Specificity and complexity of the Caenorhabditis elegans innate immune response

The MAB-5/Hox family transcription factor is important for Caenorhabditis elegans innate immune response to Staphylococcus epidermidis infection

Innate immunity functions as a rapid defense against broad classes of pathogenic agents. While the mechanisms of innate immunity in response to antigen exposure are well-studied, how pathogen exposure activates the innate immune responses and the role of genetic variation in immune activity is currently being investigated. Previously, we showed significant survival differences between the N2 and the CB4856 Caenorhabditis elegans isolates in response to Staphylococcus epidermidis infection. One of those differences was expression of the mab-5 Hox family transcription factor, which was induced in N2, but not CB4856, after infection. In this study, we use survival assays and RNA-sequencing to better understand the role of mab-5 in response to S. epidermidis. We found that mab-5 loss-of-function (LOF) mutants were more susceptible to S. epidermidis infection than N2 or mab-5 gain-of-function (GOF) mutants, but not as susceptible as CB4856 animals. We then conducted transcriptome analysis of infected worms and found considerable differences in gene expression profiles when comparing animals with mab-5 LOF to either N2 or mab-5 GOF. N2 and mab-5 GOF animals showed a significant enrichment in expression of immune genes and C-type lectins, whereas mab-5 LOF mutants did not. Overall, gene expression profiling in mab-5 mutants provided insight into MAB-5 regulation of the transcriptomic response of C. elegans to pathogenic bacteria and helps us to understand mechanisms of innate immune activation and the role that transcriptional regulation plays in organismal health.

The Caenorhabditis elegans proteome response to two protective Pseudomonas symbionts

The Caenorhabditis elegans natural microbiota isolates Pseudomonas lurida MYb11 and Pseudomonas fluorescens MYb115 protect the host against pathogens through distinct mechanisms. While P. lurida produces an antimicrobial compound and directly inhibits pathogen growth, P. fluorescens MYb115 protects the host without affecting pathogen growth. It is unknown how these two protective microbes affect host biological processes. We used a proteomics approach to elucidate the C. elegans response to MYb11 and MYb115. We found that both Pseudomonas isolates increase vitellogenin protein production in young adults, which confirms previous findings on the effect of microbiota on C. elegans reproductive timing. Moreover, the C. elegans responses to MYb11 and MYb115 exhibit common signatures with the response to other vitamin B12-producing bacteria, emphasizing the importance of vitamin B12 in C. elegans-microbe metabolic interactions. We further analyzed signatures in the C. elegans response specific to MYb11 or MYb115. We provide evidence for distinct modifications in lipid metabolism by both symbiotic microbes. We could identify the activation of host-pathogen defense responses as an MYb11-specific proteome signature and provide evidence that the intermediate filament protein IFB-2 is required for MYb115-mediated protection. These results indicate that MYb11 not only produces an antimicrobial compound but also activates host antimicrobial defenses, which together might increase resistance to infection. In contrast, MYb115 affects host processes such as lipid metabolism and cytoskeleton dynamics, which might increase host tolerance to infection. Overall, this study pinpoints proteins of interest that form the basis for additional exploration into the mechanisms underlying C. elegans microbiota-mediated protection from pathogen infection and other microbiota-mediated traits.IMPORTANCESymbiotic bacteria can defend their host against pathogen infection. While some protective symbionts directly interact with pathogenic bacteria, other protective symbionts elicit a response in the host that improves its own pathogen defenses. To better understand how a host responds to protective symbionts, we examined which host proteins are affected by two protective Pseudomonas bacteria in the model nematode Caenorhabditis elegans. We found that the C. elegans response to its protective symbionts is manifold, which was reflected in changes in proteins that are involved in metabolism, the immune system, and cell structure. This study provides a foundation for exploring the contribution of the host response to symbiont-mediated protection from pathogen infection.

BMP signaling to pharyngeal muscle in the C. elegans response to a bacterial pathogen regulates anti-microbial peptide expression and pharyngeal pumping

Host response to pathogens recruits multiple tissues in part through conserved cell signaling pathways. In Caenorhabditis elegans, the bone morphogenetic protein (BMP) like DBL-1 signaling pathway has a role in the response to infection in addition to other roles in development and postdevelopmental functions. In the regulation of body size, the DBL-1 pathway acts through cell autonomous signal activation in the epidermis (hypodermis). We have now elucidated the tissues that respond to DBL-1 signaling upon exposure to two bacterial pathogens. The receptors and Smad signal transducers for DBL-1 are expressed in pharyngeal muscle, intestine, and epidermis. We demonstrate that expression of receptor-regulated Smad (R-Smad) gene sma-3 in the pharynx is sufficient to improve the impaired survival phenotype of sma-3 mutants and that expression of sma-3 in the intestine has no effect when exposing worms to bacterial infection of the intestine. We also show that two antimicrobial peptide genes - abf-2 and cnc-2 - are regulated by DBL-1 signaling through R-Smad SMA-3 activity in the pharynx. Finally, we show that pharyngeal pumping activity is reduced in sma-3 mutants and that other pharynx-defective mutants also have reduced survival on a bacterial pathogen. Our results identify the pharynx as a tissue that responds to BMP signaling to coordinate a systemic response to bacterial pathogens.

Antibacterial activity of cinnamon essential oil and its main component of cinnamaldehyde and the underlying mechanism

Background: Plant essential oils have long been regarded as repositories of antimicrobial agents. In recent years, they have emerged as potential alternatives or supplements to antimicrobial drugs. Although literature reviews and previous studies have indicated that cinnamon essential oil (CIEO) and its major component, cinnamaldehyde (CID), possess potent antibacterial activities, their antibacterial mechanisms, especially the in vivo antibacterial mechanisms, remain elusive. Methods: In this study, we utilized the in vivo assessment system of Caenorhabditis elegans (C. elegans) to investigate the effects and mechanisms of high dose (100 mg/L) and low dose (10 mg/L) CIEO and CID in inhibiting Pseudomonas aeruginosa (P. aeruginosa). In addition, we also examined the in vitro antibacterial abilities of CIEO and CID against other common pathogens including P. aeruginosa and 4 other strains. Results: Our research revealed that both high (100 mg/L) and low doses (10 mg/L) of CIEO and CID treatment significantly alleviated the reduction in locomotion behavior, lifespan, and accumulation of P. aeruginosa in C. elegans infected with the bacteria. During P. aeruginosa infection, the transcriptional expression of antimicrobial peptide-related genes (lys-1 and lys-8) in C. elegans was upregulated with low-dose CIEO and CID treatment, while this trend was suppressed at high doses. Further investigation suggested that the PMK-1 mediated p38 signaling pathway may be involved in the regulation of CIEO and CID during nematode defense against P. aeruginosa infection. Furthermore, in vitro experimental results also revealed that CIEO and CID exhibit good antibacterial effects, which may be associated with their antioxidant properties. Conclusion: Our results indicated that low-dose CIEO and CID treatment could activate the p38 signaling pathway in C. elegans, thereby regulating antimicrobial peptides, and achieving antimicrobial effects. Meanwhile, high doses of CIEO and CID might directly participate in the internal antimicrobial processes of C. elegans. Our study provides research basis for the antibacterial properties of CIEO and CID both in vivo and in vitro.

Pathogen-induced dormancy in liquid limits gastrointestinal colonization of Caenorhabditis elegans

Colonization is generally considered a prerequisite for infection, but this event is context-dependent, as evidenced by the differing ability of the human pathogen Pseudomonas aeruginosa to efficiently colonize Caenorhabditis elegans on agar but not in liquid . In this study, we examined the impact of the environment, pathogen, host, and their interactions on host colonization. We found that the transition to a liquid environment reduces food uptake by about two-fold. Also expression of specific adhesins was significantly altered in liquid-based assays for P. aeruginosa, suggesting that it may be one factor driving diminished colonization. Unexpectedly, host immune pathways did not appear to play a significant role in decreased colonization in liquid. Although knocking down key immune pathways (e.g. daf-16 or zip-2), either alone or in combination, significantly reduced survival, the changes in colonization were very small. In spite of the limited bacterial accumulation in the liquid setting, pathogenic colonization was still required for the virulence of Enterococcus faecalis. In addition, we found that a pathogen-induced dormancy was displayed by C. elegans in liquid medium after pathogen exposure, resulting in cessation of pharyngeal pumping and a decrease in bacterial intake. We conclude that poor colonization in liquid is likely due to a combination of environmental factors and host-pathogen interactions. These results provide new insights into mechanisms for colonization in different models, enabling pathogenesis models to be fine-tuned to more accurately represent the conditions seen in human infections so that new tools for curbing bacterial and fungal infections can be developed.

The DBL-1/TGF-β signaling pathway tailors behavioral and molecular host responses to a variety of bacteria in Caenorhabditis elegans

Generating specific, robust protective responses to different bacteria is vital for animal survival. Here, we address the role of transforming growth factor β (TGF-β) member DBL-1 in regulating signature host defense responses in Caenorhabditis elegans to human opportunistic Gram-negative and Gram-positive pathogens. Canonical DBL-1 signaling is required to suppress avoidance behavior in response to Gram-negative, but not Gram-positive bacteria. We propose that in the absence of DBL-1, animals perceive some bacteria as more harmful. Animals activate DBL-1 pathway activity in response to Gram-negative bacteria and strongly repress it in response to select Gram-positive bacteria, demonstrating bacteria-responsive regulation of DBL-1 signaling. DBL-1 signaling differentially regulates expression of target innate immunity genes depending on the bacterial exposure. These findings highlight a central role for TGF-β in tailoring a suite of bacteria-specific host defenses.

TGF-β pathways in aging and immunity: lessons from Caenorhabditis elegans

The Transforming Growth Factor-β (TGF-β) superfamily of signaling molecules plays critical roles in development, differentiation, homeostasis, and disease. Due to the conservation of these ligands and their signaling pathways, genetic studies in invertebrate systems including the nematode Caenorhabditis elegans have been instrumental in identifying signaling mechanisms. C. elegans is also a premier organism for research in longevity and healthy aging. Here we summarize current knowledge on the roles of TGF-β signaling in aging and immunity.

A novel N6-Deoxyadenine methyltransferase METL-9 modulates C. elegans immunity via dichotomous mechanisms

N6-Methyldeoxyadenine (6mA) has been rediscovered as a DNA modification with potential biological function in metazoans. However, the physiological function and regulatory mechanisms regarding the establishment, maintenance and removal of 6mA in eukaryotes are still poorly understood. Here we show that genomic 6mA levels change in response to pathogenic infection in Caenorhabditis elegans (C. elegans). We further identify METL-9 as the methyltransferase that catalyzes DNA 6mA modifications upon pathogen infection. Deficiency of METL-9 impairs the induction of innate immune response genes and renders the animals more susceptible to pathogen infection. Interestingly, METL-9 functions through both 6mA-dependent and -independent mechanisms to transcriptionally regulate innate immunity. Our findings reveal that 6mA is a functional DNA modification in immunomodulation in C. elegans.

GABAergic signaling between enteric neurons and intestinal smooth muscle promotes innate immunity and gut defense in Caenorhabditis elegans

The nervous system is critical for intestinal homeostasis and function, but questions remain regarding its impact on gut immune defense. By screening the major neurotransmitters of C. elegans, we found that γ-aminobutyric acid (GABA) deficiency enhanced susceptibility to pathogenic Pseudomonas aeruginosa PA14 infection. GABAergic signaling between enteric neurons and intestinal smooth muscle promoted gut defense in a PMK-1/p38-dependent, but IIS/DAF-16- and DBL-1/TGF-β-independent, pathway. Transcriptomic profiling revealed that the neuropeptide, FLP-6, acted downstream of enteric GABAergic signaling. Further data determined that FLP-6 was expressed and secreted by intestinal smooth muscle cells and functioned as a paracrine molecule on the intestinal epithelium. FLP-6 suppressed the transcription factors ZIP-10 and KLF-1 that worked in parallel and converged to the PMK-1/p38 pathway in the intestinal epithelia for innate immunity and gut defense. Collectively, these findings uncover an enteric neuron-muscle-epithelium axis that may be evolutionarily conserved in higher organisms.

Paeoniflorin increases the survival of Pseudomonas aeruginosa infected Caenorhabditis elegans at the immunosuppression stage by activating PMK-1, BAR-1, and EGL-1 signals

Paeoniflorin is the major active compound of total glycoside of paeony in Paeonia lactiflora Pall. Although several aspects of beneficial effects of paeoniflorin have been described, whether the paeoniflorin treatment is helpful for inhibiting the pathogen infection-induced immunosuppression remains largely unclear. Using the immunosuppression model in Caenorhabditis elegans induced by Pseudomonas aeruginosa infection, we here examined the beneficial effect of paeoniflorin treatment against the immunosuppression induced by bacterial pathogen infection. In this immunosuppression model, we observed that the survival rate of P. aeruginosa infected nematodes at the immunosuppression stage could be significantly increased by 25-100 mg/L paeoniflorin treatment. P. aeruginosa accumulation in intestinal lumen of nematodes at the immunosuppression stage was reduced by paeoniflorin treatment. Paeoniflorin could activate the expressions of antimicrobial genes (lys-1 and lys-8) in nematodes at the immunosuppression stage. Moreover, at the immunosuppression stage, paeoniflorin treatment increased the expressions of bar-1, pmk-1, and egl-1 required for the control of innate immunity against bacterial infection. Meanwhile, RNAi of bar-1, pmk-1, and egl-1 inhibited the beneficial effect of paeoniflorin treatment in increasing the survival, reducing the P. aeruginosa accumulation in intestinal lumen, and activating the expressions of antimicrobial genes (lys-1 and lys-8) in nematodes at the immunosuppression stage. Therefore, paeoniflorin treatment could effectively inhibit the immunosuppression induced by bacterial pathogen infection in the hosts.

The nematode worm Caenorhabditis elegans as an animal experiment replacement for assessing the virulence of different Salmonella enterica strains

Caenorhabditis (C.) elegans has become a popular toxicological and biological test organism in the last two decades. Furthermore, the role of C. elegans as an alternative for replacing or reducing animal experiments is continuously discussed and investigated. In the current study, we investigated whether C. elegans survival assays can help in determining differences in the virulence of Salmonella enterica strains and to what extent C. elegans assays could replace animal experiments for this purpose. We focused on three currently discussed examples where we compared the longevity of C. elegans when fed (i) with S. enterica serovar Enteritidis vaccination or wild-type strains, (ii) with lipopolysaccharide (LPS) deficient rough or LPS forming smooth S. enterica serovar Enteritidis, and (iii) with an S. enterica subsp. diarizonae strain in the presence or absence of the typical pSASd plasmid encoding a bundle of putative virulence factors. We found that the C. elegans survival assay could indicate differences in the longevity of C. elegans when fed with the compared strain pairs to a certain extent. Putatively higher virulent S. enterica strains reduced the lifespan of C. elegans to a greater extent than putatively less virulent strains. The C. elegans survival assay is an effective and relatively easy method for classifying the virulence of different bacterial isolates in vivo, but it has some limitations. The assay cannot replace animal experiments designed to determine differences in the virulence of Salmonella enterica strains. Instead, we recommend using the described method for pre-screening bacterial strains of interest to select the most promising candidates for further animal experiments. The C. elegans assay possesses the potential to reduce the number of animal experiments. Further development of the C. elegans assay in conjunction with omics technologies, such as transcriptomics, could refine results relating to the estimation of the virulent potential of test organisms.

Transcriptome Analysis Reveals the Defense Mechanism of Cotton against Verticillium dahliae Induced by Hypovirulent Fungus Gibellulopsis nigrescens CEF08111

Verticillium wilt is a kind of plant vascular disease caused by the soilborne fungus Verticillium dahliae, which severely limits cotton production. Our previous studies showed that the endophytic fungus Gibellulopsis nigrescens CEF08111 can effectively control Verticillium wilt and induce a defense response in cotton plants. However, the comprehensive molecular mechanism governing this response is not yet clear. To study the signaling mechanism induced by strain CEF08111, the transcriptome of cotton seedlings pretreated with CEF08111 was sequenced. The results revealed 249, 3559 and 33 differentially expressed genes (DEGs) at 3, 12 and 48 h post inoculation with CEF08111, respectively. At 12 h post inoculation with CEF08111, Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that the DEGs were enriched mainly in the plant−pathogen interaction, mitogen-activated protein kinase (MAPK) signaling pathway-plant, and plant hormone signal transduction pathways. Gene ontology (GO) analysis revealed that these DEGs were enriched mainly in the following terms: response to external stimulus, systemic acquired resistance, kinase activity, phosphotransferase activity, xyloglucan: xyloglucosyl transferase activity, xyloglucan metabolic process, cell wall polysaccharide metabolic process and hemicellulose metabolic process. Moreover, many genes, such as calcium-dependent protein kinase (CDPK), flagellin-sensing 2 (FLS2), resistance to Pseudomonas syringae pv. maculicola 1(RPM1) and myelocytomatosis protein 2 (MYC2), that regulate crucial points in defense-related pathways were identified and may contribute to V. dahliae resistance in cotton. Seven DEGs of the pathway phenylpropanoid biosynthesis were identified by weighted gene co-expression network analysis (WGCNA), and these genes are related to lignin synthesis. The above genes were compared and analyzed, a total of 710 candidate genes that may be related to the resistance of cotton to Verticillium wilt were identified. These results provide a basis for understanding the molecular mechanism by which the biocontrol fungus CEF08111 increases the resistance of cotton to Verticillium wilt.

TENT5 cytoplasmic noncanonical poly(A) polymerases regulate the innate immune response in animals

Innate immunity is the first line of host defense against pathogens. Here, through global transcriptome and proteome analyses, we uncover that newly described cytoplasmic poly(A) polymerase TENT-5 (terminal nucleotidyltransferase 5) enhances the expression of secreted innate immunity effector proteins in Caenorhabditis elegans. Direct RNA sequencing revealed that multiple mRNAs with signal peptide-encoding sequences have shorter poly(A) tails in tent-5-deficient worms. Those mRNAs are translated at the endoplasmic reticulum where a fraction of TENT-5 is present, implying that they represent its direct substrates. Loss of tent-5 makes worms more susceptible to bacterial infection. Notably, the role of TENT-5 in innate immunity is evolutionarily conserved. Its orthologs, TENT5A and TENT5C, are expressed in macrophages and induced during their activation. Analysis of macrophages devoid of TENT5A/C revealed their role in the regulation of secreted proteins involved in defense response. In summary, our study reveals cytoplasmic polyadenylation to be a previously unknown component of the posttranscriptional regulation of innate immunity in animals.

Making "Sense" of Ecology from a Genetic Perspective: Caenorhabditis elegans, Microbes and Behavior

Our knowledge of animal and behavior in the natural ecology is based on over a century's worth of valuable field studies. In this post-genome era, however, we recognize that genes are the underpinning of ecological interactions between two organisms. Understanding how genes contribute to animal ecology, which is essentially the intersection of two genomes, is a tremendous challenge. The bacterivorous nematode Caenorhabditis elegans, one of the most well-known genetic animal model experimental systems, experiences a complex microbial world in its natural habitat, providing us with a window into the interplay of genes and molecules that result in an animal-microbial ecology. In this review, we will discuss C. elegans natural ecology, how the worm uses its sensory system to detect the microbes and metabolites that it encounters, and then discuss some of the fascinating ecological dances, including behaviors, that have evolved between the nematode and the microbes in its environment.

Long-Term Dominance of Carbapenem-Non-Susceptible Pseudomonas aeruginosa ST111 in Hematologic Malignancy Patients and Hematopoietic Cell Transplant Recipients

An epidemiological study uncovered that fluoroquinolone (FQ) neutropenic prophylaxis in hematopoietic cell transplant and hematologic malignancy (HCT/HM) patients was associated with breakthrough Pseudomonas aeruginosa bloodstream infections (BSIs) with isolates non-susceptible to both FQs and meropenem. The molecular epidemiology of the FQ/meropenem-non-susceptible P. aeruginosa isolates causing FQ-breakthrough BSIs in the HCT/HM patients remains unclear. Through whole genome sequencing on 57 P. aeruginosa isolates from 54 patients diagnosed with HM or receiving an HCT, we found that ST111 strains predominated, accounting for 22 (38.6%) of the isolates. 17 of 33 (51.5%) FQ-breakthrough BSIs were caused by ST111 strains, of which 15 (88.2%) were meropenem non-susceptible. ST111 strains, but not other oprD-deficient, meropenem-non-susceptible clinical strains, were found to have a colonization advantage over P. aeruginosa strain PA14 in C. elegans and to outcompete PA14 in in vitro co-culture assays. Together, we found that breakthrough P. aeruginosa BSIs during FQ prophylaxis in HCT/HM patients are dominated by clonally-related FQ/meropenem non-susceptible strains, predominantly ST111 type, and that the dominance of ST111 strains may be explained by a relative fitness advantage over other clinical strains. Additional work is necessary to better understand the factors driving the dominance and persistence of these ST111 strains.

Genetic analysis of Caenorhabditis elegans pry-1/Axin suppressors identifies genes involved in reproductive structure development, stress responses, and aging

The Axin family of scaffolding proteins regulates a wide array of developmental and post-developmental processes in eukaryotes. Studies in the nematode Caenorhabditis elegans have shown that the Axin homolog PRY-1 plays essential roles in multiple tissues. To understand the genetic network of pry-1, we focused on a set of genes that are differentially expressed in the pry-1-mutant transcriptome and are linked to reproductive structure development. Knocking down eight of the genes (spp-1, clsp-1, ard-1, rpn-7, cpz-1, his-7, cdk-1, and rnr-1) via RNA interference efficiently suppressed the multivulva phenotype of pry-1 mutants. In all cases, the ectopic induction of P3.p vulval precursor cell was also inhibited. The suppressor genes are members of known gene families in eukaryotes and perform essential functions. Our genetic interaction experiments revealed that in addition to their role in vulval development, these genes participate in one or more pry-1-mediated biological events. Whereas four of them (cpz-1, his-7, cdk-1, and rnr-1) function in both stress response and aging, two (spp-1 and ard-1) are specific to stress response. Altogether, these findings demonstrate the important role of pry-1 suppressors in regulating developmental and post-developmental processes in C. elegans. Given that the genes described in this study are conserved, future investigations of their interactions with Axin and their functional specificity promises to uncover the genetic network of Axin in metazoans.

Why and how do protective symbionts impact immune priming with pathogens in invertebrates?

Growing evidence demonstrates that invertebrates display adaptive-like immune abilities, commonly known as "immune priming". Immune priming is a process by which a host improves its immune defences following an initial pathogenic exposure, leading to better protection after a subsequent infection with the same - or different - pathogens. Nevertheless, beneficial symbionts can enhance similar immune priming processes in hosts, such as when they face repeated infections with pathogens. This "symbiotic immune priming" protects the host against pathogenic viruses, bacteria, fungi, or eukaryotic parasites. In this review, we explore the extent to which protective symbionts interfere and impact immune priming against pathogens from both a mechanical (proximal) and an evolutionary (ultimate) point of view. We highlight that the immune priming of invertebrates is the cornerstone of the tripartite interaction of hosts/symbionts/pathogens. The main shared mechanism of immune priming (induced by symbionts or pathogens) is the sustained immune response at the beginning of host-microbial interactions. However, the evolutionary outcome of immune priming leads to a specific discrimination, which provides enhanced tolerance or resistance depending on the type of microbe. Based on several studies testing immune priming against pathogens in the presence or absence of protective symbionts, we observed that both types of immune priming could overlap and affect each other inside the same hosts. As protective symbionts could be an evolutionary force that influences immune priming, they may help us to better understand the heterogeneity of pathogenic immune priming across invertebrate populations and species.

The bZIP Transcription Factor ZIP-11 Is Required for the Innate Immune Regulation in Caenorhabditis elegans

Innate immunity is the first line of host defense against pathogen infection in metazoans. However, the molecular mechanisms of the complex immune regulatory network are not fully understood. Based on a transcriptome profiling of the nematode Caenorhabditis elegans, we found that a bZIP transcription factor ZIP-11 was up-regulated upon Pseudomonas aeruginosa PA14 infection. The tissue specific RNAi knock-down and rescue data revealed that ZIP-11 acts in intestine to promote host resistance against P. aeruginosa PA14 infection. We further showed that intestinal ZIP-11 regulates innate immune response through constituting a feedback loop with the conserved PMK-1/p38 mitogen-activated protein signaling pathway. Intriguingly, ZIP-11 interacts with a CCAAT/enhancer-binding protein, CEBP-2, to mediate the transcriptional response to P. aeruginosa PA14 infection independently of PMK-1/p38 pathway. In addition, human homolog ATF4 can functionally substitute for ZIP-11 in innate immune regulation of C. elegans. Our findings indicate that the ZIP-11/ATF4 genetic program activates local innate immune response through conserved PMK-1/p38 and CEBP-2/C/EBPγ immune signals in C. elegans, raising the possibility that a similar process may occur in other organisms.

Biosafety assessment of Acinetobacter strains isolated from the Three Gorges Reservoir region in nematode Caenorhabditis elegans

Acinetobacter has been frequently detected in backwater areas of the Three Gorges Reservoir (TGR) region. We here employed Caenorhabditis elegans to perform biosafety assessment of Acinetobacter strains isolated from backwater area in the TGR region. Among 21 isolates and 5 reference strains of Acinetobacter, exposure to Acinetobacter strains of AC1, AC15, AC18, AC21, A. baumannii ATCC 19606T, A. junii NH88-14, and A. lwoffii DSM 2403T resulted in significant decrease in locomotion behavior and reduction in lifespan of Caenorhabditis elegans. In nematodes, exposure to Acinetobacter strains of AC1, AC15, AC18, AC21, A. baumannii, A. junii and A. lwoffii also resulted in significant reactive oxygen species (ROS) production. Moreover, exposure to Acinetobacter isolates of AC1, AC15, AC18, and AC21 led to significant increase in expressions of both SOD-3::GFP and some antimicrobial genes (lys-1, spp-12, lys-7, dod-6, spp-1, dod-22, lys-8, and/or F55G11.4) in nematodes. The Acinetobacter isolates of AC1, AC15, AC18, and AC21 had different morphological, biochemical, phylogenetical, and virulence gene properties. Our results suggested that exposure risk of some Acinetobacter strains isolated from the TGR region exists for environmental organisms and human health. In addition, C. elegans is useful to assess biosafety of Acinetobacter isolates from the environment.

Take a Walk to the Wild Side of Caenorhabditis elegans-Pathogen Interactions

Microbiomes form intimate functional associations with their hosts. Much has been learned from correlating changes in microbiome composition to host organismal functions. However, in-depth functional studies require the manipulation of microbiome composition coupled with the precise interrogation of organismal physiology-features available in few host study systems. Caenorhabditis elegans has proven to be an excellent genetic model organism to study innate immunity and, more recently, microbiome interactions. The study of C. elegans-pathogen interactions has provided in depth understanding of innate immune pathways, many of which are conserved in other animals. However, many bacteria were chosen for these studies because of their convenience in the lab setting or their implication in human health rather than their native interactions with C. elegans In their natural environment, C. elegans feed on a variety of bacteria found in rotting organic matter, such as rotting fruits, flowers, and stems. Recent work has begun to characterize the native microbiome and has identified a common set of bacteria found in the microbiome of C. elegans While some of these bacteria are beneficial to C. elegans health, others are detrimental, leading to a complex, multifaceted understanding of bacterium-nematode interactions. Current research on nematode-bacterium interactions is focused on these native microbiome components, both their interactions with each other and with C. elegans We will summarize our knowledge of bacterial pathogen-host interactions in C. elegans, as well as recent work on the native microbiome, and explore the incorporation of these bacterium-nematode interactions into studies of innate immunity and pathogenesis.

Neuro-immune communication in C. elegans defense against pathogen infection

The innate immune system is a complex collection of physical barriers and physiological defense responses to internal and external environmental assaults. Recent studies in the model organism Caenorhabditis elegans have highlighted how the nervous system interacts with the innate immune system to generate coordinated protective responses. Indeed, studies on neuro-immune interaction pathways have provided mechanistic insights into the roles of neuro-immune communication in modulating both immune and behavioral responses to pathogen attacks. The nervous system releases a variety of neurotransmitters, peptides, and hormones that regulate the innate immune response, while the innate immune system also relays information to the nervous system to affect learning and behavioral responses. Although these interactions still need further investigation, the knowledge that we have gained thus far has improved our understanding of how separate biological systems can act collectively for the survival and well-being of an organism. Here, we review recent studies on neuro-immune communication related to the survival and defense of C. elegans against pathogens.

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Neuro-immune communication is essential for generating coordinated defense against pathogen infection to improve host survival.

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Neuro-immune communication modulates both immune and behavioral responses to pathogen attacks.

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C.elegans is an excellent model system for studying neuro-immune communication.

GABAergic synapses suppress intestinal innate immunity via insulin signaling in Caenorhabditis elegans

GABAergic neurotransmission constitutes a major inhibitory signaling mechanism that plays crucial roles in central nervous system physiology and immune cell immunomodulation. However, its roles in innate immunity remain unclear. Here, we report that deficiency in the GABAergic neuromuscular junctions (NMJs) of Caenorhabditis elegans results in enhanced resistance to pathogens, whereas pathogen infection enhances the strength of GABAergic transmission. GABAergic synapses control innate immunity in a manner dependent on the FOXO/DAF-16 but not the p38/PMK-1 pathway. Our data reveal that the insulin-like peptide INS-31 level was dramatically decreased in the GABAergic NMJ GABA_AR-deficient unc-49 mutant compared with wild-type animals. C. elegans with ins-31 knockdown or loss of function exhibited enhanced resistance to Pseudomonas aeruginosa PA14 exposure. INS-31 may act downstream of GABAergic NMJs and in body wall muscle to control intestinal innate immunity in a cell-nonautonomous manner. Our results reveal a signaling axis of synapse-muscular insulin-intestinal innate immunity in vivo.

Ubiquitin-related processes and innate immunity in C. elegans

Innate immunity is an evolutionary ancient defence strategy that serves to eliminate infectious agents while maintaining host health. It involves a complex network of sensors, signaling proteins and immune effectors that detect the danger, then relay and execute the immune programme. Post-translational modifications relying on conserved ubiquitin and ubiquitin-like proteins are an integral part of the system. Studies using invertebrate models of infection, such as the nematode Caenorhabditis elegans, have greatly contributed to our understanding of how ubiquitin-related processes act in immune sensing, regulate immune signaling pathways, and participate to host defence responses. This review highlights the interest of working with a genetically tractable model organism and illustrates how C. elegans has been used to identify ubiquitin-dependent immune mechanisms, discover novel ubiquitin-based resistance strategies that mediate pathogen clearance, and unravel the role of ubiquitin-related processes in tolerance, preserving host fitness during pathogen attack. Special emphasis is placed on processes that are conserved in mammals.

Host Immunity Alters Community Ecology and Stability of the Microbiome in a Caenorhabditis elegans Model

A growing body of data suggests that the microbiome of a species can vary considerably from individual to individual, but the reasons for this variation—and the consequences for the ecology of these communities—remain only partially explained. In mammals, the emerging picture is that the metabolic state and immune system status of the host affect the composition of the microbiome, but quantitative ecological microbiome studies are challenging to perform in higher organisms. Here, we show that these phenomena can be quantitatively analyzed in the tractable nematode host Caenorhabditis elegans. Mutants in innate immunity, in particular the DAF-2/insulin growth factor (IGF) pathway, are shown to contain a microbiome that differs from that of wild-type nematodes. We analyzed the underlying basis of these differences from the perspective of community ecology by comparing experimental observations to the predictions of a neutral sampling model and concluded that fundamental differences in microbiome ecology underlie the observed differences in microbiome composition. We tested this hypothesis by introducing a minor perturbation into the colonization conditions, allowing us to assess stability of communities in different host strains. Our results show that altering host immunity changes the importance of interspecies interactions within the microbiome, resulting in differences in community composition and stability that emerge from these differences in host-microbe ecology.

IMPORTANCE Here, we used a Caenorhabditis elegans microbiome model to demonstrate how genetic differences in innate immunity alter microbiome composition, diversity, and stability by changing the ecological processes that shape these communities. These results provide insight into the role of host genetics in controlling the ecology of the host-associated microbiota, resulting in differences in community composition, successional trajectories, and response to perturbation.

Effector and regulator: Diverse functions of C. elegans C-type lectin-like domain proteins

In C. elegans, 283 clec genes encode a highly diverse family of C-type lectin-like domain (CTLD) proteins. Since vertebrate CTLD proteins have characterized functions in defense responses against pathogens and since expression of C. elegans clec genes is pathogen-dependent, it is generally assumed that clec genes function in C. elegans immune defenses. However, little is known about the relative contribution and exact function of CLEC proteins in C. elegans immunity. Here, we focused on the C. elegans clec gene clec-4, whose expression is highly upregulated by pathogen infection, and its paralogs clec-41 and clec-42. We found that, while mutation of clec-4 resulted in enhanced resistance to the Gram-positive pathogen Bacillus thuringiensis MYBt18247 (Bt247), inactivation of clec-41 and clec-42 by RNAi enhanced susceptibility to Bt247. Further analyses revealed that enhanced resistance of clec-4 mutants to Bt247 was due to an increase in feeding cessation on the pathogen and consequently a decrease in pathogen load. Moreover, clec-4 mutants exhibited feeding deficits also on non-pathogenic bacteria that were in part reflected in the clec-4 gene expression profile, which overlapped with gene sets affected by starvation or mutation in nutrient sensing pathways. However, loss of CLEC-4 function only mildly affected life-history traits such as fertility, indicating that clec-4 mutants are not subjected to dietary restriction. While CLEC-4 function appears to be associated with the regulation of feeding behavior, we show that CLEC-41 and CLEC-42 proteins likely function as bona fide immune effector proteins that have bacterial binding and antimicrobial capacities. Together, our results exemplify functional diversification within clec gene paralogs.

C-type lectin-like domain (CTLD) containing proteins fulfill various and fundamental tasks in the human and mouse immune system. Genes encoding CTLD proteins are present in all animal genomes, in some cases in very large numbers and highly diversified. While the function of several vertebrate CTLD proteins is well characterized, experimental evidence of an immune function of most invertebrate CTLD proteins is missing, although their role in immunity is usually assumed. We here explore the immune function of three related CTLD proteins in the model nematode Caenorhabditis elegans. We find that they play diverse roles in C. elegans immunity, functioning as antimicrobial immune effector proteins that are important for defense against pathogen infection and probably directly interact with bacteria, but also regulators of feeding behavior that more indirectly affect C. elegans pathogen resistance. Such insight into the functional consequence of invertebrate CTLD protein diversification contributes to our understanding of the evolution of innate and invertebrate immune systems.

Lactobacillus Regulates Caenorhabditis elegans Cell Signaling to Combat Salmonella Infection

Salmonella typhimurium DT104 infection causes the death of Caenorhabditis elegans, which can be prevented by certain Lactobacillus isolates. However, the molecular mechanisms of both the host response to the infection and the protection by Lactobacillus are largely unclear. The present study has investigated the life-span and gene expression of both wild-type (WT) and mutants in some key components of cell signaling in response to S. typhimurium infection and protection from Lactobacillus zeae. The results indicated that the gene expression of daf-16 in the DAF/ insulin-like growth factor (DAF/IGF) pathway, ced-3 and ced-9 in the programmed cell death (PCD) pathway, lys-7, spp-1, and abf-3 for antimicrobial peptide production, and bar-1 involved in the production of other defense molecules was all significantly upregulated when the wild-type (WT) was subjected to DT104 infection. On the contrary, the gene expression of tir-1, sek-1, and pmk-1 in the p38 mitogen-activated protein kinase (MAPK) pathway and clec-60, sod-3, and skn-1 for the production of other defense molecules was significantly suppressed by DT104. Pretreatment of the worms with L. zeae LB1 significantly upregulated the expression of almost all the tested genes except for ced-3, ced-9, abf-2, age-1, and dbl-1 compared with the nematode infected with DT104 only. Mutants defective in the cell signaling or other defense molecules of C. elegans were either more susceptible (defective in nsy-1, sek-1, pmk-1, ced-3, ced-9, skn-1, or daf-16) or more resistant (defective in age-1 or dbl-1) to DT104 infection than the WT except for the mutant defective in sod-3. Mutants defective in antimicrobial peptides (lys-7 or abf-3) were also more susceptible than the WT. In contrast, the mutant defective in spp-1 became more resistant. When all the mutants were pretreated with L. zeae LB1, five mutants that are defective in nsy-1, sek-1, pmk-1, abf-3, or lys-7 showed no response to the protection from LB1. These results suggest that L. zeae LB1 can regulate C. elegans cell signaling including the p38 MAPK pathway and downstream production of antimicrobial peptides and defense molecules to combat Salmonella infection.

In Vitro and In Vivo Effect of Peptides Derived from 14-3-3 Paracoccidioides spp. Protein

Background: Paracoccidioidomycosis (PCM) is a chronic disease that causes sequelae and requires prolonged treatment; therefore, new therapeutic approaches are necessary. In view of this, three peptides from Paracoccidioides brasiliensis 14-3-3 protein were selected based on its immunogenicity and therapeutic potential. Methods: The in vitro antifungal activity and cytotoxicity of the 14-3-3 peptides were evaluated. The influence of the peptides in immunological and survival aspects was evaluated in vivo, using Galleria mellonella and the expression of antimicrobial peptide genes in Caenorhabditis elegans. Results: None of the peptides were toxic to HaCaT (skin keratinocyte), MRC-5 (lung fibroblast), and A549 (pneumocyte) cell lines, and only P1 exhibited antifungal activity against Paracoccidioides spp. The peptides could induce an immune response in G. mellonella. Moreover, the peptides caused a delay in the death of Paracoccidioides spp. infected larvae. Regarding C. elegans, the three peptides were able to increase the expression of the antimicrobial peptides. These peptides had essential effects on different aspects of Paracoccidioides spp. infection showing potential for a therapeutic vaccine. Future studies using mammalian methods are necessary to validate our findings.

IGLR-2, a Leucine-Rich Repeat Domain Containing Protein, Is Required for the Host Defense in Caenorhabditis elegans

Enterohemorrhagic Escherichia coli (EHEC), a human pathogen, also infects Caenorhabditis elegans. We demonstrated previously that C. elegans activates the p38 MAPK innate immune pathway to defend against EHEC infection. However, whether a C. elegans pattern recognition receptor (PRR) exists to regulate the immune pathway remains unknown. PRRs identified in other metazoans contain several conserved domains, including the leucine-rich repeat (LRR). By screening a focused RNAi library, we identified the IGLR-2, a transmembrane protein containing the LRR domain, as a potential immune regulator in C. elegans. Our data showed that iglr-2 regulates the host susceptibility to EHEC infection. Moreover, iglr-2 is required for pathogen avoidance to EHEC. The iglr-2 overexpressed strain, which was more resistant to EHEC originally, showed hypersusceptibility to EHEC upon knockdown of the p38 MAPK pathway. Together, our data suggested that iglr-2 plays an important role in C. elegans to defend EHEC by regulating pathogen-avoidance behavior and the p38 MAPK pathway.

The PP2A/4/6 subfamily of phosphoprotein phosphatases regulates DAF-16 and confers resistance to environmental stress in postreproductive adult C. elegans

Insulin and insulin-like growth factors are longevity determinants that negatively regulate Forkhead box class O (FoxO) transcription factors. In C. elegans mutations that constitutively activate DAF-16, the ortholog of mammalian FoxO3a, extend lifespan by two-fold. While environmental insults induce DAF-16 activity in younger animals, it also becomes activated in an age-dependent manner in the absence of stress, modulating gene expression well into late adulthood. The mechanism by which DAF-16 activity is regulated during aging has not been defined. Since phosphorylation of DAF-16 generally leads to its inhibition, we asked whether phosphatases might be necessary for its increased transcriptional activity in adult C. elegans. We focused on the PP2A/4/6 subfamily of phosphoprotein phosphatases, members of which had been implicated to regulate DAF-16 under low insulin signaling conditions but had not been investigated during aging in wildtype animals. Using reverse genetics, we functionally characterized all C. elegans orthologs of human catalytic, regulatory, and scaffolding subunits of PP2A/4/6 holoenzymes in postreproductive adults. We found that PP2A complex constituents PAA-1 and PPTR-1 regulate DAF-16 transcriptional activity during aging and that they cooperate with the catalytic subunit LET-92 to protect adult animals from ultraviolet radiation. PP4 complex members PPH-4.1/4.2, and SMK-1 also appear to regulate DAF-16 in an age-dependent manner, and together with PPFR-2 they contribute to innate immunity. Interestingly, SUR-6 but no other subunit of the PP2A complex was necessary for the survival of pathogen-infected animals. Finally, we found that PP6 complex constituents PPH-6 and SAPS-1 contribute to host defense during aging, apparently without affecting DAF-16 transcriptional activity. Our studies indicate that a set of PP2A/4/6 complexes protect adult C. elegans from environmental stress, thus preserving healthspan. Therefore, along with their functions in cell division and development, the PP2A/4/6 phosphatases also appear to play critical roles later in life.

Caenorhabditis elegans saposin-like spp-9 is involved in specific innate immune responses

Animals counter specific environmental challenges with a combination of broad and tailored host responses. One protein family enlisted in the innate immune response includes the saposin-like antimicrobial proteins. We investigated the expression of a Caenorhabditis elegans saposin-like gene, spp-9, in response to different stresses. spp-9 expression was detected in the intestine and six amphid neurons, including AWB and AWC. spp-9 expression is increased in response to starvation stress. In addition, we discovered pathogen-specific regulation of spp-9 that was not clearly demarcated by Gram nature of the bacterial challenge. Multiple molecular innate immune response pathways, including DBL-1/TGF-β-like, insulin-like, and p38/MAPK, regulate expression of spp-9. Our results suggest spp-9 is involved in targeted responses to a variety of abiotic and bacterial challenges that are coordinated by multiple signaling pathways.

Identification and characterization of differentially expressed genes in Caenorhabditis elegans in response to pathogenic and nonpathogenic Stenotrophomonas maltophilia

Background

Stenotrophomonas maltophilia is an emerging nosocomial pathogen that causes infection in immunocompromised patients. S. maltophilia isolates are genetically diverse, contain diverse virulence factors, and are variably pathogenic within several host species. Members of the Stenotrophomonas genus are part of the native microbiome of C. elegans, being found in greater relative abundance within the worm than its environment, suggesting that these bacteria accumulate within C. elegans. Thus, study of the C. elegans-Stenotrophomonas interaction is of both medical and ecological significance. To identify host defense mechanisms, we analyzed the C. elegans transcriptomic response to S. maltophilia strains of varying pathogenicity: K279a, an avirulent clinical isolate, JCMS, a virulent strain isolated in association with soil nematodes near Manhattan, KS, and JV3, an even more virulent environmental isolate.

Results

Overall, we found 145 genes that are commonly differentially expressed in response to pathogenic S. maltophilia strains, 89% of which are upregulated, with many even further upregulated in response to JV3 as compared to JCMS. There are many more JV3-specific differentially expressed genes (225, 11% upregulated) than JCMS-specific differentially expressed genes (14, 86% upregulated), suggesting JV3 has unique pathogenic mechanisms that could explain its increased virulence. We used connectivity within a gene network model to choose pathogen-specific and strain-specific differentially expressed candidate genes for functional analysis. Mutations in 13 of 22 candidate genes caused significant differences in C. elegans survival in response to at least one S. maltophilia strain, although not always the strain that induced differential expression, suggesting a dynamic response to varying levels of pathogenicity.

Conclusions

Variation in observed pathogenicity and differences in host transcriptional responses to S. maltophilia strains reveal that strain-specific mechanisms play important roles in S. maltophilia pathogenesis. Furthermore, utilizing bacteria closely related to strains found in C. elegans natural environment provides a more realistic interaction for understanding host-pathogen response.

DAF-16 and SMK-1 Contribute to Innate Immunity During Adulthood in Caenorhabditis elegans

Aging is accompanied by a progressive decline in immune function termed "immunosenescence". Deficient surveillance coupled with the impaired function of immune cells compromises host defense in older animals. The dynamic activity of regulatory modules that control immunity appears to underlie age-dependent modifications to the immune system. In the roundworm Caenorhabditis elegans levels of PMK-1 p38 MAP kinase diminish over time, reducing the expression of immune effectors that clear bacterial pathogens. Along with the PMK-1 pathway, innate immunity in C. elegans is regulated by the insulin signaling pathway. Here we asked whether DAF-16, a Forkhead box (FOXO) transcription factor whose activity is inhibited by insulin signaling, plays a role in host defense later in life. While in younger C. elegans DAF-16 is inactive unless stimulated by environmental insults, we found that even in the absence of acute stress the transcriptional activity of DAF-16 increases in an age-dependent manner. Beginning in the reproductive phase of adulthood, DAF-16 upregulates a subset of its transcriptional targets, including genes required to kill ingested microbes. Accordingly, DAF-16 has little to no role in larval immunity, but functions specifically during adulthood to confer resistance to bacterial pathogens. We found that DAF-16-mediated immunity in adults requires SMK-1, a regulatory subunit of the PP4 protein phosphatase complex. Our data suggest that as the function of one branch of the innate immune system of C. elegans (PMK-1) declines over time, DAF-16-mediated immunity ramps up to become the predominant means of protecting adults from infection, thus reconfiguring immunity later in life.

Organism dual RNA-seq reveals the importance of BarA/UvrY in Vibrio parahaemolyticus virulence

Elucidation of host-pathogen interaction is essential for developing effective strategies to combat bacterial infection. Dual RNA-Seq using cultured cells or tissues/organs as the host of pathogen has emerged as a novel strategy to understand the responses concurrently from both pathogen and host at cellular level. However, bacterial infection mostly causes systematic responses from the host at organism level where the interplay is urgently to be understood but inevitably being neglected by the current practice. Here, we developed an approach that simultaneously monitor the genome-wide infection-linked transcriptional alterations in both pathogenic Vibrio parahaemolyticus and the infection host nematode Caenorhabditis elegans. Besides the dynamic alterations in transcriptomes of both C. elegans and V. parahaemolyticus during infection, we identify a two-component system, BarA/UvrY, that is important for virulence in host. BarA/UvrY not only controls the virulence factors in V. parahaemolyticus including Type III and Type VI secretion systems, but also attenuates innate immune responses in C. elegans, including repression on the MAP kinase-mediated cascades. Thus, our study exemplifies the use of dual RNA-Seq at organism level to uncover previously unrecognized interplay between host and pathogen.

Measurements of Innate Immune Function in C. elegans

The microscopic nematode Caenorhabditis elegans has emerged as a powerful system to characterize evolutionarily ancient mechanisms of pathogen sensing, innate immune activation, and protective host responses. Experimentally, C. elegans can be infected with a wide variety of human pathogens, as well as with natural pathogens of worms that were isolated from wild-caught nematodes. Here, we focus on an experimental model of bacterial pathogenesis that utilizes the human opportunistic bacterial pathogen Pseudomonas aeruginosa and present an algorithm that can be used to study mechanisms of immune function in nematodes. An initial comparison of the susceptibility of a C. elegans mutant to P. aeruginosa infection with its normal lifespan permits an understanding of a mutant's effect on pathogen susceptibility in the context of potential pleotropic consequences on general worm fitness. Assessing the behavior of nematodes in the presence of P. aeruginosa can also help determine if a gene of interest modulates pathogen susceptibility by affecting the host's ability to avoid a pathogen. In addition, quantification of the pathogen load in the C. elegans intestine during infection, characterization of immune effector transcription that are regulated by host defense pathways and an initial assessment of tissue specificity of immune gene function can refine hypotheses about the mechanism of action of a gene of interest. Together, these protocols offer one approach to characterize novel host defense mechanisms in a simple metazoan host.

Pheromones Modulate Learning by Regulating the Balanced Signals of Two Insulin-like Peptides

Social environment modulates learning through unknown mechanisms. Here, we report that a pheromone mixture that signals overcrowding inhibits C. elegans from learning to avoid pathogenic bacteria. We find that learning depends on the balanced signaling of two insulin-like peptides (ILPs), INS-16 and INS-4, which act respectively in the pheromone-sensing neuron ADL and the bacteria-sensing neuron AWA. Pheromone exposure inhibits learning by disrupting this balance: it activates ADL and increases expression of ins-16, and this cellular effect reduces AWA activity and AWA-expressed ins-4. The activities of the sensory neurons are required for learning and the expression of the ILPs. Interestingly, pheromones also promote the ingestion of pathogenic bacteria while increasing resistance to the pathogen. Thus, the balance of the ILP signals integrates social information into the learning process as part of a coordinated adaptive response that allows consumption of harmful food during times of high population density.

Caenorhabditis Elegans and Probiotics Interactions from a Prolongevity Perspective

Probiotics exert beneficial effects on host health through different mechanisms of action, such as production of antimicrobial substances, competition with pathogens, enhancement of host mucosal barrier integrity and immunomodulation. In the context of ageing, which is characterized by several physiological alterations leading to a low grade inflammatory status called inflammageing, evidences suggest a potential prolongevity role of probiotics. Unraveling the mechanisms underlying anti-ageing effects requires the use of simple model systems. To this respect, the nematode Caenorhabditis elegans represents a suitable model organism for the study of both host-microbe interactions and for ageing studies, because of conserved signaling pathways and host defense mechanisms involved in the regulation of its lifespan. Therefore, this review analyses the impact of probiotics on C. elegans age-related parameters, with particular emphasis on oxidative stress, immunity, inflammation and protection from pathogen infections. The picture emerging from our analysis highlights that several probiotic strains are able to exert anti-ageing effects in nematodes by acting on common molecular pathways, such as insulin/insulin-like growth factor-1 (IIS) and p38 mitogen-activated protein kinase (p38 MAPK). In this perspective, C. elegans appears to be advantageous for shedding light on key mechanisms involved in host prolongevity in response to probiotics supplementation.

In vitro and in vivo efficacy of Caenorhabditis elegans recombinant antimicrobial protein against Gram-negative bacteria

Antimicrobial peptides (AMPs) are short, positively charged host defense peptides, found in various life forms from microorganisms to humans. AMPs are gaining more attention as substitutes for antibiotics in order to combat the risk posed by multi-drug- resistant pathogens. The nematode Caenorhabditis elegans relies solely on its innate immune defense to cope with its challenging life-style. Bacterial infection in C. elegans leads to induction of antimicrobial proteins, defensins, nemapores, cecropins, and neuropeptide-like proteins, which act to limit bacterial proliferation. This study reports how the C. elegans recombinant antibacterial factor (ABF-1) rapidly inhibited bacterial growth (Salmonella Typhi, Klebsiella pneumonia, Shigella sonnei and Vibrio alginolyticus). The ABF-1 exposure on S. Typhi, showed differential regulation in cell-cycle, DNA repair mechanism, membrane stability, and stress related proteins. The exogenous supply of ABF-1 protein has extended C. elegans survival by reducing the bacterial colony forming units on the nematode intestine. Together, these findings indicate the valuable and potential therapeutic applications of ABF-1 protein as antimicrobial agents against intracellular pathogens.

Growth and Virulence of Salmonella Typhimurium Mutants Deficient in Iron Uptake

The present study investigated the effects of iron, iron chelators, and mutations of tonB or iroN fepA genes on the growth and virulence of Salmonella Typhimurium. Results indicated that organic iron (ferric citrate and ferrous-l-ascorbate) supported better growth of Salmonella compared to inorganic iron. Among tested chelators, 2,2'-bipyridyl at 500 μM showed the highest inhibition of Salmonella growth with 5 μM ferrous sulfate. Deletion of genes (tonB^- and iroN^- fepA^- ) in the iron uptake system attenuated Salmonella invasion of Caco-2 cells and its ability to damage the epithelial monolayer. The expression of all tested host genes in Caco-2 was not affected under the iron-poor condition. However, claudin 3, tight junction protein 1, tumor necrosis factor α (TNF-α), and interleukin-8 (IL-8) were altered under the iron-rich condition depending on individual mutations. In Caenorhabditis elegans, a significant down-regulation of ferritin 1 expression was observed when the nematode was infected by the wild-type (WT) strain.

Resisting resistance: gearing up for war

Where do we stand in our fight against antimicrobial resistance (AMR)? Many antimicrobials may lose their clinical efficacy, particularly due to the rise of multidrug-resistant (MDR) and extended drug-resistant (XDR) pathogens, including bacteria, fungi, and parasites. We need weapons against them all. Society must come together against these pathogens, just like we did against HIV, cancer, and heart disease. This opinion piece is, first and foremost, a call to arms for all partners involved in the war against AMR. Even more so, it is an attempt to highlight the positives in a seemingly long line of failures, and to identify the current set of challenges we must work on. So, how do we win the war against AMR? We must learn from the past, so we can act in the present, to save the future.

The Caenorhabditis elegans Model of Legionella Infection

Caenorhabditis elegans can serve as a simple genetic host to study interactions between Legionellaceae and their hosts and to examine the contribution of specific gene products to virulence and immunity. C. elegans nematodes have several appealing attributes as a host organism; they are inexpensive, have robust genetic analysis tools, have a simple anatomy yet display a wide range of complex behaviors, and, as invertebrates, do not require animal ethics protocols. Use of C. elegans as a host model complements cell-based models, providing additional support and consistency of the experimental data obtained from multiple models. The C. elegans innate immune system functions similarly to that of the alveolar macrophage including the apoptosis [a.k.a. programmed cell death (PCD)] pathway located within the germline. The digestive tract of C. elegans is a primary interface between the innate immune system and bacterial pathogens. Thus, the C. elegans host model provides an alternative approach to investigate L. pneumophila immunopathogenesis, particularly in the view of the recent discovery of Legionella-containing vacuoles within the gonadal tissues of Legionella-colonized nematodes supporting the plausible evolutionary origin of the strategies employed by L. pneumophila to counteract macrophage cellular responses.

The intestinal intermediate filament network responds to and protects against microbial insults and toxins

The enrichment of intermediate filaments in the apical cytoplasm of intestinal cells is evolutionarily conserved, forming a sheath that is anchored to apical junctions and positioned below the microvillar brush border, which suggests a protective intracellular barrier function. To test this, we used Caenorhabditis elegans, the intestinal cells of which are endowed with a particularly dense intermediate filament-rich layer that is referred to as the endotube. We found alterations in endotube structure and intermediate filament expression upon infection with nematicidal B. thuringiensis or treatment with its major pore-forming toxin crystal protein Cry5B. Endotube impairment due to defined genetic mutations of intermediate filaments and their regulators results in increased Cry5B sensitivity as evidenced by elevated larval arrest, prolonged time of larval development and reduced survival. Phenotype severity reflects the extent of endotube alterations and correlates with reduced rescue upon toxin removal. The results provide in vivo evidence for a major protective role of a properly configured intermediate filament network as an intracellular barrier in intestinal cells. This notion is further supported by increased sensitivity of endotube mutants to oxidative and osmotic stress.

Caenorhabditis elegans as a Model Host to Monitor the Candida Infection Processes

C. elegans has several advantages as an experimental host for the study of infectious diseases. Worms are easily maintained and propagated on bacterial lawns. The worms can be frozen for long term storage and still maintain viability years later. Their short generation time and large brood size of thousands of worms grown on a single petri dish, makes it relatively easy to maintain at a low cost. The typical wild type adult worm grows to approximately 1.5 mm in length and are transparent, allowing for the identification of several internal organs using an affordable dissecting microscope. A large collection of loss of function mutant strains are readily available from the C. elegans genetic stock center, making targeted genetic studies in the nematode possible. Here we describe ways in which this facile model host has been used to study Candida albicans, an opportunistic fungal pathogen that poses a serious public health threat.

Evaluation of Caenorhabditis elegans as a host model for Paracoccidioides brasiliensis and Paracoccidioides lutzii

Paracoccidioidomycosis is a systemic fungal infection affecting mainly Latin American countries that is caused by Paracoccidioides brasiliensis and Paracoccidioides lutzii. During the study of fungal pathogenesis, in vivo studies are crucial to understand the overall mechanisms involving the infection as well as to search for new therapeutic treatments and diagnosis. Caenorhabditis elegans is described as an infection model for different fungi species and a well-characterized organism to study the innate immune response. This study evaluates C. elegans as an infection model for Paracoccidioides spp. It was observed that both species do not cause infection in C. elegans, as occurs with Candida albicans, and one possible explanation is that the irregular size and shape of Paracoccidioides spp. difficult the ingestion of these fungi by the nematode. Besides this difficulty in the infection, we could observe that the simple exposition of C. elegans to Paracoccidioides species was able to trigger a distinct pattern of expression of antimicrobial peptide genes. The expression of cnc-4, nlpl-27 and nlp-31 was superior after the exposure to P. brasiliensis in comparison to P. lutzii (P < 0.05), and these findings demonstrate important differences regarding innate immune response activation caused by the two species of the Paracoccidioides genus.

Cell Signaling of Caenorhabditis elegans in Response to Enterotoxigenic Escherichia coli Infection and Lactobacillus zeae Protection

Enterotoxigenic Escherichia coli (ETEC) infection causes the death of Caenorhabditis elegans, which can be prevented by certain Lactobacillus isolates. The host response of C. elegans to ETEC infection and its regulation by the isolates are, however, largely unclear. This study has revealed that, in agreement with the results of life-span assays, the expression of the genes encoding p38 mitogen-activated protein kinase (MAPK) pathway (nsy-1, sek-1, and pmk-1), insulin/insulin-like growth factor (DAF/IGF) pathway (daf-16), or antimicrobial peptides (lys-7, spp-1, and abf-3) and other defensing molecules (abf-2, clec-85) was upregulated significantly when the wild-type nematode (N2) was subjected to ETEC infection. This upregulation was further enhanced by the pretreatment with Lactobacillus zeae LB1, but not with L. casei CL11. Mutants defective in the cell signaling of C. elegans were either more susceptible (defective in NSY-1, SEK-1, PMK-1, or DAF16) or more resistant (defective in AGE-1, DBL-1, SKN-1, or SOD-3) to ETEC infection compared with the wild-type. Mutants defective in antimicrobial peptides (LYS-7, SPP1, or ABF-3) were also more susceptible. In addition, mutants that are defective in NSY-1, SEK-1, PMK-1, DAF16, ABF-3, LYS-7, or SPP1 showed no response to the protection from L. zeae LB1. The expression of the genes encoding antimicrobial peptides (lys-7, spp-1, and abf-3) and other defensing molecules (abf-2, clec-60, and clec-85) were almost all upregulated in AGE-1- or DBL-1-defective mutant compared with the wild-type, which was further enhanced by the pretreatment of L. zeae LB1. The expression of these genes was, however, mostly downregulated in NSY-1- or DAF-16-defective mutant. These results suggest that L. zeae LB1 regulates C. elegans signaling through the p38 MAPK and DAF/IGF pathways to control the production of antimicrobial peptides and defensing molecules to combat ETEC infection.

Transcriptomic, Functional, and Network Analyses Reveal Novel Genes Involved in the Interaction Between Caenorhabditis elegans and Stenotrophomonas maltophilia

The bacterivorous nematode Caenorhabditis elegans is an excellent model for the study of innate immune responses to a variety of bacterial pathogens, including the emerging nosocomial bacterial pathogen Stenotrophomonas maltophilia. The study of this interaction has ecological and medical relevance as S. maltophilia is found in association with C. elegans and other nematodes in the wild and is an emerging opportunistic bacterial pathogen. We identified 393 genes that were differentially expressed when exposed to virulent and avirulent strains of S. maltophilia and an avirulent strain of E. coli. We then used a probabilistic functional gene network model (WormNet) to determine that 118 of the 393 differentially expressed genes formed an interacting network and identified a set of highly connected genes with eight or more predicted interactions. We hypothesized that these highly connected genes might play an important role in the defense against S. maltophila and found that mutations of six of seven highly connected genes have a significant effect on nematode survival in response to these bacteria. Of these genes, C48B4.1, mpk-2, cpr-4, clec-67, and lys-6 are needed for combating the virulent S. maltophilia JCMS strain, while dod-22 was solely involved in response to the avirulent S. maltophilia K279a strain. We further found that dod-22 and clec-67 were up regulated in response to JCMS vs. K279a, while C48B4.1, mpk-2, cpr-4, and lys-6 were down regulated. Only dod-22 had a documented role in innate immunity, which demonstrates the merit of our approach in the identification of novel genes that are involved in combating S. maltophilia infection.

Effects of liquid cultivation on gene expression and phenotype of C. elegans

BACKGROUND

Liquid cultures have been commonly used in space, toxicology, and pharmacology studies of Caenorhabditis elegans. However, the knowledge about transcriptomic alterations caused by liquid cultivation remains limited. Moreover, the impact of different genotypes in rapid adaptive responses to environmental changes (e.g., liquid cultivation) is often overlooked. Here, we report the transcriptomic and phenotypic responses of laboratory N2 and the wild-isolate AB1 strains after culturing P0 worms on agar plates, F1 in liquid cultures, and F2 back on agar plates.

RESULTS

Significant variations were found in the gene expressions between the N2 and AB1 strains in response to liquid cultivation. The results demonstrated that 8-34% of the environmental change-induced transcriptional responses are transmitted to the subsequent generation. By categorizing the gene expressions for genotype, environment, and genotype-environment interactions, we identified that the genotype has a substantial impact on the adaptive responses. Functional analysis of the transcriptome showed correlation with phenotypical changes. For example, the N2 strain exhibited alterations in both phenotype and gene expressions for germline and cuticle in axenic liquid cultivation. We found transcript evidence to approximately 21% of the computationally predicted genes in C. elegans by exposing the worms to environmental changes.

CONCLUSIONS

The presented study reveals substantial differences between N2 and AB1 strains for transcriptomic and phenotypical responses to rapid environmental changes. Our data can provide standard controls for future studies for the liquid cultivation of C. elegans and enable the discovery of condition-specific genes.

Changes in Caenorhabditis elegans gene expression following exposure to Photorhabdus luminescens strain TT01

Photorhabdus bacteria enter into a mutualistic symbiosis with Heterorhabditis nematodes to infect insect larvae. However, they rapidly kill the model nematode Caenorhabditis elegans. One hypothesis for these divergent outcomes is that the nematode defense responses differ. To begin testing this hypothesis, we have systematically analyzed available data on the transcriptional response of C. elegans to P. luminescens strain Hb. From a starting pool of over 7000 differentially expressed genes, we carefully chose 21 Heterorhabditis-conserved genes to develop as comparative markers. Using newly designed and validated qRT-PCR primers, we measured expression of these genes in C. elegans exposed to the sequenced TT01 strain of P. luminescens, on two different media types. Almost all (18/21) of the genes showed a significant response to P. luminescens strain TT01. One response is dependent on media type, and a subset of genes may respond differentially to distinct strains. Overall, we have established useful resources and generated new hypotheses regarding how C. elegans responds to P. luminescens infection.

What Can We Learn About Human Disease from the Nematode C. elegans?

Numerous approaches have been taken in the hunt for human disease genes. The identification of such genes not only provides a great deal of information about the mechanism of disease development, but also provides potential avenues for better diagnosis and treatment. In this chapter, we review the use of the nonmammalian model organism C. elegans for the identification of human disease genes. Studies utilizing this relatively simple organism offer a good balance between the ability to recapitulate many aspects of human disease, while still offering an abundance of powerful cell biological, genetic, and genomic tools for disease gene discovery. C. elegans and other nonmammalian models have produced, and will continue to produce, key insights into human disease pathogenesis.

Immune Signaling Networks: Sources of Robustness and Constrained Evolvability during Coevolution

Defense against infection incurs costs as well as benefits that are expected to shape the evolution of optimal defense strategies. In particular, many theoretical studies have investigated contexts favoring constitutive versus inducible defenses. However, even when one immune strategy is theoretically optimal, it may be evolutionarily unachievable. This is because evolution proceeds via mutational changes to the protein interaction networks underlying immune responses, not by changes to an immune strategy directly. Here, we use a theoretical simulation model to examine how underlying network architectures constrain the evolution of immune strategies, and how these network architectures account for desirable immune properties such as inducibility and robustness. We focus on immune signaling because signaling molecules are common targets of parasitic interference but are rarely studied in this context. We find that in the presence of a coevolving parasite that disrupts immune signaling, hosts evolve constitutive defenses even when inducible defenses are theoretically optimal. This occurs for two reasons. First, there are relatively few network architectures that produce immunity that is both inducible and also robust against targeted disruption. Second, evolution toward these few robust inducible network architectures often requires intermediate steps that are vulnerable to targeted disruption. The few networks that are both robust and inducible consist of many parallel pathways of immune signaling with few connections among them. In the context of relevant empirical literature, we discuss whether this is indeed the most evolutionarily accessible robust inducible network architecture in nature, and when it can evolve.

A Marine Actinomycete Rescues Caenorhabditis elegans from Pseudomonas aeruginosa Infection through Restitution of Lysozyme 7

The resistance of Pseudomonas aeruginosa to conventional antimicrobial treatment is a major scourge in healthcare. Therefore, it is crucial that novel potent anti-infectives are discovered. The aim of the present study is to screen marine actinomycetes for chemical entities capable of overcoming P. aeruginosa infection through mechanisms involving anti-virulence or host immunity activities. A total of 18 actinomycetes isolates were sampled from marine sediment of Songsong Island, Kedah, Malaysia. Upon confirming that the methanolic crude extract of these isolates do not display direct bactericidal activities, they were tested for capacity to rescue Caenorhabditis elegans infected with P. aeruginosa strain PA14. A hexane partition of the extract from one isolate, designated as Streptomyces sp. CCB-PSK207, could promote the survival of PA14 infected worms by more than 60%. Partial 16S sequence analysis on this isolate showed identity of 99.79% with Streptomyces sundarbansensis. This partition did not impair feeding behavior of C. elegans worms. Tested on PA14, the partition also did not affect bacterial growth or its ability to colonize host gut. The production of biofilm, protease, and pyocyanin in PA14 were uninterrupted, although there was an increase in elastase production. In lys-7::GFP worms, this partition was shown to induce the expression of lysozyme 7, an important innate immunity defense molecule that was repressed during PA14 infection. GC-MS analysis of the bioactive fraction of Streptomyces sp. CCB-PSK207 revealed the presence of methyl esters of branched saturated fatty acids. In conclusion, this is the first report of a marine actinomycete producing metabolites capable of rescuing C. elegans from PA14 through a lys-7 mediated activity.