Efficacy of olive mill wastewater for protecting Bacillus thuringiensis formulation from UV radiations

Impacts of UV radiation on Bacillus biocontrol agents and their resistance mechanisms

Bacillus biocontrol agent(s) BCA(s) such as Bacillus cereus, Bacillus thuringiensis and Bacillus subtilis have been widely applied to control insects' pests of plants and pathogenic microbes, improve plant growth, and facilitate their resistance to environmental stresses. In the last decade, researchers have shown that, the application of Bacillus biocontrol agent(s) BCA(s) optimized agricultural production yield, and reduced disease risks in some crops. However, these bacteria encountered various abiotic stresses, among which ultraviolet (UV) radiation severely decrease their efficiency. Researchers have identified several strategies by which Bacillus biocontrol agents resist the negative effects of UV radiation, including transcriptional response, UV mutagenesis, biochemical and artificial means (addition of protective agents). These strategies are governed by distinct pathways, triggered by UV radiation. Herein, the impact of UV radiation on Bacillus biocontrol agent(s) BCA(s) and their mechanisms of resistance were discussed.

UV-C and wet heat resistance of Bacillus thuringiensis biopesticide endospores compared to foodborne Bacillus cereus endospores

Bacillus endospores (spores) are generally resistant to environmental and food processing-related stress including thermal and non-thermal processing in the food industry, such as pasteurization, and UV-C inactivation. Bacillus thuringiensis insecticidal crystals and spores as the active substances in commercial biopesticides can also be introduced to vegetable foods and their food processing environment due to pre-harvest treatment of edible crops. The resistance of B. thuringiensis biopesticide spores in comparison to the genetically closely related foodborne B. cereus against heat and UV-C treatment is investigated in this study. The results show that B. thuringiensis biopesticide spores with the commercial granulated product formulation are better protected and as such more resistant to both wet heat (D values at 90 °C: 50.1-79.5 min) and UV-C treatment (D values at 0.6 mW/cm2: 7.5-8.9 min) than the pure spore suspension. The enhanced UV-C resistance properties of B. thuringiensis-formulated spores also indicate that the B. thuringiensis spores in powder or granule formulation applied in the field might not be effectively inactivated by solar radiation (UV-A and UV-B) in a short period. Furthermore, the spores of one emetic B. cereus toxin-producing strain (LFMFP 254; a Belgian outbreak strain) were found more resistant to the wet heat at 90 °C (D90-value = 71.2 min) than other tested pure spore suspensions, although the spores of B. cereus 254 did not show different behavior against UV-C treatment. This result suggests that UV-C treatment can be applied as an effective inactivation method against B. cereus 254 spores.

Effects of UV-B radiation on epiphytic bacterial communities on male and female Sargassum thunbergii

The effects of increased UV-B radiation on macroalgae have been widely studied, but knowledge concerning the response of communities of algal epiphytic bacteria to increased UV-B radiation and differences between male and female algae is still lacking. Via 16S rDNA high-throughput sequencing technology, changes in the epiphytic bacterial communities on male and female S. thunbergii under increased UV-B radiation were studied in the lab. Under different UV-B radiation intensities, although the α diversity and community composition of epiphytic bacteria changed little, the β diversity indicated that the community structure of bacteria on S. thunbergii was obviously clustered, and the relative abundance of dominant bacteria and indicator species changed considerably. There were unique bacteria in each experimental group, and the bacteria whose abundance obviously changed were members of groups related to environmental resistance or adaptability. The variation in the abundance of epiphytic bacteria was different in male and female S. thunbergii, and the bacteria whose abundance greatly changed were mainly related to algal growth and metabolism. The abundance of genes with predicted functions related to metabolism, genetic information processing, environmental adaptation and infectious diseases changed with increased UV-B radiation, and those variations differed between epiphytic bacteria on male and female S. thunbergii. This study found that the algal epiphytic bacteria were influenced by the increase in UV-B radiation and underwent certain adaptations through adjustments to community structure and function, and this response was also affected by the sex of the macroalgae. These results are expected to serve as experimental basis and provide reference for further understanding of the response of algae epiphytic bacteria to enhanced UV-B radiation caused by the thinning of the ozone layer and the resulting changes in the relationship between algae and bacteria, which may change the community of the marine ecosystem and affect important marine ecological process.

Deletion of the novel gene mother cell lysis X results in Cry1Ac encapsulation in the Bacillus thuringiensis HD73

The novel protein MclX (mother cell lysis X) in Bacillus thuringiensis subsp. kurstaki strain HD73 (B. thuringiensis HD73) was characterized in this work. MclX has no known domain and its gene deletion in HD73 resulted in Cry1Ac encapsulation in the mother cell and did not influence Cry1Ac protein production or insecticidal activity. In vitro cell wall hydrolysis experiments showed that MclX cannot hydrolyze the cell wall. In mclX deletion mutants, the expression of cwlC (which encodes a key cell wall hydrolase) was significantly decreased, as shown by the β-galactosidase activity assay. MclX cannot directly bind to the cwlC promoter, based on the electrophoretic mobility shift assay (EMSA). The cwlC was reported to be regulated by σK and GerE. However, the transcriptional activities of sigK and gerE showed no difference between HD73 and the mclX deletion mutant. It is indicated that MclX influenced cwlC expression independently of σK or GerE, through a new pathway to regulate cwlC expression. mclX deletion could be a new approach for insecticidal protein encapsulation in Bacillus thuringiensis.

Creation of an Industrial Bacillus thuringiensis Strain With High Melanin Production and UV Tolerance by Gene Editing

Bacillus thuringiensis produces insecticidal crystal proteins (ICPs) which exhibit strong insecticidal toxicity. But when used in the field, ICPs would be destroyed by ultraviolet (UV) radiation in sunlight, thus decreasing the insecticidal activity and shortening the persistence. To improve the duration of B. thuringiensis preparations, we endowed a highly toxic industrial B. thuringiensis HD-1 with UV tolerance by making it produce melanin, a pigment that absorbs UV radiation. In B. thuringiensis, melanin is derived from homogentisate (HGA), an intermediate in the tyrosine pathway. And the absence of homogentisate-1,2-dioxygenase (HmgA) will lead to the formation of melanin. In this study, we used the CRISPR/Cas9 system to knock out the hmgA gene and obtained a melanin-producing mutant HD-1-ΔhmgA from strain HD-1. The melanin yield by mutant HD-1-ΔhmgA reached 3.60 mg/mL. And the anti-UV test showed that melanin serves as a protection to both the organism and the ICPs. After UV irradiation for 3 h, mutant HD-1-ΔhmgA still had an 80% insecticidal activity against the cotton bollworm, Helicoverpa armigera, while the control line only had about 20%. This study creates a light-stable biopesticide prototype based on a classic industrial strain that can be applied directly and takes the melanin-producing strain as a concept to improve the preparation validity.

Characterization of CwlC, an autolysin, and its role in mother cell lysis of Bacillus thuringiensis subsp. israelensis

Bacillus thuringiensis subsp. israelensis (Bti) has been proven to efficiently control mosquitoes, of which many species are important vectors of human disease. The larvicidal action is attributed to the parasporal crystals formed in the sporulating cells and released upon cell autolysis. In this study, a sporulation-specific cwlC gene that encodes an N-acetylmuramoyl-_L -alanine amidase was characterized in Bti strain Bt-59. CwlC was the only cell wall hydrolase in Bti found to contain both MurNAc-LAA and Amidase02_C domains. A recombinant CwlC-His protein was able to digest the Bacillus cell wall. Deletion of the cwlC gene delayed Bti mother cell lysis without impacting vegetative growth or insecticidal efficacy. Transcriptional analyses indicated that cwlC was expressed at the late sporulation stage and was controlled by SigK. Two other cell wall hydrolase genes, cwlB and cwlE, with high expression levels at T₁₄ in Bt-59, were also identified. Like cwlC, cwlB expression was controlled by SigK; in contrast, cwlE was found not to be under the control of this sigma factor and unlike the other two, its gene was found to be plasmid encoded.

Ultraviolet protection of Bacillus thuringiensis through microencapsulation with Pickering emulsion method

An encapsulated formulation of Bacillus thuringiensis (Bt) was produced by the Pickering emulsion technique to improve its activity and stability under UV-A radiation. In this technique latex particles, GO nanosheets, olive oil, ethanol, and water were used to encapsulate Bt in colloidosomes. The protective efficacy of this formulation in protecting Bt subsp. Kurstaki against deactivation by UV-A irradiation was measured, so that spore viability and mortality on Ephestia kuehniella (E. kuehniella) Zeller larvae under UV-A radiation are investigated. According to the results of both tests, encapsulated formulation at a concentration of 0.045% has the highest protection of viability. Hence, colloidosome microcapsule formulations successfully provide good protection against UV radiation.

A novel method for biosynthesis of different polymorphs of TiO2 nanoparticles as a protector for Bacillus thuringiensis from Ultra Violet

Bacillus thuringiensis (Bt) were used for biosynthesis of amorphous TiO2 converted to distinct polymorphs (anatase, rutile, mix) under different temperature conditions. Characterizations of TiO2 nanoparticles were performed by using X-ray diffraction spectroscopy (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and, energy-dispersive X-ray spectroscopy (EDX) analysis. Stability of five formulations under ultraviolet (UV) radiation with spore viability and mortality test on Ephestia kuehniella Zeller larvae were investigated. TiO2(mix) showed the highest viabilities of 79.76% after exposure to ultraviolet (UVA385 nm), while viabilities of non-protected spores under these conditions were 41.32%. The mortality of TiO2(mix), TiO2(anatase), TiO2(rutile), TiO2(amorphous) and free spore formulations on second-instar larvae of Ephestia kuehniella were 73.76%, 71.24%, 57.12%, 51.32%, and 50.32%, respectively on the 10th day of the experiment. The obtained results suggest that TiO2(amorphous) does not increase Bt resistance, but both phases of TiO2 nanoparticles synthesized (anatase and rutile) through the Bacillus thuringiensis and phase mixture can increase the persistence of Bt to the UV light. Furthermore, the combination of both crystalline phases of TiO2(mix) has the highest performance in improving the Bt resistance.

Single Amino Acid Substitution in Homogentisate Dioxygenase Affects Melanin Production in Bacillus thuringiensis

Bacillus thuringiensis formulation losing its activity under field conditions due to UV radiation and photoprotection of B. thuringiensis based on melanin has attracted the attention of researchers for many years. Here, a single amino acid substitution (G272E) in homogentisate 1,2-dioxygenase was found to be responsible for pigment overproduction in B. thuringiensis BMB181, a derivative of BMB171. Disrupting the gene encoding homogentisate dioxygenase in BMB171 induced the accumulation of the homogentisic acid and provoked an increased pigment formation. To gain insights into homogentisate 1,2-dioxygenase in B. thuringiensis, we constructed a total of 14 mutations with a single amino acid substitution, and six of the mutant proteins were found to affect the melanin production when substituted by alanine. This study provides a new way to construct pigment-overproducing strains by impairing the homogentisate dioxygenase with a single mutation in B. thuringiensis, and the findings will facilitate a better understanding of this enzyme.

Noble UV protective agent for Bacillus thuringiensis based on a combination of graphene oxide and olive oil

The focus of this study is investigating the performance of graphene oxide (GO) in the protective effect of olive oil on Bacillus thuringiensis (Bt) after being exposed to UV radiations. Biological pesticides Bt subsp. Kurstaki is one of the most important biological control agents. We compared the protective effect of two UV protectant; GO and olive oil and also the combination of both, on the stability of the formulation of Bt after exposure to UV radiations. Spore viability was measured for protective effect and bioassay test was performed on the formulations of Bt. The combination of GO and olive oil revealed the highest viabilities of 50.62% after 96 h exposure to UV radiation, while viabilities of free spore, olive oil formulation and GO formulation were 32.54%, 37.19%,and 45.20%, respectively. The mortality of irradiated combination formulation on second-instar larvae Ephestia Kuehniella was 68.89%, while the same parameter for free spore, olive oil formulation and GO formulation were 40%, 46.66%,and 56%, respectively.