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

Zebrafish Larvae Microinjection and Automated Fluorescence Microscopy for Studying Klebsiella pneumoniae Infection and the Host Immune Response

Studying host-pathogen interactions is essential for understanding infectious diseases and developing possible treatments, especially for priority pathogens with increased virulence and antibiotic resistance, such as Klebsiella pneumoniae. Over time, this subject has been approached from different perspectives, often using mammal host models and invasive endpoint measurements (e.g., sacrifice and organ extraction). However, taking advantage of technological advances, it is now possible to follow the infective process by noninvasive visualization in real time, using optically amenable surrogate hosts. In this line, this chapter describes a live-cell imaging approach to monitor the interaction of K. pneumoniae and potentially other bacterial pathogens with zebrafish larvae in vivo. This methodology is based on the microinjection of fluorescent bacteria into the otic vesicle, followed by time-lapse observation by automated fluorescence microscopy with environmental control, monitoring the dynamics of immune cell recruitment, bacterial load, and larvae survival.

RpoS activates formation of Salmonella Typhi biofilms and drives persistence in the gall bladder

The development of strategies for targeting the asymptomatic carriage of Salmonella Typhi in chronic typhoid patients has suffered owing to our basic lack of understanding of the molecular mechanisms that enable the formation of S. Typhi biofilms. Traditionally, studies have relied on cholesterol-attached biofilms formed by a closely related serovar, Typhimurium, to mimic multicellular Typhi communities formed on human gallstones. In long-term infections, S. Typhi adopts the biofilm lifestyle to persist in vivo and survive in the carrier state, ultimately leading to the spread of infections via the fecal-oral route of transmission. In the present work, we studied S. Typhi biofilms directly, applied targeted as well as genome-wide genetic approaches to uncover unique biofilm components that do not conform to the CsgD-dependent pathway established in S. Typhimurium. We undertook a genome-wide Tn5 mutation screen in H58, a clinically relevant multidrug resistance strain of S. Typhi, in gallstone-mimicking conditions. We generated New Generation Sequencing libraries based on the ClickSeq technology to identify the key regulators, IraP and RpoS, and the matrix components Sth fimbriae, Vi capsule and lipopolysaccharide. We discovered that the starvation sigma factor, RpoS, was required for the transcriptional activation of matrix-encoding genes in vitro, and for S. Typhi colonization in persistent infections in vivo, using a heterologous fish larval model. An rpoS null mutant failed to colonize the gall bladder in chronic zebrafish infections. Overall, our work uncovered a novel RpoS-driven, CsgD-independent paradigm for the formation of cholesterol-attached Typhi biofilms, and emphasized the role(s) of stress signaling pathways for adaptation in chronic infections. Our identification of the biofilm regulators in S. Typhi paves the way for the development of drugs against typhoid carriage, which will ultimately control the increased incidence of gall bladder cancer in typhoid carriers.

A zebrafish gene with sequence similarities to human uromodulin and GP2 displays extensive evolutionary diversification among teleost and confers resistance to bacterial infection

In the process of investigating synaptic changes happening to mutants lacking postsynaptic receptors in the neuromuscular junction, we focused on a hitherto uncharacterized zebrafish gene zgc153932 whose expression was increased in the RNAseq and droplet digital PCR (ddPCR) analysis of a paralyzed mutant sofa potato. The zgc153932 gene which we named omcin5 (omc5) showed amino acid sequence similarity to human uromodulin and GP2, which are expressed in epithelial cells of the kidney and the gut respectively and bind to bacteria pili. omc5 had 14 paralogues in a ∼400 KB region on the chromosome 12 of the zebrafish genome. These genes, named omcin1 through 15, constitute a gene cluster which presumably arose from recent gene duplications in the zebrafish lineage. An antibody raised against the epitope common to 6-9 genes in the omcin family revealed expression in the cloaca of 1 day post fertilization (dpf) embryos which broadened to the urinary and digestive tracts by 5 dpf. Expression of omc5 was increased by exposure of embryos to Escherichia coli (E. coli). Survival of omc5 mutant embryos was shortened in the presence of E. coli, or when they were not maintained in germ-free conditions. Adults omc5 mutants also exhibited susceptibility to infection. Other teleost species which had omcin-like genes in their genomes showed a range of gene duplication, resulting in clusters of 1 to >15 omcin-like genes. We hereby identified a new gene family specific to teleost that include a microbial induced gene which confers resistance to bacterial infection.

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.

Detection of Aeromonas salmonicida subsp. salmonicida infection in zebrafish by labelling bacteria with GFP and a fluorescent probe based on the siderophore amonabactin

Zebrafish (Danio rerio) is an excellent model to study bacterial infections in fish and their treatment. We used zebrafish as a model of infection for Aeromonas salmonicida subsp. salmonicida (hereinafter A. salmonicida), the causative agent of fish furunculosis. The infection process of A. salmonicida was studied by immersion of zebrafish larvae in 2 different doses of the bacteria and the fish mortality was monitored for three days. The bacterium caused a high mortality (65 %) in zebrafish larvae only when they were exposed to a high bacterial concentration (107 bacterial cells/mL). To evaluate the use of fluorescence microscopy to follow A. salmonicida infection in vivo, two different fluorescent strains generated by labeling an A. salmonicida strain with either, the green fluorescent protein (GFP), or with a previously reported siderophore amonabactin-sulforhodamine B conjugate (AMB-SRB), were used. The distribution of both labeled bacterial strains in the larvae tissues was evaluated by conventional and confocal fluorescence microscopy. The fluorescent signal showed a greater intensity with the GFP-labeled bacteria, so it could be observed using conventional fluorescence microscopy. Since the AMB-SRB labeled bacteria showed a weaker signal, the larvae were imaged using a laser scanning confocal microscope after 48 h of exposure to the bacteria. Both fluorescent signals were mainly observed in the larvae digestive tract, suggesting that this is the main colonization route of zebrafish for waterborne A. salmonicida. This is the first report of the use of a siderophore-fluorophore conjugate to study a bacterial infection in fish. The use of a siderophore-fluorophore conjugate has the advantage that it is a specific marker and that does not require genetic manipulation of the bacteria.

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.

Transgenic zebrafish larvae as a non-rodent alternative model to assess pro-inflammatory (neutrophil) responses to nanomaterials

Hazard studies for nanomaterials (NMs) commonly assess whether they activate an inflammatory response. Such assessments often rely on rodents, but alternative models are needed to support the implementation of the 3Rs principles. Zebrafish (Danio rerio) offer a viable alternative for screening NM toxicity by investigating inflammatory responses. Here, we used non-protected life stages of transgenic zebrafish (Tg(mpx:GFP)ⁱ¹¹⁴) with fluorescently-labeled neutrophils to assess inflammatory responses to silver (Ag) and zinc oxide (ZnO) NMs using two approaches. Zebrafish were exposed to NMs via water following a tail fin injury, or NMs were microinjected into the otic vesicle. Zebrafish were exposed to NMs at 3 days post-fertilization (dpf) and neutrophil accumulation at the injury or injection site was quantified at 0, 4, 6, 8, 24, and 48 h post-exposure. Zebrafish larvae were also exposed to fMLF, LTB₄, CXCL-8, C5a, and LPS to identify a suitable positive control for inflammation induction. Aqueous exposure to Ag and ZnO NMs stimulated an enhanced and sustained neutrophilic inflammatory response in injured zebrafish larvae, with a greater response observed for Ag NMs. Following microinjection, Ag NMs stimulated a time-dependent neutrophil accumulation in the otic vesicle which peaked at 48 h. LTB₄ was identified as a positive control for studies investigating inflammatory responses in injured zebrafish following aqueous exposure, and CXCL-8 for microinjection studies that assess responses in the otic vesicle. Our findings support the use of transgenic zebrafish to rapidly screen the pro-inflammatory effects of NMs, with potential for wider application in assessing chemical safety (e.g. pharmaceuticals).

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.

Klebsiella virus UPM2146 lyses multiple drug-resistant Klebsiella pneumoniae in vitro and in vivo

Klebsiella pneumoniae are opportunistic bacteria found in the gut. In recent years they have been associated with nosocomial infections. The increased incidence of multiple drug-resistant K. pneumoniae makes it necessary to find new alternatives to treat the disease. In this study, phage UPM2146 was isolated from a polluted lake which can lyse its host K. pneumoniae ATCC BAA-2146. Observation from TEM shows that UPM2146 belongs to Caudoviriales (Order) based on morphological appearance. Whole genome analysis of UPM2146 showed that its genome comprises 160,795 bp encoding for 214 putative open reading frames (ORFs). Phylogenetic analysis revealed that the phage belongs to Ackermannviridae (Family) under the Caudoviriales. UPM2146 produces clear plaques with high titers of 1010 PFU/ml. The phage has an adsorption period of 4 min, latent period of 20 min, rise period of 5 min, and releases approximately 20 PFU/ bacteria at Multiplicity of Infection (MOI) of 0.001. UPM2146 has a narrow host-range and can lyse 5 out of 22 K. pneumoniae isolates (22.72%) based on spot test and efficiency of plating (EOP). The zebrafish larvae model was used to test the efficacy of UPM2146 in lysing its host. Based on colony forming unit counts, UPM2146 was able to completely lyse its host at 10 hours onwards. Moreover, we show that the phage is safe to be used in the treatment against K. pneumoniae infections in the zebrafish model.

Leveraging the zebrafish to model organ transplantation

PURPOSE OF REVIEW

The availability of organs for transplant fails to meet the demand and this shortage is growing worse every year. As the cost of not getting a suitable donor organ can mean death for patients, new tools and approaches that allows us to make advances in transplantation faster and provide a different vantage point are required. To address this need, we introduce the concept of using the zebrafish (Danio rerio) as a new model system in organ transplantation. The zebrafish community offers decades of research experience in disease modeling and a rich toolbox of approaches for interrogating complex pathological states. We provide examples of how already existing zebrafish assays/tools from cancer, regenerative medicine, immunology, and others, could be leveraged to fuel new discoveries in pursuit of solving the organ shortage.

RECENT FINDINGS

Important innovations have enabled several types of transplants to be successfully performed in zebrafish, including stem cells, tumors, parenchymal cells, and even a partial heart transplant. These innovations have been performed against a backdrop of an expansive and impressive list of tools designed to uncover the biology of complex systems that include a wide array of fluorescent transgenic fish that label specific cell types and mutant lines that are transparent, immune-deficient. Allogeneic transplants can also be accomplished using immune suppressed and syngeneic fish. Each of these innovations within the zebrafish community would provide several helpful tools that could be applied to transplant research.

SUMMARY

We highlight some examples of existing tools and assays developed in the zebrafish community that could be leveraged to overcome barriers in organ transplantation, including ischemia-reperfusion, short preservation durations, regeneration of marginal grafts, and acute and chronic rejection.

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 vivo Host-Pathogen Interaction as Revealed by Global Proteomic Profiling of Zebrafish Larvae

The outcome of a host-pathogen interaction is determined by the conditions of the host, the pathogen, and the environment. Although numerous proteomic studies of in vitro-grown microbial pathogens have been performed, in vivo proteomic approaches are still rare. In addition, increasing evidence supports that in vitro studies inadequately reflect in vivo conditions. Choosing the proper host is essential to detect the expression of proteins from the pathogen in vivo. Numerous studies have demonstrated the suitability of zebrafish (Danio rerio) embryos as a model to in vivo studies of Pseudomonas aeruginosa infection. In most zebrafish-pathogen studies, infection is achieved by microinjection of bacteria into the larvae. However, few reports using static immersion of bacterial pathogens have been published. In this study we infected 3 days post-fertilization (DPF) zebrafish larvae with P. aeruginosa PAO1 by immersion and injection and tracked the in vivo immune response by the zebrafish. Additionally, by using non-isotopic (Q-exactive) metaproteomics we simultaneously evaluated the proteomic response of the pathogen (P. aeruginosa PAO1) and the host (zebrafish). We found some zebrafish metabolic pathways, such as hypoxia response via HIF activation pathway, were exclusively enriched in the larvae exposed by static immersion. In contrast, we found that inflammation mediated by chemokine and cytokine signaling pathways was exclusively enriched in the larvae exposed by injection, while the integrin signaling pathway and angiogenesis were solely enriched in the larvae exposed by immersion. We also found important virulence factors from P. aeruginosa that were enriched only after exposure by injection, such as the Type-III secretion system and flagella-associated proteins. On the other hand, P. aeruginosa proteins involved in processes like biofilm formation, and cellular responses to antibiotic and starvation were enriched exclusively after exposure by immersion. We demonstrated the suitability of zebrafish embryos as a model for in vivo host-pathogen based proteomic studies in P. aeruginosa. Our global proteomic profiling identifies novel molecular signatures that give systematic insight into zebrafish-Pseudomonas interaction.

Salmonella Typhimurium induces cloacitis-like symptomsin zebrafish larvae

Pathogenic Salmonella strains have a set of virulence factors allowing them to generate systemic infections and damage in a variety of hosts. Among these factors, bacterial proteins secreted by specialized systems are used to penetrate the host's intestinal mucosa, through the invasion and destruction of specialized epithelial M cells in the intestine. On the other hand, numerous studies have demonstrated that humans, as well as experimental animal hosts, respond to Salmonella infection by activating both innate and adaptive immune responses. Here, through live cell imaging of S. Typhimurium infection of zebrafish larvae, we showed that besides the intestinal colonization, a deformed cloacae region and a concomitant accumulation of S. Typhimurium cells was observed upon bacterial infection. The swelling led to a persistent inflammation of infected larvae, although the infection was non-lethal. The in vivo inflammation process was confirmed by the co-localization of GFP-tagged S. Typhimurium with mCherry-tagged neutrophils at 72 h post exposition. Our live-cell analyses suggest that Salmonella Typhimurium induce cloacitis-like symptoms in zebrafish larvae.