Zebrafish embryos as a model host for the real time analysis of Salmonella typhimurium infections

3-Hydroxykynurenine targets kainate receptors to promote defense against infection

Bacterial infection involves a complex interaction between the pathogen and host where the outcome of infection is not solely determined by pathogen eradication. To identify small molecules that promote host survival by altering the host-pathogen dynamic, we conducted an in vivo chemical screen using zebrafish embryos and found that treatment with 3-hydroxykynurenine (3-HK) protects from lethal bacterial infection. 3-HK, a metabolite produced through host tryptophan metabolism, has no direct antibacterial activity but enhances host survival by restricting bacterial expansion in macrophages through a systemic mechanism that targets kainate-sensitive glutamate receptors. These findings reveal a new pathway by which tryptophan metabolism and kainate-sensitive glutamate receptors function and interact to modulate immunity, with important implications for the coordination between the immune and nervous systems in pathological conditions.

Diving into drug-screening: zebrafish embryos as an in vivo platform for antimicrobial drug discovery and assessment

The rise of multidrug-resistant bacteria underlines the need for innovative treatments, yet the introduction of new drugs has stagnated despite numerous antimicrobial discoveries. A major hurdle is a poor correlation between promising in vitro data and in vivo efficacy in animal models, which is essential for clinical development. Early in vivo testing is hindered by the expense and complexity of existing animal models. Therefore, there is a pressing need for cost-effective, rapid preclinical models with high translational value. To overcome these challenges, zebrafish embryos have emerged as an attractive model for infectious disease studies, offering advantages such as ethical alignment, rapid development, ease of maintenance, and genetic manipulability. The zebrafish embryo infection model, involving microinjection or immersion of pathogens and potential antibiotic hit compounds, provides a promising solution for early-stage drug screening. It offers a cost-effective and rapid means of assessing the efficacy, toxicity and mechanism of action of compounds in a whole-organism context. This review discusses the experimental design of this model, but also its benefits and challenges. Additionally, it highlights recently identified compounds in the zebrafish embryo infection model and discusses the relevance of the model in predicting the compound's clinical potential.

Copper regulates the host innate immune response against bacterial infection via activation of ALPK1 kinase

Copper is an essential trace element for the human body, and its requirement for optimistic immune functions has been recognized for decades. How copper is involved in the innate immune pathway, however, remains to be clarified. Here, we report that copper serves as a signal molecule to regulate the kinase activity of alpha-kinase 1 (ALPK1), a cytosolic pattern-recognition receptor (PRR), and therefore promotes host cell defense against bacterial infection. We show that in response to infection, host cells actively accumulate copper in the cytosol, and the accumulated cytosolic copper enhances host cell defense against evading pathogens, including intracellular and, unexpectedly, extracellular bacteria. Subsequently, we demonstrate that copper activates the innate immune pathway of host cells in an ALPK1-dependent manner. Further mechanistic studies reveal that copper binds to ALPK1 directly and is essential for the kinase activity of this cytosolic PRR. Moreover, the binding of copper to ALPK1 enhances the sensitivity of ALPK1 to the bacterial metabolite ADP-heptose and eventually prompts host cells to elicit an enhanced immune response during bacterial infection. Finally, using a zebrafish in vivo model, we show that a copper-treated host shows an increased production of proinflammatory cytokines, enhanced recruitment of phagosome cells, and promoted bacterial clearance. Our findings uncover a previously unrecognized role of copper in the modulation of host innate immune response against bacterial pathogens and advance our knowledge on the cross talk between cytosolic copper homeostasis and immune system.

The streptococcal phase-variable type I restriction modification system SsuCC20p dictates the methylome of Streptococcus suis impacting the transcriptome and virulence in a zebrafish larvae infection model

IMPORTANCE

Phase variation allows a single strain to produce phenotypic diverse subpopulations. Phase-variable restriction modification (RM) systems are systems that allow for such phase variation via epigenetic regulation of gene expression levels. The phase-variable RM system SsuCC20p was found in multiple streptococcal species and was acquired by an emerging zoonotic lineage of Streptococcus suis. We show that the phase variability of SsuCC20p is dependent on a recombinase encoded within the SsuCC20p locus. We characterized the genome methylation profiles of the different phases of SsuCC20p and demonstrated the consequential impact on the transcriptome and virulence in a zebrafish infection model. Acquiring mobile genetic elements containing epigenetic regulatory systems, like phase-variable RM systems, enables bacterial pathogens to produce diverse phenotypic subpopulations that are better adapted to specific (host) environments encountered during infection.

Dynamics of macrophage polarization support Salmonella persistence in a whole living organism

Numerous intracellular bacterial pathogens interfere with macrophage function, including macrophage polarization, to establish a niche and persist. However, the spatiotemporal dynamics of macrophage polarization during infection within host remain to be investigated. Here, we implement a model of persistent Salmonella Typhimurium infection in zebrafish, which allows visualization of polarized macrophages and bacteria in real time at high resolution. While macrophages polarize toward M1-like phenotype to control early infection, during later stages, Salmonella persists inside non-inflammatory clustered macrophages. Transcriptomic profiling of macrophages showed a highly dynamic signature during infection characterized by a switch from pro-inflammatory to anti-inflammatory/pro-regenerative status and revealed a shift in adhesion program. In agreement with this specific adhesion signature, macrophage trajectory tracking identifies motionless macrophages as a permissive niche for persistent Salmonella. Our results demonstrate that zebrafish model provides a unique platform to explore, in a whole organism, the versatile nature of macrophage functional programs during bacterial acute and persistent infections.

Sequential infection of human norovirus and Salmonella enterica resulted in higher mortality and ACOD1/IRG1 upregulation in zebrafish larvae

Human norovirus (HNoVs) and Salmonella are both very important foodborne pathogens with mixed infection of HNoV and Salmonella reported clinically. With the use of model organism zebrafish (Danio rerio), it was observed that the sequential infection of HNoVs and Salmonella caused lower survival rates (12.5 ± 4.2%) than the single-pathogen infection by Salmonella (31.6 ± 7.3%, P < 0.05) or HNoVs (no mortality observed). Gene expression study with the use of RT-PCR and global transcriptomic analysis revealed that the mortality of zebrafish larvae was very likely due to the harmful inflammatory responses. Specifically, it was noted that the genes encoding aconitate decarboxylase 1 (ACOD1), also known as immunoresponsive gene 1 (IRG1), were significantly upregulated in the sequentially infected zebrafish larvae. The expression of acod1 could lead to mitochondrial reactive oxygen species (ROS) production. The ROS levels were indeed higher in sequentially infected zebrafish larvae than the single-pathogen infected ones (P < 0.05). An immersion treatment of glutathione or citraconate did not affect the microbial loads of HNoVs and Salmonella but significantly reduced the ROS levels and protected the zebrafish larvae by inducing higher survival rates in the sequentially infected zebrafish larvae (P < 0.05). Taken together, this study accumulated new knowledge over the function of ACOD1/IRG1 pathway in infectious diseases, and proposed possible treatment strategies accordingly.

Transgenic expression in zebrafish embryos with an intact chorion by electroporation and microinjection

Electroporation is regularly used to deliver agents into cells, including transgenic materials, but it is not used for mutating zebrafish embryos due to the lack of suitable systems, information on appropriate operating parameters, and the challenges posed by the protective chorion. Here, a novel method for gene delivery in zebrafish embryos was developed by combining microinjection into the space between the chorion and the embryo followed by electroporation. This method eliminates the need for chorion removal and injecting into the space between the chorion and embryo eliminates the need for finding and identifying key cell locations before performing an injection, making the process much simpler and more automatable. We also developed a microfluidic electroporation system and optimized electric pulse parameters for transgenesis of embryos. The study provided a novel method for gene delivery in zebrafish embryos that can be potentially implemented in a high throughput transgenesis or mutagenesis system.

Unlocking the Potential of Zebrafish Research with Artificial Intelligence: Advancements in Tracking, Processing, and Visualization

Zebrafish have become a widely accepted model organism for biomedical research due to their strong cortisol stress response, behavioral strain differences, and sensitivity to both drug treatments and predators. However, experimental zebrafish studies generate substantial data that must be analyzed through objective, accurate, and repeatable analysis methods. Recently, advancements in artificial intelligence (AI) have enabled automated tracking, image recognition, and data analysis, leading to more efficient and insightful investigations. In this review, we examine key AI applications in zebrafish research, including behavior analysis, genomics, and neuroscience. With the development of deep learning technology, AI algorithms have been used to precisely analyze and identify images of zebrafish, enabling automated testing and analysis. By applying AI algorithms in genomics research, researchers have elucidated the relationship between genes and biology, providing a better basis for the development of disease treatments and gene therapies. Additionally, the development of more effective neuroscience tools could help researchers better understand the complex neural networks in the zebrafish brain. In the future, further advancements in AI technology are expected to enable more extensive and in-depth medical research applications in zebrafish, improving our understanding of this important animal model. This review highlights the potential of AI technology in achieving the full potential of zebrafish research by enabling researchers to efficiently track, process, and visualize the outcomes of their experiments.

A tryptophan metabolite modulates the host response to bacterial infection via kainate receptors

Bacterial infection involves a complex interaction between the pathogen and host where the outcome of infection is not solely determined by pathogen eradication. To identify small molecules that promote host survival by altering the host-pathogen dynamic, we conducted an in vivo chemical screen using zebrafish embryos and found that treatment with 3-hydroxy-kynurenine protects from lethal gram-negative bacterial infection. 3-hydroxy-kynurenine, a metabolite produced through host tryptophan metabolism, has no direct antibacterial activity but enhances host survival by restricting bacterial expansion in macrophages by targeting kainate-sensitive glutamate receptors. These findings reveal new mechanisms by which tryptophan metabolism and kainate-sensitive glutamate receptors function and interact to modulate immunity, with significant implications for the coordination between the immune and nervous systems in pathological conditions.

Impact of Drug Administration Routes on the In Vivo Efficacy of the Natural Product Sorangicin a Using a Staphylococcus aureus Infection Model in Zebrafish Embryos

Staphylococcus aureus causes a wide range of infections, and it is one of the leading pathogens responsible for deaths associated with antimicrobial resistance, the rapid spread of which among S. aureus urges the discovery of new antibiotics. The evaluation of in vivo efficacy of novel drug candidates is usually performed using animal models. Recently, zebrafish (Danio rerio) embryos have become increasingly attractive in early drug discovery. Herein, we established a zebrafish embryo model of S. aureus infection for evaluation of in vivo efficacy of novel potential antimicrobials. A local infection was induced by microinjecting mCherry-expressing S. aureus Newman followed by treatment with reference antibiotics via microinjection into different injection sites as well as via waterborne exposure to study the impact of the administration route on efficacy. We successfully used the developed model to evaluate the in vivo activity of the natural product sorangicin A, for which common mouse models were not successful due to fast degradation in plasma. In conclusion, we present a novel screening platform for assessing in vivo activity at the antibiotic discovery stage. Furthermore, this work provides consideration for the choice of an appropriate administration route based on the physicochemical properties of tested drugs.

Characterization of the innate immune response to Streptococcus pneumoniae infection in zebrafish

Streptococcus pneumoniae (pneumococcus) is one of the most frequent causes of pneumonia, sepsis and meningitis in humans, and an important cause of mortality among children and the elderly. We have previously reported the suitability of the zebrafish (Danio rerio) larval model for the study of the host-pathogen interactions in pneumococcal infection. In the present study, we characterized the zebrafish innate immune response to pneumococcus in detail through a whole-genome level transcriptome analysis and revealed a well-conserved response to this human pathogen in challenged larvae. In addition, to gain understanding of the genetic factors associated with the increased risk for severe pneumococcal infection in humans, we carried out a medium-scale forward genetic screen in zebrafish. In the screen, we identified a mutant fish line which showed compromised resistance to pneumococcus in the septic larval infection model. The transcriptome analysis of the mutant zebrafish larvae revealed deficient expression of a gene homologous for human C-reactive protein (CRP). Furthermore, knockout of one of the six zebrafish crp genes by CRISPR-Cas9 mutagenesis predisposed zebrafish larvae to a more severe pneumococcal infection, and the phenotype was further augmented by concomitant knockdown of a gene for another Crp isoform. This suggests a conserved function of C-reactive protein in anti-pneumococcal immunity in zebrafish. Altogether, this study highlights the similarity of the host response to pneumococcus in zebrafish and humans, gives evidence of the conserved role of C-reactive protein in the defense against pneumococcus, and suggests novel host genes associated with pneumococcal infection.

Localized Infections with P. aeruginosa Strains Defective in Zinc Uptake Reveal That Zebrafish Embryos Recapitulate Nutritional Immunity Responses of Higher Eukaryotes

The innate immune responses of mammals to microbial infections include strategies based on manipulating the local concentration of metals such as iron (Fe) and zinc (Zn), commonly described as nutritional immunity. To evaluate whether these strategies are also present in zebrafish embryos, we have conducted a series of heart cavity-localized infection experiments with Pseudomonas aeruginosa strains characterized by a different ability to acquire Zn. We have found that, 48 h after infection, the bacterial strains lacking critical components of the Zn importers ZnuABC and ZrmABCD have a reduced colonization capacity compared to the wild-type strain. This observation, together with the finding of a high level of expression of Zur-regulated genes, suggests the existence of antimicrobial mechanisms based on Zn sequestration. However, we have observed that strains lacking such Zn importers have a selective advantage over the wild-type strain in the early stages of infection. Analysis of the expression of the gene that encodes for a Zn efflux pump has revealed that at short times after infection, P. aeruginosa is exposed to high concentrations of Zn. At the same time, zebrafish respond to the infection by activating the expression of the Zn transporters Slc30a1 and Slc30a4, whose mammalian homologs mediate a redistribution of Zn in phagocytes aimed at intoxicating bacteria with a metal excess. These observations indicate that teleosts share similar nutritional immunity mechanisms with higher vertebrates, and confirm the usefulness of the zebrafish model for studying host-pathogen interactions.

Meningococcal virulence in zebrafish embryos depends on capsule polysaccharide structure

Neisseria meningitidis or the meningococcus, can cause devasting diseases such as sepsis and meningitis. Its polysaccharide capsule, on which serogrouping is based, is the most important virulence factor. Non-encapsulated meningococci only rarely cause disease, due to their sensitivity to the host complement system. How the capsular polysaccharide structure of N. meningitidis relates to virulence is largely unknown. Meningococcal virulence can be modeled in zebrafish embryos as the innate immune system of the zebrafish embryo resembles that of mammals and is fully functional two days post-fertilization. In contrast, the adaptive immune system does not develop before 4 weeks post-fertilization. We generated isogenic meningococcal serogroup variants to study how the chemical composition of the polysaccharide capsule affects N. meningitidis virulence in the zebrafish embryo model. H44/76 serogroup B killed zebrafish embryos in a dose-dependent manner, whereas the non-encapsulated variant was completely avirulent. Neutrophil depletion was observed after infection with encapsulated H44/76, but not with its non-encapsulated variant HB-1. The survival of embryos infected with isogenic capsule variants of H44/76 was capsule specific. The amount of neutrophil depletion differed accordingly. Both embryo killing capacity and neutrophil depletion after infection correlated with the number of carbons used per repeat unit of the capsule polysaccharide during its biosynthesis (indicative of metabolic cost).

Conclusion

Meningococcal virulence in the zebrafish embryo largely depends on the presence of the polysaccharide capsule but the extent of the contribution is determined by its structure. The observed differences between the meningococcal isogenic capsule variants in zebrafish embryo virulence may depend on differences in metabolic cost.

Covalent Proteomimetic Inhibitor of the Bacterial FtsQB Divisome Complex

The use of antibiotics is threatened by the emergence and spread of multidrug-resistant strains of bacteria. Thus, there is a need to develop antibiotics that address new targets. In this respect, the bacterial divisome, a multi-protein complex central to cell division, represents a potentially attractive target. Of particular interest is the FtsQB subcomplex that plays a decisive role in divisome assembly and peptidoglycan biogenesis in E. coli. Here, we report the structure-based design of a macrocyclic covalent inhibitor derived from a periplasmic region of FtsB that mediates its binding to FtsQ. The bioactive conformation of this motif was stabilized by a customized cross-link resulting in a tertiary structure mimetic with increased affinity for FtsQ. To increase activity, a covalent handle was incorporated, providing an inhibitor that impedes the interaction between FtsQ and FtsB irreversibly. The covalent inhibitor reduced the growth of an outer membrane-permeable E. coli strain, concurrent with the expected loss of FtsB localization, and also affected the infection of zebrafish larvae by a clinical E. coli strain. This first-in-class inhibitor of a divisome protein–protein interaction highlights the potential of proteomimetic molecules as inhibitors of challenging targets. In particular, the covalent mode-of-action can serve as an inspiration for future antibiotics that target protein–protein interactions.

Methods to Evaluate Bacterial Motility and Its Role in Bacterial–Host Interactions

Bacterial motility is a widespread characteristic that can provide several advantages for the cell, allowing it to move towards more favorable conditions and enabling host-associated processes such as colonization. There are different bacterial motility types, and their expression is highly regulated by the environmental conditions. Because of this, methods for studying motility under realistic experimental conditions are required. A wide variety of approaches have been developed to study bacterial motility. Here, we present the most common techniques and recent advances and discuss their strengths as well as their limitations. We classify them as macroscopic or microscopic and highlight the advantages of three-dimensional imaging in microscopic approaches. Lastly, we discuss methods suited for studying motility in bacterial–host interactions, including the use of the zebrafish model.

Stapling of Peptides Potentiates: The Antibiotic Treatment of Acinetobacter baumannii In Vivo

The rising incidence of multidrug resistance in Gram-negative bacteria underlines the urgency for novel treatment options. One promising new approach is the synergistic combination of antibiotics with antimicrobial peptides. However, the use of such peptides is not straightforward; they are often sensitive to proteolytic degradation, which greatly limits their clinical potential. One approach to increase stability is to apply a hydrocarbon staple to the antimicrobial peptide, thereby fixing them in an α-helical conformation, which renders them less exposed to proteolytic activity. In this work we applied several different hydrocarbon staples to two previously described peptides shown to act on the outer membrane, L6 and L8, and tested their activity in a zebrafish embryo infection model using a clinical isolate of Acinetobacter baumannii as a pathogen. We show that the introduction of such a hydrocarbon staple to the peptide L8 improves its in vivo potentiating activity on antibiotic treatment, without increasing its in vivo antimicrobial activity, toxicity or hemolytic activity.

An Overview of Zebrafish Modeling Methods in Drug Discovery and Development

Animal studies are recognized as a significant step forward in the bridging between drug discovery and clinical applications. Animal models, due to their relative genetic, molecular, physiological, and even anatomical similarities to humans, can provide a suitable platform for unraveling the mechanisms underlying human diseases and discovering new therapeutic approaches as well. Recently, zebrafish has attracted attention as a valuable experimental and pharmacological model in drug discovery and development studies due to its prominent characteristics such as the high degree of genetic similarity with humans, genetic manipulability, and prominent clinical features. Since advancing a theory to a valid and reliable observation requires the manipulation of animals, it is, therefore, essential to use efficient modeling methods appropriate to the different aspects of experimental conditions. In this context, applying several various approaches such as using chemicals, pathogens, and genetic manipulation approaches allows zebrafish development into a preferable model that mimics some human disease pathophysiology. Thus, such modeling approaches not only can provide a framework for a comprehensive understanding of the human disease mechanisms that have a counterpart in zebrafish but also can pave the way for discovering new drugs that are accompanied by higher amelioration effects on different human diseases.

The Role of Galanin during Bacterial Infection in Larval Zebrafish

Galanin is a peptide that is conserved among different species and plays various roles in an organism, although its entire role is not completely understood. For many years, galanin has been linked mainly with the neurotransmission in the nervous system; however, recent reports underline its role in immunity. Zebrafish (Danio rerio) is an intensively developing animal model to study infectious diseases. In this study, we used larval zebrafish to determine the role of galanin in bacterial infection. We showed that knockout of galanin in zebrafish leads to a higher bacterial burden and mortality during Mycobacterium marinum and Staphylococcus aureus infection, whereas administration of a galanin analogue, NAX 5055, improves the ability of fish to control the infection caused by both pathogens. Moreover, the transcriptomics data revealed that a lower number of genes were regulated in response to mycobacterial infection in gal−/− mutants compared with their gal+/+ wild-type counterparts. We also found that galanin deficiency led to significant changes in immune-related pathways, mostly connected with cytokine and chemokine functions. The results show that galanin acts not only as a neurotransmitter but is also involved in immune response to bacterial infections, demonstrating the complexity of the neuroendocrine system and its possible connection with immunity.

Brain-localized and Intravenous Microinjections in the Larval Zebrafish to Assess Innate Immune Response

Creating a robust and controlled infection model is imperative for studying the innate immune response. Leveraging the particular strengths of the zebrafish model system, such as optical transparency, ex utero development, and large clutch size, allows for the development of methods that yield consistent and reproducible results. We created a robust model for activation of innate immunity by microinjecting bacterial particles or live bacteria into larval zebrafish, unlike previous studies which largely restricted such manipulations to embryonic stages of zebrafish. The ability to introduce stimuli locally or systemically at larval stages provides significant advantages to examine host response in more mature tissues as well as the possibility to interrogate adaptive immunity at older larval stages. This protocol describes two distinct modes of microinjection to introduce lipopolysaccharide (LPS) or bacteria into the living larval zebrafish: one localized to the brain, and another into the bloodstream via the caudal vein plexus. Graphic abstract: Schematic shows the two distinct modes of larval zebrafish microinjection, either in the brain parenchyma or in the blood stream intravenously. Reagents introduced into the zebrafish to assess immune response are depicted in the "injection components" as described in the protocol.

Gut bacteria Vibrio sp. and Aeromonas sp. trigger the expression levels of proinflammatory cytokine: First evidence from the germ-free zebrafish

Gut microbiota plays a central part in the regulation of multiple host metabolic pathways, such as homeostasis, immunostasis, mucosa permeability, and even brain development. Though, slight known about the function of an individual gut bacterium in zebrafish. In this study, germ-free (GF) and conventionally reared (CV) zebrafish models utilized for studying the role of gut bacteria Vibrio sp. and Aeromonas sp. After the analysis of gut microbial profile in zebrafish male and female at three-month age, Proteobacteria and Fusobacteria dominated the main composition of zebrafish intestinal microflora. However, the relative richness of them was different base on gender variance. Aeromonas sp. and Vibrio sp. belonging to Proteobacteria phylum of bacteria were isolated from zebrafish gut, and their potential capacities to trigger innate immunity were investigated. In gut microbiota absence, the expression levels of the innate immunity genes in the GF group were not significantly changed compared to the CV group. After exposure to Aeromonas sp. and Vibrio sp., the expression levels of myd88, TLRs-, and inflammation-related genes were increased in both GF and CV groups, except tlr2 and NLRs-related genes. However, the expression level of NF-κB and JNK/AP-1 pathway genes were all decreased after exposure to Aeromonas sp. and Vibrio sp. in both GF and CV groups. Interestingly, inflammation-related genes (tnfa, tnfb, and il1β) were activated in the CV group, and there were not significantly changed in the GF group, indicating that other bacteria were indispensable for Aeromonas sp. or Vibrio sp. to activate the inflammation response. Taken together, this is the first study of gut bacteria Vibrio sp. and Aeromonas sp. prompting the innate immune response using the GF and CV zebrafish model.

Autophagy and Lc3-Associated Phagocytosis in Zebrafish Models of Bacterial Infections

Modeling human infectious diseases using the early life stages of zebrafish provides unprecedented opportunities for visualizing and studying the interaction between pathogens and phagocytic cells of the innate immune system. Intracellular pathogens use phagocytes or other host cells, like gut epithelial cells, as a replication niche. The intracellular growth of these pathogens can be counteracted by host defense mechanisms that rely on the autophagy machinery. In recent years, zebrafish embryo infection models have provided in vivo evidence for the significance of the autophagic defenses and these models are now being used to explore autophagy as a therapeutic target. In line with studies in mammalian models, research in zebrafish has shown that selective autophagy mediated by ubiquitin receptors, such as p62, is important for host resistance against several bacterial pathogens, including Shigella flexneri, Mycobacterium marinum, and Staphylococcus aureus. Furthermore, an autophagy related process, Lc3-associated phagocytosis (LAP), proved host beneficial in the case of Salmonella Typhimurium infection but host detrimental in the case of S. aureus infection, where LAP delivers the pathogen to a replication niche. These studies provide valuable information for developing novel therapeutic strategies aimed at directing the autophagy machinery towards bacterial degradation.

Macrophage-Derived IL-1β Regulates Emergency Myelopoiesis via the NF-κB and C/ebpβ in Zebrafish

Myeloid phagocytes, neutrophils in particular, are easily consumed when they fight against a large number of invading microbes. Hence, they require efficient and constant replenishment from their progenitors via the well-orchestrated emergency myelopoiesis in the hematopoietic organs. The cellular and molecular details of the danger-sensing and warning processes to activate the emergency myelopoiesis are still under debate. In this study, we set up a systemic infection model in zebrafish (Danio rerio) larvae via circulative administration of LPS. We focused on the cross-talk of macrophages with myeloid progenitors in the caudal hematopoietic tissue. We revealed that macrophages first detected LPS and sent out the emergency message via il1β The myeloid progenitors, rather than hematopoietic stem and progenitor cells, responded and fulfilled the demand to adapt myeloid expansion through the synergistic cooperation of NF-κB and C/ebpβ. Our study unveiled a critical role of macrophages as the early "whistle blowers" to initiate emergency myelopoiesis.

EvpP inhibits neutrophils recruitment via Jnk-caspy inflammasome signaling in vivo

Innate immunity is regulated by phagocytic cells and is critical for host control of bacterial infection. In many bacteria, the type VI secretion system (T6SS) can affect bacterial virulence in certain environments, but little is known about the mechanisms underlying T6SS regulation of innate immune responses during infection in vivo. Here, we developed an infection model by microinjecting bacteria into the tail vein muscle of 3-day-post-fertilized zebrafish larvae, and found that both macrophages and neutrophils are essential for bacterial clearance. Further study revealed that EvpP plays a critical role in promoting the pathogenesis of Edwardsiella piscicida (E. piscicida) via inhibiting the phosphorylation of Jnk signaling to reduce the expression of chemokine (CXC motif) ligand 8 (cxcl8a), matrix metallopeptidase 13 (mmp13) and interleukin-1β (IL-1β) in vivo. Subsequently, by utilizing Tg (mpo:eGFP^+/+) zebrafish larvae for E. piscicida infection, we found that the EvpP-inhibited Jnk-caspy (caspase-1 homolog) inflammasome signaling axis significantly suppressed the recruitment of neutrophils to infection sites, and the caspy- or IL-1β-morpholino (MO) knockdown larvae were more susceptible to infection and failed to restrict bacterial colonization in vivo. taken together, this interaction improves our understanding about the complex and contextual role of a bacterial T6SS effector in modulating the action of neutrophils during infection, and offers new insights into the warfare between bacterial weapons and host immunological surveillance.

Rubicon-Dependent Lc3 Recruitment to Salmonella-Containing Phagosomes Is a Host Defense Mechanism Triggered Independently From Major Bacterial Virulence Factors

Intracellular pathogens such as Salmonella depend on their molecular virulence factors to evade host defense responses like autophagy. Using a zebrafish systemic infection model, we have previously shown that phagocytes, predominantly macrophages, target Salmonella Typhimurium by an autophagy-related pathway known as Lc3-associated phagocytosis (LAP), which is dependent on the host protein Rubicon. Here, we explore the influence of Salmonella virulence factors on pathogenicity in the zebrafish model and induction of LAP as a defense response. We investigated five mutant strains that all could trigger GFP-Lc3 recruitment as puncta or rings around single bacteria or bacterial clusters, in a Rubicon-dependent manner. We found that S. Typhimurium strains carrying mutations in PhoP or PurA, responsible for adaptation to the intracellular environment and efficient metabolism of purines, respectively, are attenuated in the zebrafish model. However, both strains show increased virulence when LAP is inhibited by knockdown of Rubicon. Mutations in type III secretion systems 1 and 2, SipB and SsrB, which are important for invading and replicating in non-phagocytic cells, did not affect the ability to establish successful infection in the zebrafish model. This observation is in line with our previous characterization of this infection model revealing that macrophages actively phagocytose the majority of S. Typhimurium. In contrast to SipB mutants, SsrB mutants were unable to become more virulent in Rubicon-deficient hosts, suggesting that type III system 2 effectors are important for intracellular replication of Salmonella in the absence of LAP. Finally, we found that mutation of FlhD, required for production of flagella, renders S. Typhimurium hypervirulent both in wild type zebrafish embryos and in Rubicon-deficient hosts. FlhD mutation also led to lower levels of GFP-Lc3 recruitment compared with the wild type strain, indicating that recognition of flagellin by the host innate immune system promotes the LAP response. Together, our results provide new evidence that the Rubicon-dependent LAP process is an important defense mechanism against S. Typhimurium.

Macrophages target Salmonella by Lc3-associated phagocytosis in a systemic infection model

Innate immune defense against intracellular pathogens, like Salmonella, relies heavily on the autophagy machinery of the host. This response is studied intensively in epithelial cells, the target of Salmonella during gastrointestinal infections. However, little is known of the role that autophagy plays in macrophages, the predominant carriers of this pathogen during systemic disease. Here we utilize a zebrafish embryo model to study the interaction of S. enterica serovar Typhimurium with the macroautophagy/autophagy machinery of macrophages in vivo. We show that phagocytosis of live but not heat-killed Salmonella triggers recruitment of the autophagy marker GFP-Lc3 in a variety of patterns labeling tight or spacious bacteria-containing compartments, also revealed by electron microscopy. Neutrophils display similar GFP-Lc3 associations, but genetic modulation of the neutrophil/macrophage balance and ablation experiments show that macrophages are critical for the defense response. Deficiency of atg5 reduces GFP-Lc3 recruitment and impairs host resistance, in contrast to atg13 deficiency, indicating that Lc3-Salmonella association at this stage is independent of the autophagy preinitiation complex and that macrophages target Salmonella by Lc3-associated phagocytosis (LAP). In agreement, GFP-Lc3 recruitment and host resistance are impaired by deficiency of Rubcn/Rubicon, known as a negative regulator of canonical autophagy and an inducer of LAP. We also found strict dependency on NADPH oxidase, another essential factor for LAP. Both Rubcn and NADPH oxidase are required to activate a Salmonella biosensor for reactive oxygen species inside infected macrophages. These results identify LAP as the major host protective autophagy-related pathway responsible for macrophage defense against Salmonella during systemic infection. Abbreviations: ATG: autophagy related gene; BECN1: Beclin 1; CFU: colony forming units; CYBA/P22PHOX: cytochrome b-245, alpha chain; CYBB/NOX2: cytochrome b-245 beta chain; dpf: days post fertilization; EGFP: enhanced green fluorescent protein; GFP: green fluorescent protein; hfp: hours post fertilization; hpi: hours post infection; IRF8: interferon regulatory factor 8; Lcp1/L-plastin: lymphocyte cytosolic protein 1; LAP: LC3-associated phagocytosis; MAP1LC3/LC3: microtubule-associated protein 1A/1B-light chain 3; mCherry: red fluorescent protein; mpeg1: macrophage expressed gene 1; mpx: myeloid specific peroxidase; NADPH oxidase: nicotinamide adenine dinucleotide phosphate oxidase; NCF4/P40PHOX: neutrophil cytosolic factor 4; NTR-mCherry: nitroreductase-mCherry fusion; PTU: phenylthiourea; PtdIns3K: class III phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol 3-phosphate; RB1CC1/FIP200: RB-1 inducible coiled coin 1; ROS: reactive oxygen species; RT-PCR: reverse transcriptase polymerase chain reaction; RUBCN/RUBICON: RUN and cysteine rich domain containing BECN1-interacting protein; SCV: Salmonella-containing vacuole; S. Typhimurium/S.T: Salmonella enterica serovar Typhimurium; TEM: transmission electron microscopy; Tg: transgenic; TSA: tyramide signal amplification; ULK1/2: unc-51-like autophagy activating kinase 1/2; UVRAG: UVRAG: UV radiation resistance associated; wt: wild type.

Real-time functional analysis of Hv1 channel in neutrophils: a new approach from zebrafish model

Voltage-gated proton channel (Hv1) has been studied in various immune cells, including neutrophils. However, most studies have taken an in vitro approach using isolated cells or primary cultured cells of mammals; therefore, limited evidence is available on the function of Hv1 in a physiological context. In this study, we have developed the in vivo system that enables real-time functional analysis of Hv1 using zebrafish embryos (Danio rerio). Hvcn1-deficiency (hvcn1^-/-) in zebrafish completely abolished voltage-gated proton current, which is typically observed in wild-type neutrophils. Importantly, hvcn1-deficiency significantly reduced reactive oxygen species production and calcium response of zebrafish neutrophils, comparable to the results observed in mammalian models. These findings verify zebrafish Hv1 (DrHv1) as the primary contributor for native Hv1-derived proton current in neutrophils and suggest the conserved function of Hv1 in the immune cells across vertebrate animals. Taking advantage of Hv1 zebrafish model, we compared real-time behaviors of neutrophils between wild-type and hvcn1^-/- zebrafish in response to tissue injury and acute bacterial infection. Notably, we observed a significant increase in the number of phagosomes in hvcn1^-/- neutrophils, raising a possible link between Hv1 and phagosomal maturation. Furthermore, survival analysis of zebrafish larvae potentially supports a protective role of Hv1 in the innate immune response against systemic bacterial infection. This study represents the influence of Hv1 on neutrophil behaviors and highlights the benefits of in vivo approach toward the understanding of Hv1 in a physiological context.

Dopamine-induced sulfatase and its regulator are required for Salmonella enterica serovar Typhimurium pathogenesis

Catecholamine hormones enhance the virulence of pathogenic bacteria. Studies in the 1980s made intriguing observations that catecholamines were required for induction of sulfatase activity in many enteric pathogens, including Salmonella enterica serovar Typhimurium. In this report, we show that STM3122 and STM3124, part of horizontally acquired Salmonella pathogenesis island 13, encode a catecholamine-induced sulfatase and its regulator, respectively. Induction of sulfatase activity was independent of the well-studied QseBC and QseEF two-component regulatory systems. S. Typhimurium 14028S mutants lacking STM3122 or STM3124 showed reduced virulence in zebrafish. Because catecholamines are inactivated by sulfation in the mammalian gut, S. Typhimurium could utilize CA-induced sulfatase to access free catecholamines for growth and virulence.

A Review of Automated Microinjection of Zebrafish Embryos

Cell microinjection is a technique of precise delivery of substances into cells and is widely used for studying cell transfection, signaling pathways, and organelle functions. Microinjection of the embryos of zebrafish, the third most important animal model, has become a very useful technique in bioscience. However, factors such as the small cell size, high cell deformation tendency, and transparent zebrafish embryo membrane make the microinjection process difficult. Furthermore, this process has strict, specific requirements, such as chorion softening, avoiding contacting the first polar body, and high-precision detection. Therefore, highly accurate control and detection platforms are critical for achieving the automated microinjection of zebrafish embryos. This article reviews the latest technologies and methods used in the automated microinjection of zebrafish embryos and provides a detailed description of the current developments and applications of robotic microinjection systems. The review covers key areas related to automated embryo injection, including cell searching and location, cell position and posture adjustment, microscopic visual servoing control, sensors, actuators, puncturing mechanisms, and microinjection.

Dual Transcriptomics of Host-Pathogen Interaction of Cystic Fibrosis Isolate Pseudomonas aeruginosa PASS1 With Zebrafish

Pseudomonas aeruginosa is a significant cause of mortality in patients with cystic fibrosis (CF). To explore the interaction of the CF isolate P. aeruginosa PASS1 with the innate immune response, we have used Danio rerio (zebrafish) as an infection model. Confocal laser scanning microscopy (CLSM) enabled visualization of direct interactions between zebrafish macrophages and P. aeruginosa PASS1. Dual RNA-sequencing of host-pathogen was undertaken to profile RNA expression simultaneously in the pathogen and the host during P. aeruginosa infection. Following establishment of infection in zebrafish embryos with PASS1, 3 days post infection (dpi), there were 6739 genes found to be significantly differentially expressed in zebrafish and 176 genes in PASS1. A range of virulence genes were upregulated in PASS1, including genes encoding pyoverdine biosynthesis, flagellin, non-hemolytic phospholipase C, proteases, superoxide dismutase and fimbrial subunits. Additionally, iron and phosphate acquisition genes were upregulated in PASS1 cells in the zebrafish. Transcriptional changes in the host immune response genes highlighted phagocytosis as a key response mechanism to PASS1 infection. Transcriptional regulators of neutrophil and macrophage phagocytosis were upregulated alongside transcriptional regulators governing response to tissue injury, infection, and inflammation. The zebrafish host showed significant downregulation of the ribosomal RNAs and other genes involved in translation, suggesting that protein translation in the host is affected by PASS1 infection.

The intra- and extracellular functions of ASC specks

Inflammasomes are the central signaling hubs of the inflammatory response. They process cytosolic evidence of infection, cell damage, or metabolic disturbances, and elicit a pro-inflammatory response mediated by members of the interleukin-1 family of cytokines and pyroptotoic cell death. On the molecular level, this is accomplished by the sensor-nucleated recruitment and oligomerization of the adapter protein ASC. Once a tunable threshold is reached, cooperative assembly of ASC into linear filaments and their condensation into macromolecular ASC specks promotes an all-or-none response. These structures are highly regulated and provide a unique signaling platform or compartment to control the activity of caspase-1 and likely other effectors. Emerging evidence indicates that ASC specks are also released from inflammasome-activated cells and accumulate in inflamed tissues, where they can continue to mature cytokines or be internalized by surrounding cells to further nucleate ASC specks in their cytosol. Little is known about the mechanisms governing ASC speck release, uptake, and endosomal escape, as well as its contribution to inflammation and disease. Here, we describe the different outcomes of inflammasome activation and discuss the potential function of extracellular ASC specks. We highlight gaps in our understanding of this central process of inflammation, which may have direct consequences on the modulation of host responses and chronic inflammation.

Flagella-mediated secretion of a novel Vibrio cholerae cytotoxin affecting both vertebrate and invertebrate hosts

Using Caenorhabditis elegans as an infection host model for Vibrio cholerae predator interactions, we discovered a bacterial cytotoxin, MakA, whose function as a virulence factor relies on secretion via the flagellum channel in a proton motive force-dependent manner. The MakA protein is expressed from the polycistronic makDCBA (motility-associated killing factor) operon. Bacteria expressing makDCBA induced dramatic changes in intestinal morphology leading to a defecation defect, starvation and death in C. elegans. The Mak proteins also promoted V. cholerae colonization of the zebrafish gut causing lethal infection. A structural model of purified MakA at 1.9 Å resolution indicated similarities to members of a superfamily of bacterial toxins with unknown biological roles. Our findings reveal an unrecognized role for V. cholerae flagella in cytotoxin export that may contribute both to environmental spread of the bacteria by promoting survival and proliferation in encounters with predators, and to pathophysiological effects during infections.

Zebrafish nephrosin helps host defence against Escherichia coli infection

Neutrophils play important roles in innate immunity and are mainly dependent on various enzyme-containing granules to kill engulfed microorganisms. Zebrafish nephrosin (npsn) is specifically expressed in neutrophils; however, its function is largely unknown. Here, we generated an npsn mutant (npsn^smu5) via CRISPR/Cas9 to investigate the in vivo function of Npsn. The overall development and number of neutrophils remained unchanged in npsn-deficient mutants, whereas neutrophil antibacterial function was defective. Upon infection with Escherichia coli, the npsn^smu5 mutants exhibited a lower survival rate and more severe bacterial burden, as well as augmented inflammatory response to challenge with infection when compared with wild-type embryos, whereas npsn-overexpressing zebrafish exhibited enhanced host defence against E. coli infection. These findings demonstrated that zebrafish Npsn promotes host defence against bacterial infection. Furthermore, our findings suggested that npsn-deficient and -overexpressing zebrafish might serve as effective models of in vivo innate immunity.

Whole-organism phenotypic screening for anti-infectives promoting host health

To date, antibiotics have been identified on the basis of their ability to kill bacteria or inhibit their growth rather than directly for their capacity to improve clinical outcomes of infected patients. Although historically successful, this approach has led to the development of an antibiotic armamentarium that suffers from a number of shortcomings, including the inevitable emergence of resistance and, in certain infections, suboptimal efficacy leading to long treatment durations, infection recurrence, or high mortality and morbidity rates despite apparent bacterial sterilization. Conventional antibiotics fail to address the complexities of in vivo bacterial physiology and virulence, as well as the role of the host underlying the complex, dynamic interactions that cause disease. New interventions are needed, aimed at host outcome rather than microbiological cure. Here we review the role of screening models for cellular and whole-organism infection, including worms, flies, zebrafish, and mice, to identify novel therapeutic strategies and discuss their future implications.

The novel antibiotic rhodomyrtone traps membrane proteins in vesicles with increased fluidity

The acylphloroglucinol rhodomyrtone is a promising new antibiotic isolated from the rose myrtle Rhodomyrtus tomentosa, a plant used in Asian traditional medicine. While many studies have demonstrated its antibacterial potential in a variety of clinical applications, very little is known about the mechanism of action of rhodomyrtone. Preceding studies have been focused on intracellular targets, but no specific intracellular protein could be confirmed as main target. Using live cell, high-resolution, and electron microscopy we demonstrate that rhodomyrtone causes large membrane invaginations with a dramatic increase in fluidity, which attract a broad range of membrane proteins. Invaginations then form intracellular vesicles, thereby trapping these proteins. Aberrant protein localization impairs several cellular functions, including the respiratory chain and the ATP synthase complex. Being uncharged and devoid of a particular amphipathic structure, rhodomyrtone did not seem to be a typical membrane-inserting molecule. In fact, molecular dynamics simulations showed that instead of inserting into the bilayer, rhodomyrtone transiently binds to phospholipid head groups and causes distortion of lipid packing, providing explanations for membrane fluidization and induction of membrane curvature. Both its transient binding mode and its ability to form protein-trapping membrane vesicles are unique, making it an attractive new antibiotic candidate with a novel mechanism of action.

Bacterial antibiotic resistance constitutes a major public healthcare issue and deaths caused by antimicrobial resistance are expected to soon exceed the number of cancer-related fatalities. In order to fight resistance, new antibiotics have to be developed that are not affected by existing microbial resistance strategies. Thus, antibiotics with novel or multiple targets are urgently needed. Rhodomyrtone displays excellent antibacterial activity, has been safely used in traditional Asian medicine for a long time, and resistance against this promising antibiotic candidate could not be detected in multiple passaging experiments. Here we demonstrate that rhodomyrtone possesses a completely novel mechanism of action, which is opposed to that of existing cell envelope-targeting drugs, minimizing the risk of cross-resistance, and in fact rhodomyrtone is highly active against e.g. vancomycin-resistant Staphylococcus aureus. Thus, rhodomyrtone is an extremely interesting compound for further antibacterial drug development.

Evaluating Different Virulence Traits of Klebsiella pneumoniae Using Dictyostelium discoideum and Zebrafish Larvae as Host Models

Multiresistant and invasive hypervirulent Klebsiella pneumoniae strains have become one of the most urgent bacterial pathogen threats. Recent analyses revealed a high genomic plasticity of this species, harboring a variety of mobile genetic elements associated with virulent strains, encoding proteins of unknown function whose possible role in pathogenesis have not been addressed. K. pneumoniae virulence has been studied mainly in animal models such as mice and pigs, however, practical, financial, ethical and methodological issues limit the use of mammal hosts. Consequently, the development of simple and cost-effective experimental approaches with alternative host models is needed. In this work we described the use of both, the social amoeba and professional phagocyte Dictyostelium discoideum and the fish Danio rerio (zebrafish) as surrogate host models to study K. pneumoniae virulence. We compared three K. pneumoniae clinical isolates evaluating their resistance to phagocytosis, intracellular survival, lethality, intestinal colonization, and innate immune cells recruitment. Optical transparency of both host models permitted studying the infective process in vivo, following the Klebsiella-host interactions through live-cell imaging. We demonstrated that K. pneumoniae RYC492, but not the multiresistant strains 700603 and BAA-1705, is virulent to both host models and elicits a strong immune response. Moreover, this strain showed a high resistance to phagocytosis by D. discoideum, an increased ability to form biofilms and a more prominent and irregular capsule. Besides, the strain 700603 showed the unique ability to replicate inside amoeba cells. Genomic comparison of the K. pneumoniae strains showed that the RYC492 strain has a higher overall content of virulence factors although no specific genes could be linked to its phagocytosis resistance, nor to the intracellular survival observed for the 700603 strain. Our results indicate that both zebrafish and D. discoideum are advantageous host models to study different traits of K. pneumoniae that are associated with virulence.

In vitro antagonistic activity and the protective effect of probiotic Bacillus licheniformis Dahb1 in zebrafish challenged with GFP tagged Vibrio parahaemolyticus Dahv2

In vitro antagonistic activity and the protective effect of probiotic Bacillus licheniformis Dahb1 in zebrafish (Danio rerio) challenged with GFP tagged Vibrio parahaemolyticus Dahv2 was studied. The cell free extract of probiotic B. licheniformis Dahb1 at 100 μg mL^-1 showed growth inhibition of V. parahaemolyticus Dahv2 in vitro. B. licheniformis Dahb1 also inhibited the biofilm growth of GFP tagged V. parahaemolyticus Dahv2 at 100 μg mL^-1 in vitro. The growth and survival of zebrafish was tested using probiotic B. licheniformis Dahb1. Weight (1.28 g) of zebrafish that received the cell free extract was much higher than in control (1.04 g). The mortality of zebrafish infected with GFP tagged V. parahaemolyticus Dahv2 at 10⁷ Cfu mL^-1 (Group IV) was 100%, whereas a complete survival of zebrafish that received the cell free extract of B. licheniformis Dahb1 at 10⁷ Cfu mL^-1 (Group VII) was observed after 30 days. The number of GFP tagged V. parahaemolyticus Dahv2 colonies in the intestine and gills significantly reduced after treatment with the cell free extract of B. licheniformis Dahb1. Furthermore, a significant decrease in the fluorescent colonies of GFP tagged V. parahaemolyticus Dahv2 was observed after treatment with the cell free extract of B. licheniformis Dahb1 under confocal laser scanning microscopy (CLSM). In conclusion, the cell free extract of B. licheniformis Dahb1 could prevent Vibrio infection by enhancing the growth and survival of zebrafish.

Macrophage-Microbe Interactions: Lessons from the Zebrafish Model

Macrophages provide front line defense against infections. The study of macrophage-microbe interplay is thus crucial for understanding pathogenesis and infection control. Zebrafish (Danio rerio) larvae provide a unique platform to study macrophage-microbe interactions in vivo, from the level of the single cell to the whole organism. Studies using zebrafish allow non-invasive, real-time visualization of macrophage recruitment and phagocytosis. Furthermore, the chemical and genetic tractability of zebrafish has been central to decipher the complex role of macrophages during infection. Here, we discuss the latest developments using zebrafish models of bacterial and fungal infection. We also review novel aspects of macrophage biology revealed by zebrafish, which can potentiate development of new therapeutic strategies for humans.

Goldfish, Carassius auratus, as an infection model for studying the pathogenesis of Edwardsiella piscicida

This study demonstrates the feasibility of using goldfish as an infection model to investigate the pathogenesis of Edwardsiella piscicida. Goldfish were found to be susceptible to acute E. piscicida-induced disease and died in a dose-dependent manner. E. piscicida was further shown to replicate rapidly in the head kidneys and livers of infected goldfish from 1 d post-injection, and bacteria numbers were significantly decreased 5 d post-injection. Immune responses were successfully induced in goldfish injected with E. piscicida strains and 60% of goldfish inoculated with an attenuated E. piscicida strain were found to survive subsequent injection with a pathogenic strain. The results of differential leukocyte count experiments suggested that leukocytes were immediately recruited as an innate immune response against the infection. Thus, this well-characterized goldfish species is a suitable infection model for studying E. piscicida pathogenesis, and might be applicable to research on other fish diseases.

Human genetic variation in VAC14 regulates Salmonella invasion and typhoid fever through modulation of cholesterol

Risk, severity, and outcome of infection depend on the interplay of pathogen virulence and host susceptibility. Systematic identification of genetic susceptibility to infection is being undertaken through genome-wide association studies, but how to expeditiously move from genetic differences to functional mechanisms is unclear. Here, we use genetic association of molecular, cellular, and human disease traits and experimental validation to demonstrate that genetic variation affects expression of VAC14, a phosphoinositide-regulating protein, to influence susceptibility to Salmonella enterica serovar Typhi (S Typhi) infection. Decreased VAC14 expression increased plasma membrane cholesterol, facilitating Salmonella docking and invasion. This increased susceptibility at the cellular level manifests as increased susceptibility to typhoid fever in a Vietnamese population. Furthermore, treating zebrafish with a cholesterol-lowering agent, ezetimibe, reduced susceptibility to S Typhi. Thus, coupling multiple genetic association studies with mechanistic dissection revealed how VAC14 regulates Salmonella invasion and typhoid fever susceptibility and may open doors to new prophylactic/therapeutic approaches.

Cortisol plays a role in the high environmental ammonia associated suppression of the immune response in zebrafish

Exposure to high environmental ammonia (HEA) levels increases the vulnerability of fishes to parasitic, viral and bacterial diseases. We tested the hypothesis that elevated plasma cortisol levels play a role in the HEA-mediated immunosuppression in fishes. To this end, we tested the effect of exogenous cortisol treatment on the lipopolysaccharide (LPS)-induced immune response in zebrafish (Danio rerio). Also, to test whether glucocorticoid receptor (GR) signaling is involved in HEA-mediated immunosuppression, zebrafish were treated with mifepristone, a GR antagonist, and the LPS-induced immune response assessed after HEA exposure. We evaluated a panel of important immunity-related genes including interleukin 1β (il1b) and suppressor of cytokine signaling (socs-1a, 2, 3) and acute phase response genes [serum amyloid A (saa), transferrin (tfa), leukocyte cell-derived chemotaxin 2-like (lect2l), haptoglobin (hp), hepcidin (=hepatic anti-microbial peptide hamp), and complement component 3b (c3b)] by real-time quantitative PCR. Our results demonstrate that exogenous cortisol administration as well as elevated cortisol levels in response to HEA exposure modulate mRNA transcript levels of key mediators of the innate immune response in zebrafish. Mifepristone treatment reduced whole body cortisol levels and eliminated the HEA-mediated changes in transcript abundance of socs1a, il1b, as well as APR genes. Together, these results suggest that the HEA effect on the innate immune response is in part mediated by cortisol signaling, while the mode of action, including the receptors involved remains to be elucidated.

Two forms of death in ageing Caenorhabditis elegans

Ageing generates senescent pathologies, some of which cause death. Interventions that delay or prevent lethal pathologies will extend lifespan. Here we identify life-limiting pathologies in Caenorhabditis elegans with a necropsy analysis of worms that have died of old age. Our results imply the presence of multiple causes of death. Specifically, we identify two classes of corpse: early deaths with a swollen pharynx (which we call 'P deaths'), and later deaths with an atrophied pharynx (termed 'p deaths'). The effects of interventions on lifespan can be broken down into changes in the frequency and/or timing of either form of death. For example, glp-1 mutation only delays p death, while eat-2 mutation reduces P death. Combining pathology and mortality analysis allows mortality profiles to be deconvolved, providing biological meaning to complex survival and mortality profiles.

Live-cell imaging of Salmonella Typhimurium interaction with zebrafish larvae after injection and immersion delivery methods

The zebrafish model has been used to determine the role of vertebrate innate immunity during bacterial infections. Here, we compare the in vivo immune response induced by GFP-tagged Salmonella Typhimurium inoculated by immersion and microinjection in transgenic zebrafish larvae. Our novel infection protocols in zebrafish allow live-cell imaging of Salmonella colonization.

Escherichia coli O78 isolated from septicemic lambs shows high pathogenicity in a zebrafish model

The pathogenicity of Escherichia coli O78 strain K46, originally isolated from an outbreak of septicemia in neonatal lambs, was investigated in zebrafish embryo and murine models of infection. Its biofilm potential, cellulose production, and the expression of type 1 pili and curli fimbriae were measured by in vitro assays. The strain was highly pathogenic in the zebrafish embryo model of infection, where it killed all embryos within 24 h post inoculation (hpi) at doses as low as 1000 colony forming units. Zebrafish embryos inoculated with similar doses of commensal E. coli strains showed no signs of disease, and cleared the bacteria within 24 h. E. coli K46 colonized the murine gut at the same level as the uropathogenic E. coli (UPEC) reference strain CFT073 in CBA/J mice after oral inoculation, but infected the murine bladder significantly less than CFT073 after transurethral inoculation. Type 1 pili were clearly expressed by E. coli K46, while curli fimbriae and cellulose production were weakly expressed. The ability to produce biofilm varied in different growth media, but overall E. coli K46 was a poorer biofilm producer compared to the reference strain E. coli UTI89. In conclusion, the zebrafish lethality model provides further evidence that E. coli K46 is highly pathogenic and might be useful in future studies to identify bacterial virulence factors.

Immunotoxic effects of atrazine and its main metabolites at environmental relevant concentrations on larval zebrafish (Danio rerio)

Atrazine (ATZ) and its main metabolites, i.e., diaminochlorotriazine (DACT), deisopropylatrazine (DIP), and deethylatrazine (DE), have been widely detected in surface water around the world. In the present study, to determine their immunotoxic effects, zebrafish during the early developmental stage were exposed to ATZ and its main metabolites at environmental concentrations (30, 100, 300 μg L^-1). It was observed that ATZ, DACT, DIP and DE selectively induced the transcription of immunotoxic related genes including Tnfα, Il-1β, Il-6, Il-8, Cxcl-clc and Cc-chem in larval zebrafish. Pretreatment with ATZ and its metabolites also changed the immune response of larval zebrafish to LPS and E. coli challenge, which was indicated by the alternation in the mRNA levels of some cytokines. In addition, 300 μg L^-1 ATZ and DACT exposure could also increase the release of tryptase into water, indicating that they increased the anaphylactoid reaction in the larval zebrafish. According to these results, both of ATZ and its metabolites exposure could cause the immunotoxicity in larval zebrafish. Thus, we thought that the ecological risks of the metabolites of ATZ on aquatic organisms could not be ignored.

The Zebrafish as a Model for Human Bacterial Infections

The development of the zebrafish (Danio rerio) infectious disease model has provided new insights and information into pathogenesis. Many of these new discoveries would not have been possible using a typical mammalian model. The advantages of using this model are many and in the last 15 years the model has been exploited for the analysis of many different pathogens. Here, we describe in detail how to perform a bacterial infection using either the adult zebrafish or zebrafish larvae using microinjection. Multiple methods of analysis are described that can be used to address specific questions pertaining to disease progression and the interactions with the immune system.

Modeling Infectious Diseases in the Context of a Developing Immune System

Zebrafish has been used for over a decade to study the mechanisms of a wide variety of inflammatory disorders and infections, with models ranging from bacterial, viral, to fungal pathogens. Zebrafish has been especially relevant to study the differentiation, specialization, and polarization of the two main innate immune cell types, the macrophages and the neutrophils. The optical accessibility and the early appearance of myeloid cells that can be tracked with fluorescent labels in zebrafish embryos and the ability to use genetics to selectively ablate or expand immune cell populations have permitted studying the interaction between infection, development, and metabolism. Additionally, zebrafish embryos are readily colonized by a commensal flora, which facilitated studies that emphasize the requirement for immune training by the natural microbiota to properly respond to pathogens. The remarkable conservation of core mechanisms required for the recognition of microbial and danger signals and for the activation of the immune defenses illustrates the high potential of the zebrafish model for biomedical research. This review will highlight recent insight that the developing zebrafish has contributed to our understanding of host responses to invading microbes and the involvement of the microbiome in several physiological processes. These studies are providing a mechanistic basis for developing novel therapeutic approaches to control infectious diseases.

A Zebrafish Model for Chlamydia Infection with the Obligate Intracellular Pathogen Waddlia chondrophila

Obligate intracellular chlamydial bacteria of the Planctomycetes-Verrucomicrobia-Chlamydiae (PVC) superphylum are important pathogens of terrestrial and marine vertebrates, yet many features of their pathogenesis and host specificity are still unknown. This is particularly true for families such as the Waddliacea which, in addition to epithelia, cellular targets for nearly all Chlamydia, can infect and replicate in macrophages, an important arm of the innate immune system or in their free-living amoebal counterparts. An ideal pathogen model system should include both host and pathogen, which led us to develop the first larval zebrafish model for chlamydial infections with Waddlia chondrophila. By varying the means and sites of application, epithelial cells of the swim bladder, endothelial cells of the vasculature and phagocytosing cells of the innate immune system became preferred targets for infection in zebrafish larvae. Through the use of transgenic zebrafish, we could observe recruitment of neutrophils to the infection site and demonstrate for the first time that W. chondrophila is taken up and replicates in these phagocytic cells and not only in macrophages. Furthermore, we present evidence that myeloid differentiation factor 88 (MyD88) mediated signaling plays a role in the innate immune reaction to W. chondrophila, eventually by Toll-like receptor (TLRs) recognition. Infected larvae with depleted levels of MyD88 showed a higher infection load and a lower survival rate compared to control fish. This work presents a new and potentially powerful non-mammalian experimental model to study the pathology of chlamydial virulence in vivo and opens up new possibilities for investigation of other members of the PVC superphylum.