Identification of novel antimicrobials using a live-animal infection model

Insights into the novel Enterococcus faecalis phage: A comprehensive genome analysis

Enterococcus faecalis, a Gram-positive bacterium, poses a significant clinical challenge owing to its intrinsic resistance to a broad spectrum of antibiotics, warranting urgent exploration of innovative therapeutic strategies. This study investigated the viability of phage therapy as an alternative intervention for antibiotic-resistant E. faecalis, with a specific emphasis on the comprehensive genomic analysis of bacteriophage SAM-E.f 12. The investigation involved whole-genome sequencing of SAM-E.f 12 using Illumina technology, resulting in a robust dataset for detailed genomic characterization. Bioinformatics analyses were employed to predict genes and assign functional annotations. The bacteriophage SAM-E.f 12, which belongs to the Siphoviridae family, exhibited substantial potential, with a burst size of 5.7 PFU/infected cells and a latent period of 20 min. Host range determination experiments demonstrated its effectiveness against clinical E. faecalis strains, positioning SAM-E.f 12 as a precise therapeutic agent. Stability assays underscore resilience across diverse environmental conditions. This study provides a comprehensive understanding of SAM-E.f 12 genomic composition, lytic lifecycle parameters, and practical applications, particularly its efficacy in murine wound models. These results emphasize the promising role of phage therapy, specifically its targeted approach against antibiotic-resistant E. faecalis strains. The nuanced insights derived from this research will contribute to the ongoing pursuit of efficacious phage therapies and offer valuable implications for addressing the clinical challenges associated with E. faecalis infections.

Nematicidal activity of paucimannose-type glycoconjugates from acacia honey

Natural honey contains glycoconjugates as minor components. We characterized acacia honey glycoconjugates with molecular masses in the range of 2-5 kDa. The glycoconjugates were separated by RP-HPLC into three peaks (termed RP-2-5 k-I, RP-2-5 k-II, and RP-2-5 k-III) which demonstrated paralyzing effects on the model nematode C. elegans (ED50 of 50 ng glycoconjugates/μL). To examine molecular mechanisms underlying the nematicidal effects of honey glycoconjugates, expressional analyses of genes that are essential for the growth, development, reproduction, and movement of C. elegans were carried out. Quantitative PCR-based assays showed that these molecules moderately regulate the expression of genes involved in the citric acid cycle (mdh-1 and idhg-1) and cytoskeleton (act-1 and act-2). MALDI-ToF-MS/MS analysis of RP-HPLC peaks revealed the presence of paucimannose-like N-glycans which are known to play important roles in invertebrates e.g., worms and flies. These findings provided novel information regarding the structure and nematicidal function of honey glycoconjugates.

A Methylazanediyl Bisacetamide Derivative Sensitizes Staphylococcus aureus Persisters to a Combination of Gentamicin And Daptomycin

Infections caused by Staphylococcus aureus, notably methicillin-resistant S. aureus (MRSA), pose treatment challenges due to its ability to tolerate antibiotics and develop antibiotic resistance. The former, a mechanism independent of genetic changes, allows bacteria to withstand antibiotics by altering metabolic processes. Here, a potent methylazanediyl bisacetamide derivative, MB6, is described, which selectively targets MRSA membranes over mammalian membranes without observable resistance development. Although MB6 is effective against growing MRSA cells, its antimicrobial activity against MRSA persisters is limited. Nevertheless, MB6 significantly potentiates the bactericidal activity of gentamicin against MRSA persisters by facilitating gentamicin uptake. In addition, MB6 in combination with daptomycin exhibits enhanced anti-persister activity through mutual reinforcement of their membrane-disrupting activities. Crucially, the "triple" combination of MB6, gentamicin, and daptomycin exhibits a marked enhancement in the killing of MRSA persisters compared to individual components or any double combinations. These findings underscore the potential of MB6 to function as a potent and selective membrane-active antimicrobial adjuvant to enhance the efficacy of existing antibiotics against persister cells. The molecular mechanisms of MB6 elucidated in this study provide valuable insights for designing anti-persister adjuvants and for developing new antimicrobial combination strategies to overcome the current limitations of antibiotic treatments.

Antimicrobial evaluation of Tripleurospermum callosum (Boiss. & Heldr.) E. Hossain extracts using in vitro and in vivo Caenorhabditis elegans model against urinary system pathogens

Tripleurospermum callosum (Boiss. & Heldr.) E. Hossain was recorded in Turkish ethnobotanical data for its use against urinary and respiratory system ailments. Infusion, decoction and 96% ethanol extracts of T. callosum aerial parts were prepared for in vitro antimicrobial activity against urinary system pathogens Escherichia coli ATCC 8739, Staphylococcus aureus ATCC 6538, Pseudomonas aeruginosa ATCC 9027, Klebsiella aerogenes ATCC 1348 and Candida albicans ATCC 10231. The non-toxic concentrations of extracts and in vivo antimicrobial assay were performed using C. elegans. The extracts were analysed by Liquid Chromatography Mass Spectrometry (LC-MS/MS) for phytochemical composition. The water extracts were non-toxic at between 5000 and 312 µg/mL, while 96% ethanol extract at 312 µg/mL for C. elegans. The infusion extract showed in vivo anti-infective effect 5000-312 μg/mL against Gram-negative strains. The results indicate a potential role of plant extracts with relatively non-toxic and anti-infective effects against urinary system pathogens.

Current Development of Thiazole-Containing Compounds as Potential Antibacterials against Methicillin-Resistant Staphylococcus aureus

The emergence of multi-drug-resistant bacteria is threatening to human health and life around the world. In particular, methicillin-resistant Staphylococcus aureus (MRSA) causes fatal injuries to human beings and serious economic losses to animal husbandry due to its easy transmission and difficult treatment. Currently, the development of novel, highly effective, and low-toxicity antimicrobials is important to combat MRSA infections. Thiazole-containing compounds with good biological activity are widely used in clinical practice, and appropriate structural modifications make it possible to develop new antimicrobials. Here, we review thiazole-containing compounds and their antibacterial effects against MRSA reported in the past two decades and discuss their structure-activity relationships as well as the corresponding antimicrobial mechanisms. Some thiazole-containing compounds exhibit potent antibacterial efficacy in vitro and in vivo after appropriate structural modifications and could be used as antibacterial candidates. This Review provides insights into the development of thiazole-containing compounds as antimicrobials to combat MRSA infections.

Design and synthesis of etrasimod derivatives as potent antibacterial agents against Gram-positive bacteria

The emergence of multidrug-resistant bacteria along with a declining pipeline of clinically useful antibiotics has led to the urgent need for the development of more effective antibacterial agents. Inspired by our recent report on the antibacterial activity of etrasimod, an immunomodulating drug candidate, we prepared a series of etrasimod derivatives by varying substituents on the phenyl ring, altering the central tricyclic aromatic ring, and modifying the carboxyl group. From this series of compounds, indole derivative 24f was identified as the most potent antibacterial compound, with the minimum inhibitory concentration (MIC) values between 2.5 and 10 μM against various Gram-positive bacteria, including methicillin-resistant Staphylococcus aureus (MRSA), S. epidermidis and enterococci. Moreover, 24f exhibited rapid bactericidal activity against S. aureus, low toxicity and hemolytic activity, and a synergistic effect with gentamicin against S. aureus, MRSA, and Enterococcus faecalis. Furthermore, it was shown that neither etrasimod nor 24f affects S. aureus cell membranes. Importantly, 24f did not induce resistance in S. aureus, representing a significant improvement compared to etrasimod. Finally, the antibacterial activity of etrasimod and 24f against S. aureus and MRSA was confirmed in vivo in a Caenorhabditis elegans infection model. Taken together, our study highlights the value of etrasimod and its derivatives as potential antibacterial candidates for combating infections caused by Gram-positive bacteria.

Recent Advances in Drug Discovery Toxicology

Major advances in scientific discovery and insights that stem from the development and use of new techniques and models can bring remarkable progress to conventional toxicology. Although animal testing is still considered as the "gold standard" in traditional toxicity testing, there is a necessity for shift from animal testing to alternative methods regarding the drug safety testing owing to the emerging state-of-art techniques and the proposal of 3Rs (replace, reduce, and refine) towards animal welfare. This review describes some recent research methods in drug discovery toxicology, including in vitro cell and organ-on-a-chip, imaging systems, model organisms (C. elegans, Danio rerio, and Drosophila melanogaster), and toxicogenomics in modern toxicology testing.

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

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

Caenorhabditis elegans as an In Vivo Model for the Discovery and Development of Natural Plant-Based Antimicrobial Compounds

Antimicrobial resistance (AMR) due to the prevalence of multidrug-resistant (MDR) pathogens is rapidly increasing worldwide, and the identification of new antimicrobial agents with innovative mechanisms of action is urgently required. Medicinal plants that have been utilised for centuries with minor side effects may hold great promise as sources of effective antimicrobial products. The free-living nematode Caenorhabditis elegans (C. elegans) is an excellent live infection model for the discovery and development of new antimicrobial compounds. However, while C. elegans has widely been utilised to explore the effectiveness and toxicity of synthetic antibiotics, it has not been used to a comparable extent for the analysis of natural products. By screening the PubMed database, we identified articles reporting the use of the C. elegans model for the identification of natural products endowed with antibacterial and antifungal potential, and we critically analysed their results. The studies discussed here provide important information regarding "in vivo" antimicrobial effectiveness and toxicity of natural products, as evaluated prior to testing in conventional vertebrate models, thereby supporting the relevance of C. elegans as a highly proficient model for their identification and functional assessment. However, their critical evaluation also underlines that the characterisation of active phytochemicals and of their chemical structure, and the unravelling of their mechanisms of action represent decisive challenges for future research in this area.

High-Throughput Screening of Natural Product and Synthetic Molecule Libraries for Antibacterial Drug Discovery

Due to the continued emergence of resistance and a lack of new and promising antibiotics, bacterial infection has become a major public threat. High-throughput screening (HTS) allows rapid screening of a large collection of molecules for bioactivity testing and holds promise in antibacterial drug discovery. More than 50% of the antibiotics that are currently available on the market are derived from natural products. However, with the easily discoverable antibiotics being found, finding new antibiotics from natural sources has seen limited success. Finding new natural sources for antibacterial activity testing has also proven to be challenging. In addition to exploring new sources of natural products and synthetic biology, omics technology helped to study the biosynthetic machinery of existing natural sources enabling the construction of unnatural synthesizers of bioactive molecules and the identification of molecular targets of antibacterial agents. On the other hand, newer and smarter strategies have been continuously pursued to screen synthetic molecule libraries for new antibiotics and new druggable targets. Biomimetic conditions are explored to mimic the real infection model to better study the ligand-target interaction to enable the designing of more effective antibacterial drugs. This narrative review describes various traditional and contemporaneous approaches of high-throughput screening of natural products and synthetic molecule libraries for antibacterial drug discovery. It further discusses critical factors for HTS assay design, makes a general recommendation, and discusses possible alternatives to traditional HTS of natural products and synthetic molecule libraries for antibacterial drug discovery.

In vivo quantitative high-throughput screening for drug discovery and comparative toxicology

Quantitative high-throughput screening (qHTS) pharmacologically evaluates chemical libraries for therapeutic uses, toxicological risk and, increasingly, for academic probe discovery. Phenotypic high-throughput screening assays interrogate molecular pathways, often relying on cell culture systems, historically less focused on multicellular organisms. Caenorhabditis elegans has served as a eukaryotic model organism for human biology by virtue of genetic conservation and experimental tractability. Here, a paradigm enabling C. elegans qHTS using 384-well microtiter plate laser-scanning cytometry is described, in which GFP-expressing organisms revealing phenotype-modifying structure-activity relationships guide subsequent life-stage and proteomic analyses, and Escherichia coli bacterial ghosts, a non-replicating nutrient source, allow compound exposures over two life cycles, mitigating bacterial overgrowth complications. We demonstrate the method with libraries of anti-infective agents, or substances of toxicological concern. Each was tested in seven-point titration to assess the feasibility of nematode-based in vivo qHTS, and examples of follow-up strategies were provided to study organism-based chemotype selectivity and subsequent network perturbations with a physiological impact. We anticipate that this qHTS approach will enable analysis of C. elegans orthologous phenotypes of human pathologies to facilitate drug library profiling for a range of therapeutic indications.

Bacterial envelope stress responses: Essential adaptors and attractive targets

Millions of deaths a year across the globe are linked to antimicrobial resistant infections. The need to develop new treatments and repurpose of existing antibiotics grows more pressing as the growing antimicrobial resistance pandemic advances. In this review article, we propose that envelope stress responses, the signaling pathways bacteria use to recognize and adapt to damage to the most vulnerable outer compartments of the microbial cell, are attractive targets. Envelope stress responses (ESRs) support colonization and infection by responding to a plethora of toxic envelope stresses encountered throughout the body; they have been co-opted into virulence networks where they work like global positioning systems to coordinate adhesion, invasion, microbial warfare, and biofilm formation. We highlight progress in the development of therapeutic strategies that target ESR signaling proteins and adaptive networks and posit that further characterization of the molecular mechanisms governing these essential niche adaptation machineries will be important for sparking new therapeutic approaches aimed at short-circuiting bacterial adaptation.

High-throughput small molecule screen identifies inhibitors of microsporidia invasion and proliferation in C. elegans

Microsporidia are a diverse group of fungal-related obligate intracellular parasites that infect most animal phyla. Despite the emerging threat that microsporidia represent to humans and agricultural animals, few reliable treatment options exist. Here, we develop a high-throughput screening method for the identification of chemical inhibitors of microsporidia infection, using liquid cultures of Caenorhabditis elegans infected with the microsporidia species Nematocida parisii. We screen a collection of 2560 FDA-approved compounds and natural products, and identify 11 candidate microsporidia inhibitors. Five compounds prevent microsporidia infection by inhibiting spore firing, whereas one compound, dexrazoxane, slows infection progression. The compounds have in vitro activity against several other microsporidia species, including those known to infect humans. Together, our results highlight the effectiveness of C. elegans as a model host for drug discovery against intracellular pathogens, and provide a scalable high-throughput system for the identification and characterization of microsporidia inhibitors.

A novel antibiotic combination of linezolid and polymyxin B octapeptide PBOP against clinical Pseudomonas aeruginosa strains

Background

Antibiotic-resistant Gram-negative bacteria are becoming a major public health threat such as the important opportunistic pathogen Pseudomonas aeruginosa (P. aeruginosa). The present study investigated enhancement of the linezolid spectrum, which is normally used to treat Gram-positive bacteria, at inhibiting P. aeruginosa growth.

Methods

The checkerboard test or time-kill assay were carried out to determine the antibacterial effects of linezolid in cooperation with polymyxin B octapeptide PBOP (LP) against P. aeruginosa based on in vitro model. The protective effect of LP against P. aeruginosa infection was assessed based on a Caenorhabditis elegans (C. elegans) model.

Results

The synergistic activity and antibacterial effects were significantly increased against P. aeruginosa by LP treatment, while linezolid and PBOP as monotherapies exhibited no remarkably bactericidal activity against the clinical strains. Additionally, LP treatment modified biofilm production, morphology, swimming motility of P. aeruginosa, and protected C. elegans from P. aeruginosa infection.

Conclusions

This research demonstrates that LP combination has significant synergistic activity against P. aeruginosa, and PBOP is potential to be an activity enhancer. Notably, this strategy improved the antibacterial activity spectrum of linezolid and other anti-Gram-positive agents and represents an effective choice to surmount the antibiotic resistance of bacteria in the long term.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12941-022-00531-5.

An iron-chelating sulfonamide identified from Drosophila-based screening for antipathogenic discovery

We exploited bacterial infection assays using the fruit fly Drosophila melanogaster to identify anti-infective compounds that abrogate the pathological consequences in the infected hosts. Here, we demonstrated that a pyridine-3-N-sulfonylpiperidine derivative (4a) protects Drosophila from the acute infections caused by bacterial pathogens including Pseudomonas aeruginosa. 4a did not inhibit the growth of P. aeruginosa in vitro, but inhibited the production of secreted toxins such as pyocyanin and hydrogen cyanide, while enhancing the production of pyoverdine and pyochelin, indicative of iron deprivation. Based on its catechol moiety, 4a displayed iron-chelating activity in vitro toward both iron (II) and iron (III), more efficiently than the approved iron-chelating drugs such as deferoxamine and deferiprone, concomitant with more potent antibacterial efficacy in Drosophila infections and unique transcriptome profile. Taken together, these results delineate a Drosophila-based strategy to screen for antipathogenic compounds, which interfere with iron uptake crucial for bacterial virulence and survival in host tissues.

A Multiwell-Plate Caenorhabditis elegans Assay for Assessing the Therapeutic Potential of Bacteriophages against Clinical Pathogens

In order to establish phage therapy as a standard clinical treatment for bacterial infections, testing of every phage to ensure the suitability and safety of the biological compound is required. While some issues have been addressed over recent years, standard and easy-to-use animal models to test phages are still rare. Testing of phages in highly suitable mammalian models such as mice is subjected to strict ethical regulations, while insect larvae such as the Galleria mellonella model suffer from batch-to-batch variations and require manual operator skills to inject bacteria, resulting in unreliable experimental outcomes. A much simpler model is the nematode Caenorhabditis elegans, which feeds on bacteria, a fast growing and easy to handle organism that can be used in high-throughput screening. In this study, two clinical bacterial strains of Escherichia coli, one Klebsiella pneumoniae, and one Enterobacter cloacae strain were tested on the model system together with lytic bacteriophages that we isolated previously. We developed a liquid-based assay, in which the efficiency of phage treatment was evaluated using a scoring system based on microscopy and counting of the nematodes, allowing increasing statistical significance compared to other assays such as larvae or mice. Our work demonstrates the potential to use Caenorhabditis elegans to test the virulence of strains of Klebsiella pneumoniae, Enterobacter cloacae, and EHEC/EPEC as well as the efficacy of bacteriophages to treat or prevent infections, allowing a more reliable evaluation for the clinical therapeutic potential of lytic phages. IMPORTANCE Validating the efficacy and safety of phages prior to clinical application is crucial to see phage therapy in practice. Current animal models include mice and insect larvae, which pose ethical or technical challenges. This study examined the use of the nematode model organism C. elegans as a quick, reliable, and simple alternative for testing phages. The data show that all the four tested bacteriophages can eliminate bacterial pathogens and protect the nematode from infections. Survival rates of the nematodes increased from <20% in the infection group to >90% in the phage treatment group. Even the nematodes with poly-microbial infections recovered during phage cocktail treatment. The use of C. elegans as a simple whole-animal infection model is a rapid and robust way to study the efficacy of phages before testing them on more complex model animals such as mice.

Cinnamomum cassia and Syzygium aromaticum Essential Oils Reduce the Colonization of Salmonella Typhimurium in an In Vivo Infection Model Using Caenorhabditis elegans

The regulation of intestinal colonization in livestock by means of non-bactericidal additives is an important management lever for zoonotic bacteria such as Salmonella spp. Caenorhabditis elegans is proposed here as a model for the evaluation of five essential oils (EOs) as anti-colonization products against Salmonella Typhimurium. An evaluation of the toxicity of EOs for C. elegans showed LD50 values ranging from 74.5 ± 9.6 µg/mL for Cinnamomum cassia (CEO) to 271.6 ± 14.9 µg/mL for Syzygium aromaticum (SyEO). Both EOs significantly inhibited bacterial colonization in the digestive tract of C. elegans with reductions of 0.88 and 0.70 log CFU/nematode at nontoxic concentrations of 50 µg/mL and 150 µg/mL, respectively. With the minimal bactericidal concentrations of CEO and SyEO against S. Typhimurium being 312.5 µg/mL and 625 µg/mL, respectively, an antibacterial effect can be excluded to explain the inhibition of the bacterial load. The anti-colonizing activity of these two EOs could, however, be related to an inhibition of the swimming motility, which was significantly reduced by 23.47% for CEO at 50 µg/mL and 19.56% for SyEO at 150 µg/mL. This study shows the potential of C. elegans as a predictive in vivo model of anti-colonizing activities that is suitable for the evaluation of essential oils.

Identification of simple arylfluorosulfates as potent agents against resistant bacteria

Sulfur fluoride exchange (SuFEx), a next generation of click chemistry, opens an avenue for drug discovery. We report here the discovery and structure-activity relationship studies of a series of arylfluorosulfates, synthesized via SuFEx, as antibacterial agents. Arylfluorosulfates 3, 81, and 101 showed potency to overcome multidrug resistance and were not susceptible to the generation of resistance. They exhibited rapid bactericidal potency and selectively killed gram-positive bacterial strains. These compounds also exhibited the ability to disrupt established bacterial biofilm and kill persisters derived from biofilm. Furthermore, arylfluorosulfate 3 had a synergistic effect with streptomycin and gentamicin. In addition, their anti-MRSA potency was evaluated and determined by the Caenorhabditis elegans model.

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

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

Thioredoxin Reductase Is a Valid Target for Antimicrobial Therapeutic Development Against Gram-Positive Bacteria

There is a drought of new antibacterial compounds that exploit novel targets. Thioredoxin reductase (TrxR) from the Gram-positive bacterial antioxidant thioredoxin system has emerged from multiple screening efforts as a potential target for auranofin, ebselen, shikonin, and allicin. Auranofin serves as the most encouraging proof of concept drug, demonstrating TrxR inhibition can result in bactericidal effects and inhibit Gram-positive bacteria in both planktonic and biofilm states. Minimal inhibitory concentrations are on par or lower than gold standard medications, even among drug resistant isolates. Importantly, existing drug resistance mechanisms that challenge treatment of infections like Staphylococcus aureus do not confer resistance to TrxR targeting compounds. The observed inhibition by multiple compounds and inability to generate a bacterial genetic mutant demonstrate TrxR appears to play an essential role in Gram-positive bacteria. These findings suggest TrxR can be exploited further for drug development. Examining the interaction between TrxR and these proof of concept compounds illustrates that compounds representing a new antimicrobial class can be developed to directly interact and inhibit the validated target.

Drug discovery: Insights from the invertebrate Caenorhabditis elegans

Therapeutic drug development is a long, expensive, and complex process that usually takes 12-15 years. In the early phases of drug discovery, in particular, there is a growing need for animal models that ensure the reduction in both cost and time. Caenorhabditis elegans has been traditionally used to address fundamental aspects of key biological processes, such as apoptosis, aging, and gene expression regulation. During the last decade, with the advent of large-scale platforms for screenings, this invertebrate has also emerged as an essential tool in the pharmaceutical research industry to identify novel drugs and drug targets. In this review, we discuss the reasons why C. elegans has been positioned as an outstanding cost-effective option for drug discovery, highlighting both the advantages and drawbacks of this model. Particular attention is paid to the suitability of this nematode in large-scale genetic and pharmacological screenings. High-throughput screenings in C. elegans have indeed contributed to the breakthrough of a wide variety of candidate compounds involved in extensive fields including neurodegeneration, pathogen infections and metabolic disorders. The versatility of this nematode, which enables its instrumentation as a model of human diseases, is another attribute also herein underscored. As illustrative examples, we discuss the utility of C. elegans models of both human neurodegenerative diseases and parasitic nematodes in the drug discovery industry. Summing up, this review aims to demonstrate the impact of C. elegans models on the drug discovery pipeline.

Realgar increases defenses against infection by Enterococcus faecalis in Caenorhabditis elegans

ETHNOPHARMACOLOGICAL RELEVANCE

Realgar has been used in traditional remedies for a long history in China and India. It is clinically used to treat diverse cancers, especially acute promyelocytic leukemia (APL), chronic myelogenous leukemia (CML) in China. However, paradoxic roles of realgar to increase or decrease immunity are reported. It is urgent to address this question, due to immune depression can be strongly benefit to cancer development, but detrimental to patients.

AIM OF THE STUDY

This present work is to explore whether realgar promote or suppress immune responses, and shed light on its mode of action. Our results should provide cues for rational strategy to explore realgar for clinical use.

MATERIAL AND METHODS

Infection model in vivo was established by using Enterococcus faecalis to attack Caenorhabditis elegans, then realgar was used to treat the infected worms to investigate its effects on infectivity and the underlying mechanism. Killing analysis was carried out to test whether realgar can mitigate worm infection. Thermotolerance resistance analysis was used to evaluate if realgar functions hormetic effect. Quantification of live E. faecalis in nematode intestine was employed to ascertain if realgar alleviate the bacterial load in worm gut. Quantitative real-time PCR (qRT-PCR) was used to test the expression of antibacterial effectors. Western blot was used to test the effect of realgar on the expressions of p38 and phospho-p38 in worms infected by E. faecalis.

RESULTS

Realgar alleviated the infected worms in strains of N2, glp-4, and daf-2, but failed in sek-1, glp-4; sek-1, and daf-2; daf-16 when p38 MAPK or daf-16 was blocked or inactivated. Western blot assay demonstrated that realgar increased the expression of phosph-p38. Thermotolerance assay showed that realgar played a hormetic role on nemtodes, triggered protective response and reduced bacterial load after realgar treatment for 120 h qRT-PCR demonstrated that realgar significantly increased antibacterial effectors, thus leading to pathogen elimination.

CONCLUSION

Realgar increased defenses against E. faecalis in C. elegans by inducing both immune responses and protective responses. It was regulated by p38 MAPK pathway and DAF-16.

Characterization of an Antibacterial Agent Targeting Ferrous Iron Transport Protein FeoB against Staphylococcus aureus and Gram-Positive Bacteria

The emergence of multidrug-resistant Staphylococcus aureus strains has become a serious clinical problem. Iron is absolutely required for the bacterial growth, virulence associated with colonization, and survival from the host immune system. The FeoB protein is a major iron permease in bacterial ferrous iron transport systems (Feo) that has been shown to play a crucial role in virulence of some pathogenic bacteria. However, FeoB is still uncharacterized in Gram-positive pathogens, and its effects on S. aureus pathogenesis are unknown. In this study, we identified a novel inhibitor, GW3965·HCl, that targets FeoB in S. aureus. The molecule effectively inhibited FeoB in vitro enzyme activity, bacterial growth, and virulence factor expression. Genome-editing and metabolomic analyses revealed that GW3965·HCl inhibited FeoB function and affected the associated mechanisms with reduced iron availability in S. aureus. Gentamicin resistance and Caenorhabditis elegans infection assays further demonstrated the power of GW3965·HCl as a safe and efficient antibacterial agent. In addition to S. aureus, GW3965·HCl also presented its effectiveness on inhibition of the FeoB activity and growth of Gram-positive bacteria. This novel inhibitor will provide new insight for developing a next-generation antibacterial therapy.

Novel Immune Modulators Enhance Caenorhabditis elegans Resistance to Multiple Pathogens

Trends moving in opposite directions (increasing antimicrobial resistance and declining novel antimicrobial development) have precipitated a looming crisis: a nearly complete inability to safely and effectively treat bacterial infections. To avert this, new approaches are needed.

Traditionally, treatments for bacterial infection have focused on killing the microbe or preventing its growth. As antimicrobial resistance becomes more ubiquitous, the feasibility of this approach is beginning to wane and attention has begun to shift toward disrupting the host-pathogen interaction by improving the host defense. Using a high-throughput, fragment-based screen to identify compounds that alleviate Pseudomonas aeruginosa-mediated killing of Caenorhabditis elegans, we identified over 20 compounds that stimulated host defense gene expression. Five of these molecules were selected for further characterization. Four of five compounds showed little toxicity against mammalian cells or worms, consistent with their identification in a phenotypic, high-content screen. Each of the compounds activated several host defense pathways, but the pathways were generally dispensable for compound-mediated rescue in liquid killing, suggesting redundancy or that the activation of unknown pathway(s) may be driving compound effects. A genetic mechanism was identified for LK56, which required the Mediator subunit MDT-15/MED15 and NHR-49/HNF4 for its function. Interestingly, LK32, LK34, LK38, and LK56 also rescued C. elegans from P. aeruginosa in an agar-based assay, which uses different virulence factors and defense mechanisms. Rescue in an agar-based assay for LK38 entirely depended upon the PMK-1/p38 MAPK pathway. Three compounds—LK32, LK34, and LK56—also conferred resistance to Enterococcus faecalis, and the two lattermost, LK34 and LK56, also reduced pathogenesis from Staphylococcus aureus. This study supports a growing role for MDT-15 and NHR-49 in immune response and identifies five molecules that have significant potential for use as tools in the investigation of innate immunity.

IMPORTANCE Trends moving in opposite directions (increasing antimicrobial resistance and declining novel antimicrobial development) have precipitated a looming crisis: the nearly complete inability to safely and effectively treat bacterial infections. To avert this, new approaches are needed. One idea is to stimulate host defense pathways to improve the clearance of bacterial infection. Here, we describe five small molecules that promote resistance to infectious bacteria by activating C. elegans’ innate immune pathways. Several are effective against both Gram-positive and Gram-negative pathogens. One of the compounds was mapped to the action of MDT-15/MED15 and NHR-49/HNF4, a pair of transcriptional regulators more generally associated with fatty acid metabolism, potentially highlighting a new link between these biological functions. These studies pave the way for future characterization of the anti-infective activity of the molecules in higher organisms and highlight the compounds’ potential utility for further investigation of immune modulation as a novel therapeutic approach.

Traditional Chinese Medicine Tanreqing Inhibits Quorum Sensing Systems in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that can infect a wide variety of hosts including humans, plants, and animals. The production of virulence factors is the determinant of the infection paradigm and is under orchestrated regulation via cell-to-cell communication process called quorum sensing (QS). To disable QS circuits and prevent bacterial infections, a large battery of anti-QS agents, particularly from traditional Chinese medicine have been developed. Here, we used P. aeruginosa as a model microorganism to investigate the effect of traditional Chinese medicine Tanreqing (TRQ) formula on bacterial pathogenicity. Phenotypic analysis showed that TRQ treatment could completely inhibit the production of phenazine pyocyanin and moderately inhibit the production of virulence factors such as rhamnolipids, elastase, and alkaline protease. Further transcriptomic analyses revealed that TRQ treatment could significantly attenuate the expression of QS-regulated genes in P. aeruginosa and TRQ-treated P. aeruginosa regulon shared a large overlap with QS regulon. Component contribution to QS inhibition shed light on the indispensable role of all five components in TRQ formula. Further genetic analysis indicated that upstream regulators of QS systems, including two-component systems GacS/GacA and PprA/PprB, were both inhibited by TRQ treatment. Finally, our TRQ formula could efficiently protect Caenorhabditis elegans from killing by P. aeruginosa. Altogether, we have proved TRQ formula as an effective and specific agent to attenuate bacterial virulence and combat bacterial infections.

The Antioxidant Capacity In Vitro and In Vivo of Polysaccharides From Bergenia emeiensis

Polysaccharides from Bergenia emeiensis (PBE) showed a robust antioxidant ability on scavenging free radicals in vitro. However, the further antioxidant potential in cell level and in vivo was still unknown. Therefore, in this present study, the protective effect of PBE on human cervical carcinoma cell (Hela) cells and Caenorhabditis elegans against oxidative stress was evaluated. The results showed PBE could reduce the reactive oxygen species (ROS) level in Hela cells and promote the mitochondrial membrane potential. Then, the cell apoptosis was reduced. Moreover, PBE could enhance the survival of C. elegans under thermal stress to 13.44%, and significantly reduce the ROS level, which was connected with the overexpression of sod-3 and the increased nuclear localization of daf-16 transcription factor. Therefore, PBE exhibited a strong antioxidant capacity in the cellular level and for a whole organism. Thus, polysaccharides from B. emeiensis have natural potential to be a safe antioxidant.

Lactobacillus gastricus BTM 7 prevents intestinal colonization by biofilm forming Cronobacter sakazakii in Caenorhabditis elegans model host

The study reports protective role of potential probiotic cultures against infection by biofilm forming Cronobacter sakazakii in Caenorhabditis elegans model system. Among the fifteen indigenous potential probiotics, the cell free supernatant of Lactobacillus gastricus BTM7 possessed highest antimicrobial action and biofilm inhibition against C. sakazakii. The competitive exclusion assays revealed that preconditioning with probiotics resulted in increased mean life span of the nematode to 12-13 days as compared to 5-6 days when the pathogen was administered alone. Enhanced expression of the marker genes (pmk-1, daf-16 and skn-1) was observed during the administration of probiotic cultures. The highest expression of pmk-1 (2.5 folds) was observed with administration of L. gastricus BTM7. The principal component analysis on selected variables revealed that L. gastricus BTM7 has the potential to limit the infection of C. sakazakii in C. elegans and enhance the expression of key genes involved in extending life span of the worm.

Characterization of Five Novel Anti-MRSA Compounds Identified Using a Whole-Animal Caenorhabditis elegans/Galleria mellonella Sequential-Screening Approach

There is a significant need to combat the growing challenge of antibacterial drug resistance. We have previously developed a whole-animal dual-screening platform that first used the nematode Caenorhabditis elegans, to identify low-toxicity antibacterial hits in a high-throughput format. The hits were then evaluated in the wax moth caterpillar Galleria mellonella infection model to confirm efficacy and low toxicity at a whole animal level. This multi-host approach is a powerful tool for revealing compounds that show antibacterial effects and relatively low toxicity at the whole organism level. This paper reports the use of the multi-host approach to identify and validate five new anti-staphylococcal compounds: (1) 4,4′,4″-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol(PPT), (2) (1S,2S)-2-[2-[[3-(1H-benzimidazol-2-yl)propyl]methylamino]ethyl]-6-fluoro-1,2,3,4-tetrahydro-1-(1-methylethyl)-2-naphthalenyl cyclopropanecarboxylate dihydrochloride(NNC), (3) 4,5,6,7-tetrabromobenzotriazole (TBB), (4) 3-[2-[2-chloro-4-[3-(2,6-dichlorophenyl)-5-(1-methylethyl)-4-isoxazolyl]methoxy]phenyl]ethenyl] benzoic acid(GW4064), and (5) N-(cyclopropylmethoxy)-3,4,5-trifluoro-2-[(4-iodo-2-methylphenyl)amino] benzamide(PD198306). The compounds reduced the severity of methicillin-resistant Staphylococcus aureus (MRSA, strain MW2) infections in both C. elegans and G. mellonella and showed minimal inhibitory concentrations (MICs) in the range of 2–8 µg/mL. Compounds NNC, PPT, and TBB permeabilized MRSA-MW2 cells to SYTOX green, suggesting that they target bacterial membranes. Compound TBB showed synergistic activity with doxycycline and oxacillin against MRSA-MW2, and compounds PPT, NNC, GW4064, and PD198306 synergized with doxycycline, polymyxin-B, gentamicin, and erythromycin, respectively. The study demonstrates the utility of the multi-host approach with follow-up hit characterization for prioritizing anti-MRSA compounds for further evaluation.

Repurposing bioactive compounds for treating multidrug-resistant pathogens

Introduction. Antimicrobial development is being outpaced by the rising rate of antimicrobial resistance in the developing and industrialized world. Drug repurposing, where novel antibacterial functions can be found for known molecular entities, reduces drug development costs, reduces regulatory hurdles, and increases rate of success.Aim. We sought to characterize the antimicrobial properties of five known bioactives (DMAQ-B1, carboplatin, oxaliplatin, CD437 and PSB-069) that were discovered in a high-throughput phenotypic screen for hits that extend Caenorhabditis elegans survival during exposure to Pseudomonas aeruginosa PA14.Methodology. c.f.u. assays, biofilm staining and fluorescence microscopy were used to assay the compounds' effect on various virulence determinants. Checkerboard assays were used to assess synergy between compounds and conventional antimicrobials. C. elegans-based assays were used to test whether the compounds could also rescue against Enterococcus faecalis and Staphyloccus aureus. Finally, toxicity was assessed in C. elegans and mammalian cells.Results. Four of the compounds rescued C. elegans from a second bacterial pathogen and two of them (DMAQ-B1, a naturally occurring insulin mimetic, and CD437, an agonist of the retinoic acid receptor) rescued against all three. The platinum complexes displayed increased antimicrobial activity against P. aeruginosa. Of the molecules tested, only CD437 showed slight synergy with ampicillin. The two most effective compounds, DMAQ-B1 and CD437, showed toxicity to mammalian cells.Conclusion. Although these compounds' potential for repurposing is limited by their toxicity, our results contribute to this growing field and provide a simple road map for using C. elegans for preliminary testing of known bioactive compounds with predicted antimicrobial activity.

The Neutrally Charged Diarylurea Compound PQ401 Kills Antibiotic-Resistant and Antibiotic-Tolerant Staphylococcus aureus

Membrane-damaging antimicrobial agents have great potential to treat multidrug-resistant or multidrug-tolerant bacteria against which conventional antibiotics are not effective. However, their therapeutic applications are often hampered due to their low selectivity to bacterial over mammalian membranes or their potential for cross-resistance to a broad spectrum of cationic membrane-active antimicrobial agents. We discovered that the diarylurea derivative compound PQ401 has antimicrobial potency against multidrug-resistant and multidrug-tolerant Staphylococcus aureus. PQ401 selectively disrupts bacterial membrane lipid bilayers in comparison to mammalian membranes. Unlike cationic membrane-active antimicrobials, the neutral form of PQ401 rather than its cationic form exhibits maximum membrane activity. Overall, our results demonstrate that PQ401 could be a promising lead compound that overcomes the current limitations of membrane selectivity and cross-resistance. Also, this work provides deeper insight into the design and development of new noncharged membrane-targeting therapeutics to combat hard-to-cure bacterial infections.

Resistance or tolerance to traditional antibiotics is a challenging issue in antimicrobial chemotherapy. Moreover, traditional bactericidal antibiotics kill only actively growing bacterial cells, whereas nongrowing metabolically inactive cells are tolerant to and therefore “persist” in the presence of legacy antibiotics. Here, we report that the diarylurea derivative PQ401, previously characterized as an inhibitor of the insulin-like growth factor I receptor, kills both antibiotic-resistant and nongrowing antibiotic-tolerant methicillin-resistant Staphylococcus aureus (MRSA) by lipid bilayer disruption. PQ401 showed several beneficial properties as an antimicrobial lead compound, including rapid killing kinetics, low probability for resistance development, high selectivity to bacterial membranes compared to mammalian membranes, and synergism with gentamicin. In contrast to well-studied membrane-disrupting cationic antimicrobial low-molecular-weight compounds and peptides, molecular dynamic simulations supported by efficacy data demonstrate that the neutral form of PQ401 penetrates and subsequently embeds into bacterial lipid bilayers more effectively than the cationic form. Lastly, PQ401 showed efficacy in both the Caenorhabditis elegans and Galleria mellonella models of MRSA infection. These data suggest that PQ401 may be a lead candidate for repurposing as a membrane-active antimicrobial and has potential for further development as a human antibacterial therapeutic for difficult-to-treat infections caused by both drug-resistant and -tolerant S. aureus.

Anti-MRSA agent discovery using Caenorhabditis elegans-based high-throughput screening

Staphylococcus aureus is a leading cause of hospital- and community-acquired infections. Despite current advances in antimicrobial chemotherapy, the infections caused by S. aureus remain challenging due to their ability to readily develop resistance. Indeed, antibiotic resistance, exemplified by methicillin-resistant S. aureus (MRSA) is a top threat to global health security. Furthermore, the current rate of antibiotic discovery is much slower than the rate of antibiotic-resistance development. It seems evident that the conventional in vitro bacterial growth-based screening strategies can no longer effectively supply new antibiotics at the rate needed to combat bacterial antibiotic-resistance. To overcome this antibiotic resistance crisis, screening assays based on host-pathogen interactions have been developed. In particular, the free-living nematode Caenorhabditis elegans has been used for drug screening against MRSA. In this review, we will discuss the general principles of the C. elegans-based screening platform and will highlight its unique strengths by comparing it with conventional antibiotic screening platforms. We will outline major hits from high-throughput screens of more than 100,000 small molecules using the C. elegans-MRSA infection assay and will review the mode-of-action of the identified hit compounds. Lastly, we will discuss the potential of a C. elegans-based screening strategy as a paradigm shift screening platform.

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.

Pediococcus acidilactici P25 Protected Caenorhabditis elegans against Enterotoxigenic Escherichia coli K88 Infection and Transcriptomic Analysis of Its Potential Mechanisms

Enterotoxigenic Escherichia coli (ETEC) K88 is a zoonotic pathogen. Previous studies have shown that lactic acid bacteria (LAB) have great potential in promoting health and resisting pathogenic infections; however, relatively little research has been done on the Pediococcus genus of LAB. This study is aimed at exploring the mechanisms imparted by Pediococcus acidilactici P25 against ETEC K88 in Caenorhabditis elegans. The probiotic performance of P25 was investigated in vitro. Colonization of K88 in the intestinal tract of C. elegans and abundance of enterotoxin genes were measured. In addition, the transcriptome of C. elegans infected by K88 was analyzed. The result showed that P25 possessed the ability to produce acid, as well as high tolerances to acidic and high bile salt concentrations. Coculture revealed that the growth of ETEC K88 was significantly inhibited by the presence of P25. The median survival of C. elegans fed P25 was 2 days longer than the group infected with K88 alone (P < 0.01). At the same time, the number of colonizing K88 and the abundances of estB and elt were reduced by up to 71.70% and 2.17 times, respectively, by P25. Transcriptome data indicated that P25 affected expression of genes relative to innate immune response and upregulated the abundance of genes in multiple pathways of C. elegans, including peroxisome, longevity, and mitogen-activated protein kinase (MAPK) pathways. These results demonstrated that in the presence of P25, K88 colonization and their expression of enterotoxin genes were reduced. This was accomplished through the alteration of environmental parameters (pH and bile salt) as well as through the promotion of the innate immune response processes, increased longevity, and increased antipathogenic bacteria-related pathways. This work highlights the potential application of P. acidilactici P25 as a probiotic resistant to ETEC K88.

Disrupting Membrane Adaptation Restores In Vivo Efficacy of Antibiotics Against Multidrug-Resistant Enterococci and Potentiates Killing by Human Neutrophils

Daptomycin resistance in enterococci is often mediated by the LiaFSR system, which orchestrates the cell membrane stress response. Activation of LiaFSR through the response regulator LiaR generates major changes in cell membrane function and architecture (membrane adaptive response), permitting the organism to survive the antibiotic attack. Here, using a laboratory strain of Enterococcus faecalis, we developed a novel Caenorhabditis elegans model of daptomycin therapy and showed that disrupting LiaR-mediated cell membrane adaptation restores the in vivo activity of daptomycin. The LiaR effect was also seen in a clinical strain of daptomycin-resistant Enterococcus faecium, using a murine model of peritonitis. Furthermore, alteration of the cell membrane response increased the ability of human polymorphonuclear neutrophils to readily clear both E. faecalis and multidrug-resistant E. faecium. Our results provide proof of concept that targeting the cell membrane adaptive response restores the in vivo activity of antibiotics, prevents resistance, and enhances the ability of the innate immune system to kill infecting bacteria.

Black sea cucumber (Holothuria atra Jaeger, 1833) rescues Pseudomonas aeruginosa-infected Caenorhabditis elegans via reduction of pathogen virulence factors and enhancement of host immunity

A strategy to circumvent the problem of multidrug resistant pathogens is the discovery of anti-infectives targeting bacterial virulence or host immunity. Black sea cucumber (Holothuria atra) is a tropical sea cucumber species traditionally consumed as a remedy for many ailments. There is a paucity of knowledge on the anti-infective capacity of H. atra and the underlying mechanisms involved. The objective of this study is to utilize the Caenorhabditis elegans-P. aeruginosa infection model to elucidate the anti-infective properties of H. atra. A bioactive H. atra extract and subsequently its fraction were shown to have the capability of promoting the survival of C. elegans during a customarily lethal P. aeruginosa infection. The same entities also attenuate the production of elastase, protease, pyocyanin and biofilm in P. aeruginosa. The treatment of infected transgenic lys-7::GFP worms with this H. atra fraction restores the repressed expression of the defense enzyme lys-7, indicating an improved host immunity. QTOF-LCMS analysis revealed the presence of aspidospermatidine, an indole alkaloid, and inosine in this fraction. Collectively, our findings show that H. atra possesses anti-infective properties against P. aeruginosa infection, by inhibiting pathogen virulence and, eventually, reinstating host lys-7 expression.

Pathogenicity of Enterococci

Enterococci are unusually well adapted for survival and persistence in a variety of adverse environments, including on inanimate surfaces in the hospital environment and at sites of infection. This intrinsic ruggedness undoubtedly played a role in providing opportunities for enterococci to interact with other overtly drug-resistant microbes and acquire additional resistances on mobile elements. The rapid rise of antimicrobial resistance among hospital-adapted enterococci has rendered hospital-acquired infections a leading therapeutic challenge. With about a quarter of a genome of additional DNA conveyed by mobile elements, there are undoubtedly many more properties that have been acquired that help enterococci persist and spread in the hospital setting and cause diseases that have yet to be defined. Much remains to be learned about these ancient and rugged microbes, particularly in the area of pathogenic mechanisms involved with human diseases.

A Novel Virulence Phenotype Rapidly Assesses Candida Fungal Pathogenesis in Healthy and Immunocompromised Caenorhabditis elegans Hosts

Opportunistic pathogens are commensals capable of causing disease and are serious threats to human health. It is critical to understand the mechanisms and host contexts under which opportunistic pathogens become virulent. In this work, we present a novel assay to quickly and quantitatively measure pathogen virulence in healthy and immunocompromised nematode hosts. We found that Candida species, one of the most prominent fungal opportunistic pathogens of humans, decrease host fitness by reducing survival and impacting host reproduction. Most importantly, by measuring virulence in hosts that have intact or compromised immune function, we can reveal the pathogenic potential of opportunistic fungal pathogens.

The yeast Candida albicans is an opportunistic pathogen of humans, meaning that despite commensal interactions with its host, it can transition to a harmful pathogen. While C. albicans is the predominant species isolated in the human gastrointestinal mycobiome and is implicated in fungal infection, infections due to non-albicans Candida species are rapidly rising. Studying the factors that contribute to virulence is often challenging and frequently depends on many contexts, including host immune status and pathogen genetic background. Here, we utilize the nematode Caenorhabditis elegans as a perspicuous and efficient model host system to study fungal infections of Candida pathogens. We find that, in addition to reducing lifetime host survival, exposure to C. albicans results in delayed reproduction, which significantly reduced lineage growth over multiple generations. Furthermore, we assessed fungal pathogen virulence in C. elegans hosts compromised for innate immune function and detected increased early mortality, reduced brood sizes, and delayed reproduction relative to infected healthy hosts. Importantly, by assessing virulence in both healthy and immunocompromised host backgrounds, we reveal the pathogen potential in non-albicans Candida species. Taken together, we present a novel lineage growth assay to measure reduction in host fitness associated with fungal infection and demonstrate significant interactions between pathogen and host immune function that contribute to virulence.

IMPORTANCE Opportunistic pathogens are commensals capable of causing disease and are serious threats to human health. It is critical to understand the mechanisms and host contexts under which opportunistic pathogens become virulent. In this work, we present a novel assay to quickly and quantitatively measure pathogen virulence in healthy and immunocompromised nematode hosts. We found that Candida species, one of the most prominent fungal opportunistic pathogens of humans, decrease host fitness by reducing survival and impacting host reproduction. Most importantly, by measuring virulence in hosts that have intact or compromised immune function, we can reveal the pathogenic potential of opportunistic fungal pathogens.

Genetically Compromising Phospholipid Metabolism Limits Candida albicans' Virulence

Perturbing ergosterol synthesis has been previously shown to reduce the virulence of Candida albicans. We tested the hypothesis that further altering cell membrane composition by limiting phospholipid synthesis or remodeling will have the same effect. To model partial inhibition, C. albicans strains independently harboring heterozygous deletion of four genes that encode for enzymes that mediate phospholipid synthesis or modification were generated. Quantitative PCR determined that heterozygous deletion routinely caused a nearly 50% reduction in the respective gene's transcript abundance. Compensatory increased transcript abundance was only found with the deletion of LRO1, a homolog of phospholipid diacylglycerol acyltransferases. Virulence of the mutants was assayed in a Caenorhabditis elegans host model. Even modestly reduced expression of LRO1, phosphatidylserine synthase (CHO1), and lysophospholipid acyltransferase (LPT1) significantly reduced virulence by 23-38%. Reintroducing a second functional allele, respectively, to all three mutants restored virulence. Heterozygous deletion of SLC1, a homolog of 1-acylglycerol-3-phosphate O-acyltransferases, did not significantly reduce virulence. Electrospray ionization tandem mass spectrometry analysis of phospholipid composition followed by principal component analysis identified comprehensive changes in the LRO1 and CHO1 deletion heterozygotes. Strikingly (p < 0.001), univariate comparisons found that both deletion heterozygotes had 20% more phosphatidylinositol, 75% less lysophosphatidylcholine, and 35% less lysophosphatidylethanolamine compared to wild type. Heterozygous deletion of LPT1 also significantly increased phosphatidylinositol abundance. No growth phenotype, including filamentation, was affected by any mutation. Together, these data predict that even partial pharmacological inhibition of Lro1p, Cho1p, and Lpt1p will limit C. albicans virulence through altering phospholipid composition.

Chlorogenic acid attenuates virulence factors and pathogenicity of Pseudomonas aeruginosa by regulating quorum sensing

Quorum sensing (QS) is a cell-to-cell communication that is used by bacteria to regulate collective behaviors. Quorum sensing controls virulence factor production in many bacterial species and it is regarded as an attractive target to combat bacterial pathogenicity, especially against antibiotic-resistant bacteria. Chlorogenic acid (CA), abundant in fruits, vegetables, and Chinese herbs, processes multiple activities. In this research, we explored its quorum sensing quenching activity. In Pseudomonas aeruginosa, CA significantly inhibited the formation of biofilm, the ability of swarming, and virulence factors including protease and elastase activities and rhamnolipid and pyocyanin production. CA showed similar inhibitory effects in Chromobacterium violaceum on its biofilm formation, swarming motility, chitinolytic activity and violacein production. We examined the expression of QS-related genes in P.aeruginosa  and found these genes were all downregulated by CA treatment. Computational modeling revealed that CA can form hydrogen bonds with all three QS receptors. Caenorhabditis elegans and mouse infection models were employed to explore the anti-virulence ability of CA and its effect on pathogenesis process in vivo. CA extended the survival period and reduced the quantity of P. aeruginosa in nematode gut, showing a moderate protective effect on C. elegans. In mice wound model, CA-treated groups showed an accelerating healing rate and the bacteria number in wound area was also decreased by CA treatment. It is suggested by our research that CA has potential to be used as an anti-virulence factor in P. aeruginosa infection.

Immune and xenobiotic response crosstalk to chemical exposure by PA01 infection in the nematode Caenorhabditis elegans

Most organisms simultaneously face various chemical and biological stresses in the environment. Herein, we investigated how pathogen infection modifies an organism's response to chemical exposure. To explore this phenomenon, we conducted a toxicity study combined with pathogen infection by using the nematode Caenorhabditis elegans, the pathogen Pseudomonas aeruginosa, and various environmental chemicals. C. elegans preinfected with PA01, when subsequently exposed to chemicals, became sensitized to the toxicity of nonylphenol (NP) and cadmium (Cd), whereas they became tolerant to the toxicity of silver nanoparticles (AgNPs); this led us to conduct a mechanistic study focusing on AgNP exposure. A gene expression profiling study revealed that most of the immune response genes activated by PA01 infection remained activated after subsequent exposure to AgNPs, thereby suggesting that the acquired tolerance of C. elegans to AgNP exposure may be due to boosted immunity resulting from PA01 preinfection. Further, a functional genetic analysis revealed that the immune response pathway (i.e., PMK-1/p38 MAPK) was involved in defense against AgNP exposure in PA01-preinfected C. elegans, thus suggesting immune and stress response crosstalk to xenobiotic exposure. This study will aid in the elucidation of how pathogen infection impacts the way the defense system responds to subsequent xenobiotic exposure.

A High-throughput, High-content, Liquid-based C. elegans Pathosystem

The number of new drugs identified by traditional, in vitro screens has waned, reducing the success of this approach in the search for new weapons to combat multiple drug resistance. This has led to the conclusion that researchers do not only need to find new drugs, but also need to develop new ways of finding them. Amongst the most promising candidate methods are whole-organism, in vivo assays that use high-throughput, phenotypic readouts and hosts that range from Caenorhabditis elegans to Danio rerio. These hosts have several powerful advantages, including dramatic reductions in false positive hits, as compounds that are toxic to the host and/or biounavailable are typically dropped in the initial screen, prior to costly follow up. Here we show how our assay has been used to interrogate host variation in the well-documented C. elegans-Pseudomonas aeruginosa liquid killing pathosystem. We also demonstrate several extensions of this well-worked out technique. For example, we are able to carry out high-throughput genetic screens using RNAi in 24- or 96-well plate formats to query host factors in this host-pathogen interaction. Using this assay, whole genome screens can be completed in only a few months, which can dramatically simplify the task of identifying drug targets, potentially without the need for laborious biochemical purification approaches. We also report here a variation of our method that substitutes the gram-positive bacterium Enterococcus faecalis for the gram-negative pathogen P. aeruginosa. Much as is the case for P. aeruginosa, killing by E. faecalis is time-dependent. Unlike previous C. elegans-E. faecalis assays, our assay for E. faecalis does not require preinfection, improving its safety profile and reducing the chances of contaminating liquid-handling equipment. The assay is highly robust, showing ~95% death rates 96 h post infection.

Caenorhabditis elegans in high-throughput screens for anti-infective compounds

New classes of antimicrobials that are effective therapies for infections with multi-drug resistant pathogens are urgently needed. The nematode Caenorhabditis elegans has been incorporated into small molecule screening platforms to identify anti-infective compounds that provide protection of a host during infection. The use of a live animal in these screening systems offers several advantages, including the ability to identify molecules that boost innate immune responses in a manner advantageous to host survival and compounds that disrupt bacterial virulence mechanisms. In addition, new classes of antimicrobials that target the pathogen have been uncovered, as well as interesting chemical probes that can be used to dissect new mechanisms of host-pathogen interactions.