Resistance to antimicrobial peptides contributes to persistence of Salmonella typhimurium in the C. elegans intestine

LysR-type transcriptional regulators: state of the art

The LysR-type transcriptional regulators (LTTRs) are DNA-binding proteins present in bacteria, archaea, and in algae. Knowledge about their distribution, abundance, evolution, structural organization, transcriptional regulation, fundamental roles in free life, pathogenesis, and bacteria-plant interaction has been generated. This review focuses on these aspects and provides a current picture of LTTR biology.

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.

Immunological pathogenesis of Bovine E. coli infection in a model of C. elegans

BACKGROUND

Cattle industry is critical for China's livestock industry, whereas E. coli infection and relevant diseases could lead huge economic loss. Traditional mammalian models would be costly, time consuming and complicated to study pathological changes of bovine E. coli. There is an urgent need for a simple but efficient animal model to quantitatively evaluate the pathological changes of bovine-derived E. coli in vivo. Caenorhabditis elegans (C. elegans) has a broad host range of diverse E. coli strains with advantages, including a short life cycle, a simple structure, a transparent body which is easily visualized, a well-studied genetic map, an intrinsic immune system which is conservable with more complicated mammalians.

RESULTS

Here, we considered that O126 was the dominant serotype, and a total of 19 virulence factors were identified from 41 common E. coli virulence factors. Different E. coli strains with diverse pathogenicity strengths were tested in C. elegans in E. coli with higher pathogenicity (EC3/10), Nsy-1, Sek-1 and Pmk-1 of the p38 MAPK signaling pathway cascade and the expression of the antimicrobial peptides Abf-3 and Clec-60 were significantly up-regulated comparing with other groups. E. coli with lower pathogenicity (EC5/13) only activated the expression of Nsy-1 and Sek-1 genes in the p38 MAPK signaling pathway, Additionally, both groups of E. coli strains caused significant upregulation of the antimicrobial peptide Spp-1.

CONCLUSION

Thirteen E. coli strains showed diverse pathogenicity in nematodes and the detection rate of virulence factors did not corresponding to the virulence in nematodes, indicating complex pathogenicity mechanisms. We approved that C. elegans is a fast and convenient detection model for pathogenic bacteria virulence examinations.

Salmonella Typhimurium fepB negatively regulates C. elegans behavioral plasticity

OBJECTIVES

Dauer is an alternative developmental stage of Caenorhabditis elegans (C. elegans) that gives survival benefits under unfavorable environmental conditions. Our study aims to decipher C. elegans dauer larvae development upon Salmonella Typhimurium infection and how the bacterial gene regulating the worm's behavioural plasticity for better survival.

METHODS

Age-synchronized L4 C. elegans worms were infected with Salmonella Typhimurium 14028s (WT-STM) strain and mutant strains to check the dauer larvae development using 1% SDS. Besides, bacterial load in animals' gut, pharyngeal pumping rate and viability were checked. Worm's immune genes (e.g., ilys-3, lys-7, pmk-1, abf-2, clec-60) and dauer regulatory genes (e.g., daf-7, daf-11, daf-12, daf-16, daf-3) were checked by performing qRT-PCR under infection conditions.

RESULTS

We found that deletion of the fepB gene in S. Typhimurium strain became less pathogenic with reduced flagellar motility and biofilm-forming ability. Besides, there was decreased bacterial burden in the worm's gut with no damage to their pharynx. The fepB mutant strain was also able to enhance the immune responses for better survival of worms. Infection with mutant strain could activate dauer signaling via the TGF-β pathway leading to a significant increase in dauer formation than WT-STM infection.

CONCLUSION

Our study indicated that the bacteria act as a food source for the growth of C. elegans and development and can act as a signal that might be playing an essential role in regulating the host physiology for their survival. Such a study can help us in understanding the complex host-pathogen interaction benefiting pathogen in host dissemination.

Identification of the Genes of the Plant Pathogen Pseudomonas syringae MB03 Required for the Nematicidal Activity Against Caenorhabditis elegans Through an Integrated Approach

Nematicidal potential of the common plant pathogen Pseudomonas syringae has been recently identified against Caenorhabditis elegans. The current study was designed to investigate the detailed genetic mechanism of the bacterial pathogenicity by applying comparative genomics, transcriptomics, mutant library screening, and protein expression. Results showed that P. syringae strain MB03 could kill C. elegans in the liquid assay by gut colonization. The genome of P. syringae MB03 was sequenced and comparative analysis including multi locus sequence typing, and genome-to-genome distance placed MB03 in phylogroup II of P. syringae. Furthermore, comparative genomics of MB03 with nematicidal strains of Pseudomonas aeruginosa (PAO1 and PA14) predicted 115 potential virulence factors in MB03. However, genes for previously reported nematicidal metabolites, such as phenazine, pyochelin, and pyrrolnitrin, were found absent in the MB03 genome. Transcriptomics analysis showed that the growth phase of the pathogen considerably affected the expression of virulence factors, as genes for the flagellum, glutamate ABC transporter, phoP/phoQ, fleS/fleR, type VI secretion system, and serralysin were highly up-regulated when stationary phase MB03 cells interacted with C. elegans. Additionally, screening of a transposon insertion mutant library led to the identification of other nematicidal genes such as acnA, gltP, oprD, and zapE. Finally, the nematicidal activity of selected proteins was confirmed by heterologous expression in Escherichia coli.

How the PhoP/PhoQ System Controls Virulence and Mg2+ Homeostasis: Lessons in Signal Transduction, Pathogenesis, Physiology, and Evolution

The PhoP/PhoQ two-component system governs virulence, Mg2+ homeostasis, and resistance to a variety of antimicrobial agents, including acidic pH and cationic antimicrobial peptides, in several Gram-negative bacterial species. Best understood in Salmonella enterica serovar Typhimurium, the PhoP/PhoQ system consists o-regulated gene products alter PhoP-P amounts, even under constant inducing conditions. PhoP-P controls the abundance of hundreds of proteins both directly, by having transcriptional effects on the corresponding genes, and indirectly, by modifying the abundance, activity, or stability of other transcription factors, regulatory RNAs, protease regulators, and metabolites. The investigation of PhoP/PhoQ has uncovered novel forms of signal transduction and the physiological consequences of regulon evolution.

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.

TOR functions as a molecular switch connecting an iron cue with host innate defense against bacterial infection

As both host and pathogen require iron for survival, iron is an important regulator of host-pathogen interactions. However, the molecular mechanism by which how the availability of iron modulates host innate immunity against bacterial infections remains largely unknown. Using the metazoan Caenorhabditis elegans as a model, we demonstrate that infection with a pathogenic bacterium Salmonella enterica serovar Typhimurium induces autophagy by inactivating the target of rapamycin (TOR). Although the transcripts of ftn-1 and ftn-2 encoding two H-ferritin subunits are upregulated upon S. Typhimurium infection, the ferritin protein is kept at a low level due to its degradation mediated by autophagy. Autophagy, but not ferritin, is required for defense against S. Typhimurium infection under normal circumstances. Increased abundance of iron suppresses autophagy by activating TOR, leading to an increase in the ferritin protein level. Iron sequestration, but not autophagy, becomes pivotal to protect the host from S. Typhimurium infection in the presence of exogenous iron. Our results show that TOR acts as a regulator linking iron availability with host defense against bacterial infection.

Iron, an essential nutrient for both hosts and pathogens, can impact host-pathogen interactions. Thus, depending on availability of iron, hosts may use distinct strategies to defend against bacterial infections. Using the model organism Caenorhabditis elegans, we show that autophagy is activated and required for resistance to S. Typhimurium infection under normal conditions. In the presence of exogenous iron, autophagy is suppressed, leading to an increase in the protein level of ferritin, which in turn protects worms against S. Typhimurium infection. Finally, our data demonstrate that TOR is a molecular switch for turning autophagy and ferritin on and off. Thus, our study reveals a previously unknown role of TOR in modulating the transition between autophagy and iron sequestration in response to a bacterial infection via sensing an iron cue.

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.

A neurotransmitter produced by gut bacteria modulates host sensory behaviour

Animals coexist in commensal, pathogenic or mutualistic relationships with complex communities of diverse organisms, including microorganisms1. Some bacteria produce bioactive neurotransmitters that have previously been proposed to modulate nervous system activity and behaviours of their hosts2,3. However, the mechanistic basis of this microbiota-brain signalling and its physiological relevance are largely unknown. Here we show that in Caenorhabditis elegans, the neuromodulator tyramine produced by commensal Providencia bacteria, which colonize the gut, bypasses the requirement for host tyramine biosynthesis and manipulates a host sensory decision. Bacterially produced tyramine is probably converted to octopamine by the host tyramine β-hydroxylase enzyme. Octopamine, in turn, targets the OCTR-1 octopamine receptor on ASH nociceptive neurons to modulate an aversive olfactory response. We identify the genes that are required for tyramine biosynthesis in Providencia, and show that these genes are necessary for the modulation of host behaviour. We further find that C. elegans colonized by Providencia preferentially select these bacteria in food choice assays, and that this selection bias requires bacterially produced tyramine and host octopamine signalling. Our results demonstrate that a neurotransmitter produced by gut bacteria mimics the functions of the cognate host molecule to override host control of a sensory decision, and thereby promotes fitness of both the host and the microorganism.

Molecular mechanism of the smart attack of pathogenic bacteria on nematodes

Nematode-bacterial associations are far-reaching subjects in view of their impact on ecosystems, economies, agriculture and human health. There is still no conclusion regarding which pathogenic bacteria sense nematodes. Here, we found that the pathogenic bacterium Bacillus nematocida B16 was sensitive to C. elegans and could launch smart attacks to kill the nematodes. Further analysis revealed that the spores of B. nematocida B16 are essential virulence factors. Once gaseous molecules (morpholine) produced from C. elegans were sensed, the sporulation of B16 was greatly accelerated. Then, B16 showed maximum attraction to C. elegans during the spore-forming process but had no attraction until all the spores were formed. The disruption of either the spore formation gene spo0A or the germination gene gerD impaired colonization and attenuated infection in B16. In contrast, complementation with the intact genes restored most of the above-mentioned deficient phenotypes, which indicated that the spo0A gene was a key factor in the smart attack of B16 on C. elegans. Further, transcriptome, molecular simulations and quantitative PCR analysis showed that morpholine from C. elegans could promote sporulation and initiate infection by increasing the transcription of the spo0A gene by decreasing the transcription of the rapA and spo0E genes. The overexpression of rapA or spo0E decreased the induced sporulation effect, and morpholine directly reduced the level of phosphorylation of purified recombinant RapA and Spo0E compared to that of Spo0A. Collectively, these findings further support a 'Trojan horse-like' infection model. The significance of our paper is that we showed that the soil-dwelling bacterium B. nematocida B16 has the ability to actively detect, attract and attack their host C. elegans. These studies are the first report on the behaviours, signalling molecules and mechanism of the smart attack of B16 on nematodes and also reveal new insights into microbe-host interactions.

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.

Caenorhabditis elegans: a model to understand host-microbe interactions

Host-microbe interactions within the gut are fundamental to all higher organisms. Caenorhabditis elegans has been in use as a surrogate model to understand the conserved mechanisms in host-microbe interactions. Morphological and functional similarities of C. elegans gut with the human have allowed the mechanistic investigation of gut microbes and their effects on metabolism, development, reproduction, behavior, pathogenesis, immune responses and lifespan. Recent reports suggest their suitability for functional investigations of human gut bacteria, such as gut microbiota of healthy and diseased individuals. Our knowledge on the gut microbial diversity of C. elegans in their natural environment and the effect of host genetics on their core gut microbiota is important. Caenorhabditis elegans, as a model, is continuously bridging the gap in our understanding the role of genetics, environment, and dietary factors on physiology of the host.

Signal pathways involved in microbe-nematode interactions provide new insights into the biocontrol of plant-parasitic nematodes

Plant-parasitic nematodes (PPNs) cause severe damage to agricultural crops worldwide. As most chemical nematicides have negative environmental side effects, there is a pressing need for developing efficient biocontrol methods. Nematophagous microbes, the natural enemies of nematodes, are potential biocontrol agents against PPNs. These natural enemies include both bacteria and fungi and they use diverse methods to infect and kill nematodes. For instance, nematode-trapping fungi can sense host signals and produce special trapping devices to capture nematodes, whereas endo-parasitic fungi can kill nematodes by spore adhesion and invasive growth to break the nematode cuticle. By contrast, nematophagous bacteria can secrete virulence factors to kill nematodes. In addition, some bacteria can mobilize nematode-trapping fungi to kill nematodes. In response, nematodes can also sense and defend against the microbial pathogens using strategies such as producing anti-microbial peptides regulated by the innate immunity system. Recent progresses in our understanding of the signal pathways involved in microbe-nematode interactions are providing new insights in developing efficient biological control strategies against PPNs. This article is part of the theme issue 'Biotic signalling sheds light on smart pest management'.

The Intestinal Roundworm Ascaris suum Releases Antimicrobial Factors Which Interfere With Bacterial Growth and Biofilm Formation

Ascariasis is a widespread soil-transmitted helminth infection caused by the intestinal roundworm Ascaris lumbricoides in humans, and the closely related Ascaris suum in pigs. Progress has been made in understanding interactions between helminths and host immune cells, but less is known concerning the interactions of parasitic nematodes and the host microbiota. As the host microbiota represents the direct environment for intestinal helminths and thus a considerable challenge, we studied nematode products, including excretory-secretory products (ESP) and body fluid (BF), of A. suum to determine their antimicrobial activities. Antimicrobial activities against gram-positive and gram-negative bacterial strains were assessed by the radial diffusion assay, while effects on biofilm formation were assessed using the crystal violet static biofilm and macrocolony assays. In addition, bacterial neutralizing activity was studied by an agglutination assay. ESP from different A. suum life stages (in vitro-hatched L3, lung-stage L3, L4, and adult) as well as BF from adult males were analyzed by mass spectrometry. Several proteins and peptides with known and predicted roles in nematode immune defense were detected in ESP and BF samples, including members of A. suum antibacterial factors (ASABF) and cecropin antimicrobial peptide families, glycosyl hydrolase enzymes such as lysozyme, as well as c-type lectin domain-containing proteins. Native, unconcentrated nematode products from intestine-dwelling L4-stage larvae and adults displayed broad-spectrum antibacterial activity. Additionally, adult A. suum ESP interfered with biofilm formation by Escherichia coli, and caused bacterial agglutination. These results indicate that A. suum uses a variety of factors with broad-spectrum antibacterial activity to affirm itself within its microbe-rich environment in the gut.

Relative Quantitative Proteomic Analysis of Brucella abortus Reveals Metabolic Adaptation to Multiple Environmental Stresses

Brucella spp. are facultative intracellular pathogens that cause chronic brucellosis in humans and animals. The virulence of Brucella primarily depends on its successful survival and replication in host cells. During invasion of the host tissue, Brucella is simultaneously subjected to a variety of harsh conditions, including nutrient limitation, low pH, antimicrobial defenses, and extreme levels of reactive oxygen species (ROS) via the host immune response. This suggests that Brucella may be able to regulate its metabolic adaptation in response to the distinct stresses encountered during its intracellular infection of the host. An investigation into the differential proteome expression patterns of Brucella grown under the relevant stress conditions may contribute toward a better understanding of its pathogenesis and adaptive response. Here, we utilized a mass spectrometry-based label-free relative quantitative proteomics approach to investigate and compare global proteomic changes in B. abortus in response to eight different stress treatments. The 3 h short-term in vitro single-stress and multi-stress conditions mimicked the in vivo conditions of B. abortus under intracellular infection, with survival rates ranging from 3.17 to 73.17%. The proteomic analysis identified and quantified a total of 2,272 proteins and 74% of the theoretical proteome, thereby providing wide coverage of the B. abortus proteome. By including eight distinct growth conditions and comparing these with a control condition, we identified a total of 1,221 differentially expressed proteins (DEPs) that were significantly changed under the stress treatments. Pathway analysis revealed that most of the proteins were involved in oxidative phosphorylation, ABC transporters, two-component systems, biosynthesis of secondary metabolites, the citrate cycle, thiamine metabolism, and nitrogen metabolism; constituting major response mechanisms toward the reconstruction of cellular homeostasis and metabolic balance under stress. In conclusion, our results provide a better understanding of the global metabolic adaptations of B. abortus associated with distinct environmental stresses. The identification of proteins necessary for stress resistance is crucial toward elucidating the infectious process in order to control brucellosis, and may facilitate the discovery of novel therapeutic targets and effective vaccines.

Antimicrobial effectors in the nematode Caenorhabditis elegans: an outgroup to the Arthropoda

Nematodes and arthropods likely form the taxon Ecdysozoa. Information on antimicrobial effectors from the model nematode Caenorhabditis elegans may thus shed light on the evolutionary origin of these defences in arthropods. This nematode species possesses an extensive armory of putative antimicrobial effector proteins, such as lysozymes, caenopores (or saposin-like proteins), defensin-like peptides, caenacins and neuropeptide-like proteins, in addition to the production of reactive oxygen species and autophagy. As C. elegans is a bacterivore that lives in microbe-rich environments, some of its effector peptides and proteins likely function in both digestion of bacterial food and pathogen elimination. In this review, we provide an overview of C. elegans immune effector proteins and mechanisms. We summarize the experimental evidence of their antimicrobial function and involvement in the response to pathogen infection. We further evaluate the microbe-induced expression of effector genes using WormExp, a recently established database for C. elegans gene expression analysis. We emphasize the need for further analysis at the protein level to demonstrate an antimicrobial activity of these molecules both in vitro and in vivoThis article is part of the themed issue 'Evolutionary ecology of arthropod antimicrobial peptides'.

The Shift of the Intestinal Microbiome in the Innate Immunity-Deficient Mutant rde-1 Strain of C. elegans upon Orsay Virus Infection

The status of intestinal microbiota is a determinant of host health. However, the alteration of the gut microbiota caused by the innate immune response to virus infection is unclear. Caenorhabditis elegans and its natural virus Orsay provide an excellent model of host–virus interactions. We evaluated the intestinal microbial community complexity of the wild-type N2 and the innate immunity-deficient mutant rde-1 (ne219) strains of C. elegans upon Orsay virus infection. The gut microbiota diversity was decreased in rde-1 (ne219) mutant animals, and a large number of genes were associated with the difference between infected and uninfected rde-1 (ne219) mutant animals. Therefore, this study provides the first evaluation of the alterations caused by Orsay virus on intestinal microbiota in wildtype and innate immunity-deficient animals using C. elegans as the model species. Our findings indicate that virus infection may alters the microbiome in animals with defective immune response.

Reciprocal Interactions between Nematodes and Their Microbial Environments

Parasitic nematode infections are widespread in nature, affecting humans as well as wild, companion, and livestock animals. Most parasitic nematodes inhabit the intestines of their hosts living in close contact with the intestinal microbiota. Many species also have tissue migratory life stages in the absence of severe systemic inflammation of the host. Despite the close coexistence of helminths with numerous microbes, little is known concerning these interactions. While the environmental niche is considerably different, the free-living nematode Caenorhabditis elegans (C. elegans) is also found amongst a diverse microbiota, albeit on decaying organic matter. As a very well characterized model organism that has been intensively studied for several decades, C. elegans interactions with bacteria are much more deeply understood than those of their parasitic counterparts. The enormous breadth of understanding achieved by the C. elegans research community continues to inform many aspects of nematode parasitology. Here, we summarize what is known regarding parasitic nematode-bacterial interactions while comparing and contrasting this with information from work in C. elegans. This review highlights findings concerning responses to bacterial stimuli, antimicrobial peptides, and the reciprocal influences between nematodes and their environmental bacteria. Furthermore, the microbiota of nematodes as well as alterations in the intestinal microbiota of mammalian hosts by helminth infections are discussed.

Role of GATA transcription factor ELT-2 and p38 MAPK PMK-1 in recovery from acute P. aeruginosa infection in C. elegans

Infectious diseases caused by bacterial pathogens reduce the fitness of their associated host but are generally limited in duration. In order for the diseased host to regain any lost fitness upon recovery, a variety of molecular, cellular, and physiological processes must be employed. To better understand mechanisms underlying the recovery process, we have modeled an acute Pseudomonas aeruginosa infection in C. elegans using brief exposures to this pathogen and subsequent antibiotic treatment. To identify host genes altered during recovery from P. aeruginosa infection, we performed whole genome expression profiling. The analysis of this dataset indicated that the activity of the host immune system is down-regulated upon recovery and revealed shared and pathogen-specific host responses during recovery. We determined that the GATA transcription factor ELT-2 and the p38 MAP kinase PMK-1 are necessary for animals to successfully recover from an acute P. aeruginosa infection. In addition, we found that ELT-2 plays a more prominent and earlier role than PMK-1 during recovery. Our data sheds further light on the molecular mechanisms and transcriptional programs involved in recovery from an acute bacterial infection, which provides a better understanding of the entire infectious disease process.

The Pathogenicity of Pseudomonas syringae MB03 against Caenorhabditis elegans and the Transcriptional Response of Nematicidal Genes upon Different Nutritional Conditions

Different species of the Pseudomonas genus have been reported for their pathogenic potential against animal cells. However, the pathogenicity of Pseudomonas syringae against Caenorhabditis elegans has never been reported. In this study, the interaction of P. syringae MB03 with C. elegans was studied. Different bioassays such as killing assay, lawn leaving assay, food preference assay, L4 growth assay and newly developed “secretion assay” were performed to evaluate the pathogenic potential of P. syringae on different growth media. The results of the killing assay showed that P. syringae MB03 was able to kill C. elegans under specific conditions, as the interaction between the host and the pathogen varied from non-pathogenic (assay on NGM medium) to pathogenic (assay on PG medium). The lawn leaving assay and the food preference assay illustrated that C. elegans identified P. syringae MB03 as a pathogen when assays were performed on PG medium. Green fluorescent protein was used as the reporter protein to study gut colonization by P. syringae MB03. Our results suggested that MB03 has the ability to colonize the gut of C. elegans. Furthermore, to probe the role of selected virulence determinants, qRT-PCR was used. The genes for pyoverdine, phoQ/phoP, phoR/phoB, and flagella were up regulated during the interaction of P. syringae MB03 and C. elegans on PG medium. Other than these, the genes for some proteases, such as pepP, clpA, and clpS, were also up regulated. On the other hand, kdpD and kdpB were down regulated more than threefold in the NGM – C. elegans interaction model. The deletion of the kdpD and kdpE genes altered the pathogenicity of the bacterial strain against C. elegans. Overall, our results suggested that the killing of C. elegans by P. syringae requires a prolonged interaction between the host and pathogen in an agar-based assay. Moreover, it seemed that some toxic metabolites were secreted by the bacterial strain that were sensed by C. elegans. Previously, it was believed that P. syringae could not damage animal cells. However, this study provides evidence of the pathogenic behavior of P. syringae against C. elegans.

Red Seaweeds Sarcodiotheca gaudichaudii and Chondrus crispus down Regulate Virulence Factors of Salmonella Enteritidis and Induce Immune Responses in Caenorhabditis elegans

Red seaweeds are a rich source of unique bioactive compounds and secondary metabolites that are known to improve human and animal health. S. Enteritidis is a broad range host pathogen, which contaminates chicken and poultry products that end into the human food chain. Worldwide, Salmonella outbreaks have become an important economic and public health concern. Moreover, the development of resistance in Salmonella serovars toward multiple drugs highlights the need for alternative control strategies. This study evaluated the antimicrobial property of red seaweeds extracts against Salmonella Enteritidis using the Caenorhabditis elegans infection model. Six red seaweed species were tested for their antimicrobial activity against S. Enteritidis and two, Sarcodiotheca gaudichaudii (SG) and Chondrus crispus (CC), were found to exhibit such properties. Spread plate assay revealed that SG and CC (1%, w/v) significantly reduced the growth of S. Enteritidis. Seaweed water extracts (SWE) of SG and CC, at concentrations from 0.4 to 2 mg/ml, significantly reduced the growth of S. Enteritidis (log CFU 4.5–5.3 and log 5.7–6.0, respectively). However, methanolic extracts of CC and SG did not affect the growth of S. Enteritidis. Addition of SWE (0.2 mg/ml, CC and SG) significantly decreased biofilm formation and reduced the motility of S. Enteritidis. Quantitative real-time PCR analyses showed that SWE (CC and SG) suppressed the expression of quorum sensing gene sdiA and of Salmonella Pathogenesis Island-1 (SPI-1) associated genes sipA and invF, indicating that SWE might reduce the invasion of S. Enteritidis in the host by attenuating virulence factors. Furthermore, CC and SG water extracts significantly improved the survival of infected C. elegans by impairing the ability of S. Enteritidis to colonize the digestive tract of the nematode and by enhancing the expression of C. elegans immune responsive genes. As the innate immune response pathways of C. elegans and mammals show a high degree of conservation, these results suggest that these SWE may also impart beneficial effects on animal and human health.

Changes in intestinal microflora of Caenorhabditis elegans following Bacillus nematocida B16 infection

The effect of pathogenic bacteria on a host and its symbiotic microbiota is vital and widespread in the biotic world. The soil-dwelling opportunistic bacterium Bacillus nematocida B16 uses a "Trojan horse" mechanism to kill Caenorhabditis elegans. The alterations in the intestinal microflora that occur after B16 infection remain unknown. Here, we analyzed the intestinal bacteria presented in normal and infected worms. The gut microbial community experienced a complex change after B16 inoculation, as determined through marked differences in species diversity, structure, distribution and composition between uninfected and infected worms. Regardless of the worm's origin (i.e., from soil or rotten fruits), the diversity of the intestinal microbiome decreased after infection. Firmicutes increased sharply, whereas Proteobacteria, Actinobacteria, Cyanobacteria and Acidobacteria decreased to different degrees. Fusobacteria was only present 12 h post-infection. After 24 h of infection, 1228 and 1109 bacterial species were identified in the uninfected and infected groups, respectively. The shared species reached 21.97%. The infected group had a greater number of Bacillus species but a smaller number of Pediococcus, Halomonas, Escherichia and Shewanella species (P < 0.01). Therefore, this study provides the first evaluation of the alterations caused by pathogenic bacteria on symbiotic microbiota using C. elegans as the model species.

Caenorhabditis elegans: What We Can and Cannot Learn from Aging Worms

SIGNIFICANCE

The nematode Caenorhabditis elegans is a widely used model organism for research into aging. However, nematodes diverged from other animals between 600 and 1300 million years ago. Beyond the intuitive impression that some aspects of aging appear to be universal, is there evidence that insights into the aging process of nematodes may be applicable to humans?

RECENT ADVANCES

There have been a number of results in nematodes that appear to contradict long-held beliefs about mechanisms and causes of aging. For example, ablation of several key antioxidant systems has often failed to result in lifespan shortening in C. elegans.

CRITICAL ISSUES

While it is clear that some central signaling pathways controlling lifespan are broadly conserved across large evolutionary distances, it is less clear to what extent downstream molecular mechanisms of aging are conserved. In this review we discuss the biology of C. elegans and mammals in the context of aging and age-dependent diseases. We consider evidence from studies that attempt to investigate basic, possibly conserved mechanisms of aging especially in the context of the free radical theory of aging. Practical points, such as the need for blinding of lifespan studies and for appropriate biomarkers, are also considered.

FUTURE DIRECTIONS

As data on the aging process(es) in different organisms increase, it is becoming increasingly clear that there are both conserved (public) and private aspects to aging. It is important to explore the dividing lines between these two aspects and to be aware of the large gray areas in-between.

Knockout of the adp gene related with colonization in Bacillus nematocida B16 using customized transcription activator-like effectors nucleases

Bacillus nematocida B16 is able to dominate in the intestines of the worm Caenorhabditis elegans in 'Trojan horse' pathogenic mechanism. The adp is one candidate gene which potentially play a vital role in the colonization from our previous random mutagenesis screening results. To analyse the functional role of this gene, we constructed the adp knockout mutant through customized transcription activator-like effectors nucleases (TALEN), which has been successfully used in yeasts, nematodes, zebrafish and human pluripotent cells. Here, we first time report this knockout method in bacteria on this paper. Bioassay experiments demonstrated that the adp knockout mutant of B16 showed considerably lower colonization activity, reduced numbers of intestines and less than 80% nematocidal activity compared with the wild-type strain when infected for 48 h. However, no obvious change on proteolytic activity was observed in the mutant. Conversely, the complementation of adp gene restored most of the above deficient phenotypes. These results indicated that the adp gene was involved in surface adhesion and played a comparatively important role in colonizing host nematodes. Moreover, TALENs successfully disrupt target genes in bacteria.

Host Antimicrobial Peptides in Bacterial Homeostasis and Pathogenesis of Disease

Innate immune responses function as a first line of host defense against the development of bacterial infection, and in some cases to preserve the sterility of privileged sites in the human host. Bacteria that enter these sites must counter host responses for colonization. From the host's perspective, the innate immune system works expeditiously to minimize the bacterial threat before colonization and subsequent dysbiosis. The multifactorial nature of disease further challenges predictions of how each independent variable influences bacterial pathogenesis. From bacterial colonization to infection and through disease, the microenvironments of the host are in constant flux as bacterial and host factors contribute to changes at the host-pathogen interface, with the host attempting to eradicate bacteria and the bacteria fighting to maintain residency. A key component of this innate host response towards bacterial infection is the production of antimicrobial peptides (AMPs). As an early component of the host response, AMPs modulate bacterial load and prevent establishment of infection. Under quiescent conditions, some AMPs are constitutively expressed by the epithelium. Bacterial infection can subsequently induce production of other AMPs in an effort to maintain sterility, or to restrict colonization. As demonstrated in various studies, the absence of a single AMP can influence pathogenesis, highlighting the importance of AMP concentration in maintaining homeostasis. Yet, AMPs can increase bacterial virulence through the co-opting of the peptides or alteration of bacterial virulence gene expression. Further, bacterial factors used to subvert AMPs can modify host microenvironments and alter colonization of the residential flora that principally maintain homeostasis. Thus, the dynamic interplay between host defense peptides and bacterial factors produced to quell peptide activity play a critical role in the progression and outcome of disease.

Antinematode activity of Violacein and the role of the insulin/IGF-1 pathway in controlling violacein sensitivity in Caenorhabditis elegans

The purple pigment violacein is well known for its numerous biological activities including antibacterial, antiviral, antiprotozoan, and antitumor effects. In the current study we identify violacein as the antinematode agent produced by the marine bacterium Microbulbifer sp. D250, thereby extending the target range of this small molecule. Heterologous expression of the violacein biosynthetic pathway in E. coli and experiments using pure violacein demonstrated that this secondary metabolite facilitates bacterial accumulation in the nematode intestine, which is accompanied by tissue damage and apoptosis. Nematodes such as Caenorhabditis elegans utilise a well-defined innate immune system to defend against pathogens. Using C. elegans as a model we demonstrate the DAF-2/DAF-16 insulin/IGF-1 signalling (IIS) component of the innate immune pathway modulates sensitivity to violacein-mediated killing. Further analysis shows that resistance to violacein can occur due to a loss of DAF-2 function and/or an increased function of DAF-16 controlled genes involved in antimicrobial production (spp-1) and detoxification (sod-3). These data suggest that violacein is a novel candidate antinematode agent and that the IIS pathway is also involved in the defence against metabolites from non-pathogenic bacteria.

Intestinal autophagy activity is essential for host defense against Salmonella typhimurium infection in Caenorhabditis elegans

Salmonella typhimurium infects both intestinal epithelial cells and macrophages. Autophagy is a lysosomal degradation pathway that is present in all eukaryotes. Autophagy has been reported to limit the Salmonella replication in Caenorhabditis elegans and in mammals. However, it is unknown whether intestinal autophagy activity plays a role in host defense against Salmonella infection in C. elegans. In this study, we inhibited the autophagy gene bec-1 in different C. elegans tissues and examined the survival of these animals following Salmonella infection. Here we show that inhibition of the bec-1 gene in the intestine but not in other tissues confers susceptibility to Salmonella infection, which is consistent with recent studies in mice showing that autophagy is involved in clearance of Salmonella in the intestinal epithelial cells. Therefore, the intestinal autophagy activity is essential for host defense against Salmonella infection from C. elegans to mice, perhaps also in humans.

A protocol to infect Caenorhabditis elegans with Salmonella typhimurium

In the last decade, C. elegans has emerged as an invertebrate organism to study interactions between hosts and pathogens, including the host defense against gram-negative bacterium Salmonella typhimurium. Salmonella establishes persistent infection in the intestine of C. elegans and results in early death of infected animals. A number of immunity mechanisms have been identified in C. elegans to defend against Salmonella infections. Autophagy, an evolutionarily conserved lysosomal degradation pathway, has been shown to limit the Salmonella replication in C. elegans and in mammals. Here, a protocol is described to infect C. elegans with Salmonella typhimurium, in which the worms are exposed to Salmonella for a limited time, similar to Salmonella infection in humans. Salmonella infection significantly shortens the lifespan of C. elegans. Using the essential autophagy gene bec-1 as an example, we combined this infection method with C. elegans RNAi feeding approach and showed this protocol can be used to examine the function of C. elegans host genes in defense against Salmonella infection. Since C. elegans whole genome RNAi libraries are available, this protocol makes it possible to comprehensively screen for C. elegans genes that protect against Salmonella and other intestinal pathogens using genome-wide RNAi libraries.

Activated and inactivated immune responses in Caenorhabditis elegans against Photorhabdus luminescens TT01

The Gram-negative bacterium Photorhabdus luminescens which symbiotically associates with the entomopathogenic nematode Heterorhabditis bacteriophora, has a broad insecticidal and nematicidal activity. The virulence of P. luminescens toward the non-mutualistic nematode Caenorhabditis elegans has not been described. We showed that when fed on P. luminescens, the intestinal cells of C. elegans worms become delicate and some crystal-like structure was developed within the intestinal lumen. Next, we examined the requirement of the p38 mitogen-activated protein kinase (MAPK) and insulin/IGF-1 signaling pathway against P. luminescens. Depletion of pmk-1 by RNAi enhances susceptibility to P. luminescens, and numerous downstream targets regulated by the p38 MAPK pathway were induced when fed on P. luminescens. On the other hand, knockdown of daf-16 has no effects on C. elegans lifespan, but knockdown of daf-2 dramatically increased resistance to P. luminescens in a daf-16-dependent manner. We also revealed one of the daf-2 ligands ins-7 was induced and ins-7 deletion mutant survived longer when fed on P. luminescens. These results suggest the p38 MAPK pathway is activated and required for the host defense against P. luminescens. Insulin/IGF-1 signaling pathway is inactivated by P. luminescens through the overexpression of insulin-like gene.

Helicobacter and salmonella persistent infection strategies

Some host-adapted bacterial pathogens are capable of causing persistent infections in humans. For example, Helicobacter pylori inhabits the human gastric mucosa and persistence can be lifelong. Salmonella enterica serovar Typhi causes systemic infections that involve colonization of the reticuloendothelial system and some individuals become lifelong carriers. In this review, I compare and contrast the different lifestyles of Helicobacter and Salmonella within the host and the strategies they have evolved to persist in mammalian hosts. Persistently infected carriers serve as the reservoirs for these pathogens, and the carrier state is an essential feature that is required for survival of the bacteria within a restricted host population. Therefore, investigating the chronic carrier state should provide insight into bacterial survival strategies, as well as new therapeutic approaches for treatments.

A novel antisense RNA from the Salmonella virulence plasmid pSLT expressed by non-growing bacteria inside eukaryotic cells

Bacterial small RNAs (sRNAs) are regulatory molecules playing relevant roles in response to environmental changes, stressful conditions and pathogenesis. The intracellular bacterial pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium) is known to regulate expression of some sRNAs during colonization of fibroblasts. Here, we characterize a previously unknown sRNA encoded in the S. Typhimurium pSLT virulence plasmid that is specifically up-regulated by non-growing dormant bacteria persisting inside fibroblasts. This sRNA was inferred in microarray expression analyses, which unraveled enhanced transcriptional activity in the PSLT047- PSLT046 (mig5) intergenic region. The sRNA transcript was further identified as a 597-nucleotide molecule, which we named IesR-1, for ‘Intracellular-expressed-sRNA-1′. IesR-1 expression is low in bacteria growing in axenic cultures across a variety of experimental conditions but displays a marked increase (∼200–300 fold) following bacterial entry into fibroblasts. Remarkably, induction of IesR-1 expression is not prominent in bacteria proliferating within epithelial cells. IesR-1 deletion affects the control of bacterial growth in defined fibroblast cell lines and impairs virulence in a mouse infection model. Expression analyses performed in the PSLT047-iesR-1-PSLT046 (mig5) region support a cis-acting regulatory mechanism of IesR-1 as antisense RNA over the PSLT047 transcript involving interaction at their respective 3′ ends and modulation of PSLT047 protein levels. This model is sustained by the scarce production of PSLT047 protein observed in non-growing intracellular bacteria and the high amount of PSLT047 protein produced by bacteria carrying a truncated IesR-1 version with separated 5′ and 3′ regions. Taken together, these data reveal that S. Typhimurium sRNAs encoded in the pSLT virulence plasmid respond to a state of persistence inside the host cell. As exemplified by IesR-1, some of these sRNAs may contribute to diminish the relative levels of proteins, such as PSLT047, which are probably dispensable for the intracellular lifestyle.

Identification of virulence properties in Salmonella Typhimurium DT104 using Caenorhabditis elegans

Salmonella enterica serover Typhimurium definitive phage type DT104, resistant to multiple antibiotics, is one of the most widespread Salmonella species in human infection worldwide. Although several cohort studies indicate that DT104 carrying the multidrug resistance (MDR) locus on salmonella genomic island 1 is a possible hyper-virulent strain compared to DT104 strains without MDR, or other Salmonella enterica serotypes, existing experimental evidence regarding virulence properties associated with the MDR region is controversial. To address this question, we constructed an isogenic MDR deletion (∆MDR) mutant strain of DT104, SNS12, by allelic exchange and used Caenorhabditis elegans as a host model to assess differences in virulence between these two strains. SNS12 exhibited decreased virulence in C. elegans, and we observed increased colonization and proliferation of the intestine of C. elegans by DT104. The immune response against MDR-carrying DT104 appears to function through a non-canonical Unfolded Protein Response (UPR) pathway, namely prion-like-(QN-rich)-domain-bearing protein pathway (PQN), in a ced-1 dependent manner in C. elegans. Further, we also demonstrate that genes of the PQN pathway and antimicrobial peptide gene abf-2, are expressed at higher transcriptional levels in worms immediately following exposure to DT104, in comparison with worms exposed to SNS12. Altogether, our results suggest that the MDR region of Salmonella Typhimurium DT104 has a direct role in virulence against Caenorhabditis elegans.

Salmonella typhimurium intercepts Escherichia coli signaling to enhance antibiotic tolerance

Bacterial communication plays an important role in many population-based phenotypes and interspecies interactions, including those in host environments. These interspecies interactions may prove critical to some infectious diseases, and it follows that communication between pathogenic bacteria and commensal bacteria is a subject of growing interest. Recent studies have shown that Escherichia coli uses the signaling molecule indole to increase antibiotic tolerance throughout its population. Here, we show that the intestinal pathogen Salmonella typhimurium increases its antibiotic tolerance in response to indole, even though S. typhimurium does not natively produce indole. Increased antibiotic tolerance can be induced in S. typhimurium by both exogenous indole added to clonal S. typhimurium populations and indole produced by E. coli in mixed-microbial communities. Our data show that indole-induced tolerance in S. typhimurium is mediated primarily by the oxidative stress response and, to a lesser extent, by the phage shock response, which were previously shown to mediate indole-induced tolerance in E. coli. Further, we find that indole signaling by E. coli induces S. typhimurium antibiotic tolerance in a Caenorhabditis elegans model for gastrointestinal infection. These results suggest that the intestinal pathogen S. typhimurium can intercept indole signaling from the commensal bacterium E. coli to enhance its antibiotic tolerance in the host intestine.

Effect of Caenorhabditis elegans age and genotype on horizontal gene transfer in intestinal bacteria

Horizontal gene transfer (HGT) between bacteria occurs in the intestinal tract of their animal hosts and facilitates both virulence and antibiotic resistance. A model in which both the pathogen and the host are genetically tractable facilitates developing insight into mechanistic processes enabling or restricting the transfer of antibiotic resistance genes. Here we develop an in vivo experimental system to study HGT in bacteria using Caenorhabditis elegans as a model host. Using a thermosensitive conjugative system, we provide evidence that conjugation between two Escherichia coli strains can take place in the intestinal lumen of N2 wild-type worms at a rate of 10(-3) and 10(-2) per donor. We also show that C. elegans age and genotype are important determinants of the frequency of conjugation. Whereas ∼1 transconjugant for every 100 donor cells could be recovered from the intestine of N2 C. elegans, for the age-1 and tol-1 mutants, the detected rate of transconjugation (10(-3) and 10(-4) per donor cell, respectively) was significantly lower. This work demonstrates that increased recombination among lumenal microbial populations is a phenotype associated with host aging, and the model provides a framework to study the dynamics of bacterial horizontal gene transfer within the intestinal environment.

Burkholderia pseudomallei kills Caenorhabditis elegans through virulence mechanisms distinct from intestinal lumen colonization

The nematode Caenorhabditis elegans is hypersusceptible to Burkholderia pseudomallei infection. However, the virulence mechanisms underlying rapid lethality of C. elegans upon B. pseudomallei infection remain poorly defined. To probe the host-pathogen interaction, we constructed GFP-tagged B. pseudomallei and followed bacterial accumulation within the C. elegans intestinal lumen. Contrary to slow-killing by most bacterial pathogens, B. pseudomallei caused fairly limited intestinal lumen colonization throughout the period of observation. Using grinder-defective mutant worms that allow the entry of intact bacteria also did not result in full intestinal lumen colonization. In addition, we observed a significant decline in C. elegans defecation and pharyngeal pumping rates upon B. pseudomallei infection. The decline in defecation rates ruled out the contribution of defecation to the limited B. pseudomallei colonization. We also demonstrated that the limited intestinal lumen colonization was not attributed to slowed host feeding as bacterial loads did not change significantly when feeding was stimulated by exogenous serotonin. Both these observations confirm that B. pseudomallei is a poor colonizer of the C. elegans intestine. To explore the possibility of toxin-mediated killing, we examined the transcription of the C. elegans ABC transporter gene, pgp-5, upon B. pseudomallei infection of the ppgp-5::gfp reporter strain. Expression of pgp-5 was highly induced, notably in the pharynx and intestine, compared with Escherichia coli-fed worms, suggesting that the host actively thwarted the pathogenic assaults during infection. Collectively, our findings propose that B. pseudomallei specifically and continuously secretes toxins to overcome C. elegans immune responses.

Control of intestinal bacterial proliferation in regulation of lifespan in Caenorhabditis elegans

Background

A powerful approach to understanding complex processes such as aging is to use model organisms amenable to genetic manipulation, and to seek relevant phenotypes to measure. Caenorhabditis elegans is particularly suited to studies of aging, since numerous single-gene mutations have been identified that affect its lifespan; it possesses an innate immune system employing evolutionarily conserved signaling pathways affecting longevity. As worms age, bacteria accumulate in the intestinal tract. However, quantitative relationships between worm genotype, lifespan, and intestinal lumen bacterial load have not been examined. We hypothesized that gut immunity is less efficient in older animals, leading to enhanced bacterial accumulation, reducing longevity. To address this question, we evaluated the ability of worms to control bacterial accumulation as a functional marker of intestinal immunity.

Results

We show that as adult worms age, several C. elegans genotypes show diminished capacity to control intestinal bacterial accumulation. We provide evidence that intestinal bacterial load, regulated by gut immunity, is an important causative factor of lifespan determination; the effects are specified by bacterial strain, worm genotype, and biologic age, all acting in concert.

Conclusions

In total, these studies focus attention on the worm intestine as a locus that influences longevity in the presence of an accumulating bacterial population. Further studies defining the interplay between bacterial species and host immunity in C. elegans may provide insights into the general mechanisms of aging and age-related diseases.

Salmonella's long-term relationship with its host

Host-adapted strains of Salmonella enterica cause systemic infections and have the ability to persist systemically for long periods of time and pose significant public-health problems. Multidrug-resistant S. enterica serovar Typhi (S. Typhi) and nontyphoidal Salmonella (NTS) are on the increase and are often associated with HIV infection. Chronically infected hosts are often asymptomatic and transmit disease to naïve hosts via fecal shedding of bacteria, thereby serving as a critical reservoir for disease. Salmonella utilizes multiple ways to evade and modulate host innate and adaptive immune responses in order to persist in the presence of a robust immune response. Survival in macrophages and modulation of immune cells migration allow Salmonella to evade various immune responses. The ability of Salmonella to persist depends on a balance between immune responses that lead to the clearance of the pathogen and avoidance of damage to host tissues.

Competition and resilience between founder and introduced bacteria in the Caenorhabditis elegans gut

The microbial communities that reside within the intestinal tract in vertebrates are complex and dynamic. In this report, we establish the utility of Caenorhabditis elegans as a model system for identifying the factors that contribute to bacterial persistence and for host control of gut luminal populations. We found that for N2 worms grown on mixed lawns of bacteria, Salmonella enterica serovar Typhimurium substantially outcompeted Escherichia coli, even when E. coli was initially present at 100-fold-higher concentrations. To address whether innate immunity affects the competition, the daf-2 and daf-16 mutants were studied; their total gut bacterial levels reflect overall capacity for colonization, but Salmonella outcompeted E. coli to an extent similar to wild-type worms. To address the role of virulence properties, Salmonella Δspi-1 Δspi-2 was used to compete with E. coli. The net differential was significantly less than that for wild-type Salmonella; thus, spi-1 spi-2 encodes C. elegans colonization factors. An E. coli strain with repeated in vivo passage had an enhanced ability to compete against an in vitro-passed E. coli strain and against Salmonella. Our data provide evidence of active competition for colonization niches in the C. elegans gut, as determined by bacterial factors and subject to in vivo selection.

Caenorhabditis elegans, a model organism for investigating immunity

The nematode Caenorhabditis elegans has been a powerful experimental organism for almost half a century. Over the past 10 years, researchers have begun to exploit the power of C. elegans to investigate the biology of a number of human pathogens. This work has uncovered mechanisms of host immunity and pathogen virulence that are analogous to those involved during pathogenesis in humans or other animal hosts, as well as novel immunity mechanisms which appear to be unique to the worm. More recently, these investigations have uncovered details of the natural pathogens of C. elegans, including the description of a novel intracellular microsporidian parasite as well as new nodaviruses, the first identification of viral infections of this nematode. In this review, we consider the application of C. elegans to human infectious disease research, as well as consider the nematode response to these natural pathogens.

The two-component sensor kinase KdpD is required for Salmonella typhimurium colonization of Caenorhabditis elegans and survival in macrophages

The ability of enteric pathogens to perceive and adapt to distinct environments within the metazoan intestinal tract is critical for pathogenesis; however, the preponderance of interactions between microbe- and host-derived factors remain to be fully understood. Salmonella enterica serovar Typhimurium is a medically important enteric bacterium that colonizes, proliferates and persists in the intestinal lumen of the nematode Caenorhabditis elegans. Several Salmonella virulence factors important in murine and tissue culture models also contribute to worm mortality and intestinal persistence. For example, PhoP and the virulence plasmid pSLT are virulence factors required for resistance to the C. elegans antimicrobial peptide SPP-1. To uncover additional determinants required for Salmonella typhimurium pathogenesis in vivo, we devised a genetic screen to identify bacterial mutants defective in establishing a persistent infection in the intestine of C. elegans. Here we report on identification of 14 loci required for persistence in the C. elegans intestine and characterization of KdpD, a sensor kinase of a two-component system in S. typhimurium pathogenesis. We show that kdpD mutants are profoundly attenuated in intestinal persistence in the nematode and in macrophage survival. These findings may be attributed to the essential role KdpD plays in promoting resistance to osmotic, oxidative and antimicrobial stresses.

Innate immunity in C. elegans

The nematode Caenorhabditis elegans is proving to be a powerful invertebrate model to study host-pathogen interactions. In common with other invertebrates, C. elegans relies solely on its innate immune system to defend itself against pathogens. Studies of the nematode response to infection with various fungal and bacterial pathogens have revealed that the innate immune system of C. elegans employs evolutionary conserved signalling pathways. They regulate the expression of various effectors molecules, some of which are also conserved. Here, we summarize the current knowledge of the pathways and effector molecules involved in the nematode immune response, with a particular focus on the antifungal immune response of the C. elegans epidermis.

C. elegans: model host and tool for antimicrobial drug discovery

For almost four decades, the nematode Caenorhabditis elegans has been of great value in many fields of biological research. It is now used extensively in studies of microbial pathogenesis and innate immunity. The worm lacks an adaptive immune system and relies solely on its innate immune defences to cope with pathogen attack. Infectious microbes, many of which are of clinical interest, trigger specific mechanisms of innate immunity, and provoke the expression of antifungal or antibacterial polypeptides. In this review, we highlight some of these families of antimicrobial peptides (AMPs) and proteins that are candidates for the development of novel antibiotics. In addition, we describe how systems of C. elegans infection provide an increasing number of possibilities for large-scale in vivo screens for the discovery of new antimicrobial drugs. These systems open promising perspectives for innovative human therapies.

It takes nerves to fight infections: insights on neuro-immune interactions from C. elegans

The innate immune response is evoked as a consequence of interactions between invading foreign infectious agents and host immune cells. A successful innate immune response is pivotal in maintaining the delicate balance between health and disease; an insufficient response results in infection, whereas an excessive response results in prolonged inflammation and tissue damage. Alterations in the state and function of the nervous system influence the immune response. The nervous system regulates innate immune responses through the release of neurotransmitters, neuropeptides and neurohormones. However, many questions related to the molecular and cellular mechanisms involved, the physiological role of the link between the immune and the nervous system, and the biological significance of neuro-immune interactions remain unresolved. The interactions between the nematode Caenorhabditis elegans and its pathogens provide insights into mechanisms of neuroendocrine regulation of immunity and address many outstanding issues related to neuro-immune interactions.

Interkingdom signaling between pathogenic bacteria and Caenorhabditis elegans

Investigators have recently turned to the soil nematode Caenorhabditis elegans as a small animal infection model to study infectious disease. To extrapolate findings concerning bacterial pathogenesis from non-mammals to mammals, virulence factors should be conserved in function, independent of the infection model. Emerging from these studies is the observation that bacterial virulence regulatory networks function in a conserved manner across multiple hosts, including nematodes, mice and plants. Several regulatory networks have been implicated in nematode innate immune function and are being exploited in the C. elegans infection model to develop novel chemical therapies against bacterial pathogens.

Phenotypic covariance of longevity, immunity and stress resistance in the caenorhabditis nematodes

BACKGROUND

Ageing, immunity and stresstolerance are inherent characteristics of all organisms. In animals, these traits are regulated, at least in part, by forkhead transcription factors in response to upstream signals from the Insulin/Insulin-like growth factor signalling (IIS) pathway. In the nematode Caenorhabditis elegans, these phenotypes are molecularly linked such that activation of the forkhead transcription factor DAF-16 both extends lifespan and simultaneously increases immunity and stress resistance. It is known that lifespan varies significantly among the Caenorhabditis species but, although DAF-16 signalling is highly conserved, it is unclear whether this phenotypic linkage occurs in other species. Here we investigate this phenotypic covariance by comparing longevity, stress resistance and immunity in four Caenorhabditis species.

METHODOLOGY/PRINCIPAL FINDINGS

We show using phenotypic analysis of DAF-16 influenced phenotypes that among four closely related Caenorhabditis nematodes, the gonochoristic species (Caenorhabditis remanei and Caenorhabditis brenneri) have diverged significantly with a longer lifespan, improved stress resistance and higher immunity than the hermaphroditic species (C. elegans and Caenorhabditis briggsae). Interestingly, we also observe significant differences in expression levels between the daf-16 homologues in these species using Real-Time PCR, which positively correlate with the observed phenotypes. Finally, we provide additional evidence in support of a role for DAF-16 in regulating phenotypic coupling by using a combination of wildtype isolates, constitutively active daf-16 mutants and bioinformatic analysis.

CONCLUSIONS

The gonochoristic species display a significantly longer lifespan (p<0.0001) and more robust immune and stress response (p<0.0001, thermal stress; p<0.01, heavy metal stress; p<0.0001, pathogenic stress) than the hermaphroditic species. Our data suggests that divergence in DAF-16 mediated phenotypes may underlie many of the differences observed between these four species of Caenorhabditis nematodes. These findings are further supported by the correlative higher daf-16 expression levels among the gonochoristic species and significantly higher lifespan, immunity and stress tolerance in the constitutively active daf-16 hermaphroditic mutants.

Molecular mechanisms of Salmonella virulence and host resistance

Salmonella species can cause typhoid fever and gastroenteritis in humans and pose a global threat to human health. In order to establish a successful infection, Salmonella utilize a large number of genes encoding a variety of virulence factors. Different animal models of infection have been used to better understand the mechanisms underlying each disease including cattle, rodents, and nematodes. To date, a number of different bacterial virulence factors have been identified using such animal models, most of which are secreted by two type three secretion systems (T3SS) encoded within Salmonella pathogenicity islands (SPI) 1 and 2. These proteins alter various host cell pathways, facilitating the invasion of epithelial cells during infection, as well as the survival and replication of Salmonella inside phagocytic cells. On the other hand, host genetics and resistance also play a role in the susceptibility to Salmonella infection. The natural resistance-associated macrophage protein 1 (Nramp1), for example, is critical for host defense, since mice lacking Nramp1 fail to control bacterial replication and succumb to low doses of S. Typhimurium. In this chapter, we analyze the different pathogen and host factors that play a role in the dynamic interaction between Salmonella and its host and their impact on disease.

Caenopores are antimicrobial peptides in the nematode Caenorhabditis elegans instrumental in nutrition and immunity

For the soil nematode Caenorhabditis elegans, microbes are both food source and potential pathogens. Intrinsic antibiotic agents such as antimicrobial peptides (AMP) are important to protect the worm against infection. Here, we show that among potential antimicrobial peptides of C. elegans, with respect to gene number, the majority belongs to the SPP-protein family which we named caenopores as they resemble structurally and functionally amoebapores. SPP-5 kills bacteria by permeabilizing their cytoplasmic membrane and displays pore-forming activity as judged by liposome depolarization. The antimicrobial polypeptide is required to cope with Escherichia coli, the regular food source of C. elegans in the laboratory, as worms devoid of this weapon develop poorly, permitting a substantial number of bacteria to survive in the intestine. As numerous caenopores exert their activity in the intestinal lumen, an environment mimicking with its acidic pH and the presence of hydrolytic enzymes, the interior of phagolysosomes, individual members may be operative in eliminating distinct groups of microorganisms that enter this tract by food consumption. Individual spp genes are induced upon contact with particular bacteria, whereas others are expressed regardless of the bacteria they live on. The multifarious caenopore family of antimicrobial peptides may have been a key event that enables C. elegans to live and survive in its natural habitat, on rotting organic material.