The role of midgut nonspecific esterase in the susceptibility of Galleria mellonella larvae to Bacillus thuringiensis

Abridgement of Microbial Esterases and Their Eminent Industrial Endeavors

Esterases are hydrolases that contribute to the hydrolysis of ester bonds into both water-soluble acyl esters and emulsified glycerol-esters containing short-chain acyl groups. They have garnered significant attention from biotechnologists and organic chemists due to their immense commercial value. Esterases, with their diverse and significant properties, have become highly sought after for various industrial applications. Synthesized ubiquitously by a wide range of living organisms, including animals, plants, and microorganisms, these enzymes have found microbial esterases to be the preferred choice in industrial settings. The cost-effective production of microbial esterases ensures higher yields, unaffected by seasonal variations. Their applications span diverse sectors, such as food manufacturing, leather tanneries, paper and pulp production, textiles, detergents, cosmetics, pharmaceuticals, biodiesel synthesis, bioremediation, and waste treatment. As the global trend shifts toward eco-friendly and sustainable practices, industrial processes are evolving with reduced waste generation, lower energy consumption, and the utilization of biocatalysts derived from renewable and unconventional raw materials. This review explores the background, structural characteristics, thermostability, and multifaceted roles of bacterial esterases in crucial industries, aiming to optimize and analyze their properties for continued successful utilization in diverse industrial processes. Additionally, recent advancements in esterase research are overviewed, showcasing novel techniques, innovations, and promising areas for further exploration.

Virulent and necrotrophic strategies of Bacillus thuringiensis in susceptible and resistant insects, Galleria mellonella

Bacillus thuringiensis (Bt) is one of the most common entomopathogenic bacteria used as a biopesticide, and source of endotoxin genes for generating insect-resistant transgenic plants. The mechanisms underpinning an insect's susceptibility or resistance to B. thuringiensis are diverse. The bacterial lifecycle does not end with the death of a host, they continue to exploit the cadaver to reproduce and sporulate. Herein, we studied the progression of B. thuringiensis subsp. galleriae infection in two populations of wax moth larvae (Galleria mellonella) to gain further insight into the "arms race" between B. thuringiensis virulence and insect defences. Two doses of B. thuringiensis subsp. galleriae (spore and crystalline toxin mixtures) were administered orally to compare the responses of susceptible (S) and resistant (R) populations at ∼30% mortality each. To investigate B. thuringiensis-insect antibiosis, we used a combination of in vivo infection trials, bacterial microbiome analysis, and RNAi targeting the antibacterial peptide gloverin. Within 48 h post-inoculation, B. thuringiensis-resistant insects purged the midgut of bacteria, i.e., colony forming unit numbers fell below detectable levels. Second, B. thuringiensis rapidly modulated gene expression to initiate sporulation (linked to quorum sensing) when exposed to resistant insects in contrast to susceptible G. mellonella. We reinforce earlier findings that elevated levels of antimicrobial peptides, specifically gloverin, are found in the midgut of resistant insects, which is an evolutionary strategy to combat B. thuringiensis infection via its main portal of entry. A sub-population of highly virulent B. thuringiensis can survive the enhanced immune defences of resistant G. mellonella by disrupting the midgut microbiome and switching rapidly to a necrotrophic strategy, prior to sporulation in the cadaver.

Citrobacter freundii, a natural associate of the Colorado potato beetle, increases larval susceptibility to Bacillus thuringiensis

BACKGROUND

We assume that certain representatives of gut microflora mediate immune changes during dysbiosis, accelerating septicemia caused by Bacillus thuringiensis.

RESULTS

Co-introduction of Citrobacter freundii with Bacillus thuringiensis var. tenebrionis (morrisoni) (Bt) led to an increase in Colorado potato beetle (CPB) larval mortality to 69.0% (1.3-5×) and a synergistic effect was observed from day 1 to day 6. Ultrathin sections of the CPB midgut showed autophagosome formation and partial destruction of gut microvilli under the influence of Bt, which was accompanied by pronounced hypersecretion of the endoplasmic reticulum with apocrine vesicle formation and oncotic changes in cells under the action of C. freundii. The destruction of gut tissues was accompanied by suppression of detoxification processes under the action of the bacteria and a decrease (2.8-3.5×) in the concentration of lipid oxidation products during Bt infection. In the first hours post combined treatment, we registered a slight increase in the total hemocyte count (THC) especially a predomination (1.4×) of immune-competent plasmatocytes. Oral administration of symbiotic and entomopathogenic bacteria to the CPB larvae significantly decreased the THC (1.4×) after 24 h and increased (1.1-1.5×) the detoxifying enzymes level in the lymph. These changes are likely to be associated with the destruction of hemocytes and the need to remove the toxic products of reactive oxygen species.

CONCLUSION

The obtained results indicate that feeding of C. freundii and B. thuringiensis to the CPB larvae is accompanied by tissue changes that significantly affect the cellular and humoral immunity of the insect, increasing its susceptibility to Bt. © 2022 Society of Chemical Industry.

Insecticidal Serralysin of Serratia marcescens Is Detoxified in M3 Midgut Region of Riptortus pedestris

Riptortus pedestris insect indiscriminately acquires not only the symbiotic bacterium Burkholderia insecticola, but also entomopathogens that are abundant in the soil via feeding. However, it is unclear how the host insect survives oral infections of entomopathogens. A previous study suggested that serralysin, a potent virulence factor produced by Serratia marcescens, suppresses cellular immunity by degrading adhesion molecules, thereby contributing to bacterial pathogenesis. Here, we observed that S. marcescens orally administered to R. pedestris stably colonized the insect midgut, while not exhibiting insecticidal activity. Additionally, oral infection with S. marcescens did not affect the host growth or fitness. When co-incubated with the midgut lysates of R. pedestris, serralysin was remarkably degraded. The detoxification activity against serralysin was enhanced in the midgut extract of gut symbiont-colonizing insects. The mRNA expression levels of serralysin genes were negligible in M3-colonizing S. marcescens. M3-colonizing S. marcescens did not produce serralysin toxin. Immunoblot analyses revealed that serralysin was not detected in the M3 midgut region. The findings of our study suggest that orally infected S. marcescens lose entomopathogenicity through host-derived degrading factors and suppression of serralysin.

The Effect of Silicon Dioxide Nanoparticles Combined with Entomopathogenic Bacteria or Fungus on the Survival of Colorado Potato Beetle and Cabbage Beetles

Three types of modified silicon dioxide nanoparticles (SiO2, 10−20 nm) with additives of epoxy, silane and amino groups, used independently and in combination with the entomopathogenic bacteria Bacillus thuringiensis subsp. morrisoni and fungus Metarhizium robertsii were tested against Colorado potato beetle (Leptinotarsa decemlineata) and cabbage beetles (Phyllotreta spp.). All three nanoparticles were found to have an entomocidal effect on Colorado potato beetle larvae and crucifer flea beetles when ingested. Increased susceptibility of insects to B. thuringiensis or M. robertsii blastospores and their metabolites was shown after exposure to the modified silicon dioxide nanoparticles. The potential of modified silicon dioxide nanoparticles to enhance the efficiency of biopesticides based on the bacteria B. thuringiensis and fungi M. robertsii is considered in the paper.

Bacillus thuringiensis Spores and Cry3A Toxins Act Synergistically to Expedite Colorado Potato Beetle Mortality

The insect integument (exoskeleton) is an effective physiochemical barrier that limits disease-causing agents to a few portals of entry, including the gastrointestinal and reproductive tracts. The bacterial biopesticide Bacillus thuringiensis (Bt) enters the insect host via the mouth and must thwart gut-based defences to make its way into the body cavity (haemocoel) and establish infection. We sought to uncover the main antibacterial defences of the midgut and the pathophysiological features of Bt in a notable insect pest, the Colorado potato beetle Leptinotarsa decemlineata (CPB). Exposing the beetles to both Bt spores and their Cry3A toxins (crystalline δ-endotoxins) via oral inoculation led to higher mortality levels when compared to either spores or Cry3A toxins alone. Within 12 h post-exposure, Cry3A toxins caused a 1.5-fold increase in the levels of reactive oxygen species (ROS) and malondialdehyde (lipid peroxidation) within the midgut - key indicators of tissue damage. When Cry3A toxins are combined with spores, gross redox imbalance and 'oxidation stress' is apparent in beetle larvae. The insect detoxification system is activated when Bt spores and Cry3A toxins are administered alone or in combination to mitigate toxicosis, in addition to elevated mRNA levels of candidate defence genes (pattern-recognition receptor, stress-regulation, serine proteases, and prosaposin-like protein). The presence of bacterial spores and/or Cry3A toxins coincides with subtle changes in microbial community composition of the midgut, such as decreased Pseudomonas abundance at 48 h post inoculation. Both Bt spores and Cry3A toxins have negative impacts on larval health, and when combined, likely cause metabolic derangement, due to multiple tissue targets being compromised.

Persistent pesticides: effects of endosulfan at the molecular level on the aquatic invertebrate Chironomus riparius

Although banned in multiple areas, due to its persistence in the environment, endosulfan constitutes a significant environmental concern. In this work, fourth instar Chironomus riparius larvae were exposed at environmentally relevant endosulfan concentrations of 0.1, 1, and 10 μg/L for 24 h to analyze the possible effects of this acaricide on gene expression and enzymatic activity. Transcriptional changes were studied through the implementation of a real-time polymerase chain reaction array with 42 genes related to several metabolic pathways (endocrine system, detoxification response, stress response, DNA reparation, and immune system). Moreover, glutathione-S-transferase (GST), phenoloxidase (PO), and acetylcholinesterase (AChE) activities were assessed. The five pathways were differentially altered by endosulfan exposure with significant changes in the E93, Dis, MAPR, Met, InR, GSTd3, GSTt3, MRP1, hsp70, hsp40, hsp24, ATM, PARP, Proph, and Def genes. Besides, all of the measured enzymatic activities were modified, with increased activity of GST, followed by PO and AChE. In summary, the results reflected the effects provoked in C. riparius at molecular level despite the absence of lethality. These data raise concerns about the strong alteration on different metabolic routes despite the low concentrations used. Therefore, new risk assessment strategies should consider include the effects at the sub-organismal level as endpoints in addition to the classical ecologically relevant parameters (such as survival). This endeavor will facilitate a comprehensive evaluation of toxicants in the environment.

Gut-specific arylphorin mediates midgut regenerative response against Cry-induced damage in Achaea janata

Development of insect resistance to biopesticides is a current and pertinent global issue. Earlier, it was established that lepidopteran larvae can recover from Bt intoxication via a midgut regenerative response and subsequently generate resistance. Molecular aspects of restoration of the midgut integrity following toxin exposure are emerging recently. In the present study, we bring out the pivotal role of gut arylphorin in mediating the midgut regenerative response following sublethal Bt exposure in Achaea janata. Bt-induced midgut damage was characterized by microscopic analysis using differential interference contrast (DIC) and immunofluorescence (IF). Extensive disruption of brush-border membrane, associated with the underlying cytoskeletal alterations including F-actin, α-actin and β-tubulin was observed. Single-photon fluorescence microscopy combined with fluorescence lifetime imaging (FLIM) established the metabolic state associated with enhanced stem cell proliferation and migration from the basal side towards the luminal side following the damage. In-silico analysis revealed the phylogenetic relationship of gut arylphorin with closely related insect species and indicated the presence of two different subunits. Transient RNAi knockdown of the arylphorin resulted in diminished expression of mitotic Cyclin B mRNA levels. Human monoclonal Cyclin B antibody cross-reactivity with the Cyclin B located in the stem cells further validate the role of arylphorin as the mitogenic factor responsible for stem cell proliferation and epithelial regeneration. An in-depth understanding of resistance mechanisms will aid in the design of new strategies for the long-term usage and efficacy of Bt technology against pest control.

Fungus Metarhizium robertsii and neurotoxic insecticide affect gut immunity and microbiota in Colorado potato beetles

Fungal infections and toxicoses caused by insecticides may alter microbial communities and immune responses in the insect gut. We investigated the effects of Metarhizium robertsii fungus and avermectins on the midgut physiology of Colorado potato beetle larvae. We analyzed changes in the bacterial community, immunity- and stress-related gene expression, reactive oxygen species (ROS) production, and detoxification enzyme activity in response to topical infection with the M. robertsii fungus, oral administration of avermectins, and a combination of the two treatments. Avermectin treatment led to a reduction in microbiota diversity and an enhancement in the abundance of enterobacteria, and these changes were followed by the downregulation of Stat and Hsp90, upregulation of transcription factors for the Toll and IMD pathways and activation of detoxification enzymes. Fungal infection also led to a decrease in microbiota diversity, although the changes in community structure were not significant, except for the enhancement of Serratia. Fungal infection decreased the production of ROS but did not affect the gene expression of the immune pathways. In the combined treatment, fungal infection inhibited the activation of detoxification enzymes and prevented the downregulation of the JAK-STAT pathway caused by avermectins. The results of this study suggest that fungal infection modulates physiological responses to avermectins and that fungal infection may increase avermectin toxicosis by blocking detoxification enzymes in the gut.

Galleria mellonella for the Evaluation of Antifungal Efficacy against Medically Important Fungi, a Narrative Review

The treatment of invasive fungal infections remains challenging and the emergence of new fungal pathogens as well as the development of resistance to the main antifungal drugs highlight the need for novel therapeutic strategies. Although in vitro antifungal susceptibility testing has come of age, the proper evaluation of therapeutic efficacy of current or new antifungals is dependent on the use of animal models. Mammalian models, particularly using rodents, are the cornerstone for evaluation of antifungal efficacy, but are limited by increased costs and ethical considerations. To circumvent these limitations, alternative invertebrate models, such as Galleria mellonella, have been developed. Larvae of G. mellonella have been widely used for testing virulence of fungi and more recently have proven useful for evaluation of antifungal efficacy. This model is suitable for infection by different fungal pathogens including yeasts (Candida, Cryptococcus, Trichosporon) and filamentous fungi (Aspergillus, Mucorales). Antifungal efficacy may be easily estimated by fungal burden or mortality rate in infected and treated larvae. The aim of the present review is to summarize the actual data about the use of G. mellonella for testing the in vivo efficacy of licensed antifungal drugs, new drugs, and combination therapies.

Galleria mellonella for the Evaluation of Antifungal Efficacy against Medically Important Fungi, a Narrative Review

The treatment of invasive fungal infections remains challenging and the emergence of new fungal pathogens as well as the development of resistance to the main antifungal drugs highlight the need for novel therapeutic strategies. Although in vitro antifungal susceptibility testing has come of age, the proper evaluation of therapeutic efficacy of current or new antifungals is dependent on the use of animal models. Mammalian models, particularly using rodents, are the cornerstone for evaluation of antifungal efficacy, but are limited by increased costs and ethical considerations. To circumvent these limitations, alternative invertebrate models, such as Galleria mellonella, have been developed. Larvae of G. mellonella have been widely used for testing virulence of fungi and more recently have proven useful for evaluation of antifungal efficacy. This model is suitable for infection by different fungal pathogens including yeasts (Candida, Cryptococcus, Trichosporon) and filamentous fungi (Aspergillus, Mucorales). Antifungal efficacy may be easily estimated by fungal burden or mortality rate in infected and treated larvae. The aim of the present review is to summarize the actual data about the use of G. mellonella for testing the in vivo efficacy of licensed antifungal drugs, new drugs, and combination therapies.