Phenotypic characterization of Bacillus thuringiensis and Bacillus cereus

The cereus matter of Bacillus endophthalmitis

Bacillus cereus (B. cereus) endophthalmitis is a devastating intraocular infection primarily associated with post-traumatic injuries. The majority of these infections result in substantial vision loss, if not loss of the eye itself, within 12-48 h. Multifactorial mechanisms that lead to the innate intraocular inflammatory response during this disease include the combination of robust bacterial replication, migration of the organism throughout the eye, and toxin production by the organism. Therefore, the window of therapeutic intervention in B. cereus endophthalmitis is quite narrow compared to that of other pathogens which cause this disease. Understanding the interaction of bacterial and host factors is critical in understanding the disease and formulating more rational therapeutics for salvaging vision. In this review, we will discuss clinical and research findings related to B. cereus endophthalmitis in terms of the organism's virulence and inflammogenic potential, and strategies for improving of current therapeutic regimens for this blinding disease.

S-layer Impacts the Virulence of Bacillus in Endophthalmitis

Purpose

Bacillus causes a sight-threating infection of the posterior segment of the eye. The robust intraocular inflammatory response in this disease is likely activated via host innate receptor interactions with components of the Bacillus cell envelope. S-layer proteins (SLPs) of some Gram-positive pathogens contribute to the pathogenesis of certain infections. The potential contributions of SLPs in eye infection pathogenesis have not been considered. Here, we explored the role of a Bacillus SLP (SlpA) in endophthalmitis pathogenesis.

Methods

The phenotypes and infectivity of wild-type (WT) and S-layer deficient (ΔslpA) Bacillus thuringiensis were compared. Experimental endophthalmitis was induced in C57BL/6J mice by intravitreally injecting 100-CFU WT or ΔslpA B. thuringiensis. Infected eyes were analyzed by bacterial counts, retinal function analysis, histology, and inflammatory cell influx. SLP-induced inflammation was also analyzed in vitro. Muller cells (MIO-M1) were treated with purified SLP. Nuclear factor-κB (NF-κB) DNA binding was measured by ELISA and expression of proinflammatory mediators from Muller cells was measured by RT-qPCR.

Results

Tested phenotypes of WT and ΔslpA B. thuringiensis were similar, with the exception of absence of the S-layer in the ΔslpA mutant. Intraocular growth of WT and ΔslpA B. thuringiensis was also similar. However, eyes infected with the ΔslpA mutant had significantly reduced inflammatory cell influx, less inflammatory damage to the eyes, and significant retention of retinal function compared with WT-infected eyes. SLP was also a potent stimulator of the NF-κB pathway and induced the expression of proinflammatory mediators (IL6, TNFα, CCL2, and CXCL-1) in human retinal Muller cells.

Conclusions

Taken together, our results suggest that SlpA contributes to the pathogenesis of Bacillus endophthalmitis, potentially by triggering innate inflammatory pathways in the retina.

Insights into the draft genome sequence of bioactives-producing Bacillus thuringiensis DNG9 isolated from Algerian soil-oil slough

Bacillus thuringiensis is widely used as a bioinsecticide due to its ability to form parasporal crystals containing proteinaceous toxins. It is a member of the Bacillus cereus sensu lato, a group with low genetic diversity but produces several promising antimicrobial compounds. B. thuringiensis DNG9, isolated from an oil-contaminated slough in Algeria, has strong antibacterial, antifungal and biosurfactant properties. Here, we report the 6.06 Mbp draft genome sequence of B. thuringiensis DNG9. The genome encodes several gene inventories for the biosynthesis of bioactive compounds such as zwittermycin A, petrobactin, insecticidal toxins, polyhydroxyalkanoates and multiple bacteriocins. We expect the genome information of strain DNG9 will provide another model system to study pathogenicity against insect pests, plant diseases, and antimicrobial compound mining and comparative phylogenesis among the Bacillus cereus sensu lato group.

Complete genome sequence of Bacillus thuringiensis strain HD521

Bacillus thuringiensis is the most widely used biological pesticide in the world. It belongs to the Bacillus cereus sensu lato group, which contains six species. Among these six species, B. thuringiensis, B. anthracis, and B. cereus have a low genetic diversity. B. thuringiensis strain HD521 shows maroon colony which is different from most of the B. thuringiensis strains. Strain HD521 also displays an ability to inhibit plant sheath blight disease pathogen (Rhizoctonia solani AG1 IB) growth and can form bipyramidal parasporal crystals consisting of three cry7 genes. These crystals have an insecticidal activity against Henosepilachna vigintioctomaculata larva (Coleoptera). Here we report the complete genome sequence of strain HD521, which has one chromosome and six circular plasmids.

Distribution of phenotypes among Bacillus thuringiensis strains

An extensive collection of Bacillus thuringiensis isolates from around the world were phenotypically profiled using standard biochemical tests. Six phenotypic traits occurred in 20-86% of the isolates and were useful in distinguishing isolates: production of urease (U; 20.5% of isolates), hydrolysis of esculin (E; 32.3% of isolates), acid production from salicin (A; 37.4% of isolates), acid production from sucrose (S; 34.0% of isolates), production of phospholipase C or lecithinase (L; 79.7% of isolates), and hydrolysis of starch (T; 85.8% of isolates). With the exception of acid production from salicin and hydrolysis of esculin, which were associated, the traits assorted independently. Of the 64 possible combinations of these six phenotypic characteristics, 15 combinations accounted for ca. 80% of all isolates, with the most common phenotype being TL (23.6% of isolates). Surprisingly, while the biochemical traits generally assorted independently, certain phenotypic traits associated with the parasporal crystal were correlated with certain combinations of biochemical traits. Crystals that remained attached to spores (which tended to be non-toxic to insects) were highly correlated with the phenotypes that included both L and S. Among the 15 most abundant phenotypes characterizing B. thuringiensis strains, amorphous crystals were associated with TLE, TL, T, and Ø (the absence of positive tested biochemical traits). Amorphous crystal types displayed a distinct bias toward toxicity to dipteran insects. Although all common phenotypes included B. thuringiensis isolates producing bipyramidal crystals toxic to lepidopteran insects, those with the highest abundance of these toxic crystals displayed phenotypes TLU, TLUA, TLUAE, and TLAE.

Conjugative transfer of insecticidal plasmid pHT73 from Bacillus thuringiensis to B. anthracis and compatibility of this plasmid with pXO1 and pXO2

Bacillus anthracis, the etiologic agent of anthrax, is genetically close to and commonly shares a giant gene pool with B. cereus and B. thuringiensis. In view of the human pathogenicity and the long persistence in the environment of B. anthracis, there is growing concern about the effects of genetic exchange with B. anthracis on public health. In this work, we demonstrate that an insecticidal plasmid, pHT73, from B. thuringiensis strain KT0 could be efficiently transferred into two attenuated B. anthracis strains, Ba63002R (pXO1(+) pXO2(-)) and Ba63605R (pXO1(-) pXO2(+)), by conjugation in liquid medium in the laboratory, with transfer rates of 2.3 x 10(-4) and 1.6 x 10(-4) CFU/donor, respectively. The B. anthracis transconjugants containing both pHT73 and pXO1 or pXO2 could produce crystal protein Cry1Ac encoded by plasmid pHT73 and had high toxicity to Helicoverpa armigera larvae. Furthermore, the compatibility and stability of pHT73 with pXO1/pXO2 were demonstrated. The data are informative for further investigation of the safety of B. thuringiensis and closely related strains in food and in the environment.

Use of 16S rRNA, 23S rRNA, and gyrB gene sequence analysis to determine phylogenetic relationships of Bacillus cereus group microorganisms

In order to determine if variations in rRNA sequence could be used for discrimination of the members of the Bacillus cereus group, we analyzed 183 16S rRNA and 74 23S rRNA sequences for all species in the B. cereus group. We also analyzed 30 gyrB sequences for B. cereus group strains with published 16S rRNA sequences. Our findings indicated that the three most common species of the B. cereus group, B. cereus, Bacillus thuringiensis, and Bacillus mycoides, were each heterogeneous in all three gene sequences, while all analyzed strains of Bacillus anthracis were found to be homogeneous. Based on analysis of 16S and 23S rRNA sequence variations, the microorganisms within the B. cereus group were divided into seven subgroups, Anthracis, Cereus A and B, Thuringiensis A and B, and Mycoides A and B, and these seven subgroups were further organized into two distinct clusters. This classification of the B. cereus group conflicts with current taxonomic groupings, which are based on phenotypic traits. The presence of B. cereus strains in six of the seven subgroups and the presence of B. thuringiensis strains in three of the subgroups do not support the proposed unification of B. cereus and B. thuringiensis into one species. Analysis of the available phenotypic data for the strains included in this study revealed phenotypic traits that may be characteristic of several of the subgroups. Finally, our results demonstrated that rRNA and gyrB sequences may be used for discriminating B. anthracis from other microorganisms in the B. 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.

Fluorescent Amplified Fragment Length Polymorphism Analysis of Norwegian Bacillus cereus and Bacillus thuringiensis Soil Isolates

We examined 154 Norwegian B. cereus and B. thuringiensis soil isolates (collected from five different locations), 8 B. cereus and 2 B. thuringiensis reference strains, and 2 Bacillus anthracis strains by using fluorescent amplified fragment length polymorphism (AFLP). We employed a novel fragment identification approach based on a hierarchical agglomerative clustering routine that identifies fragments in an automated fashion. No method is free of error, and we identified the major sources so that experiments can be designed to minimize its effect. Phylogenetic analysis of the fluorescent AFLP results reveals five genetic groups in these group 1 bacilli. The ATCC reference strains were restricted to two of the genetic groups, clearly not representative of the diversity in these bacteria. Both B. anthracis strains analyzed were closely related and affiliated with a B. cereus milk isolate (ATCC 4342) and a B. cereus human pathogenic strain (periodontitis). Across the entire study, pathogenic strains, including B. anthracis, were more closely related to one another than to the environmental isolates. Eight strains representing the five distinct phylogenetic clusters were further analyzed by comparison of their 16S rRNA gene sequences to confirm the phylogenetic status of these groups. This analysis was consistent with the AFLP analysis, although of much lower resolution. The innovation of automated genotype analysis by using a replicated and statistical approach to fragment identification will allow very large sample analyses in the future.

Rapid identification of Bacillus cereus based on the detection of a 28.5-kilodalton cell surface antigen

Conventional procedures for the identification of suspect Bacillus cereus isolated on mannitol-egg yolk-polymyxin (MYP) agar may need several days. To facilitate the identification of the bacterium, an enzyme-linked immunosorbent assay (ELISA) was developed. The assay was based on the detection of a 28.5-kDa cell surface antigen of B. cereus. Bacterial colonies grown on MYP agar or nutrient agar were suspended in phosphate-buffered saline (pH 7.2) containing 0.1% Teepol. The cell suspensions were heated at 100 degrees C for 5 min and added to the microtiter plates coated with antibodies against the 28.5-kDa antigen. After washing, the same antibodies labeled with horseradish peroxidase were used as secondary antibodies to reveal the signal of antigen-antibody reaction. For 38 strains of B. cereus and 127 strains of non-B. cereus bacteria (including 79 isolates of Bacillus spp.) tested, the sensitivity and specificity of the ELISA were 100 and 88.2%, respectively. Strains producing false-positive results were members of the B. cereus group (i.e., Bacillus anthracis, Bacillus mycoides, and Bacillus thuringiensis), which are genetically and biochemically similar to B. cereus. Similar ELISA results were obtained by using antibodies against another cell surface antigen with a molecular mass of 20 kDa. If members of the B. cereus group were recognized as a single species, the sensitivity and specificity of the ELISA were 100 and 99.1%, respectively. The ELISA could be used as a rapid method for presumptive identification of B. cereus grown on MYP agar.

The mammalian safety of Bacillus thuringiensis-based insecticides

The United States Environmental Protection Agency between the years 1961 and 1995 registered 177 products containing viable Bacillus thuringiensis (Bt). Numerous laboratory studies have demonstrated that Bt and Bt products are noninfectious and are toxic to mammals only at a dose > or =10(8) colony forming units (cfu) per mouse (a human equivalent based on the weight of >10(11) cfu). In contrast, as few as three vegetative cells of Bacillus anthracis can kill mice (a human equivalent of >10(3) cfu). There are only two literature reports of Bt infection in man between the year 1997 and the present, and all infected individuals had experienced either extensive burns or a blast injury, which predisposed them to infection. Two epidemiology studies conducted during large-scale aerial Bt serovar kurstaki spray campaigns reported no increased incidence of illness. Some recent papers have expressed concern about the production of Bacillus cereus enterotoxins by Bt isolates. Laboratory studies found no evidence of illness in rats and sheep fed Bt products, nor have epidemiology studies found increased incidence of diarrhea during Bt aerial spray campaigns. Increases in human antibody levels following exposure to Bt products have been reported but there was no increased incidence in asthma or other illness. Based on laboratory studies and field experience, Bt insecticides have an excellent safety record.

Bacillus thuringiensis and its pesticidal crystal proteins

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

Genomic variations in mosquitocidal strains of Bacillus sphaericus detected by M13 DNA fingerprinting

The genomic variation of Bacillus sphaericus reference and local strains belonging to different serotypes was examined by DNA fingerprinting. A phage M13 DNA probe detected a number of variable fragments in the restriction digests of total strain DNAs. The patterns of band distribution showed a certain homology among mosquitocidal strains, expressed by similarity index D and might be a reliable criterion for assessing the level of genomic similarity between closely related strains. An important advantage of DNA fingerprinting is the differentiation of one bacterial strain from another, both expressing common phenotype and possessing highly similar genomic portions. The strain variation revealed by the M13 probe will be useful for characterization of individual strains within a serotype. It could help as well to solve some uncertain cases based on the results obtained by other methods of identification.

Distribution, Frequency, and Diversity of Bacillus thuringiensis in an Animal Feed Mill

Bacillus thuringiensis was isolated from 36 of 50 residue samples obtained from an animal feed mill (a stored-product environment). Of 710 selected colonies having Bacillus cereus-B. thuringiensis morphology isolated from the samples, 477 were classified as B. thuringiensis because of production of parasporal δ-endotoxin crystals. There was a diverse population of B. thuringiensis , as revealed by differentiation of the isolates into 36 subgroups by using (i) their spectra of toxicity to the lepidopterans Heliothis virescens, Pieris brassicae , and Spodoptera littoralis and the dipteran Aedes aegypti and (ii) their parasporal crystal morphology. A total of 55% of the isolates were not toxic to any of these insects at the concentrations used in the bioassays; 40% of all isolates were toxic to one or more of the Lepidoptera; and 20, 1, and 1% of the isolates were toxic to only P. brassicae, H. virescens , and S. littoralis , respectively. The most frequent toxicity was toxicity to P. brassicae (36% of all isolates); 18% of the isolates were toxic to A. aegypti (5% exclusively), 10% were toxic to H. virescens , and 4% were toxic to S. littoralis . Toxicity to P. brassicae was more often linked with toxicity to H. virescens than with toxicity to S. littoralis . The frequency of toxicity was significantly greater in isolates that produced bipyramidal crystals than in isolates that produced irregular pointed, irregular spherical, rectangular, or spherical crystals.

Bacillus sphaericus as a mosquito pathogen: properties of the organism and its toxins

In the course of sporulation, Bacillus sphaericus produces an inclusion body which is toxic to a variety of mosquito larvae. In this review we discuss the general biology of this species and concentrate on the genetics and physiology of toxin production and its processing in the midgut of the larval host. The larvicide of B. sphaericus is unique in that it consists of two proteins of 51 and 42 kDa, both of which are required for toxicity to mosquito larvae. There is a low level of sequence similarity between these two proteins, which differ in their sequences from all the other known insecticidal proteins of Bacillus thuringiensis. Within the midgut the 51- and 42-kDa proteins are processed to proteins of 43 and 39 kDa, respectively. The conversion of the 42-kDa protein to a 39-kDa protein results in a major increase in toxicity; the significance of the processing of the 51-kDa protein is not known. In contrast to the results with mosquito larvae, the 39-kDa protein is alone toxic for mosquito-derived tissue culture-grown cells, and this toxicity is not affected by the 51-kDa protein or its derivative, the 43-kDa protein. Comparisons of larvae from species which differ in their susceptibility to the B. sphaericus toxin indicate that the probable difference resides in the nature of the target sites of the epithelial midgut cells and not in uptake or processing of the toxin. A similar conclusion is derived from experiments involving tissue culture-grown cells from mosquito species which differ in their susceptibility to the B. sphaericus toxin.

Plasmid-associated sensitivity of Bacillus thuringiensis to UV light

Spores and vegetative cells of Bacillus thuringiensis were more sensitive to UV light than were spores or cells of plasmid-cured B. thuringiensis strains or of the closely related Bacillus cereus. Introduction of B. thuringiensis plasmids into B. cereus by cell mating increased the UV sensitivity of the cells and spores. Protoxins encoded by one or more B. thuringiensis plasmids were not involved in spore sensitivity, since a B. thuringiensis strain conditional for protoxin accumulation was equally sensitive at the permissive and nonpermissive temperatures. In addition, introduction of either a cloned protoxin gene, the cloning vector, or another plasmid not containing a protoxin gene into a plasmid-cured strain of B. thuringiensis all increased the UV sensitivity of the spores. Although the variety of small, acid-soluble proteins was the same in the spores of all strains examined, the quantity of dipicolinic acid was about twice as high in the plasmid-containing strains, and this may account for the differences in UV sensitivity of the spores. The cells of some strains harboring only B. thuringiensis plasmids were much more sensitive than cells of any of the other strains, and the differences were much greater than observed with spores.

Plasmid transfer between strains of Bacillus thuringiensis infecting Galleria mellonella and Spodoptera littoralis

To determine the possibility of plasmid transfer occurring between strains of Bacillus thuringiensis in infected lepidopterous larvae, Galleria mellonella and Spodoptera littoralis were infected with two or more strains of B. thuringiensis and the resulting bacteria from the dead insects were examined for plasmid transfer. Transfer rates of plasmids coding for crystal production and tetracycline resistance were high, reaching levels similar to those obtained in laboratory broth cultures. Transfer was higher in G. mellonella than S. littoralis, probably due to the greater ability of B. thuringiensis to colonize the larvae. In broth cultures, B. thuringiensis was also able to transfer plasmids into sporeforming bacteria present in soil samples. The results suggest that plasmid transfer between strains of B. thuringiensis occurs in nature, resulting in the production of new combinations of delta-endotoxins within populations of the bacteria.

DNA fingerprinting of the mosquito pathogen Bacillus sphaericus with M13 DNA as a probe

Hypervariable nucleotide sequences were detected in Bacillus sphaericus by hybridization with radioactively labelled M13 DNA. Different serotypes could be distinguished by their hybridization profiles. The appearance of bands common for mosquito-pathogenic strains and their absence in an apathogenic strain opens the probability that M13 could hybridize to specific alleles, related to insect toxicity.

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.