Caenorhabditis elegans employs innate and learned aversion in response to bacterial toxic metabolites tambjamine and violacein

Microbial metabolites as engines of behavioral variation across animals

The microbiome, especially that present in the gut, has emerged as a key modulator of animal behavior. However, the extent of its influence across species and behavioral repertoires, as well as the underlying mechanisms, remains poorly understood. Increasing evidence suggests that microbial metabolites play an important role in driving behavioral variation. In this review, we synthesize findings from vertebrates to invertebrates, spanning both model and non-model organisms, to define key groups of microbial-derived metabolites involved in modulating seven distinct behaviors: nutrition, olfaction, circadian rhythms, reproduction, locomotion, aggression, and social interactions. We discuss how these microbial metabolites interact with host chemosensory systems, neurotransmitter signaling, and epigenetic modifications to shape behavior. Additionally, we highlight critical gaps in mechanistic understanding, including the need to map additional host receptors and signaling pathways, as well as the untapped potential of microbial biosynthetic gene clusters as sources for novel bioactive compounds. Advancing these areas will enhance understanding of the microbiome's role in behavioral modulation and open new avenues for microbiome-based interventions for behavioral disorders.

Behavioral adaptations of Caenorhabditis elegans against pathogenic threats

This review examines the behavioral adaptation mechanisms of Caenorhabditis elegans in response to pathogenic bacterial threats, emphasizing their ecological significance. It systematically explores how mechanisms such as avoidance behavior, transgenerational learning, and forgetting enable C. elegans to optimize its survival and reproductive strategies within dynamic microbial environments. C. elegans detects harmful signals through chemosensation and initiates avoidance behaviors. Simultaneously, it manages environmental adaptation and energy allocation through transgenerational memory and forgetting, allowing C. elegans to cope with selective pressures from environmental fluctuations. In contrast, pathogenic bacteria such as Pseudomonas aeruginosa and Salmonella influence C. elegans behavior through strategies such as toxin release and biofilm formation, highlighting the complex co-evolutionary dynamics between hosts and pathogens. Additionally, these pathogens employ "Trojan Horse-like" and "Worm Star" mechanisms to kill C. elegans, further complicating host-pathogen interactions. These processes are driven by behavioral adaptations, biochemical signaling, and evolutionary pressures, which emphasize the ecological niche of C. elegans within microbial ecosystems. C. elegans serves as a valuable model for studying host-pathogen interactions. This study provides crucial theoretical insights into adaptive evolution and ecosystem dynamics, offering valuable guidance for the development of biocontrol strategies and the effective management of microbial ecosystems.

Nonribosomal peptides protect Pseudomonas nunensis 4A2e from amoebal and nematodal predation

The rhizosphere is a highly competitive environment forcing bacteria to evolve strategies to oppose their enemies. The production of toxic secondary metabolites allows bacteria to counteract predators. In this study, we describe the anti-predator armamentarium of the soil-derived bacterium Pseudomonas nunensis 4A2e. Based on a genome mining approach, we identified several biosynthetic gene clusters coding for nonribosomal peptide synthetases. Generation of gene deletion mutants of the respective clusters shows a loss of defense capabilities. We isolated the novel lipopeptides keanumycin D and nunapeptins B and C, and fully elucidated their structures by a combination of in-depth mass spectrometry experiments, stable isotope labelling, and chemical synthesis. Additionally, investigation of the quorum sensing-dependent biosynthesis allowed us to elucidate parts of the underlying regulation of the biosynthetic machinery. Ecology-inspired bioassays highlight the role of these peptides as a defence strategy against protozoans and led us to find a previously unknown function against the bacterivorous nematode Oscheius myriophilus.

The Bacterial Gq Signal Transduction Inhibitor FR900359 Impairs Soil-Associated Nematodes

The cyclic depsipeptide FR900359 (FR) is derived from the soil bacterium Chromobacterium vaccinii and known to bind Gq proteins of mammals and insects, thereby abolishing the signal transduction of their Gq protein-coupled receptors, a process that leads to severe physiological consequences. Due to their highly conserved structure, Gq family of proteins are a superior ecological target for FR producing organisms, resulting in a defense towards a broad range of harmful organisms. Here, we focus on the question whether bacteria like C. vaccinii are important factors in soil in that their secondary metabolites impair, e.g., plant harming organisms like nematodes. We prove that the Gq inhibitor FR is produced under soil-like conditions. Furthermore, FR inhibits heterologously expressed Gαq proteins of the nematodes Caenorhabditis elegans and Heterodera schachtii in the micromolar range. Additionally, in vivo experiments with C. elegans and the plant parasitic cyst nematode H. schachtii demonstrated that FR reduces locomotion of C. elegans and H. schachtii. Finally, egg-laying of C. elegans and hatching of juvenile stage 2 of H. schachtii from its cysts is inhibited by FR, suggesting that FR might reduce nematode dispersion and proliferation. This study supports the idea that C. vaccinii and its excreted metabolome in the soil might contribute to an ecological equilibrium, maintaining and establishing the successful growth of plants.

Within- and between-host dynamics of producer and non-producer pathogens

For infections to be maintained in a population, pathogens must compete to colonize hosts and transmit between them. We use an experimental approach to investigate within-and-between host dynamics using the pathogen Pseudomonas aeruginosa and the animal host Caenorhabditis elegans. Within-host interactions can involve the production of goods that are beneficial to all pathogens in the local environment but susceptible to exploitation by non-producers. We exposed the nematode host to ‘producer’ and two ‘non-producer’ bacterial strains (specifically for siderophore production and quorum sensing), in single infections and coinfections, to investigate within-host colonization. Subsequently, we introduced infected nematodes to pathogen-naive populations to allow natural transmission between hosts. We find that producer pathogens are consistently better at colonizing hosts and transmitting between them than non-producers during coinfection and single infection. Non-producers were poor at colonizing hosts and between-host transmission, even when coinfecting with producers. Understanding pathogen dynamics across these multiple levels will ultimately help us predict and control the spread of infections, as well as contribute to explanations for the persistence of cooperative genotypes in natural populations.

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.

Multi-target drug with potential applications: violacein in the spotlight

The aim of the current review is to address updated research on a natural pigment called violacein, with emphasis on its production, biological activity and applications. New information about violacein's action mechanisms as antitumor agent and about its synergistic action in drug delivery systems has brought new alternatives for anticancer therapy. Thus, violacein is introduced as reliable drug capable of overcoming at least three cancer hallmarks, namely: proliferative signaling, cell death resistance and metastasis. In addition, antimicrobial effects on several microorganisms affecting humans and other animals turn violacein into an attractive drug to combat resistant pathogens. Emphasis is given to effects of violacein combined with different agents, such as antibiotics, anticancer agents and nanoparticles. Although violacein is well-known for many decades, it remains an attractive compound. Thus, research groups have been making continuous effort to help improving its production in recent years, which can surely enable its pharmaceutical and chemical application as multi-task compound, even in the cosmetics and food industries.

Measurement of the Effects of Metals on Taxis-to-Food Behavior in Caenorhabditis elegans

Chemosensation in nematodes is linked to processes that affect their ability to survive, such as the search for food and the avoidance of toxic substances. Since the 1970s, numerous studies have assessed chemotaxis in the nematode species Caenorhabditis elegans, focusing on a multitude of agents, including bacteria (food), ions, salts, hormones, volatile organic compounds, and, to a lesser extent, metal-contaminated medium/food. The few studies evaluating metal exposure have reported a variety of responses (neutral, attraction, avoidance), which generally appear to be contaminant and/or concentration specific. Differences in experimental designs, however, hinder appropriate comparison of the findings and attainment of firm conclusions. Therefore, we herein propose and describe a detailed protocol for the assessment of the effects of metals on taxis-to-food behavior in C. elegans. Distinct approaches are proposed in two innovative stages of testing to (1) screen metals' effects on taxis-to-food behavior and (2) classify the behavioral response as attraction/avoidance/indifference or preference. Use of such a standard protocol will allow for easy comparison across studies and direct interpretation of results. Findings using this model system can contribute to a deeper understanding of the real risks of metal contamination to nematodes and how such contaminants could impact ecosystems in general, given the key environmental roles that these organisms play. © 2021 Wiley Periodicals LLC. Basic Protocol: Assessing the effects of metal contamination on taxis-to-food behavior in Caenorhabditis elegans Support Protocol 1: Synchronization of C. elegans by hand-picking gravid worms Support Protocol 2: Synchronization of C. elegans by using a bleaching solution.

Overview of Chemotaxis Behavior Assays in Caenorhabditis elegans

Environmental pollution related to anthropogenic pressures, and the associated repercussions on public health, represent a worldwide problem. Thus, the study of the effects that environmental contaminants can pose to natural ecosystems and human health is of vital importance. Laboratory model organisms such as Caenorhabditis elegans have played a significant role in clarifying multilevel effects of those agents. Although the evaluation of contaminant effects at the behavioral level of organisms is an emerging approach in ecotoxicology, studies assessing chemotaxis behavior in C. elegans within the ecotoxicological research context are still scarce. Chemotaxis studies in C. elegans have contributed to the understanding of both the neuronal mechanisms involved in the behavioral effects triggered by environmental cues and the impact of contaminants on natural ecosystems. Its compact and well-characterized nervous system, as well as the availability of transgenic strains and molecular tools, allows a detailed examination of behavioral, molecular, and genetic chemosensation mechanisms. This overview provides a summary and general comparison of methods used to measure chemotaxis behavior in C. elegans, with the aim of helping researchers select the most suitable approach in their chemotaxis studies. We compare methods based on the type of chemical tested, advantages and drawbacks of the different approaches, and specific experimental goals. Lastly, we hope to encourage the evaluation of C. elegans chemotaxis behavior in ecotoxicology studies, as well as its potential integration in standardized protocols assessing environmental quality. © 2021 Wiley Periodicals LLC.

Microbial synthesis of violacein pigment and its potential applications

Violacein is a pigment synthesized by Gram-negative bacteria such as Chromobacterium violaceum. It has garnered significant interest owing to its unique physiological and biological activities along with its synergistic effects with various antibiotics. In addition to C. violaceum, several microorganisms, including: Duganella sp., Pseudoalteromonas sp., Iodobacter sp., and Massilia sp., are known to produce violacein. Along with the identification of violacein-producing strains, the genetic regulation, quorum sensing mechanism, and sequence of the vio-operon involved in the biosynthesis of violacein have been elucidated. From an engineering perspective, the heterologous production of violacein using the genetically engineered Escherichia coli or Citrobacter freundii host has also been attempted. Genetic engineering of host cells involves the heterologous expression of genes involved in the vio operon and the optimization of metabolic pathways and gene regulation. Further, the crystallography of VioD and VioE was revealed, and mass production by enzyme engineering has been accelerated. In this review, we highlight the biologically assisted end-use applications of violacein (such as functional fabric development, nanoparticles, functional polymer composites, and sunscreen ingredients) and violacein activation mechanisms, production strains, and the results of mass production with engineered methods. The prospects for violacein research and engineering applications have also been discussed.

The Role of Neurotransmitters in the Protection of Caenorhabditis Elegans for Salmonella Infection by Lactobacillus

Salmonellosis is a common foodborne disease. We previously reported the protection of Caenorhabditis elegans from Salmonella Typhimurium DT104 infection by Lactobacillus zeae LB1. However, the mechanism is not fully understood. C. elegans exhibits behavior plasticity when presented with diverse pathogenic or commensal bacteria. Whether it can exert approach avoidance to S. Typhimurium through altering its neurological activity remains to be determined. In the current study, both the wild type and mutants defective in serotonin or dopamine production of C. elegans were used to investigate olfactory preference of the nematode to L. zeae LB1, DT104, and Escherichia coli OP50 by choice assays, and its resistance to DT104 infection and the protection offered by L. zeae LB1 using a life-span assay. The expression of target genes in C. elegans was also examined by real-time quantitative PCR. Results showed that pre-exposure to L. zeae LB1 did not elicit aversive olfactory behavior of the nematode toward DT104. Both mutants tph-1 and cat-2 succumbed faster than the wild type when infected with DT104. While pre-exposure to L. zeae LB1 significantly increased the survival of both the wild type and mutant tph-1, it provided no protection to mutant cat-2. Supplementation of dopamine resulted in both the resistance of mutant cat-2 to S. Typhimurium infection and the protection from L. zeae LB1 to the same mutant. Gene expression data also supported the observations in the life-span assay. These results suggest that both serotonin and dopamine play a positive role in the host defense of C. elegans to S. Typhimurium infection and that the L. zeae LB1 protection is not dependent on modifying olfactory preference of the nematode but mediated by dopamine that may have involved the regulation of p38-mitogen-activated protein kinase and insulin/insulin-like growth factor signaling pathways.

Remembering your enemies: mechanisms of within-generation and multigenerational immune priming in Caenorhabditis elegans

Pathogens are abundant and drive evolution of host immunity. Whilst immune memory is classically associated with adaptive immunity, studies in diverse species now show that priming of innate immune defences can also protect against secondary infection. Remarkably, priming may also be passed on to progeny to enhance pathogen resistance and promote survival in future generations. Phenotypic changes that occur independent of DNA sequence underlie both 'within-generation' priming and 'multigenerational' priming. However, the molecular mechanisms responsible for these phenomena are still poorly understood. Caenorhabditis elegans is a simple and genetically tractable model organism that has enabled key advances in immunity and environmental epigenetics. Using both natural and human pathogens, researchers have uncovered numerous examples of innate immune priming in this animal. Viral infection models have provided key evidence for a conserved antiviral RNA silencing mechanism that is inherited in progeny. Bacterial infection models have explored mechanisms of within-generation and multigenerational priming that span chromatin modification and transcriptional changes, small RNA pathways, maternal provisioning and pathogen avoidance strategies. Together, these studies are providing novel insight into the immune reactivity of the genome and have important consequences for our understanding of health and evolution. In this review, we present the current evidence for learned protection against pathogens in C. elegans, discuss the significance and limitations of these findings and highlight important avenues of future investigation.

Consumption of Oleic Acid During Matriphagy in Free-Living Nematodes Alleviates the Toxic Effects of the Bacterial Metabolite Violacein

Maternal behaviors benefit the survival of young, contributing directly to the mother’s reproductive fitness. An extreme form of this is seen in matriphagy, when a mother performs the ultimate sacrifice and offers her body as a meal for her young. Whether matriphagy offers only a single energy-rich meal or another possible benefit to the young is unknown. Here, we characterized the toxicity of a bacterial secondary metabolite, namely, violacein, in Caenorhabditis elegans and found it is not only toxic towards adults, but also arrests growth and development of C. elegans larvae. To counteract this, C. elegans resorted to matriphagy, with the mothers holding their eggs within their bodies and hatching the young larvae internally, which eventually led to the mothers’ death. This violacein-induced matriphagy alleviated some of the toxic effects of violacein, allowing a portion of the internally-hatched young to bypass developmental arrest. Using genetic and pharmacological experiments, we found the consumption of oleate, a monounsaturated fatty acid produced by the mother, during matriphagy is partially responsible. As such, our study provides experimental evidence of why such a drastic and peculiar maternal behavior may have arisen in nematode natural habitats.

Chromobacterium violaceum delivers violacein, a hydrophobic antibiotic, to other microbes in membrane vesicles

This study describes Chromobacterium violaceum's use of extracellular membrane vesicles (MVs) to both solubilize and transport violacein to other microorganisms. Violacein is a hydrophobic bisindole with known antibiotic activities against other microorganisms. Characterization of the MVs found they carried more violacein than protein (1.37 ± 0.19-fold), suggesting they may act as a reservoir for this compound. However, MVs are not produced in response to violacein - a ΔvioA isogenic mutant, which is incapable of making violacein, actually produced significantly more MVs (3.2-fold) than the wild-type strain. Although violacein is insoluble in water (Log P_{octanol:water} = 3.34), 79.5% remained in the aqueous phase when it was present within the C. violaceum MVs, an increase in solubility of 1740-fold. Moreover, tests with a strain of Staphylococcus aureus showed MV-associated violacein is bactericidal, with 3.1 mg/l killing 90% of S. aureus in 6 h. Tests with the ΔvioA MVs found no loss in the S. aureus viability, even when its MVs were added at much higher concentrations, demonstrating violacein is the active component within the wild-type MVs. In conclusion, our study clearly demonstrates C. violaceum produces MVs and uses them as vehicles to solubilize violacein and transport this hydrophobic antibiotic to other microbes.

Diversity and Distribution of Bacteria Producing Known Secondary Metabolites

There is an increasing interest in the utilisation of marine bioactive compounds as novel biopharmaceuticals and agrichemicals; however, little is known about the environmental distribution for many of these molecules. Here, we aimed to elucidate the environmental distribution and to detect the biosynthetic gene clusters in environmental samples of four bioactive compounds, namely violacein, tropodithietic acid (TDA), tambjamine and the antibacterial protein AlpP. Our database analyses revealed high bacterial diversity for AlpP and violacein producers, while TDA-producing bacteria were mostly associated with marine surfaces and all belonged to the roseobacter group. In contrast, the tambjamine cluster was only found in the genomes of two Pseudoalteromonas species and in one terrestrial species belonging to the Cupriavidus genus. Using a PCR-based screen of different marine samples, we detected TDA and violacein genes associated with the microbiome of Ulva and Protohyale niger and tambjamine genes associated with Nodilittorina unifasciata; however, alpP was not detected. These results highlight the variable distribution of the genes encoding these four bioactive compounds, including their detection from the surface of multiple marine eukaryotic hosts. Determining the natural distribution of these gene clusters will help to understand the ecological importance of these metabolites and the bacteria that produce them.

The structure and function of pathogen disgust

Researchers have long noted that many of the multiple elicitors of disgust have some relation to infectious disease. There is an emerging consensus that disgust evolved in Animalia to direct the behaviours that reduce risk of infection, so-called 'parasite avoidance theory'. If this is correct, then the disgust motive should be structured in a manner that reflects the ways in which infectious disease can be avoided. In this study, we generated a set of items based on the epidemiology of disease transmission. These were then rated for their capacity to elicit disgust by a large, predominantly North American/UK sample and subjected to factor analysis to identify latent variables. While a number of plausible factor solutions emerged, Velicer's MAP (minimum average partial) test suggested six domains: atypical appearance, lesions, sex, hygiene, food and animals. This structure did not exactly mirror the transmission routes of infections, as we initially predicted, but it may rather reflect distinct kinds of behavioural tasks involved in avoiding disease. This finding makes sense from the perspective of a cognitive system that evolved under selection for a behavioural response to threats from the social and biological environment. We suggest that regularly occurring types of infectious disease problems have produced regularities in the domain structure of pathogen disgust and discuss the implications of these results for understanding the structure, function and measurement of motives such as disgust in humans and other animals.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Olfactory experience primes the heat shock transcription factor HSF-1 to enhance the expression of molecular chaperones in C. elegans

Learning, a process by which animals modify their behavior as a result of experience, enables organisms to synthesize information from their surroundings to acquire resources and avoid danger. We showed that a previous encounter with only the odor of pathogenic bacteria prepared Caenorhabditis elegans to survive exposure to the pathogen by increasing the heat shock factor 1 (HSF-1)-dependent expression of genes encoding molecular chaperones. Experience-mediated enhancement of chaperone gene expression required serotonin, which primed HSF-1 to enhance the expression of molecular chaperone genes by promoting its localization to RNA polymerase II-enriched nuclear loci, even before transcription occurred. However, HSF-1-dependent chaperone gene expression was stimulated only if and when animals encountered the pathogen. Thus, learning equips C. elegans to better survive environmental dangers by preemptively and specifically initiating transcriptional mechanisms throughout the whole organism that prepare the animal to respond rapidly to proteotoxic agents. These studies provide one plausible basis for the protective role of environmental enrichment in disease.

An oxytocin-dependent social interaction between larvae and adult C. elegans

Oxytocin has a conserved role in regulating animal social behaviour including parental-offspring interactions. Recently an oxytocin-like neuropeptide, nematocin, and its cognate receptors have been identified in the nematode Caenorhabditis elegans. We provide evidence for a pheromone signal produced by C. elegans larvae that modifies the behaviour of adult animals in an oxytocin-dependent manner increasing their probability of leaving a food patch which the larvae are populating. This increase is positively correlated to the size of the larval population but cannot be explained by food depletion nor is it modulated by biogenic amines, which suggest it is not an aversive behaviour. Moreover, the food-leaving behaviour is conspecific and pheromone dependent: C. elegans adults respond more strongly to C. elegans larvae compared to other nematode species and this effect is absent in C. elegans daf-22 larvae which are pheromone deficient. Neurotransmitter receptors previously implicated in C. elegans foraging decisions NPR-1 and TYRA-3, for NPY-like neuropeptides and tyramine respectively, do not appear to be involved in oxytocin-dependent adult food-leaving. We conclude oxytocin signals within a novel neural circuit that regulates parental-offspring social behaviour in C. elegans and that this provides evidence for evolutionary conservation of molecular components of a parental decision making behaviour.