated with the presence of B. thuringiensis in the fecal samples.

Refined, circular restriction map of the Bacillus thuringiensis subsp. israelensis plasmid carrying the mosquito larvicidal genes

All the genetic elements responsible for the mosquito larval toxicity of Bacillus thuringiensis subsp. israelensis are located on one of its largest plasmids, nicknamed pBtoxis. Two linkage groups (with sizes of about 75 and 55 kb) have previously been mapped partially with respect to SacI and BamHI restriction sites (Ben-Dov et al., 1996), but linking them to a single circular plasmid unambiguously was impossible with the available data. To finalize the plasmid map, another rare cutting restriction endonuclease, AlwNI, was used in addition. The two linkage groups and the fragments generated by AlwNI were aligned on the circular plasmid, and known insertion sequences were localized on the refined map. Pulsed-field electrophoresis revealed that the total size of pBtoxis (137 kb) was larger than thought before.

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.

Kinetics of conjugative transfer: a study of the plasmid pXO16 from Bacillus thuringiensis subsp. israelensis

The aggregation-mediated conjugation system of Bacillus thuringiensis subsp. israelensis, encoded by the 200-kb plasmid pXO16, is highly potent in transferring itself and efficient in mobilizing other nonconjugative plasmids. The present study reveals some salient features of this conjugation system. Our observations can be summarized as follows: (i) The conjugative transfer takes about 3(1/2) to 4 min. For a 200-kb plasmid this corresponds to about 1 kb per second. (ii) The ability to transfer the plasmid seems to be evenly distributed among the donors. (iii) Functionally, the mating complex was found to consist of one donor and one recipient cell, even though aggregates comprising thousands of interconnected cells are formed. (iv) Having donated the plasmid, the donor needs a "period of recovery" of about 10 min before it can redonate the plasmid. (v) Secondary transfer, i.e., transfer from newly formed transconjugants, is delayed about 40 min. This maturation time exceeds the generation time, and it may indicate that to display donor activity, a surface protein (the aggregation substance) has to be uniformly incorporated into the cell wall. Lastly, we found that when the experiments were sufficiently short and when the recipient cells were in excess compared with the donors, the process of conjugation could be reasonably described by a kinetic model analogous to the Michaelis-Menten model for enzyme catalysis. This allowed us to estimate (vi) the maximal conjugation rate to be about 0.05 transconjugant per donor per minute, and (vii) the Km value, i.e., the concentration of recipient that results in half of the maximal conjugation rate, to be about 4 x 10(6) recipients/ml.

A novel class of mosquitocidal delta-endotoxin, Cry19B, encoded by a Bacillus thuringiensis serovar higo gene

Partially digested HincII fragments of DNA from a mosquitocidal strain of Bacillus thuringiensis serovar higo were cloned into pBluescript II SK(+) and propagated in Escherichia coli. Recombinant cells were screened immunologically for the production of parasporal inclusion antigens. One E. coli clone harboring a recombinant plasmid exhibited larvicidal activity to Culex pipiens molestus, but not to Anopheles stephensi. Hybridization experiments revealed that the gene of the toxin protein is located on a 110 kb plasmid of B. thuringiensis serovar higo. Sequence analysis detected an open reading frame of 2046 nucleotides encoding a polypeptide of 682 amino acid residues with a predicted molecular weight of 78,467. The gene encoded five block regions commonly conserved in the insecticidal protein genes of B. thuringiensis. Amino acid sequence of the 78 kDa protein shared 49% identity and 56% similarity with that of the Cry19A protein from B. thuringiensis serovar jegathesan. A new class of delta-endotoxin protein, designated Cry19B, was established on the basis of this protein.

Bacillus thuringiensis insecticidal proteins: molecular mode of action

Growing interest in biorational pesticides has placed the Bacillus thuringiensis insecticidal crystal proteins at the forefront of pesticides for plant genetic engineering. The development of improvement pesticides, both in enhanced activity and broader host range, depends on an understanding of its mechanism of action. This review presents a complete overview of the bacterium and the group of insecticidal proteins known as Cry proteins or delta-endotoxins. The molecular mode of action is described in detail, including the mapping of receptor binding sites by site-directed mutagenesis, the known receptors, and the ion-channel activity of the toxins.

Unique regulation of crystal protein production in Bacillus thuringiensis subsp. yunnanensis is mediated by the cry protein-encoding 103-megadalton plasmid

In sporulating cultures of Bacillus thuringiensis subsp. yunnanensis HD977, two cell types are observed: cells forming only spores and cells forming only crystals. Curing analysis suggested that the crystal proteins are plasmid encoded. Through plasmid transfer experiments, it was established that a 103-MDa plasmid is involved in the crystal production. Conjugal transfer of this plasmid to Cry- recipient cells of Bacillus thuringiensis subsp. kurstaki HD73-26 conferred the ability to produce crystals exclusively on asporogenous cells of the recipient, indicating that the 103-MDa plasmid mediates the unique regulation of Cry protein production. When the dipteran-specific cryIVB gene was introduced into wild-type (Cry+) and Cry- backgrounds of B. thuringiensis subsp. yunnanensis by phage CP51ts45-mediated transduction, similar to all other B. thuringiensis strains, irregular crystals of CryIVB protein were produced by spore-forming cells in both backgrounds. However, the synthesis of the bipyramidal inclusions of B. thuringiensis subsp. yunnanensis was still limited only to asporogenous cells of the transductant. Thus, it appears that the unique property of exclusive crystal formation in asporogenous cells of B. thuringiensis subsp. yunnanensis is associated with the crystal protein gene(s) per se or its cis acting elements. As the crystals in B. thuringiensis subsp. yunnanensis were formed only in asporogenous cells, attempts were made to find out whether crystal formation had any inhibitory effect on sporulation. It was observed that both Cry+ and Cry- strains of B. thuringiensis subsp. yunnanensis (HD977 and HD977-1, respectively) exhibited comparable sporulation efficiencies. In addition, the Cry- B. thuringiensis subsp. kurstaki host (HD73-26) and its Cry+ transconjugant (HD73-26-16), expressing the B. thuringiensis subsp. yunnanensis crystal protein, were also comparable in their sporulation efficiencies, indicating that production of the crystal proteins of B. thuringiensis subsp. yunnanensis does not affect the process of sporulation.

Cloning of the nprA gene for neutral protease A of Bacillus thuringiensis and effect of in vivo deletion of nprA on insecticidal crystal protein

The nprA gene, encoding Bacillus thuringiensis neutral protease A, was cloned by the use of gene-specific oligonucleotides. The size of neutral protease A deduced from the nprA sequence was 566 amino acids (60,982 Da). The cloned nprA gene was partially deleted in vitro, and the deleted allele, designated nprA3, was used to construct an nprA3 strain (neutral protease A-deficient strain) of B. thuringiensis. Growth and sporulation of the nprA3 strain were similar to those of an isogenic nprA+ strain, although the extracellular proteolytic activity of the nprA3 strain was significantly less than that of the nprA+ strain. The nprA3 strain produced insecticidal crystal proteins that were more stable than those of the isogenic nprA+ strain after solubilization in vitro, and sporulated cultures of the nprA3 strain contained higher concentrations of full-length insecticidal crystal proteins than did those of its isogenic counterpart. The absence of neutral protease A did not affect the insecticidal activity of a lepidopteran-specific crystal protein of B. thuringiensis. These results indicate that crystal protein stability and yield may be improved by deletion of specific proteases from B. thuringiensis.

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.

The aggregation-mediated conjugation system of Bacillus thuringiensis subsp. israelensis: host range and kinetics of transfer

The aggregation-mediated conjugation system in Bacillus thuringiensis subsp. israelensis encoded on the plasmid pXO16 is characterized by the formation of aggregates when Agr+ and Agr- cells are socialized in exponential growth. Using the aggregation phenotypes, we have identified potential recipients of the aggregation-plasmid pXO16 among Bacillus cereus, Bacillus subtilis, Bacillus megaterium, Bacillus sphaericus, and 24 subspecies of B. thuringiensis. We found 14 Agr- strains, i.e., potential recipients of the aggregation system encoded by plasmid pXO16. Five strains contained a conjugative apparatus of their own and were excluded from further examinations. To monitor the transfer of plasmid pXO16, we constructed a transposon insertion of the plasmid with Tn5401. The study of the plasmid transfer of pXO16::Tn5401 indicated the secretion of bacteriocins from both donor strain and recipient strains. Only one out of the nine strains examined was unable to receive the aggregation-plasmid pXO16 and express the aggregation phenotype and the conjugative abilities. It was found that the transfer of plasmid pXO16 to Bacillus thuringiensis subsp. israelensis Agr- strains was 100%. All recipients had acquired the aggregation-plasmid pXO16 and converted to the Agr+ phenotype.

Restriction map of the 125-kilobase plasmid of Bacillus thuringiensis subsp. israelensis carrying the genes that encode delta-endotoxins active against mosquito larvae

A large plasmid containing all delta-endotoxin genes was isolated from Bacillus thuringiensis subsp. israelensis; restricted by BamHI, EcoRI, HindIII, KpnI, PstI, SacI, and SalI; and cloned as appropriate libraries in Escherichia coli. The libraries were screened for inserts containing recognition sites for BamHI, SacI, and SalI. Each was labeled with 32P and hybridized to Southern blots of gels with fragments generated by cleaving the plasmid with several restriction endonucleases, to align at least two fragments of the relevant enzymes. All nine BamHI fragments and all eight SacI fragments were mapped in two overlapping linkage groups (with total sizes of about 76 and 56 kb, respectively). The homology observed between some fragments is apparently a consequence of the presence of transposons and repeated insertion sequences. Four delta-endotoxin genes (cryIVB-D and cytA) and two genes for regulatory polypeptides (of 19 and 20 kDa) were localized on a 21-kb stretch of the plasmid; without cytA, they are placed on a single BamHI fragment. This convergence enables subcloning of delta-endotoxin genes (excluding cryIVA, localized on the other linkage group) as an intact natural fragment.

Isolation and Identification of novel toxins from a new mosquitocidal isolate from Malaysia, Bacillus thuringiensis subsp. jegathesan

A new mosquitocidal Bacillus thuringiensis subsp., jegathesan, has recently been isolated from Malaysia. Parasporal crystal inclusions were purified from this strain and bioassayed against fourth-instar larvae of Culex quinquefasciatus, Aedes aegypti, Aedes togoi, Aedes albopictus, Anopheles maculatus, and Mansonia uniformis. The 50% lethal concentration of crystal inclusions for each species was 0.34, 8.08, 0.34, 17.59, 3.91, and 120 ng/ml, respectively. These values show that parasporal inclusions from this new subspecies have mosquitocidal toxicity comparable to that of inclusions isolated from B. thuringiensis subsp. israelensis. Solubilized and chymotrypsin-activated parasporal inclusions possessed low-level hemolytic activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that the crystals were composed of polypeptides of 77, 74, 72, 68, 55, 38, 35, 27, and 23 kDa. Analysis by Western blotting (immunoblotting) with polyclonal antisera raised against toxins purified from B. thuringiensis subsp. israelensis reveals that proteins in parasporal inclusions of subsp. jegathesan are distinct, because little cross-reactivity was shown. Analysis of the plasmid content of B. thuringiensis subsp. jegathesan indicates that the genes for toxin production may be located on 105- to 120-kb plasmids. Cry- clones that have been cured of these plasmids are nontoxic. Southern blot analysis of plasmid and chromosomal DNA from subsp. jegathesan showed little or low homology to the genes coding for CryIVA, CryIVB, and CryIVD from B. thuringiensis subsp. israelensis.

Mosquito larvicidal activity of Escherichia coli with combinations of genes from Bacillus thuringiensis subsp. israelensis

The genes cryIVA and cryIVD, encoding 134- and 72-kDa proteins, respectively, and the gene for a regulatory 20-kDa polypeptide of Bacillus thuringiensis subsp. israelensis (serovar H14) were cloned in all seven possible combinations by the Escherichia coli expression vectors pT7 and pUHE. The four combinations containing cryIVA (cryIVA alone, with cryIVD, with the 20-kDa-protein gene, and with both) displayed high levels of mosquito larvicidal activity in pUHE. The toxicity of the combination of cryIVA and cryIVD, with or without the 20-kDa-protein gene, was higher than has ever been achieved with delta-endotoxin genes in recombinant E. coli. Fifty percent lethal concentrations against third-instar Aedes aegypti larvae for these clones decreased (i.e., toxicity increased) continuously to about 3 x 10(5) cells ml-1 after 4 h of induction. Larvicidal activities, obtained after 30 min of induction, were lower for clones in pT7 and decreased for an additional 3.5 h. Induction of either cryIVD or the 20-kDa-protein gene alone resulted in no larvicidal activity in either pT7 or pUHE20. Cloned together, these genes were slightly toxic in pT7 but not in pUHE20. Five minutes of induction of this combination (cryIVD with the 20-kDa-protein gene) in pT7 yielded a maximal mortality of about 40%, which decreased rapidly and disappeared completely after 50 min. CryIVD is thus apparently degraded in E. coli and partially stabilized by the 20-kDa regulatory protein. Larvicidal activity of the combination of cryIVA and cryIVD was sevenfold higher than that of cryIVA alone, probably because of the cross-stabilization of the polypeptides or the synergism between their activities.

The genetic basis of the aggregation system in Bacillus thuringiensis subsp. israelensis is located on the large conjugative plasmid pXO16

The aggregation phenotypes Agr+ and Agr- of Bacillus thuringiensis subsp. israelensis are correlated with a conjugation-like plasmid transfer and characterized by the formation of aggregates when the bacteria are socialized during exponential growth. We present evidence for the association of the Agr+ phenotype with the presence of the large (135-MDa) self-transmissible plasmid pXO16.

Factors regulating cryIVB expression in the cyanobacterium--Synechococcus PCC 7942

The expression of the larvicidal Bacillus thuringiensis subsp. israelensis cryIVB gene in cyanobacteria has been suggested to be an effective means of controlling mosquito populations. Using a variety of cryIVB constructs, in this study we have examined the effect of Synechococcus PCC 7942 culture age on intracellular toxin levels and have attempted to determine the mechanisms by which cryIVB gene expression is regulated. The data suggest that specific degradation of the cryIVB mRNA limits toxin production; however, the addition of cyanobacterial 3' untranslated DNA sequences to the cryIVB gene did not improve mRNA stability or toxin levels. An analysis of the cryIVB sequence and comparison of codon usage patterns with highly expressed cyanobacterial genes suggest that inefficient translation and intragenic ribosomal binding sites impede protein synthesis and result in rapid turnover of the toxin mRNA.

Flexibility in the protoxin composition of Bacillus thuringiensis

In at least three Bacillus thuringiensis subspecies, multiple protoxin genes are confined to just a few of the many plasmids with two or more on one of > 100 mDa and a particular gene, cryIA(b), on a 40-50 mDa plasmid. The latter is unstable but can be maintained in the population by cell mating. Cells which had lost this plasmid compensated by increasing transcription of the remaining protoxin genes resulting in the formation of inclusions which differed from those in the parental strains in their toxicity profiles for selected insects as well as their solubility. Instability of a particular protoxin-encoding plasmid appears to be a mechanism for rapidly shifting the protoxin gene complement and thus the toxicity profiles of these bacteria.

Mobilization of small plasmids in Bacillus thuringiensis subsp. israelensis is accompanied by specific aggregation

Mobilizations of pBC16 and pAND006, containing the replicon of the Bacillus thuringiensis subsp. israelensis plasmid pTX14-3, between strains of B. thuringiensis subsp. israelensis were examined. Transconjugants appeared after a few minutes and reached a maximum frequency after approximately 2 h. Plasmid pBC16 was mobilized at a frequency approximately 200 times that of pAND006. However, pAND006 was consistently transferred, suggesting that the replicon of pTX14-3 is sufficient to sustain mobilization in B. thuringiensis subsp. israelensis. A specific protease-sensitive coaggregation between strains of B. thuringiensis subsp. israelensis was found to be unambiguously correlated with plasmid transfer. Two aggregation phenotypes, Agr+ and Agr-, were identified in this subspecies. Aggregation disappeared when the optical density of the mating mixture at 600 nm exceeded approximately 1, and it did not reappear upon dilution. Aggregation was shown to involve interactions of cells with opposite aggregation phenotypes, and evidence of a proteinaceous molecule on the surface of the Agr- that is cells involved in aggregation formation is presented. Matings and selection for the presence of two antibiotic resistance plasmids followed by identification of the host cell revealed that mobilization was unidirectional, from the Agr+ cell to the Agr- cell. The aggregation phenotype was found to be transferred with high frequency (approximately 100%) in broth matings, and the appearance of Agr- isolates from Agr+ strains suggested that the loci involved in aggregation formation are located on a plasmid. No excreted aggregation-inducing signals were detected in the supernatant or culture filtrate of either the donor, the recipient, or the mating mixture.

Effect of promoter modification on mosquitocidal cryIVB gene expression in Synechococcus sp. strain PCC 7942

The impact of promoter modification on the expression of the mosquitocidal Bacillus thuringiensis subsp. israelensis cryIVB gene when used to transform the cyanobacterium Synechococcus sp. strain PCC 7942 has been examined. Maximal transcript and protein abundances were achieved by the addition of the lacZ promoter upstream of the cryIVB sequence. Replacement of the endogenous corresponding Bacillus sequences with the Synechococcus petF1 promoter, ribosome binding site, and initiation codon also resulted in increased expression of the cryIVB gene relative to the expression obtained with the Bacillus promoter alone but decreased expression relative to the expression achieved with the tandem array of the Bacillus and lacZ promoters. Synechococcus cells carrying plasmids in which the expression of the cryIVB gene was regulated by either the lacZ or the petF1 promoter were readily consumed by first-instar Culex restuans larvae and proved to be toxic for these organisms.

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.

The two faces of Bacillus thuringiensis: insecticidal proteins and post-exponential survival

Post-exponential Bacillus thuringiensis cells produce both an endospore and a variety of intracellular inclusions. The latter are comprised of protoxins, each being specific for the larvae of certain species from at least three orders of insects. Following ingestion of spores and inclusions, toxicity results in the spores gaining access to haemolymph, a source of nutrients suitable for germination and growth. Most B. thuringiensis subspecies contain multiple, plasmid-encoded protoxin genes, often with several on the same plasmid. These genes have been manipulated in order to understand the basis of toxicity and specificity, information which is important to the use of these toxins as biological control agents. Some protoxin genes are in operons, and others are in close proximity, perhaps to enhance the chances of recombination, and some are on unstable plasmids. The arrangement of these genes is probably important for flexibility in the variety of protoxins packaged into inclusions by a particular subspecies and thus the capacity to adapt to changing populations of insects. Protoxins accumulate over a prolonged period during sporulation because of the sequential transcription from two promoters, each being dependent upon a specific sporulation sigma factor, the relative stability of the messenger RNA, and the synthesis of proteins which stabilize protoxins and perhaps facilitate inclusion assembly. During the post-exponential phase, spore and inclusion formation must be balanced so as to ensure that both are available to contribute to the survival of these bacilli.

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

The cytA gene encoding the 28-kDa polypeptide of Bacillus thuringiensis subsp. israelensis crystals was disrupted in the 72-MDa resident plasmid by in vivo recombination, thus indicating that homologous recombination occurs in B. thuringiensis. The absence of the 28-kDa protein in B. thuringiensis did not affect the crystallization of the other toxic components of the parasporal body (68-, 125-, and 135-kDa polypeptides). The absence of the 28-kDa protein abolished the hemolytic activity of B. thuringiensis subsp. israelensis crystals. However, the mosquitocidal activity of the 28-kDa protein-free crystals did not differ significantly from that of the wild-type crystals when tested on Aedes aegypti and Culex pipiens larvae. The 28-kDa protein contributed slightly to the toxicity to Anopheles stephensi larvae. This indicates that the 28-kDa protein is not essential for mosquitocidal activity, at least against the three species tested.