A model of bacterial intestinal infections in Drosophila melanogaster

Diet-induced mechanical stress promotes immune and metabolic alterations in the Drosophila melanogaster digestive tract

A fundamental query in immunology is how cells recognize danger in the tissue milieu. For many years, standpoints were mainly centered around damaged cells or structures of invading pathogens, like lipopolysaccharide, being the initiators of danger signals to activate immunity. Today, rising evidence presents "biophysical signals" as potential regulators of immune cell functions too. This emerging notion of the ability of tissue mechanotransduction to tune the immunological system appears to likewise exist in other body system, among which is the metabolic system, where startling connection between mechanotransduction and enzymesknown to regulate metabolism have been also reported. Being continuously subjected to mechanical forces, and owing to its multifaceted role in not only absorbing and digesting nutrients, but also in supporting important immunological defense strategies as well as metabolic responses, attention has been lately given to organs making up the gastrointestinal (GI) tract, predominantly the intestine, with growing interest in unravelling the impact of mechanotransduction on the intestinal environment is on the rise. As such, we investigated in this study the impact of mechanical stress introduced by ingesting diet containing the indigestible fiber methylcellulose (MC) on gut immune and metabolic activities using the Drosophila melanogaster model organism. Our findings reveal that feeding on MC-containing diet causes consequential alterations in the fly gut environment manifested by enlargement of the midgut diameter, remodeling of the microbiota community, activation of immune responses, differential regulation of the tachykinin (Tk) peptide hormone expression and modulation of lipometabolism. Particularly, we show that feeding on MC-containing diet promotes a marked increase in the relative abundance of Leuconostocaceae/Leuconostoc, microbiota-dependent Reactive Oxygen Species (ROS) production, IMD pathway activation, and IMD-dependent elevation in Tk expression. We also demonstrate that maintaining flies on MC-containing diet for several days leads to a reduction in body weight and in systemic glucose and triacylglycerol levels and modulates lipid droplets accumulation and storage in the gut and fat body. Taken together, these findings provide novel insight into the effect of diet induced-mechanical forces on the intestinal physiology and pathology.

Capillary Electrophoresis Electrospray Ionization Mass Spectrometry Reveals Metabolic Perturbations During Nematode Infection in Drosophila melanogaster

Drosophila melanogaster is broadly used to model host-pathogen interactions. Entomopathogenic nematodes are excellent research tools for dissecting the molecular and functional basis of parasitism and the host's anti-parasitic response. In this work, we used discovery metabolomics to explore the differences in the metabolome composition of wild type D. melanogaster larvae that were infected with symbiotic nematodes (Steinernema carpocapsae carrying Xenorhabdus nematophila mutualistic bacteria) or axenic nematodes (S. carpocapsae lacking their bacterial partners). Benefiting from their high separation power, sensitivity, and compatibility with low amounts of the starting metabolome, we leveraged microanalytical capillary electrophoresis electrospray ionization mass spectrometry (CE-ESI-MS) to profile the small (<500 Da) polar portion of the metabolome among these experimental treatments. We detected and quantified 122 different small molecules, of which 50 were identified with high confidence. Supervised multivariate analysis revealed that the infection was paralleled with changes in amino acid biosynthesis (arginine, phenylalanine, tryptophan, and tyrosine), metabolism (alanine, arginine, aspartate, glutamate, glycine, proline, serine, and threonine), and classical signalling (aspartate, γ-aminobutyrate, glutamate, and pyridoxine). This study demonstrates the ability of high-sensitivity CE-ESI-MS to uncover metabolic perturbations during infection. The results from the metadata may facilitate the design of targeted studies to explore small biomolecules and their functions during host-pathogen interaction.

A C-type lectin of Helicoverpa armigera maintains the stability of the hemolymph microbiota by regulating the expression of lysozyme

An increasing body of evidence suggests that the insect hemolymph is not a sterile environment and that various nonpathogenic microorganisms can stably or transiently inhabit the hemolymph in many insect species. However, little is currently known about how the insect immune system maintains microbial homeostasis within the hemolymph. In this study, a C-type lectin of Helicoverpa armigera (HaCTL6) was shown to be involved in maintaining the stability of the hemolymph microbiota. The expression of H. armigera antimicrobial peptide (AMP) genes was down-regulated after RNAi of HaCTL6. Moreover, the knockdown of HaCTL6 resulted in a decrease in the antibacterial activity and an increase in the total bacterial load of the hemolymph. Transcriptome analysis showed that a lysozyme (HaLysozyme-like) was significantly down-regulated after HaCTL6 RNAi. Moreover, the knockdown of HaLysozyme-like led to a decrease in the antibacterial activity and an increase in the total bacterial load of the hemolymph. Furthermore, the injection of recombinant HaLysozyme-like into the hemocoel caused a significant reduction in the total number of bacteria in the hemolymph. These results indicate that HaCTL6 may regulate the homeostasis of bacteria in the hemolymph by utilizing HaLysozyme-like as an effector.

Evolution of resistance and disease tolerance mechanisms to oral bacterial infection in Drosophila melanogaster

Pathogens exert strong selection on hosts that evolve and deploy different defensive strategies, namely minimizing pathogen exposure (avoidance), directly promoting pathogen elimination (resistance) and/or managing the deleterious effects of illness (disease tolerance). However, how the host response partitions across these processes has not been directly tested in a single host-pathogen system, let alone in the context of known adaptive trajectories resulting from experimental evolution. Here, we compare a Drosophila melanogaster population adapted to oral infection with its natural pathogen Pseudomonas entomophila (BactOral), to its control population to find no evidence for behavioural changes but measurable differences in both resistance and disease tolerance. In BactOral, no differences were detected in bacterial intake or defecation, nor gut cell renewal. However, a measurable relative decrease in bacterial loads correlates with an increase in gut-specific anti-microbial peptide production, pointing to a strengthening in resistance. Additionally, we posit that disease tolerance also contributes to the response of BactOral through a tighter control of self- and pathogen-derived damage caused by bacteria exposure. This study reveals a genetically complex and mechanistically multi-layered response, possibly reflecting the structure of adaptation to infection in natural populations.

Research on the Expression of Immune-Related Genes at Different Stages in the Third-Instar Larvae of Spodoptera frugiperda Infected by Metarhizium rileyi

Spodoptera frugiperda is a major migratory agricultural pest that poses a significant threat to global crop safety. Metarhizium rileyi has emerged as an effective biocontrol agent against lepidopteran pests. In this study, we examined the immune responses of third-instar S. frugiperda larvae at various stages of an M. rileyi infection. Using RNA-seq and microscopic observation, we identified the immune-related pathways enriched at different infection stages, which were further validated by a qRT-PCR. Our findings revealed the following immune responses during infection: During the stage when M. rileyi penetrated the host cuticle (0-48 h), the genes related to energy metabolism, detoxification, and melanization were upregulated. Meanwhile, the TOLL and IMD signaling pathways were activated to counter the infection. During the stage of M. rileyi's internal infection (48-96 h), which was the peak expression period of the immune-related genes, cellular immunity predominated. Hemocytes encapsulated and phagocytosed the hyphal bodies. Phagocytosis was enhanced through the upregulation of the genes related to ROS and the melanization-related genes, as well as the genes involved in insect hormone biosynthesis. During the stage when M. rileyi grew from the inside to the outside of the host (96-120 h), immune system paralysis resulted in host mortality. These findings deepen our understanding of the immune interactions between M. rileyi and S. frugiperda, support the potential of M. rileyi as an effective biocontrol agent, and provide a theoretical foundation for the development of targeted biopesticides for pests using biotechnological approaches.

Anopheles gambiae phagocytic hemocytes promote Plasmodium falciparum infection by regulating midgut epithelial integrity

For successful transmission, the malaria parasite must traverse tissue epithelia and survive attack from the insect's innate immune system. Hemocytes play a multitude of roles in mosquitoes, including defense against invading pathogens. Here, we show that hemocytes of the major malaria vector Anopheles gambiae promote Plasmodium falciparum infection by maintaining midgut epithelial integrity by controlling cell proliferation upon blood feeding. The mosquito's hemocytes also control the midgut microbiota and immune gene expression. Our study unveils novel hemocyte functions that are exploited by the human malaria parasite to evade the mosquito's immune system.

Nora virus proliferates in dividing intestinal stem cells and sensitizes flies to intestinal infection and oxidative stress

The digestive tract represents the most complex interface of an organism with its biotope. Food may be contaminated by pathogens and toxicants while an abundant and complex microbiota strives in the gut lumen. The organism must defend itself against potentially noxious biotic or abiotic stresses while preserving its microbiota, provided it plays a beneficial role. The presence of intestinal viruses adds another layer of complexity. Starting from a differential sensitivity of two lines from the same Drosophila wild-type strain to ingested Pseudomonas aeruginosa, we report here that the presence of Nora virus in the gut epithelium promotes the sensitivity to this bacterial pathogen as well as to an ingested oxidizing xenobiotic. The genotype, age, nature of the ingested food and to a limited extent the microbiota are relevant parameters that influence the effects of Nora virus on host fitness. Mechanistically, we detect the initial presence of viral proteins essentially in progenitor cells. Upon stress such as infection, exposure to xenobiotics, aging or feeding on a rich-food diet, the virus is then detected in enterocytes, which correlates with a disruption of the intestinal barrier function in aged flies. Finally, we show that the virus proliferates only when ISCs are induced to divide and that blocking either enterocyte apoptosis or JAK/STAT-driven ISC division leads to a drastically reduced Nora virus titer. In conclusion, it is important to check that experimental strains are devoid of intestinal viruses when monitoring survival/life span of fly lines or when investigating the homeostasis of the intestinal epithelium as these viruses can constitute significant confounding factors.

Transcriptional induction of the IMD signaling pathway and associated antibacterial activity in the digestive tract of cat fleas (Ctenocephalides felis)

BACKGROUND

Fleas are insect vectors that transmit several Gram-negative bacterial pathogens acquired by ingesting infected vertebrate blood. To combat foodborne illness, insect midgut epithelial cells are armed with efficient microbial recognition and control systems, such as the immune deficiency (IMD) pathway that regulates the expression of antimicrobial peptides (AMPs). However, despite their medical and veterinary importance, relatively little is known about the IMD signaling pathway and production of AMPs in the digestive tract of cat fleas (Ctenocephalides felis).

METHODS

In the present study, we measured the expression of target genes comprising the IMD pathway, as well as corresponding AMP transcripts, in the digestive tract of C. felis following exposure to three different species of bacteria: Gram-negative Bartonella henselae (a flea-borne pathogen), Gram-negative Serratia marcescens (a model laboratory species), and Gram-positive Micrococcus luteus (a model laboratory species). Additionally, we examined the antibacterial activity of proteins isolated from the flea digestive tract in vitro following bacterial challenge and at different days post adult emergence to determine if feeding-induced antibacterial activity varies with age.

RESULTS

In our analysis of C. felis, we observed an increase in the expression of representative IMD pathway genes and associated AMP transcripts, indicating the activation of the IMD pathway. Furthermore, our results revealed that different bacterial species elicit distinct transcriptional profiles of IMD pathway genes, suggesting a species-specific response to bacterial invasion. We found that the gut of C. felis produces antibacterial molecules as a localized defense mechanism. Additionally, we observed that proteins with antimicrobial properties are synthesized as part of local defense mechanisms in the gut, with differential patterns of antibacterial activity related to infection status and age.

CONCLUSIONS

Our findings provide essential insights into the potential mechanisms by which cat fleas regulate immune responses in their digestive tract against different bacterial species.

Extracts ofHylotelephiumerythrostictum (miq.) H. Ohba ameliorate intestinal injury by scavenging ROS and inhibiting multiple signaling pathways in Drosophila

BACKGROUND

The intestinal epithelial barrier is the first line of defense against pathogens and noxious substances entering the body from the outside world. Through proliferation and differentiation, intestinal stem cells play vital roles in tissue regeneration, repair, and the maintenance of intestinal homeostasis. Inflammatory bowel disease (IBD) is caused by the disruption of intestinal homeostasis through the invasion of toxic compounds and pathogenic microorganisms. Hylotelephium erythrostictum (Miq.) H. Ohba (H. erythrostictum) is a plant with diverse pharmacological properties, including antioxidant, anti-inflammatory, antidiabetic, and antirheumatic properties. However, the roles of H. erythrostictum and its bioactive compounds in the treatment of intestinal injury are unknown.

METHODS

We examined the protective effects of H. erythrostictum water extract (HEWE) and H. erythrostictum butanol extract (HEBE) on Drosophila intestinal injury caused by dextran sodium sulfate (DSS) or Erwinia carotovoracarotovora 15 (Ecc15).

RESULTS

Our findings demonstrated that both HEWE and HEBE significantly prolonged the lifespan of flies fed toxic compounds, reduced cell mortality, and maintained intestinal integrity and gut acid‒base homeostasis. Furthermore, both HEWE and HEBE eliminated DSS-induced ROS accumulation, alleviated the increases in antimicrobial peptides(AMPs) and intestinal lipid droplets caused by Ecc15 infection, and prevented excessive ISC proliferation and differentiation by inhibiting the JNK, EGFR, and JAK/STAT pathways. In addition, they reversed the significant changes in the proportions of the gut microbiota induced by DSS. The bioactive compounds contained in H. erythrostictum extracts have sufficient potential for use as natural therapeutic agents for the treatment of IBD in humans.

CONCLUSION

Our results suggest that HEWE and HEBE are highly effective in reducing intestinal inflammation and thus have the potential to be viable therapeutic agents for the treatment of gut inflammation.

CLINICAL TRIAL NUMBER

Not applicable.

Protective effects of Chinese bayberry pomace wine against oxidative stress on Drosophila melanogaster

The Chinese bayberry pomace wine (CPW) was prepared with the assisted fermentation of lactic acid bacteria and acetic acid bacteria, and its antioxidant effect on Drosophila melanogaster was researched. After mixed fermentation, CPW had a better color, which means there was more retention of anthocyanins, and the functional activity of anthocyanins could enhance the antioxidant capacity of flies. We found that the lifespan of flies exposed to CPW was prolonged, and the reproductive capacity of these flies was decreased. The food intake of flies was also influenced by CPW with gender differences. Furthermore, CPW alleviated the excessive proliferation of the intestinal precursor cells of H2O2-induced flies and activated the transcription level of antibacterial peptide genes. CPW had a protective effect on H2O2-induced acute injury flies, with an increased survival rate, enhanced SOD and CAT activities, and decreased malondialdehyde (MDA) content in flies. The expression of oxidative stress-related genes including CuZn-SOD, Mn-SOD, and CAT was also significantly upregulated by CPW, but the downregulation effect of CPW on age-related gene expression such as methuselah (MTH), the target of rapamycin (TOR) and ribosomaiprotein S6 kinase (S6K) was sex-specific. These results suggested that CPW played an important role in anti-oxidative stress injury, which was beneficial to promoting the reuse of by-products from Chinese bayberry processing.

High sugar diets can increase susceptibility to bacterial infection in Drosophila melanogaster

Overnutrition with dietary sugar can worsen infection outcomes in diverse organisms including insects and humans, through generally unknown mechanisms. In the present study, we show that adult Drosophila melanogaster fed high-sugar diets became more susceptible to infection by the Gram-negative bacteria Providencia rettgeri and Serratia marcescens. We found that P. rettgeri and S. marcescens proliferate more rapidly in D. melanogaster fed a high-sugar diet, resulting in increased probability of host death. D. melanogaster become hyperglycemic on the high-sugar diet, and we find evidence that the extra carbon availability may promote S. marcescens growth within the host. However, we found no evidence that increased carbon availability directly supports greater P. rettgeri growth. D. melanogaster on both diets fully induce transcription of antimicrobial peptide (AMP) genes in response to infection, but D. melanogaster provided with high-sugar diets show reduced production of AMP protein. Thus, overnutrition with dietary sugar may impair host immunity at the level of AMP translation. Our results demonstrate that dietary sugar can shape infection dynamics by impacting both host and pathogen, depending on the nutritional requirements of the pathogen and by altering the physiological capacity of the host to sustain an immune response.

Shaping the Microbial Landscape: Parasitoid-Driven Modifications of Bactrocera dorsalis Microbiota

Koinobiont endoparasitoids regulate the physiology of their hosts through altering host immuno-metabolic responses, processes which function in tandem to shape the composition of the microbiota of these hosts. Here, we employed 16S rRNA and ITS amplicon sequencing to investigate whether parasitization by the parasitoid wasps, Diachasmimorpha longicaudata (Ashmaed) (Hymenoptera: Braconidae) and Psyttalia cosyrae (Wilkinson) (Hymenoptera: Braconidae), induces gut dysbiosis and differentially alter the gut microbial (bacteria and fungi) communities of an important horticultural pest, Bactrocera dorsalis (Hendel) (Diptera: Tephritidae). We further investigated the composition of bacterial communities of adult D. longicaudata and P. cosyrae to ascertain whether the adult parasitoids and parasitized host larvae share microbial taxa through transmission. We demonstrated that parasitism by D. longicaudata induced significant gut perturbations, resulting in the colonization and increased relative abundance of pathogenic gut bacteria. Some pathogenic bacteria like Stenotrophomonas and Morganella were detected in both the guts of D. longicaudata-parasitized B. dorsalis larvae and adult D. longicaudata wasps, suggesting a horizontal transfer of microbes from the parasitoid to the host. The bacterial community of P. cosyrae adult wasps was dominated by Arsenophonus nasoniae, whereas that of D. longicaudata adults was dominated by Paucibater spp. and Pseudomonas spp. Parasitization by either parasitoid wasp was associated with an overall reduction in fungal diversity and evenness. These findings indicate that unlike P. cosyrae which is avirulent to B. dorsalis, parasitization by D. longicaudata induces shifts in the gut bacteriome of B. dorsalis larvae to a pathobiont-dominated community. This mechanism possibly enhances its virulence against the pest, further supporting its candidacy as an effective biocontrol agent of this frugivorous tephritid fruit fly pest.

A specific innate immune response silences the virulence of Pseudomonas aeruginosa in a latent infection model in the Drosophila melanogaster host

Microbial pathogenicity often depends on the route of infection. For instance, P. aeruginosa or S. marcescens cause acute systemic infections when low numbers of bacteria are injected into D. melanogaster flies whereas flies succumb much slower to the continuous ingestion of these pathogens, even though both manage to escape from the gut compartment and reach the hemocoel. Here, we have developed a latent P. aeruginosa infection model by feeding flies on the bacteria for a short period. The bacteria stably colonize internal tissues yet hardly cause any damage since latently-infected flies live almost as long as noninfected control flies. The apparently dormant bacteria display particular characteristics in terms of bacterial colony morphology, composition of the outer cell wall, and motility. The virulence of these bacteria can however be reactivated upon wounding the host. We show that melanization but not the cellular or the systemic humoral response is the predominant host defense that establishes latency and may coerce the bacteria to a dormant state. In addition, the lasting activation of the melanization responses in latently-infected flies provides a degree of protection to the host against a secondary fungal infection. Latent infection by an ingested pathogen protects against a variety of homologous or heterologous systemic secondary infectious challenges, a situation previously described for the endosymbiotic Wolbachia bacteria, a guard against viral infections.

Duox activation in Drosophila Malpighian tubules stimulates intestinal epithelial renewal through a countercurrent flow

The gut must perform a dual role of protecting the host against toxins and pathogens while harboring mutualistic microbiota. Previous studies suggested that the NADPH oxidase Duox contributes to intestinal homeostasis in Drosophila by producing reactive oxygen species (ROS) in the gut that stimulate epithelial renewal. We find instead that the ROS generated by Duox in the Malpighian tubules leads to the production of Upd3, which enters the gut and stimulates stem cell proliferation. We describe in Drosophila the existence of a countercurrent flow system, which pushes tubule-derived Upd3 to the anterior part of the gut and stimulates epithelial renewal at a distance. Thus, our paper clarifies the role of Duox in gut homeostasis and describes the existence of retrograde fluid flow in the gut, collectively revealing a fascinating example of inter-organ communication.

Multiple toxins and a protease contribute to the aphid-killing ability of Pseudomonas fluorescens PpR24

Aphids are globally important pests causing damage to a broad range of crops. Due to insecticide resistance, there is an urgent need to develop alternative control strategies. In our previous work, we found Pseudomonas fluorescens PpR24 can orally infect and kill the insecticide-resistant green-peach aphid (Myzus persicae). However, the genetic basis of the insecticidal capability of PpR24 remains unclear. Genome sequencing of PpR24 confirmed the presence of various insecticidal toxins such as Tc (toxin complexes), Rhs (rearrangement hotspot) elements, and other insect-killing proteases. Upon aphids infection with PpR24, RNA-Seq analysis revealed 193 aphid genes were differentially expressed with down-regulation of 16 detoxification genes. In addition, 1325 PpR24 genes (542 were upregulated and 783 downregulated) were subject to differential expression, including genes responsible for secondary metabolite biosynthesis, the iron-restriction response, oxidative stress resistance, and virulence factors. Single and double deletion of candidate virulence genes encoding a secreted protease (AprX) and four toxin components (two TcA-like; one TcB-like; one TcC-like insecticidal toxins) showed that all five genes contribute significantly to aphid killing, particularly AprX. This comprehensive host-pathogen transcriptomic analysis provides novel insight into the molecular basis of bacteria-mediated aphid mortality and the potential of PpR24 as an effective biocontrol agent.

Massively parallel mutant selection identifies genetic determinants of Pseudomonas aeruginosa colonization of Drosophila melanogaster

Pseudomonas aeruginosa is recognized for its ability to colonize diverse habitats and cause disease in a variety of hosts, including plants, invertebrates, and mammals. Understanding how this bacterium is able to occupy wide-ranging niches is important for deciphering its ecology. We used transposon sequencing [Tn-Seq, also known as insertion sequencing (INSeq)] to identify genes in P. aeruginosa that contribute to fitness during the colonization of Drosophila melanogaster. Our results reveal a suite of critical factors, including those that contribute to polysaccharide production, DNA repair, metabolism, and respiration. Comparison of candidate genes with fitness determinants discovered in previous studies on P. aeruginosa identified several genes required for colonization and virulence determinants that are conserved across hosts and tissues. This analysis provides evidence for both the conservation of function of several genes across systems, as well as host-specific functions. These findings, which represent the first use of transposon sequencing of a gut pathogen in Drosophila, demonstrate the power of Tn-Seq in the fly model system and advance the existing knowledge of intestinal pathogenesis by D. melanogaster, revealing bacterial colonization determinants that contribute to a comprehensive portrait of P. aeruginosa lifestyles across habitats.IMPORTANCEDrosophila melanogaster is a powerful model for understanding host-pathogen interactions. Research with this system has yielded notable insights into mechanisms of host immunity and defense, many of which emerged from the analysis of bacterial mutants defective for well-characterized virulence factors. These foundational studies-and advances in high-throughput sequencing of transposon mutants-support unbiased screens of bacterial mutants in the fly. To investigate mechanisms of host-pathogen interplay and exploit the tractability of this model host, we used a high-throughput, genome-wide mutant analysis to find genes that enable the pathogen P. aeruginosa to colonize the fly. Our analysis reveals critical mediators of P. aeruginosa establishment in its host, some of which are required across fly and mouse systems. These findings demonstrate the utility of massively parallel mutant analysis and provide a platform for aligning the fly toolkit with comprehensive bacterial genomics.

Ingested soil bacteria breach gut epithelia and prime systemic immunity in an insect

Insects lack acquired immunity and were thought to have no immune memory, but recent studies reported a phenomenon called immune priming, wherein sublethal dose of pathogens or nonpathogenic microbes stimulates immunity and prevents subsequential pathogen infection. Although the evidence for insect immune priming is accumulating, the underlying mechanisms are still unclear. The bean bug Riptortus pedestris acquires its gut microbiota from ambient soil and spatially structures them into a multispecies and variable community in the anterior midgut and a specific, monospecies Caballeronia symbiont population in the posterior region. We demonstrate that a particular Burkholderia strain colonizing the anterior midgut stimulates systemic immunity by penetrating gut epithelia and migrating into the hemolymph. The activated immunity, consisting of a humoral and a cellular response, had no negative effect on the host fitness, but on the contrary protected the insect from subsequent infection by pathogenic bacteria. Interruption of contact between the Burkholderia strain and epithelia of the gut weakened the host immunity back to preinfection levels and made the insects more vulnerable to microbial infection, demonstrating that persistent acquisition of environmental bacteria is important to maintain an efficient immunity.

Vibrio cholerae arrests intestinal epithelial proliferation through T6SS-dependent activation of the bone morphogenetic protein pathway

To maintain an effective barrier, intestinal progenitor cells must divide at a rate that matches the loss of dead and dying cells. Otherwise, epithelial breaches expose the host to systemic infection by gut-resident microbes. Unlike most pathogens, Vibrio cholerae blocks tissue repair by arresting progenitor proliferation in the Drosophila model. At present, we do not understand how V. cholerae circumvents such a critical antibacterial defense. We find that V. cholerae blocks epithelial repair by activating the growth inhibitor bone morphogenetic protein (BMP) pathway in progenitors. Specifically, we show that interactions between V. cholerae and gut commensals initiate BMP signaling via host innate immune defenses. Notably, we find that V. cholerae also activates BMP and arrests proliferation in zebrafish intestines, indicating an evolutionarily conserved link between infection and failure in tissue repair. Our study highlights how enteric pathogens engage host immune and growth regulatory pathways to disrupt intestinal epithelial repair.

Infection risk by oral contamination does not induce immune priming in the mealworm beetle (Tenebrio molitor) but triggers behavioral and physiological responses

In invertebrates, immune priming is the ability of individuals to enhance their immune response based on prior immunological experiences. This adaptive-like immunity likely evolved due to the risk of repeated infections by parasites in the host's natural habitat. The expression of immune priming varies across host and pathogen species, as well as infection routes (oral or wounds), reflecting finely tuned evolutionary adjustments. Evidence from the mealworm beetle (Tenebrio molitor) suggests that Gram-positive bacterial pathogens play a significant role in immune priming after systemic infection. Despite the likelihood of oral infections by natural bacterial pathogens in T. molitor, it remains debated whether ingestion of contaminated food leads to systemic infection, and whether oral immune priming is possible is currently unknown. We first attempted to induce immune priming in both T. molitor larvae and adults by exposing them to food contaminated with living or dead Gram-positive and Gram-negative bacterial pathogens. We found that oral ingestion of living bacteria did not kill them, but septic wounds caused rapid mortality. Intriguingly, the consumption of either dead or living bacteria did not protect against reinfection, contrasting with injury-induced priming. We further examined the effects of infecting food with various living bacterial pathogens on variables such as food consumption, mass gain, and feces production in larvae. We found that larvae exposed to Gram-positive bacteria in their food ingested less food, gained less mass and/or produced more feces than larvae exposed to contaminated food with Gram-negative bacteria or control food. This suggests that oral contamination with Gram-positive bacteria induced both behavioral responses and peristalsis defense mechanisms, even though no immune priming was observed here. Considering that the oral route of infection neither caused the death of the insects nor induced priming, we propose that immune priming in T. molitor may have primarily evolved as a response to the infection risk associated with wounds rather than oral ingestion.

Massively parallel mutant selection identifies genetic determinants of Pseudomonas aeruginosa colonization of Drosophila melanogaster

Pseudomonas aeruginosa is recognized for its ability to colonize diverse habitats and cause disease in a variety of hosts, including plants, invertebrates, and mammals. Understanding how this bacterium is able to occupy wide-ranging niches is important for deciphering its ecology. We used transposon sequencing (Tn-Seq, also known as INSeq) to identify genes in P. aeruginosa that contribute to fitness during colonization of Drosophila melanogaster. Our results reveal a suite of critical factors, including those that contribute to polysaccharide production, DNA repair, metabolism, and respiration. Comparison of candidate genes with fitness determinants discovered in previous studies of P. aeruginosa identified several genes required for colonization and virulence determinants that are conserved across hosts and tissues. This analysis provides evidence for both the conservation of function of several genes across systems, as well as host-specific functions. These findings, which represent the first use of transposon sequencing of a gut pathogen in Drosophila, demonstrate the power of Tn-Seq in the fly model system and advance existing knowledge of intestinal pathogenesis by D. melanogaster, revealing bacterial colonization determinants that contribute to a comprehensive portrait of P. aeruginosa lifestyles across habitats.

Alteration of lipopolysaccharide O antigen leads to avirulence of gut-colonizing Serratia marcescens

The reason why the potent entomopathogen Serratia marcescens fails to kill insects through oral infection is unknown. To compare effects of septic injection and oral administration of S. marcescens, we used a model bean bug, Riptortus pedestris. Most R. pedestris insects survived oral infections, but not septic infections. Although the number of S. marcescens cells in hemolymph after oral infection, which were originated from gut-colonizing S. marcescens, was higher than the fatal number of cells used in septic injection, they did not kill host insects, suggesting a loss of virulence in gut-colonizing S. marcescens cells. When gut-colonizing S. marcescens cells were septically injected into insects, they failed to kill R. pedestris and survive in hemolymph. To understand the avirulence mechanisms in gut-colonizing bacteria, lipopolysaccharides of S. marcescens were analyzed and revealed that the O antigen was lost during gut colonization. Gut-colonizing S. marcescens cells were resistant to humoral immune responses but susceptible to cellular immune responses, easily succumbing to phagocytosis of hemocytes. When cellular immunity was suppressed, the gut-colonizing S. marcescens cells recovered their virulence and killed insects through septic injection. These results suggest that a key mechanism of avirulence in orally infected S. marcescens is the loss of the O antigen, resulting in susceptibility to host's cellular immune responses.

Transcriptome analysis of immune-related genes of Asian corn borer (Ostrinia furnacalis [Guenée]) after oral bacterial infection

The Asian corn borer (Ostrinia furnacalis) is an important agricultural pest causing serious damage to economic crops, such as corn and sorghum. The gut is the first line of defense against pathogens that enter through the mouth. Staphylococcus aureus was used to infect the O. furnacalis midgut to understand the midgut immune mechanism against exogenous pathogens to provide new ideas and methods for the prevention and control of O. furnacalis. A sequencing platform was used for genome assembly and gene expression. The unigene sequences were annotated and functionally classified by Gene Ontology and Kyoto Encyclopedia of Genes and Genomes. Significant differences were found in the induced expression profiles before and after infection. Some differentially expressed genes have important relations with lipid metabolism and immune mechanism, suggesting that they play an important role in the innate immune response of O. furnacalis. Furthermore, quantitative real-time polymerase chain reaction assay was used to identify the key genes involved in the signaling pathway, and the expression patterns of these key genes were confirmed. The results could help study the innate immune system of lepidopteran insects and provide theoretical support for the control of related pests and the protection of beneficial insects.

Fast drosophila enterocyte regrowth after infection involves a reverse metabolic flux driven by an amino acid transporter

Upon exposure to a bacterial pore-forming toxin, enterocytes rapidly purge their apical cytoplasm into the gut lumen, resulting in a thin intestinal epithelium. The enterocytes regain their original shape and thickness within 16 h after the ingestion of the bacteria. Here, we show that the regrowth of Drosophila enterocytes entails an inversion of metabolic fluxes from the organism back toward the intestine. We identify a proton-assisted transporter, Arcus, that is required for the reverse absorption of amino acids and the timely recovery of the intestinal epithelium. Arcus is required for a peak of amino acids appearing in the hemolymph shortly after infection. The regrowth of enterocytes involves the insulin signaling pathway and Myc. The purge decreases Myc mRNA levels, which subsequently remain at low levels in the arcus mutant. Interestingly, the action of arcus and Myc in the intestinal epithelium is not cell-autonomous, suggesting amino acid fluxes within the intestinal epithelium.

Intake of caffeine containing sugar diet remodels gut microbiota and perturbs Drosophila melanogaster immunity and lifespan

The diet-microbiome-immunity axis is one among the many arms that draw up the "we are what we intake" proclamation. As such, studies on the effect of food and beverage intake on the gut environment and microbiome and on modulating immunological responses and the host's susceptibility to pathogens are on the rise. A typical accompaniment in different sustenance we consume on daily basis is the trimethylxanthine alkaloid caffeine. Being a chief component in our regular aliment, a better understanding of the effect of caffeine containing food and beverages on our gut-microbiome-immunity axis and henceforth on our health is much needed. In this study, we shed more light on the effect of oral consumption of caffeine supplemented sugar diet on the gut environment, specifically on the gut microbiota, innate immunity and host susceptibility to pathogens using the Drosophila melanogaster model organism. Our findings reveal that the oral intake of a dose-specific caffeine containing sucrose/agarose sugar diet causes a significant alteration within the fly gut milieu demarcated by microbial dysbiosis and an elevation in the production of reactive oxygen species and expression of immune-deficiency (Imd) pathway-dependent antimicrobial peptide genes. The oral intake of caffeine containing sucrose/agarose sugar diet also renders the flies more susceptible to bacterial infection and shortens their lifespan in both infection and non-infection settings. Our findings set forth additional insight into the potentiality of diet to alter the gut milieu and highlight the importance of dietary control on health.

Clip-SP1 cleavage activates downstream prophenoloxidase activating protease (PAP) in Plutella xylostella

Melanization is a component of the humoral immune defense of insects and is induced by serine protease-mediated phenoloxidase (PO) catalysis. Prophenoloxidase (PPO) in the midgut of Plutella xylostella is activated by the CLIP domain serine protease (clip-SP) in response to Bacillus thuringiensis (Bt) infection, but the detailed signaling cascade following this activation is unknown. Here, we report that activation of clip-SP enhances PO activity in the P. xylostella midgut by cleaving three downstream PPO-activating proteases (PAPs). First, the expression level of clip-SP1 was increased in the midgut after Bt8010 infection of P. xylostella. Then, purified recombinant clip-SP1 was able to activate three PAPs - PAPa, PAPb and PAP3 - which in turn enhanced their PO activity in the hemolymph. Furthermore, clip-SP1 showed a dominant effect on PO activity compared to the individual PAPs. Our results indicate that Bt infection induces the expression of clip-SP1, which is upstream of a signaling cascade, to efficiently activate PO catalysis and mediate melanization in the midgut of P. xylostella. And it provides a basis for studying the complex PPO regulatory system in the midgut during Bt infection.

Drosophila melanogaster larvae are tolerant to oral infection with the bacterial pathogen Photorhabdus luminescens

The fruit fly Drosophila melanogaster is an excellent model for dissecting the molecular and functional bases of bacterial pathogenicity and host antibacterial immune response. The Gram-negative bacterium Photorhabdus luminescens is an insect-specific pathogen that forms a mutualistic relationship with the entomopathogenic nematode Heterorhabditis bacteriophora . Here we find that oral infection of D. melanogaster larvae with P. luminescens moderately reduces their survival ability while the bacteria replicate efficiently in the infected insects. This information will contribute towards understanding host gut immunity against potent bacterial pathogens.

Peroxiredoxins Play an Important Role in the Regulation of Immunity and Aging in Drosophila

Aberrant immune responses and chronic inflammation can impose significant health risks and promote premature aging. Pro-inflammatory responses are largely mediated via reactive oxygen species (ROS) and reduction-oxidation reactions. A pivotal role in maintaining cellular redox homeostasis and the proper control of redox-sensitive signaling belongs to a family of antioxidant and redox-regulating thiol-related peroxidases designated as peroxiredoxins (Prx). Our recent studies in Drosophila have shown that Prxs play a critical role in aging and immunity. We identified two important 'hubs', the endoplasmic reticulum (ER) and mitochondria, where extracellular and intracellular stress signals are transformed into pro-inflammatory responses that are modulated by the activity of the Prxs residing in these cellular organelles. Here, we found that mitochondrial Prx activity in the intestinal epithelium is required to prevent the development of intestinal barrier dysfunction, which can drive systemic inflammation and premature aging. Using a redox-negative mutant, we demonstrated that Prx acts in a redox-dependent manner in regulating the age-related immune response. The hyperactive immune response observed in flies under-expressing mitochondrial Prxs is due to a response to abiotic signals but not to changes in the bacterial content. This hyperactive response, but not reduced lifespan phenotype, can be rescued by the ER-localized Prx.

Anopheline mosquito saliva contains bacteria that are transferred to a mammalian host through blood feeding

Introduction

Malaria transmission occurs when Plasmodium sporozoites are transferred from the salivary glands of anopheline mosquitoes to a human host through the injection of saliva. The need for better understanding, as well as novel modes of inhibiting, this key event in transmission has driven intense study of the protein and miRNA content of saliva. Until now the possibility that mosquito saliva may also contain bacteria has remained an open question despite the well documented presence of a rich microbiome in salivary glands.

Methods

Using both 16S rRNA sequencing and MALDI-TOF approaches, we characterized the composition of the saliva microbiome of An. gambiae and An. stephensi mosquitoes which respectively represent two of the most important vectors for the major malaria-causing parasites P. falciparum and P. vivax.

Results

To eliminate the possible detection of non-mosquito-derived bacteria, we used a transgenic, fluorescent strain of one of the identified bacteria, Serratiamarcescens, to infect mosquitoes and detect its presence in mosquito salivary glands as well as its transfer to, and colonization of, mammalian host tissues following a mosquito bite. We also showed that Plasmodium infection modified the mosquito microbiota, increasing the presence of Serratia while diminishing the presence of Elizabethkingia and that both P. berghei and Serratia were transferred to, and colonized mammalian tissues.

Discussion

These data thus document the presence of bacteria in mosquito saliva, their transfer to, and growth in a mammalian host as well as possible interactions with Plasmodium transmission. Together they raise the possible role of mosquitoes as vectors of bacterial infection and the utility of commensal mosquito bacteria for the development of transmission-blocking strategies within a mammalian host.

Tachinid parasitoid Exorista japonica affects the utilization of diet by changing gut microbial composition in the silkworm, Bombyx mori

Changes in both intake and digestion of feed have been demonstrated in the host following parasitization. However, its regulatory mechanism has not been clarified. In this study, silkworms and Exorista japonica were used as research objects to analyze the effect of parasitism on the midgut immune system of the silkworm. After being parasitized, the expressions of antimicrobial peptide (AMP) genes of silkworms showed a fluctuating trend of first upregulation and then downregulation, while phenoloxidase and lysozyme activities were inhibited. To study the possible impact of the downregulation of AMP genes on intestinal microorganisms, the characteristics of the intestinal microbial population of silkworms on the third day of parasitism were analyzed. The relative abundance of Firmicutes, Proteobacteria, and Bacteroidota decreased, while that of Actinobacteriota increased. The increased abundance of conditionally pathogenic bacteria Serratia and Staphylococcus might lead to a decrease in the amount of silkworm ingestion. Meanwhile, the abundance of Acinetobacter, Bacillus, Pseudomonas, and Enterobacter promotes an increase in the digestion of nutrients. This study indicated that the imbalance of intestinal microbial homeostasis caused by parasitism may affect the absorption and digestion of nutrients by the host. Collectively, our findings provided a new clue for further exploring the mechanism of nutrient transport among the host, parasitoid, and intestinal microorganisms.

Regulation of gut bacteria in silkworm (Bombyx mori) after exposure to endogenous cadmium-polluted mulberry leaves

Soil cadmium (Cd) pollution presents a severe pollution burden to flora and fauna due to its non-degradability and transferability. The Cd in the soil is stressing the silkworm (Bombyx mori) out through a soil-mulberry-silkworm system. The gut microbiota of B.mori are reported to shape host health. However, earlier research had not reported the effect of endogenous Cd-polluted mulberry leaves on the gut microbiota of B.mori. In the current research, we compared the phyllosphere bacteria of endogenous Cd-polluted mulberry leaves at different concentrations. The investigation of the gut bacteria of B.mori fed with the mulberry leaves was done to evaluate the impact of endogenous Cd- polluted mulberry leaves on the gut bacteria of the silkworm. The results revealed a dramatic change in the gut bacteria of B.mori whereas, the changes in the phyllosphere bacteria of mulberry leaves in response to an increased Cd concentration were insignificant. It also increased the α-diversity and altered the gut bacterial community structure of B. mori. A significant change in the abundance of dominant phyla of gut bacteria of B.mori was recorded. At the genus level, the abundance of Enterococcus, Brachybacterium and Brevibacterium group related to disease resistance, and the abundance of Sphingomonas, Glutamicibacter and Thermus related to metal detoxification was significantly increased after Cd exposure. Meanwhile, there was a significant decrease in the abundance of the pathogenic bacteria Serratia and Enterobacter. The results demonstrated that endogenous Cd-polluted mulberry leaves caused perturbations in the gut bacterial composition of B.mori, which may driven by Cd content rather than phyllosphere bacteria. A significant variation in the specific bacterial community indicated the adaptation of B. mori gut for its role in heavy metal detoxification and immune function regulation. The results of this study help to understand the bacterial community associated with endogenous Cd-polluted resistance in the gut of B.mori, which proves to be a novel addition in describing its response in activating the detoxification mechanism and promoting its growth and development. This research work will help to explore the other mechanisms and microbiota associated with the adaptations to mitigate the Cd pollution problems.

Intestinal barrier dysfunction: an evolutionarily conserved hallmark of aging

A major challenge in the biology of aging is to understand how specific age-onset pathologies relate to the overall health of the organism. The integrity of the intestinal epithelium is essential for the wellbeing of the organism throughout life. In recent years, intestinal barrier dysfunction has emerged as an evolutionarily conserved feature of aged organisms, as reported in worms, flies, fish, rodents and primates. Moreover, age-onset intestinal barrier dysfunction has been linked to microbial alterations, elevated immune responses, metabolic alterations, systemic health decline and mortality. Here, we provide an overview of these findings. We discuss early work in the Drosophila model that sets the stage for examining the relationship between intestinal barrier integrity and systemic aging, then delve into research in other organisms. An emerging concept, supported by studies in both Drosophila and mice, is that directly targeting intestinal barrier integrity is sufficient to promote longevity. A better understanding of the causes and consequences of age-onset intestinal barrier dysfunction has significant relevance to the development of interventions to promote healthy aging.

Wolbachia Promotes Its Own Uptake by Host Cells

Wolbachia pipientis is an incredibly widespread bacterial symbiont of insects, present in an estimated 25 to 52% of species worldwide. Wolbachia is faithfully maternally transmitted both in a laboratory setting and in the wild. In an established infection, Wolbachia is primarily intracellular, residing within host-derived vacuoles that are associated with the endoplasmic reticulum. However, Wolbachia also frequently transfers between host species, requiring an extracellular stage to its life cycle. Indeed, Wolbachia has been moved between insect species for the precise goal of controlling populations. The use of Wolbachia in this application requires that we better understand how it initiates and establishes new infections. Here, we designed a novel method for live tracking Wolbachia cells during infection using a combination of stains and microscopy. We show that live Wolbachia cells are taken up by host cells at a much faster rate than dead Wolbachia cells, indicating that Wolbachia bacteria play a role in their own uptake and that Wolbachia colonization is not just a passive process. We also show that the host actin cytoskeleton must be intact for this to occur and that drugs that disrupt the actin cytoskeleton effectively abrogate Wolbachia uptake. The development of this live infection assay will assist in future efforts to characterize Wolbachia factors used during host infection.

The transcriptomic response of Hyphantria cunea (Drury) to the infection of Serratia marcescens Bizio based on full-length SMRT transcriptome sequencing

Hyphantria cunea (Drury) is a globally important forest pest. We found that the Serratia marcescens Bizio strain SM1 had insecticidal activity against H. cunea, but the transcriptomic response of H. cunea to SM1 were not clear. Therefore, we performed full-length sequencing of the transcriptomes of H. cunea larvae infected with SM1 and the control group. A total of 1,183 differentially expressed genes (DEGs) were identified by comparing the group infected with SM1 and the control group, including 554 downregulated genes and 629 upregulated genes. We found many downregulated genes in metabolic pathways. Furthermore, some of these downregulated genes were involved in cellular immunity, melanization, and detoxification enzymes, which showed that SM1 weakened H. cunea immunity. In addition, genes in the juvenile hormone synthesis pathway were upregulated, which was detrimental to the survival of H. cunea. This research analyzed the transcriptomic response of H. cunea to SM1 by high-throughput full-length transcriptome sequencing. The results provide useful information to explore the relationship between S. marcescens and H. cunea, and theoretical support for the application of S. marcescens and the control of H. cunea in the future.

Studying Plant-Insect Interactions through the Analyses of the Diversity, Composition, and Functional Inference of Their Bacteriomes

As with many other trophic interactions, the interchange of microorganisms between plants and their herbivorous insects is unavoidable. To test the hypothesis that the composition and diversity of the insect bacteriome are driven by the bacteriome of the plant, the bacteriomes of both the plant Datura inoxia and its specialist insect Lema daturaphila were characterised using 16S sRNA gene amplicon sequencing. Specifically, the bacteriomes associated with seeds, leaves, eggs, guts, and frass were described and compared. Then, the functions of the most abundant bacterial lineages found in the samples were inferred. Finally, the patterns of co-abundance among both bacteriomes were determined following a multilayer network approach. In accordance with our hypothesis, most genera were shared between plants and insects, but their abundances differed significantly within the samples collected. In the insect tissues, the most abundant genera were Pseudomonas (24.64%) in the eggs, Serratia (88.46%) in the gut, and Pseudomonas (36.27%) in the frass. In contrast, the most abundant ones in the plant were Serratia (40%) in seeds, Serratia (67%) in foliar endophytes, and Hymenobacter (12.85%) in foliar epiphytes. Indeed, PERMANOVA analysis showed that the composition of the bacteriomes was clustered by sample type (F = 9.36, p < 0.001). Functional inferences relevant to the interaction showed that in the plant samples, the category of Biosynthesis of secondary metabolites was significantly abundant (1.4%). In turn, the category of Xenobiotics degradation and metabolism was significantly present (2.5%) in the insect samples. Finally, the phyla Proteobacteria and Actinobacteriota showed a pattern of co-abundance in the insect but not in the plant, suggesting that the co-abundance and not the presence−absence patterns might be more important when studying ecological interactions.

Lipopolysaccharide -mediated resistance to host antimicrobial peptides and hemocyte-derived reactive-oxygen species are the major Providencia alcalifaciens virulence factors in Drosophila melanogaster

Bacteria from the genus Providencia are ubiquitous Gram-negative opportunistic pathogens, causing “travelers’ diarrhea”, urinary tract, and other nosocomial infections in humans. Some Providencia strains have also been isolated as natural pathogens of Drosophila melanogaster. Despite clinical relevance and extensive use in Drosophila immunity research, little is known about Providencia virulence mechanisms and the corresponding insect host defenses. To close this knowledge gap, we investigated the virulence factors of a representative Providencia species—P. alcalifaciens which is highly virulent to fruit flies and amenable to genetic manipulations. We generated a P. alcalifaciens transposon mutant library and performed an unbiased forward genetics screen in vivo for attenuated mutants. Our screen uncovered 23 mutants with reduced virulence. The vast majority of them had disrupted genes linked to lipopolysaccharide (LPS) synthesis or modifications. These LPS mutants were sensitive to cationic antimicrobial peptides (AMPs) in vitro and their virulence was restored in Drosophila mutants lacking most AMPs. Thus, LPS-mediated resistance to host AMPs is one of the virulence strategies of P. alcalifaciens. Another subset of P. alcalifaciens attenuated mutants exhibited increased susceptibility to reactive oxygen species (ROS) in vitro and their virulence was rescued by chemical scavenging of ROS in flies prior to infection. Using genetic analysis, we found that the enzyme Duox specifically in hemocytes is the source of bactericidal ROS targeting P. alcalifaciens. Consistently, the virulence of ROS-sensitive P. alcalifaciens mutants was rescued in flies with Duox knockdown in hemocytes. Therefore, these genes function as virulence factors by helping bacteria to counteract the ROS immune response. Our reciprocal analysis of host-pathogen interactions between D. melanogaster and P. alcalifaciens identified that AMPs and hemocyte-derived ROS are the major defense mechanisms against P. alcalifaciens, while the ability of the pathogen to resist these host immune responses is its major virulence mechanism. Thus, our work revealed a host-pathogen conflict mediated by ROS and AMPs.

Pathogens express special molecules or structures called virulence factors to successfully infect a host. By identifying these factors, we can learn how hosts fight and how pathogens cause infections. Here, we identified virulence factors of the human and fruit fly pathogen Providencia alcalifaciens, by infecting flies with a series of mutants of this pathogen. In this way, we detected 23 mutants that were less virulent. Some of these less virulent mutants were hypersensitive to fruit fly immune defense molecules called antimicrobial peptides (AMPs), while others were sensitive to reactive oxygen species (ROS) produced by the immune cells. Notably, AMPs-sensitive mutants remained virulent in a Drosophila mutant that lacks AMPs, while pathogens sensitive to oxidative stress retained their virulence in a fruit fly mutant devoid of oxidative species. These results suggest that the ability of P. alcalifaciens to resist two major host immune molecules, namely AMPs and ROS, is the major virulence mechanism. Overall, our systematic analysis of P. alcalifaciens virulence factors has identified the major defense mechanisms of the fruit fly against this pathogen and the bacterial mechanisms to combat these immune responses.

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.

Pre-copulatory reproductive behaviours are preserved in Drosophila melanogaster infected with bacteria

The activation of the immune system upon infection exerts a huge energetic demand on an individual, likely decreasing available resources for other vital processes, like reproduction. The factors that determine the trade-off between defensive and reproductive traits remain poorly understood. Here, we exploit the experimental tractability of the fruit fly Drosophila melanogaster to systematically assess the impact of immune system activation on pre-copulatory reproductive behaviour. Contrary to expectations, we found that male flies undergoing an immune activation continue to display high levels of courtship and mating success. Similarly, immune-challenged female flies remain highly sexually receptive. By combining behavioural paradigms, a diverse panel of pathogens and genetic strategies to induce the fly immune system, we show that pre-copulatory reproductive behaviours are preserved in infected flies, despite the significant metabolic cost of infection.

Role of Insect Gut Microbiota in Pesticide Degradation: A Review

Insect pests cause significant agricultural and economic losses to crops worldwide due to their destructive activities. Pesticides are designed to be poisonous and are intentionally released into the environment to combat the menace caused by these noxious pests. To survive, these insects can resist toxic substances introduced by humans in the form of pesticides. According to recent findings, microbes that live in insect as symbionts have recently been found to protect their hosts against toxins. Symbioses that have been formed are between the pests and various microbes, a defensive mechanism against pathogens and pesticides. Insects' guts provide unique conditions for microbial colonization, and resident bacteria can deliver numerous benefits to their hosts. Insects vary significantly in their reliance on gut microbes for basic functions. Insect digestive tracts are very different in shape and chemical properties, which have a big impact on the structure and composition of the microbial community. Insect gut microbiota has been found to contribute to feeding, parasite and pathogen protection, immune response modulation, and pesticide breakdown. The current review will examine the roles of gut microbiota in pesticide detoxification and the mechanisms behind the development of resistance in insects to various pesticides. To better understand the detoxifying microbiota in agriculturally significant pest insects, we provided comprehensive information regarding the role of gut microbiota in the detoxification of pesticides.

Phagocytosis Is the Sole Arm of Drosophila melanogaster Known Host Defenses That Provides Some Protection Against Microsporidia Infection

Microsporidia are obligate intracellular parasites able to infest specifically a large range of species, including insects. The knowledge about the biology of microsporidial infections remains confined to mostly descriptive studies, including molecular approaches such as transcriptomics or proteomics. Thus, functional data to understand insect host defenses are currently lacking. Here, we have undertaken a genetic analysis of known host defenses of the Drosophila melanogaster using an infection model whereby Tubulinosema ratisbonensis spores are directly injected in this insect. We find that phagocytosis does confer some protection in this infection model. In contrast, the systemic immune response, extracellular reactive oxygen species, thioester proteins, xenophagy, and intracellular antiviral response pathways do not appear to be involved in the resistance against this parasite. Unexpectedly, several genes such as PGRP-LE seem to promote this infection. The prophenol oxidases that mediate melanization have different functions; PPO1 presents a phenotype similar to that of PGRP-LE whereas that of PPO2 suggests a function in the resilience to infection. Similarly, eiger and Unpaired3, which encode two cytokines secreted by hemocytes display a resilience phenotype with a strong susceptibility to T. ratisbonensis.

Suppression of intestinal dysfunction in a Drosophila model of Parkinson's disease is neuroprotective

The innate immune response mounts a defense against foreign invaders and declines with age. An inappropriate induction of this response can cause diseases. Previous studies showed that mitochondria can be repurposed to promote inflammatory signaling. Damaged mitochondria can also trigger inflammation and promote diseases. Mutations in pink1, a gene required for mitochondrial health, cause Parkinson's disease, and Drosophila melanogaster pink1 mutants accumulate damaged mitochondria. Here, we show that defective mitochondria in pink1 mutants activate Relish targets and demonstrate that inflammatory signaling causes age-dependent intestinal dysfunction in pink1-mutant flies. These effects result in the death of intestinal cells, metabolic reprogramming and neurotoxicity. We found that Relish signaling is activated downstream of a pathway stimulated by cytosolic DNA. Suppression of Relish in the intestinal midgut of pink1-mutant flies restores mitochondrial function and is neuroprotective. We thus conclude that gut-brain communication modulates neurotoxicity in a fly model of Parkinson's disease through a mechanism involving mitochondrial dysfunction.

Autophagy-mediated plasma membrane removal promotes the formation of epithelial syncytia

Epithelial wound healing in Drosophila involves the formation of multinucleate cells surrounding the wound. We show that autophagy, a cellular degradation process often deployed in stress responses, is required for the formation of a multinucleated syncytium during wound healing, and that autophagosomes that appear near the wound edge acquire plasma membrane markers. In addition, uncontrolled autophagy in the unwounded epidermis leads to the degradation of endo‐membranes and the lateral plasma membrane, while apical and basal membranes and epithelial barrier function remain intact. Proper functioning of TORC1 is needed to prevent destruction of the larval epidermis by autophagy, in a process that depends on phagophore initiation and expansion but does not require autophagosomes fusion with lysosomes. Autophagy induction can also affect other sub‐cellular membranes, as shown by its suppression of experimentally induced laminopathy‐like nuclear defects. Our findings reveal a function for TORC1‐mediated regulation of autophagy in maintaining membrane integrity and homeostasis in the epidermis and during wound healing.

Biotin controls intestinal stem cell mitosis and host-microbiome interactions

Diet is a key regulator of metabolism and interacts with the intestinal microbiome. Here, we study the role of the Drosophila intestinal stem cell (ISC)-specific biotin transporter Smvt in midgut homeostasis, infection-induced regeneration, and tumorigenesis. We show that Smvt-transported biotin in ISCs is necessary for ISC mitosis. Smvt deficiency impairs intestinal maintenance, which can be rescued by the human Smvt, encoded by SLC5A6. ISC-specific, Smvt-silenced flies exhibit microbial dysbiosis, whereby the growth of Providencia sneebia, an opportunistic pathogen, is favored. Dysbiosis correlates with increased Nox expression, reactive oxygen species (ROS), and enterocyte apoptosis. Flies acquire biotin from their diet and microbiota. We show that, when dietary biotin is scarce, biotin-producing commensals, e.g., E. coli, can rescue reduced ISC mitosis. Smvt and commensals also control intestinal tumor growth. Our findings suggest that direct modification of the gut microbiome by biotin can serve as an approach for the treatment of dysbiosis-promoted diseases and tumorigenesis control.

The fliR gene contributes to the virulence of S. marcescens in a Drosophila intestinal infection model

Serratia marcescens is an opportunistic bacterium that infects a wide range of hosts including humans. It is a potent pathogen in a septic injury model of Drosophila melanogaster since a few bacteria directly injected in the body cavity kill the insect within a day. In contrast, flies do not succumb to ingested bacteria for days even though some bacteria cross the intestinal barrier into the hemolymph within hours. The mechanisms by which S. marcescens attacks enterocytes and damages the intestinal epithelium remain uncharacterized. To better understand intestinal infections, we performed a genetic screen for loss of virulence of ingested S. marcescens and identified FliR, a structural component of the flagellum, as a virulence factor. Next, we compared the virulence of two flagellum mutants fliR and flhD in two distinct S. marcescens strains. Both genes are required for S. marcescens to escape the gut lumen into the hemocoel, indicating that the flagellum plays an important role for the passage of bacteria through the intestinal barrier. Unexpectedly, fliR but not flhD is involved in S. marcescens-mediated damages of the intestinal epithelium that ultimately contribute to the demise of the host. Our results therefore suggest a flagellum-independent role for fliR in bacterial virulence.

The protective effect of safranal against intestinal tissue damage in Drosophila

Drosophila is often exposed to harmful environments, and the intestinal epithelium is the first line of defense against external infection. Intestinal stem cells (ISCs) in the Drosophila midgut play a crucial role in maintaining tissue homeostasis and compensating for cell loss caused by tissue damage. Crocus sativus L. (saffron) can protect against intestinal injury in response to inflammation; however, the specific protective components of saffron and the related mechanisms remain unclear. Safranal is one of the main components of saffron. Here, we used dextran sodium sulfate (DSS) or Erwinia carotovora carotovora 15 (Ecc15) to create an intestinal injury model and explored the protective effect of safranal against tissue damage. Excessive proliferation and differentiation of ISCs in the Drosophila midgut were observed after DSS or Ecc15 feeding; however, these phenotypes were rescued after safranal feeding. In addition, we found that this process occurred through inhibition of the c-Jun N-terminal kinase (JNK), epidermal growth factor receptor (EGFR) and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathways. Furthermore, safranal inhibited the Ecc15- and DSS-induced increases in antimicrobial peptide (AMP) and reactive oxygen species (ROS) levels and intestinal epithelial cell death, thereby protecting gut integrity. In summary, safranal was found to have a significant protective effect and maintain intestinal homeostasis in Drosophila; these findings provide a foundation for the application of safranal in clinical research and the treatment of intestinal injury.

Bacterial recognition by PGRP-SA and downstream signalling by Toll/DIF sustain commensal gut bacteria in Drosophila

The gut sets the immune and metabolic parameters for the survival of commensal bacteria. We report that in Drosophila, deficiency in bacterial recognition upstream of Toll/NF-κB signalling resulted in reduced density and diversity of gut bacteria. Translational regulation factor 4E-BP, a transcriptional target of Toll/NF-κB, mediated this host-bacteriome interaction. In healthy flies, Toll activated 4E-BP, which enabled fat catabolism, which resulted in sustaining of the bacteriome. The presence of gut bacteria kept Toll signalling activity thus ensuring the feedback loop of their own preservation. When Toll activity was absent, TOR-mediated suppression of 4E-BP made fat resources inaccessible and this correlated with loss of intestinal bacterial density. This could be overcome by genetic or pharmacological inhibition of TOR, which restored bacterial density. Our results give insights into how an animal integrates immune sensing and metabolism to maintain indigenous bacteria in a healthy gut.

Diversity of insect antimicrobial peptides and proteins - A functional perspective: A review

The innate immune response of insects provides a robust line of defense against pathogenic microbes and eukaryotic parasites. It consists of two types of overlapping immune responses, named humoral and cellular, which share protective molecules and regulatory mechanisms that closely coordinate to prevent the spread and replication of pathogens within the compromised insect hemocoel. The major feature of the humoral part of the insect immune system involves the production and secretion of antimicrobial peptides from the fat body, which is considered analogous to adipose tissue and liver in vertebrates. Previous research has identified and characterized the nature of antimicrobial peptides that are directed against various targets during the different stages of infection. Here we review this information focusing mostly on the diversity and mode of action of these host defense components, and their critical contribution to maintaining host homeostasis. Extending this knowledge is paramount for understanding the evolution of innate immune function and the physiological balance required to provide sufficient protection to the host against external enemies while avoiding overactivation signaling events that would severely undermine physiological stability.

Rapid and Direct Action of Lipopolysaccharide (LPS) on Skeletal Muscle of Larval Drosophila

The endotoxin lipopolysaccharide (LPS) from Gram-negative bacteria exerts a direct and rapid effect on tissues. While most attention is given to the downstream actions of the immune system in response to LPS, this study focuses on the direct actions of LPS on skeletal muscle in Drosophila melanogaster. It was noted in earlier studies that the membrane potential rapidly hyperpolarizes in a dose-dependent manner with exposure to LPS from Pseudomonas aeruginosa and Serratia marcescens. The response is transitory while exposed to LPS, and the effect does not appear to be due to calcium-activated potassium channels, activated nitric oxide synthase (NOS), or the opening of Cl- channels. The purpose of this study was to further investigate the mechanism of the hyperpolarization of the larval Drosophila muscle due to exposure of LPS using several different experimental paradigms. It appears this response is unlikely related to activation of the Na-K pump or Ca2+ influx. The unknown activation of a K+ efflux could be responsible. This will be an important factor to consider in treatments of bacterial septicemia and cellular energy demands.

Activation of the Host Immune Response in Hyphantria cunea (Drury) (Lepidoptera: Noctuidae) Induced by Serratia marcescens Bizio

Simple Summary

Hyphantria cunea (Drury) is a quarantine pest, due to its extensive host, leading to serious economic losses in the agricultural and forestry industries. To control this pest, it is increasingly important to use microbial pesticides because they are biologically active and ecologically safe. Serratia marcescens Bizio (SM1) is a potential biocontrol bacterium. Although SM1 has a pathogenic role in H. cunea, H. cunea self-defense reduces the pathogenic effect of SM1. In this study, immune-related differentially expressed genes (DEGs) in H. cunea were first identified after SM1 infection, and the immune regulation mode of H. cunea in response to SM1, including antimicrobial peptide synthesis pathways, melanization and cellular immunity, was revealed. According to the analysis, the immune system of H. cunea was induced by SM1. In summary, our study demonstrates how the immune systems of the H. cunea work to resist the infection of SM1, which provides the theoretical basis for researching more efficient microbial pesticides for H. cunea.

Abstract

Host–pathogen interactions are essential to our understanding of biological pesticides. Hyphantria cunea (Drury) is an important forest pest worldwide. The immune mechanism of the interaction between H. cunea and Serratia marcescens Bizio (SM1) is unclear. First, transcriptome sequencing and quantitative real-time PCR (qRT-PCR) analysis described the H. cunea immune response to SM1. A total of 234 immune-related differentially expressed genes (DEGs) were found. Many immune regulatory genes in three classical pathways were found. Antimicrobial peptides, including attacin B, cecropin A, gloverin, lebocin and diapausin, are involved in defending against SM1 challenge, and are mainly produced by Toll and immune deficiency (IMD) pathways. Some melanization genes were changed in H. cunea, which suggested that H. cunea melanization was activated by SM1. Furthermore, phagocytosis, autophagolysosome and apoptosis pathways in cellular immunity were activated in H. cunea against SM1. Finally, the expression patterns of 10 immune genes were analyzed systematically by qRT-PCR, and most of the genes were upregulated compared to the control. Our studies provide useful information about the immune response of H. cunea under the stress of SM1, which is important to understand how SM1 affects the immune system of H. cunea and provides new ideas to control H. cunea by using SM1.

Engineering Gac/Rsm Signaling Cascade for Optogenetic Induction of the Pathogenicity Switch in Pseudomonas aeruginosa

Bacterial pathogens operate by tightly controlling the pathogenicity to facilitate invasion and survival in host. While small molecule inducers can be designed to modulate pathogenicity to perform studies of pathogen-host interaction, these approaches, due to the diffusion property of chemicals, may have unintended, or pleiotropic effects that can impose limitations on their use. By contrast, light provides superior spatial and temporal resolution. Here, using optogenetics we reengineered GacS of the opportunistic pathogen Pseudomonas aeruginosa, signal transduction protein of the global regulatory Gac/Rsm cascade which is of central importance for the regulation of infection factors. The resultant protein (termed YGS24) displayed significant light-dependent activity of GacS kinases in Pseudomonas aeruginosa. When introduced in the Caenorhabditis elegans host systems, YGS24 stimulated the pathogenicity of the Pseudomonas aeruginosa strain PAO1 in a brain-heart infusion and of another strain, PA14, in slow killing media progressively upon blue-light exposure. This optogenetic system provides an accessible way to spatiotemporally control bacterial pathogenicity in defined hosts, even specific tissues, to develop new pathogenesis systems, which may in turn expedite development of innovative therapeutics.