Cloning and heterologous expression of insecticidal-protein-encoding genes from Photorhabdus luminescens TT01 in Enterobacter cloacae for termite control

Genetic Engineering of Escherichia coli BL21 (DE3) with a codon-optimized insecticidal toxin complex gene tccZ

A toxin complex consists of a high-molecular-weight group of toxins that exhibits insecticidal activity against insect pests. These toxins are a promising alternative to Bacillus thuringiensis (Bt) toxins that have been extensively utilized in insect pest control. Herein, a codon-optimized insecticidal gene (tccZ) (381 bp) identified in Pantoea ananatis strain MHSD5 (a bacterial endophyte previously isolated from Pellaea calomelanos) was ligated into the pET SUMO expression vector and expressed in Escherichia coli BL21 (DE3). We report the success of cloning the tccZ gene into the pET SUMO vector and ultimately the transformation into E. coli BL21 (DE3) competent cells. However, despite conducting a time course of expression as well as isopropyl β-d-1-thiogalactopyranoside (IPTG) dosage optimization to determine optimal conditions for expression, TccZ protein expression could not be detected on Stain-Free and Coomassie-stained SDS-PAGE gels.

Composition and Diversity of Gut Bacterial Community in Different Life Stages of a Leaf Beetle Gastrolina depressa

Insect gut bacteria have a significant impact on host biology, which has a favorable or negative impact on insect fitness. The walnut leaf beetle (Gastrolina depressa) is a notorious pest in China, causing severe damage to Juglandaceae trees including Juglans regia and Pterocarya rhoifolia. To date, however, we know surprisingly little about the gut microbiota of G. depressa. This study used a high-throughput sequencing platform to investigate the gut bacterial community of G. depressa throughout its life cycle, including the 1st, 2nd, and 3rd instar larvae, as well as male, female, and pre-pregnant female adults. Our results showed that the diversity of the gut bacterial community in larvae was generally higher than that in adults, and young larvae (1st and 2nd larvae) possessed the most diversified and abundant community. Principal coordinate analysis results showed that the gut microbiota of adults cluster together, which is independent of the 1st and 2nd instar larvae. The main phyla were Proteobacteria and Firmicutes in the microbial community of G. depressa, while the dominant genera were Enterobacter, Rosenbergiella, Erwinia, Pseudomonas, and Lactococcus. The gut bacteria of G. depressa were mostly enriched in metabolic pathways (carbohydrate metabolism and amino acid metabolism) as revealed by functional prediction. This study contributes to a better knowledge of G. depressa's gut microbiota and its potential interactions with the host insect, facilitating the development of a microbial-based pest management strategy.

Engineering insects from the endosymbiont out

Insects are an incredibly diverse group of animals with species that benefit and harm natural ecosystems, agriculture, and human health. Many insects have consequential associations with microbes: bacterial symbionts may be embedded in different insect tissues and cell types, inherited across insect generations, and required for insect survival and reproduction. Genetically engineering insect symbionts is key to understanding and harnessing these associations. We summarize different types of insect-bacteria relationships and review methods used to genetically modify endosymbiont and gut symbiont species. Finally, we discuss recent studies that use this approach to study symbioses, manipulate insect-microbe interactions, and influence insect biology. Further progress in insect symbiont engineering promises to solve societal challenges, ranging from controlling pests to protecting pollinator health.

Genetically modified entomopathogenic bacteria, recent developments, benefits and impacts: A review

Entomopathogenic bacteria (EPBs), insect pathogens that produce pest-specific toxins, are environmentally-friendly alternatives to chemical insecticides. However, the most important problem with EPBs application is their limited field stability. Moreover, environmental factors such as solar radiation, leaf temperature, and vapor pressure can affect the pathogenicity of these pathogens and their toxins. Scientists have conducted intensive research to overcome such problems. Genetic engineering has great potential for the development of new engineered entomopathogens with more resistance to adverse environmental factors. Genetically modified entomopathogenic bacteria (GM-EPBs) have many advantages over wild EPBs, such as higher pathogenicity, lower spraying requirements and longer-term persistence. Genetic manipulations have been mostly applied to members of the bacterial genera Bacillus, Lysinibacillus, Pseudomonas, Serratia, Photorhabdus and Xenorhabdus. Although many researchers have found that GM-EPBs can be used safely as plant protection bioproducts, limited attention has been paid to their potential ecological impacts. The main concerns about GM-EPBs and their products are their potential unintended effects on beneficial insects (predators, parasitoids, pollinators, etc.) and rhizospheric bacteria. This review address recent update on the significant role of GM-EPBs in biological control, examining them through different perspectives in an attempt to generate critical discussion and aid in the understanding of their potential ecological impacts.

Hype or opportunity? Using microbial symbionts in novel strategies for insect pest control

All insects, including pest species, are colonized by microorganisms, variously located in the gut and within insect tissues. Manipulation of these microbial partners can reduce the pest status of insects, either by modifying insect traits (e.g. altering the host range or tolerance of abiotic conditions, reducing insect competence to vector disease agents) or by reducing fitness. Strategies utilizing heterologous microorganisms (i.e. derived from different insect species) and genetically-modified microbial symbionts are under development, particularly in relation to insect vectors of human disease agents. There is also the potential to target microorganisms absolutely required by the insect, resulting in insect mortality or suppression of insect growth or fecundity. This latter approach is particularly valuable for insect pests that depend on nutrients from symbiotic microorganisms to supplement their nutritionally-inadequate diet, e.g. insects feeding through the life cycle on vertebrate blood (cimicid bugs, anopluran lice, tsetse flies), plant sap (whiteflies, aphids, psyllids, planthoppers, leafhoppers/sharpshooters) and sound wood (various xylophagous beetles and some termites). Further research will facilitate implementation of these novel insect pest control strategies, particularly to ensure specificity of control agents to the pest insect without dissemination of bio-active compounds, novel microorganisms or their genes into the wider environment.

Assessment of genetically engineered Trabulsiella odontotermitis as a 'Trojan Horse' for paratransgenesis in termites

Background

The Formosan subterranean termite, Coptotermes formosanus is an invasive urban pest in the Southeastern USA. Paratransgenesis using a microbe expressed lytic peptide that targets the termite gut protozoa is currently being developed for the control of Formosan subterranean termites. In this study, we evaluated Trabulsiella odontotermitis, a termite-specific bacterium, for its potential to serve as a ‘Trojan Horse’ for expression of gene products in termite colonies.

Results

We engineered two strains of T. odontotermitis, one transformed with a constitutively expressed GFP plasmid and the other engineered at the chromosome with a Kanamycin resistant gene using a non- disruptive Tn7 transposon. Both strains were fed to termites from three different colonies.

Fluorescent microscopy confirmed that T. odontotermitis expressed GFP in the gut and formed a biofilm in the termite hindgut. However, GFP producing bacteria could not be isolated from the termite gut after 2 weeks. The feeding experiment with the chromosomally engineered strain demonstrated that T. odontotermitis was maintained in the termite gut for at least 21 days, irrespective of the termite colony. The bacteria persisted in two termite colonies for at least 36 days post feeding. The experiment also confirmed the horizontal transfer of T. odontotermitis amongst nest mates.

Conclusion

Overall, we conclude that T. odontotermitis can serve as a ‘Trojan Horse’ for spreading gene products in termite colonies. This study provided proof of concept and laid the foundation for the future development of genetically engineered termite gut bacteria for paratransgenesis based termite control.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0822-4) contains supplementary material, which is available to authorized users.

Protozoacidal Trojan-Horse: use of a ligand-lytic peptide for selective destruction of symbiotic protozoa within termite guts

For novel biotechnology-based termite control, we developed a cellulose bait containing freeze-dried genetically engineered yeast which expresses a protozoacidal lytic peptide attached to a protozoa-recognizing ligand. The yeast acts as a ‘Trojan-Horse’ that kills the cellulose-digesting protozoa in the termite gut, which leads to the death of termites, presumably due to inefficient cellulose digestion. The ligand targets the lytic peptide specifically to protozoa, thereby increasing its protozoacidal efficiency while protecting non-target organisms. After ingestion of the bait, the yeast propagates in the termite's gut and is spread throughout the termite colony via social interactions. This novel paratransgenesis-based strategy could be a good supplement for current termite control using fortified biological control agents in addition to chemical insecticides. Moreover, this ligand-lytic peptide system could be used for drug development to selectively target disease-causing protozoa in humans or other vertebrates.

Biotransformation of explosives by Reticulitermes flavipes--associated termite Endosymbionts

BACKGROUND/AIMS

Termites have an important role in the carbon and nitrogen cycles despite their reputation as destructive pests. With the assistance of microbial endosymbionts, termites are responsible for the conversion of complex biopolymers into simple carbon substrates. Termites also rely on endosymbionts for fixing and recycling nitrogen. As a result, we hypothesize that termite bacterial endosymbionts are a novel source of metabolic pathways for the transformation of nitrogen-rich compounds like explosives.

METHODS

Explosives transformation capability of termite (Reticulitermes flavipes)-derived endosymbionts was determined in media containing the chemical constituents nitrotriazolone (NTO) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) that comprise new insensitive explosive formulations. Media dosed with 40 µg/ml of explosive was inoculated with surface-sterilized, macerated termites. Bacterial isolates capable of explosives transformation were characterized by 16S rRNA sequencing.

RESULTS

Termite-derived enrichment cultures demonstrated degradation activity towards the explosives NTO, RDX, as well as the legacy explosive 2,4,6-trinitrotoluene (TNT). Three isolates with high similarity to the Enterobacteriaceae(Enterobacter, Klebsiella) were able to transform TNT and NTO within 2 days, while isolates with high similarity to Serratia marcescens and Lactococcus lactis were able to transform RDX.

CONCLUSION

Termite endosymbionts harbor a range of metabolic activities and possess unique abilities to transform nitrogen-rich explosives.

The role of TcdB and TccC subunits in secretion of the Photorhabdus Tcd toxin complex

The Toxin Complex (TC) is a large multi-subunit toxin encoded by a range of bacterial pathogens. The best-characterized examples are from the insect pathogens Photorhabdus, Xenorhabdus and Yersinia. They consist of three large protein subunits, designated A, B and C that assemble in a 5∶1∶1 stoichiometry. Oral toxicity to a range of insects means that some have the potential to be developed as pest control technology. The three subunit proteins do not encode any recognisable export sequences and as such little progress has been made in understanding their secretion. We have developed heterologous TC production and secretion models in E. coli and used them to ascribe functions to different domains of the crucial B+C sub-complex. We have determined that the B and C subunits use a secretion mechanism that is either encoded by the proteins themselves or employ an as yet undefined system common to laboratory strains of E. coli. We demonstrate that both the N-terminal domains of the B and C subunits are required for secretion of the whole complex. We propose a model whereby the N-terminus of the C-subunit toxin exports the B+C sub-complex across the inner membrane while that of the B-subunit allows passage across the outer membrane. We also demonstrate that even in the absence of the B-subunit, that the C-subunit can also facilitate secretion of the larger A-subunit. The recognition of this novel export system is likely to be of importance to future protein secretion studies. Finally, the identification of homologues of B and C subunits in diverse bacterial pathogens, including Burkholderia and Pseudomonas, suggests that these toxins are likely to be important in a range of different hosts, including man.

The Toxin Complex (TC) is a large multimeric protein complex first identified in the insect pathogens Photorhabdus and Xenorhabdus. TC isolates from these pathogens exhibit oral toxicity to a diverse range of insects. As such there is significant interest in developing them as candidates for crop protection strategies. Currently all insect resistant transgenic crops rely upon the production of Bacillus thuringiensis Cry toxins. However, to minimise the risk of insect resistance development it is imperative to develop additional toxin systems employing alternative modes of action. A barrier to the further development of TCs as agrochemical tools has been the complexity of their synthesis, secretion and assembly. Little is known about how the large TC subunits are secreted across the bacterial cell wall. We present here an investigation into the roles that the different domains of the B and C-subunit proteins play in secretion of the whole TC. The significance of this goes beyond these specific insect toxins as homologues of these two subunits are encoded in the genomes of a range of human pathogens, such as Burkholderia and Yersinia, in which they have been implicated in human virulence.

Pdl1 is a putative lipase that enhances Photorhabdus toxin complex secretion

The Toxin Complex (TC) is a large multi-subunit toxin first characterized in the insect pathogens Photorhabdus and Xenorhabdus, but now seen in a range of pathogens, including those of humans. These complexes comprise three protein subunits, A, B and C which in the Xenorhabdus toxin are found in a 4∶1∶1 stoichiometry. Some TCs have been demonstrated to exhibit oral toxicity to insects and have the potential to be developed as a pest control technology. The lack of recognisable signal sequences in the three large component proteins hinders an understanding of their mode of secretion. Nevertheless, we have shown the Photorhabdus luminescens (Pl) Tcd complex has been shown to associate with the bacteria's surface, although some strains can also release it into the surrounding milieu. The large number of tc gene homologues in Pl make study of the export process difficult and as such we have developed and validated a heterologous Escherichia coli expression model to study the release of these important toxins. In addition to this model, we have used comparative genomics between a strain that releases high levels of Tcd into the supernatant and one that retains the toxin on its surface, to identify a protein responsible for enhancing secretion and release of these toxins. This protein is a putative lipase (Pdl1) which is regulated by a small tightly linked antagonist protein (Orf53). The identification of homologues of these in other bacteria, linked to other virulence factor operons, such as type VI secretion systems, suggests that these genes represent a general and widespread mechanism for enhancing toxin release in Gram negative pathogens.

Bacterial pathogens of insects deploy a range of toxins to combat the innate immune system and kill the host. There is significant interest in developing these toxins as candidates for crop protection strategies. To date, transgenic crops expressing Bacillus thuringiensis Cry toxins have been used to resist predation by pests. In order to minimize the risk of insect resistance development, current research in crop biotechnology comprises the design of new transgenic plants expressing toxins with different modes of action. The Toxin Complex (TC) gene family first identified in the insect pathogen Photorhabdus has received interest as an alternative. It remains obscure how Photorhabdus regulates, assembles, and secretes such a large toxin complex. We have identified a small lipase protein, Pdl1, which enhances secretion and leads to the release of the Toxin complex off the bacterial surface. This is of wider significance because TC toxin homologues are also found in a range of human pathogens, such as Yersinia in which they have been implicated in human virulence. Furthermore homologues of pdl are also seen tightly linked to other virulence loci such as the type VI systems of Vibrio. We speculate that this Pdl mediated secretion enhancement system is a widespread and important mechanism used by Gram negative bacterial pathogens.

Symbiosis in subterranean termites: a review of insights from molecular studies

The symbiotic relationship of termites and their eukaryotic and prokaryotic gut microbiota is a focal point of research because of the important roles symbionts play in termite nutrition. The use of molecular methods has recently provided valuable insights into the species diversity and the roles of microorganisms in the guts of termites. This paper provides a review of the current knowledge of symbiont species inventories, genome analysis, and gene expression in the guts of subterranean termites. Particular emphasis is given to the termite genera Reticulitermes and Coptotermes (Isoptera: Rhinotermitidae), because they contain pest species of global impact in their native and invasive range.

Comparative genomics of the emerging human pathogen Photorhabdus asymbiotica with the insect pathogen Photorhabdus luminescens

Background

The Gram-negative bacterium Photorhabdus asymbiotica (Pa) has been recovered from human infections in both North America and Australia. Recently, Pa has been shown to have a nematode vector that can also infect insects, like its sister species the insect pathogen P. luminescens (Pl). To understand the relationship between pathogenicity to insects and humans in Photorhabdus we have sequenced the complete genome of Pa strain ATCC43949 from North America. This strain (formerly referred to as Xenorhabdus luminescens strain 2) was isolated in 1977 from the blood of an 80 year old female patient with endocarditis, in Maryland, USA. Here we compare the complete genome of Pa ATCC43949 with that of the previously sequenced insect pathogen P. luminescens strain TT01 which was isolated from its entomopathogenic nematode vector collected from soil in Trinidad and Tobago.

Results

We found that the human pathogen Pa had a smaller genome (5,064,808 bp) than that of the insect pathogen Pl (5,688,987 bp) but that each pathogen carries approximately one megabase of DNA that is unique to each strain. The reduced size of the Pa genome is associated with a smaller diversity in insecticidal genes such as those encoding the Toxin complexes (Tc's), Makes caterpillars floppy (Mcf) toxins and the Photorhabdus Virulence Cassettes (PVCs). The Pa genome, however, also shows the addition of a plasmid related to pMT1 from Yersinia pestis and several novel pathogenicity islands including a novel Type Three Secretion System (TTSS) encoding island. Together these data suggest that Pa may show virulence against man via the acquisition of the pMT1-like plasmid and specific effectors, such as SopB, that promote its persistence inside human macrophages. Interestingly the loss of insecticidal genes in Pa is not reflected by a loss of pathogenicity towards insects.

Conclusion

Our results suggest that North American isolates of Pa have acquired virulence against man via the acquisition of a plasmid and specific virulence factors with similarity to those shown to play roles in pathogenicity against humans in other bacteria.