Combinatorial effect of Bacillus thuringiensis kurstaki and Photorhabdus luminescens against Spodoptera littoralis (Lepidoptera: Noctuidae)

Perspective on utilization of Bacillus species as plant probiotics for different crops in adverse conditions

Plant probiotic bacteria are a versatile group of bacteria isolated from different environmental sources to improve plant productivity and immunity. The potential of plant probiotic-based formulations is successfully seen as growth enhancement in economically important plants. For instance, endophytic Bacillus species acted as plant growth-promoting bacteria, influenced crops such as cowpea and lady's finger, and increased phytochemicals in crops such as high antioxidant content in tomato fruits. The present review aims to summarize the studies of Bacillus species retaining probiotic properties and compare them with the conventional fertilizers on the market. Plant probiotics aim to take over the world since it is the time to rejuvenate and restore the soil and achieve sustainable development goals for the future. Comprehensive coverage of all the Bacillus species used to maintain plant health, promote plant growth, and fight against pathogens is crucial for establishing sustainable agriculture to face global change. Additionally, it will give the latest insight into this multifunctional agent with a detailed biocontrol mechanism and explore the antagonistic effects of Bacillus species in different crops.

Pyrones Identified as LuxR Signal Molecules in Photorhabdus and Their Synthetic Analogues Can Alter Multicellular Phenotypic Behavior of Bacillus atropheaus

Individual bacteria communicate by the release and interpretation of small molecules, a phenomenon known as quorum sensing (QS). We hypothesized that QS compounds extruded by Photorhabdus could be interpreted by Bacillus-a form of interspecies communication. We interrogate the structure-activity relationship within the recently discovered pyrone QS network and reveal the exquisite structural features required for targeted phenotypic behavior. The interruption of QS is an exciting, nonbiocidal approach to tackling infection, and understanding its nuances can only be achieved by studies such as this.

Photorhabdus spp.: An Overview of the Beneficial Aspects of Mutualistic Bacteria of Insecticidal Nematodes

The current approaches to sustainable agricultural development aspire to use safer means to control pests and pathogens. Photorhabdus bacteria that are insecticidal symbionts of entomopathogenic nematodes in the genus Heterorhabditis can provide such a service with a treasure trove of insecticidal compounds and an ability to cope with the insect immune system. This review highlights the need of Photorhabdus-derived insecticidal, fungicidal, pharmaceutical, parasiticidal, antimicrobial, and toxic materials to fit into current, or emerging, holistic strategies, mainly for managing plant pests and pathogens. The widespread use of these bacteria, however, has been slow, due to cost, natural presence within the uneven distribution of their nematode partners, and problems with trait stability during in vitro culture. Yet, progress has been made, showing an ability to overcome these obstacles via offering affordable mass production and mastered genome sequencing, while detecting more of their beneficial bacterial species/strains. Their high pathogenicity to a wide range of arthropods, efficiency against diseases, and versatility, suggest future promising industrial products. The many useful properties of these bacteria can facilitate their integration with other pest/disease management tactics for crop protection.

The great potential of entomopathogenic bacteria Xenorhabdus and Photorhabdus for mosquito control: a review

The control of insects of medical importance, such as Aedes aegypti and Aedes albopictus are still the only effective way to prevent the transmission of diseases, such as dengue, chikungunya and Zika. Their control is performed mainly using chemical products; however, they often have low specificity to non-target organisms, including humans. Also, studies have reported resistance to the most commonly used insecticides, such as the organophosphate and pyrethroids. Biological control is an ecological and sustainable method since it has a slow rate of insect resistance development. Bacterial species of the genera Xenorhabdus and Photorhabdus have been the target of several research groups worldwide, aiming at their use in agricultural, pharmaceutical and industrial products. This review highlights articles referring to the use of Xenorhabdus and Photorhabdus for insects and especially for mosquito control proposing future ways for their biotechnological applicability. Approximately 24 species of Xenorhabdus and five species of Photorhabdus have been described to have insecticidal properties. These studies have shown genes that are capable of encoding low molecular weight proteins, secondary toxin complexes and metabolites with insecticide activities, as well as antibiotic, fungicidal and antiparasitic molecules. In addition, several species of Xenorhabdus and Photorhabdus showed insecticidal properties against mosquitoes. Therefore, these biological agents can be used in new control methods, and must be, urgently considered in short term, in studies and applications, especially in mosquito control.

Insecticidal Activity of Photorhabdus luminescens 0805-P2R Against Plutella xylostella

Photorhabdus luminescens is an entomopathogenic rod-shaped bacterium infected with insect nematodes of the Heterorhabditidae family. It kills insects through the secretion of high molecular weight toxin complexes. In this study, Plutella xylostella larvae were orally administered P. luminescens for bioassay. After incubation in Luria-Bertani (LB) medium for a sufficiently long period, the mortality rates of P. xylostella observed after diluting the fermentation broth 50 times and diluting the supernatant 5 times were 18.89% and 91.11%, respectively. Retentates measuring more than 70 kDa showed 88% mortality after ultrafiltration (UF) membrane treatment. Thus, the supernatant of P. luminescens had insecticidal activity, and the main insecticidal toxin complexes had a molecular weight exceeding 70 kDa. The L₉ (3⁴) Taguchi orthogonal experimental optimized medium mode-predicted insecticidal activity levels were 84% and 119% in the 50-fold diluted fermentation broth and 5-fold diluted supernatant, respectively. Moreover, the insecticidal activity was improved to 92.2% in the 100-fold diluted fermentation broth and to 97.8% in the 10-fold diluted supernatant in the experiments. All combinations tested showed clear indications of lethality, including swelling, vesicle formation, cytoplasm vacuolization, and brush border membrane lysis. Thus, these results promote the use of P. luminescens 0805-P2R as a potent biopesticide to effectively control P. xylostella.

Combinational Effect of Rumex tingitanus (Polygonaceae) Hexane Extract and Bacillus thuringiensis δ-Endotoxin against Spodoptera littoralis (Lepidoptera: Noctuidae)

The increasing insect resistance against Bacillus thuringiensis delta-endotoxins is a serious problem which makes it urgent to look for new eco-friendly strategies. Combining these toxins with other biomolecules is one of the promising strategies against insect pests. In this work, we evaluated the bioinsecticidal potential of Rumex tingitanus extracts and B. thuringiensis strain BLB250 against Spodoptera littoralis (Lepidoptera: Noctuidae) larvae. The chemical composition of the hexane extract, the most active fraction, was analyzed to validate the correlation between chemical composition and biological activity. Among the tested extracts, only the hexanic extract showed toxicity against first and second instar larvae with LC₅₀ of 2.56 and 2.95 mg g⁻¹, respectively. The Bacillus thuringiensis BLB250 delta-endotoxins showed toxicity with an LC₅₀ of 56.3 μg g⁻¹. Therefore, the investigated combinational effect of BLB250 delta-endotoxins and R. tingitanus hexane extract proved significant synergistic effect against S. littoralis larvae. The GC-MS analysis of R. tingitanus hexane extract showed the richness of this extract in phytosterols such as β and γ-sitosterol (48.91%), campesterol (6.43%), and β-amyrin (8.92%) which are known for their insecticidal activity. This novel finding highlights the potential use of this combination against insect pests to prevent the appearance of resistance problems.

Combinatorial effect of Photorhabdus luminescens TT01 and Bacillus thuringiensis Vip3Aa16 toxin against Agrotis segetum

The entomopathogenic Photorhabdus luminescens TT01 is a promoting bacterium that controls effectively many insect pests. Indeed, it exhibited a mortality rate of 32.36% against the first instar larvae of the turnip moth Agrotis segetum, when it was used at a concentration of 5 × 10⁷ cells/ml but no toxicity against the second instar larvae in the same condition. P. luminescens TT01 oral toxicity is associated to septicaemia since cells fraction exhibited the highest mortality rate of 34%. In order to enhance P. luminescens TT01 insecticidal potential, combination with Bacillus thuringiensis Vip3Aa16 toxin was tested. An improvement of insecticidal activity was shown. Indeed, 100% mortality of A. segetum first instar larvae was obtained after 2 days of treatment, when using TT01 cells and Vip3Aa16 toxin at a concentration of 5 × 10⁷ cells/ml and 9.025 ng/cm², respectively. Moreover, growth inhibition rate of 45% of the second instar larvae was observed, when using the same combination. A. segetum mortality could be the result of several alterations in the midgut epithelium caused by Vip3Aa16 toxin, allowing a rapid invasion of the hemocoel by TT01 cells as demonstrated by histopathological study. Clear symptoms of intoxication were observed for all combinations tested, including swelling, vesicle formation, cytoplasm vacuolization and brush border membrane lysis. Taken together, these results promote the use of P. luminescens TT01 as a potent bioinsecticide to control effectively A. segetum by oral treatment in a mixture with Vip3Aa16 toxin.

Isolation and identification of Xenorhabdus and Photorhabdus bacteria associated with entomopathogenic nematodes and their larvicidal activity against Aedes aegypti

BACKGROUND

Aedes aegypti is a potential vector of West Nile, Japanese encephalitis, chikungunya, dengue and Zika viruses. Alternative control measurements of the vector are needed to overcome the problems of environmental contamination and chemical resistance. Xenorhabdus and Photorhabdus are symbionts in the intestine of entomopathogenic nematodes (EPNs) Steinernema spp. and Heterorhabditis spp. These bacteria are able to produce a broad range of bioactive compounds including antimicrobial, antiparasitic, cytotoxic and insecticidal compounds. The objectives of this study were to identify Xenorhabdus and Photorhabdus isolated from EPNs in upper northern Thailand and to study their larvicidal activity against Ae. aegypti larvae.

RESULTS

A total of 60 isolates of symbiotic bacteria isolated from EPNs consisted of Xenorhabdus (32 isolates) and Photorhabdus (28 isolates). Based on recA gene sequencing, BLASTN and phylogenetic analysis, 27 isolates of Xenorhabdus were identical and closely related to X. stockiae, 4 isolates were identical to X. miraniensis, and one isolate was identical to X. ehlersii. Twenty-seven isolates of Photorhabdus were closely related to P. luminescens akhurstii and P. luminescens hainanensis, and only one isolate was identical and closely related to P. luminescens laumondii. Xenorhabdus and Photorhabdus were lethal to Ae aegypti larvae. Xenorhabdus ehlersii bMH9.2_TH showed 100% efficiency for killing larvae of both fed and unfed conditions, the highest for control of Ae. aegypti larvae and X. stockiae (bLPA18.4_TH) was likely to be effective in killing Ae. aegypti larvae given the mortality rates above 60% at 72 h and 96 h.

CONCLUSIONS

The common species in the study area are X. stockiae, P. luminescens akhurstii, and P. luminescens hainanensis. Three symbiotic associations identified included P. luminescens akhurstii-H. gerrardi, P. luminescens hainanensis-H. gerrardi and X. ehlersii-S. Scarabaei which are new observations of importance to our knowledge of the biodiversity of, and relationships between, EPNs and their symbiotic bacteria. Based on the biological assay, X. ehlersii bMH9.2_TH begins to kill Ae. aegypti larvae within 48 h and has the most potential as a pathogen to the larvae. These data indicate that X. ehlersii may be an alternative biological control agent for Ae. aegypti and other mosquitoes.

Bacillus: A Biological Tool for Crop Improvement through Bio-Molecular Changes in Adverse Environments

Crop productivity is affected by environmental and genetic factors. Microbes that are beneficial to plants are used to enhance the crop yield and are alternatives to chemical fertilizers and pesticides. Pseudomonas and Bacillus species are the predominant plant growth-promoting bacteria. The spore-forming ability of Bacillus is distinguished from that of Pseudomonas. Members of this genus also survive for a long time under unfavorable environmental conditions. Bacillus spp. secrete several metabolites that trigger plant growth and prevent pathogen infection. Limited studies have been conducted to understand the physiological changes that occur in crops in response to Bacillus spp. to provide protection against adverse environmental conditions. This review describes the current understanding of Bacillus-induced physiological changes in plants as an adaptation to abiotic and biotic stresses. During water scarcity, salinity and heavy metal accumulate in soil, Bacillus spp. produce exopolysaccharides and siderophores, which prevent the movement of toxic ions and adjust the ionic balance and water transport in plant tissues while controlling the pathogenic microbial population. In addition, the synthesis of indole-3-acetic acid, gibberellic acid and1-aminocyclopropane-1-carboxylate (ACC) deaminase by Bacillus regulates the intracellular phytohormone metabolism and increases plant stress tolerance. Cell-wall-degrading substances, such as chitosanase, protease, cellulase, glucanase, lipopeptides and hydrogen cyanide from Bacillus spp. damage the pathogenic bacteria, fungi, nematodes, viruses and pests to control their populations in plants and agricultural lands. The normal plant metabolism is affected by unfavorable environmental stimuli, which suppress crop growth and yield. Abiotic and biotic stress factors that have detrimental effects on crops are mitigated by Bacillus-induced physiological changes, including the regulation of water transport, nutrient up-take and the activation of the antioxidant and defense systems. Bacillus association stimulates plant immunity against stresses by altering stress-responsive genes, proteins, phytohormones and related metabolites. This review describes the beneficial effect of Bacillus spp. on crop plants, which improves plant productivity under unfavorable climatic conditions, and the current understanding of the mitigation mechanism of Bacillus spp. in stress-tolerant and/or stress-resistant plants.

Curing of plasmid pBMB28 from Bacillus thuringiensis YBT-020 using an unstable replication region

Bacillus thuringiensis serovar finitimus strain YBT-020 is the well-studied spore-crystal association (SCA) phenotypic strain, whose parasporal crystals adhere to spore after lysis of the mother cell. Its endogenous plasmids pBMB26 and pBMB28 were proved essential for this SCA phenotype. In our previous study, using conventional methods, pBMB26 cured derivative and both pBMB26 and pBMB28 cured derivative of YBT-020 were obtained. However, YBT-020 solely cured of pBMB28 could not be obtained. In this study, an unstable replication region of pBMB28 was identified and was used to construct an incompatible plasmid pRep28B. This incompatible plasmid was successfully used to cure plasmid pBMB28 and was easily eliminated through segregational instability under the optimum growth temperature of YBT-020. Therefore, an endogenous plasmid was cured from the B. thuringiensis strain utilizing plasmid incompatibility. Moreover, using an unstable replication region instead of a temperature sensitive (Ts) replication region is better to cure the incompatible plasmid because it can avoid culturing at higher temperature. This method provides an efficient method for plasmid curing in B. thuringiensis and other bacteria.