Quantifying Vibrio cholerae Enterotoxicity in a Zebrafish Infection Model

Animal and In Vitro Models as Powerful Tools to Decipher the Effects of Enteric Pathogens on the Human Gut Microbiota

Examining the interplay between intestinal pathogens and the gut microbiota is crucial to fully comprehend the pathogenic role of enteropathogens and their broader impact on human health. Valid alternatives to human studies have been introduced in laboratory practice to evaluate the effects of infectious agents on the gut microbiota, thereby exploring their translational implications in intestinal functionality and overall health. Different animal species are currently used as valuable models for intestinal infections. In addition, considering the recent advances in bioengineering, futuristic in vitro models resembling the intestinal environment are also available for this purpose. In this review, the impact of the main human enteropathogens (i.e., Clostridioides difficile, Campylobacter jejuni, diarrheagenic Escherichia coli, non-typhoidal Salmonella enterica, Shigella flexneri and Shigella sonnei, Vibrio cholerae, and Bacillus cereus) on intestinal microbial communities is summarized, with specific emphasis on results derived from investigations employing animal and in vitro models.

Advances in cholera research: from molecular biology to public health initiatives

The aquatic bacterium Vibrio cholerae is the etiological agent of the diarrheal disease cholera, which has plagued the world for centuries. This pathogen has been the subject of studies in a vast array of fields, from molecular biology to animal models for virulence activity to epidemiological disease transmission modeling. V. cholerae genetics and the activity of virulence genes determine the pathogenic potential of different strains, as well as provide a model for genomic evolution in the natural environment. While animal models for V. cholerae infection have been used for decades, recent advances in this area provide a well-rounded picture of nearly all aspects of V. cholerae interaction with both mammalian and non-mammalian hosts, encompassing colonization dynamics, pathogenesis, immunological responses, and transmission to naïve populations. Microbiome studies have become increasingly common as access and affordability of sequencing has improved, and these studies have revealed key factors in V. cholerae communication and competition with members of the gut microbiota. Despite a wealth of knowledge surrounding V. cholerae, the pathogen remains endemic in numerous countries and causes sporadic outbreaks elsewhere. Public health initiatives aim to prevent cholera outbreaks and provide prompt, effective relief in cases where prevention is not feasible. In this review, we describe recent advancements in cholera research in these areas to provide a more complete illustration of V. cholerae evolution as a microbe and significant global health threat, as well as how researchers are working to improve understanding and minimize impact of this pathogen on vulnerable populations.

Pandemic Vibrio cholerae acquired competitive traits from an environmental Vibrio species

Vibrio cholerae is a human pathogen that thrives in estuarine environments. Within the environment and human host, V. cholerae uses the type VI secretion system (T6SS) to inject toxic effectors into neighboring microbes and to establish its replicative niche. V. cholerae strains encode a wide variety of horizontally shared effectors, but pandemic isolates encode an identical set of distinct effectors. Effector set retention in pandemic strains despite mobility between disparate strains suggests that horizontal acquisition of these effectors was crucial for evolving pandemic V. cholerae We attempted to locate the donor of the pandemic effectors to V. cholerae To this end, we identified potential gene transfer events of the pandemic-associated T6SS clusters between a fish pathogen, Vibrio anguillarum, and V. cholerae We supported the likelihood of interaction between these species by demonstrating that homologous effector-immunity pairs from V. cholerae and V. anguillarum can cross-neutralize one another. Thus, V. anguillarum constitutes an environmental reservoir of pandemic-associated V. cholerae T6SS effectors that may have initially facilitated competition between pre-pandemic V. cholerae and V. anguillarum for an environmental niche.

Good practices in the rearing and maintenance of zebrafish (Danio rerio) in Brazilian laboratories

Abstract Good Laboratory Practice (GLP) is a management quality control system that encompasses the organizational process and conditions under which non-clinical health and environmental studies are carried out. According to the World Health Organization, GLP must contain five topics: resources, characterization, rules, results, and quality control. This work aims to address a review according to WHO standards of implementing Good Laboratory Practices in zebrafish (Danio rerio) vivariums. Considering that the promotion of one health (animal, human, and environmental) associated with an education plan, protocols, and records are fundamental to guarantee the safety and integrity of employees, animals, and the environment as well as reliability in the results generated. In a way, Brazil still needs improvements related to the well-being of aquatic organisms (national laws, international agreements, corporate programs, and others), especially concerning its use in research and technological development. In this way, the implementation of GLPs provides valuable guidance for improving animal welfare and worker safety, facilitating the standardization of research.

Boas práticas na criação e manutenção de zebrafish (Danio rerio) em laboratório no Brasil

Resumo As Boas Práticas de Laboratório (BPL) são um sistema de controle de qualidade gerencial que abrange o processo organizacional e as condições sob as quais os estudos não clínicos de saúde e meio ambiente são desenvolvidos. Conforme a Organização Mundial da Saúde (OMS) as BPL devem conter cinco tópicos: recursos, caracterização, regras, resultados e controle de qualidade. O objetivo deste trabalho foi apresentar uma revisão conforme o padrão da OMS para a implementação das BPL em biotério de zebrafish. Considerando que a promoção da saúde única (animal, humana e ambiental) associada a um plano de educação, protocolos e registros são fundamentais para garantir a segurança e a integridade dos trabalhadores/pesquisadores, animais e meio ambiente assim como confiabilidade nos resultados gerados. De certa forma o Brasil ainda necessita de melhorias relacionadas ao bem-estar de organismos aquáticos (leis nacionais, acordos internacionais, programas corporativos e outros); especialmente em relação à utilização deste na pesquisa e desenvolvimento tecnológico. Desta forma, a implementação de BPL fornece uma orientação valiosa para a melhoria do bem-estar animal, e segurança do trabalhador vindo a facilitar a padronização da pesquisa.

Intestinal colonization against Vibrio cholerae: host and microbial resistance mechanisms

Vibrio cholerae is a non-invasive enteric pathogen known to cause a major public health problem called cholera. The pathogen inhabits the aquatic environment while outside the human host, it is transmitted into the host easily through ingesting contaminated food and water containing the vibrios, thus causing diarrhoea and vomiting. V. cholerae must resist several layers of colonization resistance mechanisms derived from the host or the gut commensals to successfully survive, grow, and colonize the distal intestinal epithelium, thus causing an infection. The colonization resistance mechanisms derived from the host are not specific to V. cholerae but to all invading pathogens. However, some of the gut commensal-derived colonization resistance may be more specific to the pathogen, making it more challenging to overcome. Consequently, the pathogen has evolved well-coordinated mechanisms that sense and utilize the anti-colonization factors to modulate events that promote its survival and colonization in the gut. This review is aimed at discussing how V. cholerae interacts and resists both host- and microbe-specific colonization resistance mechanisms to cause infection.

An Experimental Adult Zebrafish Model for Shigella Pathogenesis, Transmission, and Vaccine Efficacy Studies

Shigellosis has been a menace to society for ages. The absence of an effective vaccine against Shigella, improper sanitation, and unhygienic use of food and water allow the disease to flourish. Shigella can also be transmitted via natural water bodies. In the absence of a good animal model, the actual nature of pathogenesis and transmission remains unclear. Zebrafish larvae have previously been described as a model for Shigella pathogenesis. However, larval fish lack a mature intestinal microbiota and immune system. Here, the adult zebrafish was assessed as a potential model for Shigella pathogenesis. Their well-developed innate and adaptive immune responses mimic the mammalian immune system. Shigella showed a clear dose-, time-, and temperature-dependent colonization of the adult zebrafish gut. Efficacy of a three-dose immunization regime was tested using bath immunization with heat-killed trivalent Shigella immunogen. The present study demonstrates the efficacy of an adult zebrafish model for pathogenesis, transmission, and vaccine efficacy studies.

IMPORTANCE Shigellosis is a diarrheal disease that is prevalent in developing countries and especially dangerous in young children. Currently, animal models for shigellosis are unable to model some aspects of the infectious cycle. Here, we describe a new shigellosis model in adult zebrafish, an increasingly common model organism for studying bacterial pathogens. The zebrafish model can be used to study Shigella colonization, transmission, and immune responses, as well as test vaccine efficacy.

Neutrophil-associated responses to Vibrio cholerae infection in a natural host model

Vibrio cholerae , the cause of human cholera, is an aquatic bacterium found in association with a variety of animals in the environment, including many teleost fish species. V. cholerae infection induces a pro-inflammatory response followed by a non-inflammatory convalescent phase. Neutrophils are integral to this early immune response. However, the relationship between the neutrophil-associated protein calprotectin and V. cholerae has not been investigated, nor have the effects of limiting transition metals on V. cholerae growth. Zebrafish are useful as a natural V. cholerae model as the entire infectious cycle can be recapitulated in the presence of an intact intestinal microbiome and mature immune responses. Here, we demonstrate that zebrafish produce a significant neutrophil, IL-8, and calprotectin response following V. cholerae infection. Bacterial growth was completely inhibited by purified calprotectin protein or the chemical chelator TPEN, but growth was recovered by addition of transition metals zinc and manganese. Expression of downstream calprotectin targets also significantly increased in the zebrafish. These findings illuminate the role of host calprotectin in combating V. cholerae infection. Inhibition of V. cholerae growth through metal limitation may provide new approaches in the development of anti- V. cholerae therapeutics. This study also establishes a major role for calprotectin in combating infectious diseases in zebrafish.

Characterization of the Immune Response to Vibrio cholerae Infection in a Natural Host Model

The gram-negative bacterium Vibrio cholerae causes the life-threatening diarrheal disease cholera, which is spread through the ingestion of contaminated food or water. Cholera epidemics occur largely in developing countries that lack proper infrastructure to treat sewage and provide clean water. Numerous vertebrate fish species have been found to be natural V. cholerae hosts. Based on these findings, zebrafish (Danio rerio) have been developed as a natural host model for V. cholerae. Diarrheal symptoms similar to those seen in humans are seen in zebrafish as early as 6 hours after exposure. Our understanding of basic zebrafish immunology is currently rudimentary, and no research has been done to date exploring the immune response of zebrafish to V. cholerae infection. In the present study, zebrafish were infected with either pandemic El Tor or non-pandemic, environmental V. cholerae strains and select immunological markers were assessed to determine cellular immunity and humoral immunity. Significant increases in the gene expression of two transcription factors, T-bet and GATA3, were observed in response to infection with both V. cholerae strains, as were levels of mucosal related antibodies. Additionally, the cytokine IL-13 was shown to be significantly elevated and paralleled the mucin output in zebrafish excretions, strengthening our knowledge of IL-13 induced mucin production in cholera. The data presented here further solidify the relevancy of the zebrafish model in studying V. cholerae, as well as expanding its utility in the field of cholera immunology.

Vibrio cholerae Infection Induces Strain-Specific Modulation of the Zebrafish Intestinal Microbiome

Zebrafish (Danio rerio) is an attractive model organism to use for an array of scientific studies, including host-microbe interactions. Zebrafish contain a core (i.e., consistently detected) intestinal microbiome consisting primarily of Proteobacteria. Furthermore, this core intestinal microbiome is plastic and can be significantly altered due to external factors. Zebrafish are particularly useful for the study of aquatic microbes that can colonize vertebrate hosts, including Vibrio cholerae. As an intestinal pathogen, V. cholerae must colonize the intestine of an exposed host for pathogenicity to occur. Members of the resident intestinal microbial community likely must be reduced or eliminated by V. cholerae for colonization, and subsequent disease, to occur. Many studies have explored a variety of aspects of the pathogenic effects of V. cholerae on zebrafish and other model organisms but few have researched how a V. cholerae infection changes the resident intestinal microbiome. In this study, 16S rRNA gene sequencing was used to examine how five genetically diverse V. cholerae strains alter the intestinal microbiome following an infection. We found that V. cholerae colonization induced significant changes in the zebrafish intestinal microbiome. Notably, changes in the microbial profile were significantly different from each other, based on the particular strain of V. cholerae used to infect zebrafish hosts. We conclude that V. cholerae significantly modulates the zebrafish intestinal microbiota to enable colonization and that specific microbes that are targeted depend on the V. cholerae genotype.

The Vibrio cholerae Type Six Secretion System Is Dispensable for Colonization but Affects Pathogenesis and the Structure of Zebrafish Intestinal Microbiome

Zebrafish (Danio rerio) are an attractive model organism for a variety of scientific studies, including host-microbe interactions. The organism is particularly useful for the study of aquatic microbes that can colonize vertebrate hosts, including Vibrio cholerae, an intestinal pathogen. V. cholerae must colonize the intestine of an exposed host for pathogenicity to occur. While numerous studies have explored various aspects of the pathogenic effects of V. cholerae on zebrafish and other model organisms, few, if any, have examined how a V. cholerae infection alters the resident intestinal microbiome and the role of the type six secretion system (T6SS) in that process. In this study, 16S rRNA gene sequencing was utilized to investigate how strains of V. cholerae both with and without the T6SS alter the aforementioned microbial profiles following an infection. V. cholerae infection induced significant changes in the zebrafish intestinal microbiome, and while not necessary for colonization, the T6SS was important for inducing mucin secretion, a marker for diarrhea. Additional salient differences to the microbiome were observed based on the presence or absence of the T6SS in the V. cholerae utilized for challenging the zebrafish hosts. We conclude that V. cholerae significantly modulates the zebrafish intestinal microbiome to enable colonization and that the T6SS is important for pathogenesis induced by the examined V. cholerae strains. Furthermore, the presence or absence of T6SS differentially and significantly affected the composition and structure of the intestinal microbiome, with an increased abundance of other Vibrio bacteria observed in the absence of V. cholerae T6SS.

The Interface of Vibrio cholerae and the Gut Microbiome

The bacterium Vibrio cholerae is the etiologic agent of the severe human diarrheal disease cholera. The gut microbiome, or the native community of microorganisms found in the human gastrointestinal tract, is increasingly being recognized as a factor in driving susceptibility to infection, in vivo fitness, and host interactions of this pathogen. Here, we review a subset of the emerging studies in how gut microbiome structure and microbial function are able to drive V. cholerae virulence gene regulation, metabolism, and modulate host immune responses to cholera infection and vaccination. Improved mechanistic understanding of commensal-pathogen interactions offers new perspectives in the design of prophylactic and therapeutic approaches for cholera control.

Quorum-sensing signaling by chironomid egg masses' microbiota, affects haemagglutinin/protease (HAP) production by Vibrio cholerae

Vibrio cholerae, the causative agent of cholera, is commonly isolated, along with other bacterial species, from chironomid insects (Diptera: Chironomidae). Nevertheless, its prevalence in the chironomid egg masses' microbiota is less than 0.5%. V. cholerae secretes haemagglutinin/protease (HAP) that degrades the gelatinous matrix of chironomid egg masses and prevents hatching. Quorum sensing (QS) activates HAP production in response to accumulation of bacterial autoinducers (AIs). Our aim was to define the impact of chironomid microbiota on HAP production by V. cholerae. To study QS signaling, we used V. cholerae bioluminescence reporter strains (QS-proficient O1 El-Tor wild-type and QS-deficient mutants) and different bacterial species that we isolated from chironomid egg masses. These egg mass isolates, as well as a synthetic AI-2, caused an enhancement in lux expression by a V. cholerae QS-deficient mutant. The addition of the egg mass bacterial isolate supernatant to the QS-deficient mutant also enhanced HAP production and egg mass degradation activities. Moreover, the V. cholerae wild-type strain was able to proliferate using egg masses as their sole carbon source, while the QS-deficient was not. The results demonstrate that members of the chironomid bacterial consortium produce external chemical cues that, like AI-2, induce expression of the hapA gene in V. cholerae. Understanding the interactions between V. cholerae and the insects' microbiota may help uncover the interactions between this pathogen and the human gut microbiota.

Effects of dietary exposure to ciguatoxin P-CTX-1 on the reproductive performance in marine medaka (Oryzias melastigma)

Ciguatoxins are natural compounds produced by benthic dinoflagellates Gambierdiscus and Fukuyoa spp., which cause fish intoxication by ciguatera fish poisoning. This study aimed to assess the dietary exposure effects of ciguatoxin P-CTX-1 on the reproductive performance in marine medaka (Oryzias melastigma). Fish which ingested >1.16 pg·day^-1 for 21 days exhibited abnormal behaviors including diarrhea, abnormal swimming, loss of appetite and decreased egg production. After 7-day exposure to P-CTX-1 at a dose of 1.93 pg·day^-1, significant gender-specific differences in reproductive performance and decreased hatching rate of the offspring were observed. Chemical analysis of P-CTX-1 showed that the P-CTX-1 accumulation rates were 24.1 ± 1.4% in female fish and 9.9 ± 0.4% in male fish, and 0.05 pg·egg^-1 was detected. The results illustrate that dietary exposure to P-CTX-1 affected the reproductive performance and survival of offspring, and caused bioaccumulation and maternal transfer of P-CTX-1 in marine medaka.

The zebrafish as a model for gastrointestinal tract-microbe interactions

The zebrafish (Danio rerio) has become a widely used vertebrate model for bacterial, fungal, viral, and protozoan infections. Due to its genetic tractability, large clutch sizes, ease of manipulation, and optical transparency during early life stages, it is a particularly useful model to address questions about the cellular microbiology of host-microbe interactions. Although its use as a model for systemic infections, as well as infections localised to the hindbrain and swimbladder having been thoroughly reviewed, studies focusing on host-microbe interactions in the zebrafish gastrointestinal tract have been neglected. Here, we summarise recent findings regarding the developmental and immune biology of the gastrointestinal tract, drawing parallels to mammalian systems. We discuss the use of adult and larval zebrafish as models for gastrointestinal infections, and more generally, for studies of host-microbe interactions in the gut.

Zebrafish are Resistant to Staphylococcus aureus Endophthalmitis

Gram-positive bacteria remain the leading cause of endophthalmitis, a blinding infectious disease of the eye. Murine models have been widely used for understanding the pathogenesis of bacterial endophthalmitis. In this study, we sought to develop an alternative zebrafish (Danio rerio) model for Staphylococcus aureus and compare the disease pathobiology to a murine model. Endophthalmitis was induced in zebrafish and C57BL/6 mice through the intravitreal injection of S. aureus. Disease progression was monitored by assessing corneal haze, opacity, bacterial burden, and retinal histology. Our results demonstrated that, unlike the murine models, zebrafish maintained ocular integrity, corneal transparency, and retinal architecture. We found that the zebrafish was capable of clearing S. aureus from the eye via transport through retinal vessels and the optic nerve and by mounting a monocyte/macrophage response beginning at 8 hour post-infection (hpi). The bacterial burden increased up to 8 hpi and significantly decreased thereafter. An assessment of the innate retinal response revealed the induced expression of Il-1β and Il-6 transcripts. Collectively, our study shows that unlike the murine model, zebrafish do not develop endophthalmitis and rapidly clear the pathogen. Hence, a better understanding of the zebrafish protective ocular innate response may provide new insights into the pathobiology of bacterial endophthalmitis.

Tilapia (Oreochromis niloticus) as a Putative Reservoir Host for Survival and Transmission of Vibrio cholerae O1 Biotype El Tor in the Aquatic Environment

Studies have reported the occurrence of Vibrio cholerae in fish but little is known about the interaction between fish and toxigenic V. cholerae as opposed to phytoplankton, which are well-established aquatic reservoirs for V. cholerae. The present study determined the role of tilapia (Oreochromis niloticus) as a reservoir host for survival and transmission of V. cholerae in aquatic environments. Three experiments were performed with one repetition each, where O. niloticus (∼2 g) kept in beakers were inoculated with four V. cholerae strains (5 × 10⁷ cfu/mL). Firstly, infected tilapia were kept in stagnant water and fed live brine shrimp (Artemia salina) larvae daily. Secondly, infected tilapia were kept without feeding and water was changed every 24 h. Thirdly, infected tilapia were fed and water was renewed daily. Infected tilapia and non-infected controls were sacrificed on days 1, 2, 3, 7, and 14 post-inoculation and V. cholerae were enumerated in intestinal content and water. Another experiment assessed the transmission of V. cholerae from infected to non-infected tilapia. The study revealed that El Tor biotype V. cholerae O1 and V. cholerae non-O1 colonized tilapia intestines and persisted at stable concentrations during the second week of the experiment whereas the Classical biotype was undetectable after 1 week. In stagnant water with feeding, V. cholerae counts dropped to 10⁵ cfu/ml in water and from 10⁷ to 10⁴ cfu/intestine in fish after 14 days. When water was renewed, counts in water decreased from 10⁷ to 10³ cfu/ml and intestinal counts went from 10⁶ to 10² cfu/intestine regardless of feeding. All strains were transmitted from infected to naïve fish after 24 h of cohabitation. Tilapia like other fish may play an essential role in the survival and dissemination of V. cholerae O1 in aquatic environments, e.g., the seventh pandemic strains mostly. In this study, tilapia were exposed to high concentrations of V. cholerae to ensure initial uptake and follow-up studies with lower doses resembling natural concentrations of V. cholerae in the aquatic environment are needed to confirm our findings.

Internal Versus External Pressures: Effect of Housing Systems on the Zebrafish Microbiome

Zebrafish (Danio rerio) are an attractive model organism for scientific studies, including host-microbe interactions. The organism is particularly useful for studying aquatic microbes that can colonize vertebrate hosts, including Vibrio cholerae. Previous studies have established the presence of a core zebrafish intestinal microbiome, and tracked the development of the zebrafish intestinal microbiome from the larval stage to adulthood. An unexplored matter in this host-microbe relationship is the effect of the housing system on the zebrafish intestinal and tank water microbiomes. In this study, we used 16S rRNA gene sequencing to investigate the response of zebrafish intestinal and tank water microbiomes to a change in housing conditions. Zebrafish in the separated fish tanks showed no initial differences in the structures of their intestinal microbial profiles; the same prominent bacteria were present and abundant across tanks. Immediately after the housing switch, the zebrafish intestinal microbial profiles changed in composition and structure. Within 5 days of the housing switch, the intestinal microbiome had stabilized, and changed significantly from the prehousing switch profile. This study demonstrates that although external factors can significantly perturb and alter the zebrafish intestinal microbiome, the microbiome displays a large level of selective resilience whose primary members (namely Vibrio) persist.

Surveillance and Genomics of Toxigenic Vibrio cholerae O1 From Fish, Phytoplankton and Water in Lake Victoria, Tanzania

The occurrence of toxigenic Vibrio cholerae O1 during a non- outbreak period in Lake Victoria was studied and genetic characteristics for environmental persistence and relatedness to pandemic strains were assessed. We analyzed 360 samples of carps, phytoplankton and water collected in 2017 during dry and rainy seasons in the Tanzanian basin of Lake Victoria. Samples were tested using PCR (ompW and ctxA) with DNA extracted from bacterial isolates and samples enriched in alkaline peptone water. Isolates were screened with polyvalent antiserum O1 followed by antimicrobial susceptibility testing. Whole genome sequencing and bioinformatics tools were employed to investigate the genomic characteristics of the isolates. More V. cholerae positive samples were recovered by PCR when DNA was obtained from enriched samples than from isolates (69.0% vs. 21.3%, p < 0.05), irrespectively of season. We identified ten V. cholerae O1 among 22 ctxA-positive isolates. Further studies are needed to serotype the remaining ctxA-positive non-O1 strains. Sequenced strains belonged to El Tor atypical biotype of V. cholerae O1 of MLST ST69 harboring the seventh pandemic gene. Major virulence genes, ctxA, ctxB, zot, ace, tcpA, hlyA, rtxA, ompU, toxR, T6SS, alsD, makA and pathogenicity islands VPI-1, VPI-2, VSP-1, and VSP-2 were found in all strains. The strains contained Vibrio polysaccharide biosynthesis enzymes, the mshA gene and two-component response regulator proteins involved in stress response and autoinducers for quorum sensing and biofilm formation. They carried the SXT integrative conjugative element with phenotypic and genotypic resistance to aminoglycoside, sulfamethoxazole, trimethoprim, phenicol, and quinolones. Strains contained a multidrug efflux pump component and were resistant to toxic compounds with copper homeostasis and cobalt-zinc-cadmium resistance proteins. The environmental strains belonged to the third wave of the seventh pandemic and most are genetically closely related to recent outbreak strains from Tanzania, Kenya, and Uganda with as low as three SNPs difference. Some strains have persisted longer in the environment and were more related to older outbreak strains in the region. V. cholerae O1 of outbreak potential seem to persist in Lake Victoria through interactions with fish and phytoplankton supported by the optimum water parameters and intrinsic genetic features enhancing survival in the aquatic environment.

Using the Protozoan Paramecium caudatum as a Vehicle for Food-borne Infections in Zebrafish Larvae

Due to their transparency, genetic tractability, and ease of maintenance, zebrafish (Danio rerio) have become a widely-used vertebrate model for infectious diseases. Larval zebrafish naturally prey on the unicellular protozoan Paramecium caudatum. This protocol describes the use of P. caudatum as a vehicle for food-borne infection in larval zebrafish. P. caudatum internalize a wide range of bacteria and bacterial cells remain viable for several hours. Zebrafish then prey on P. caudatum, the bacterial load is released in the foregut upon digestion of the paramecium vehicle, and the bacteria colonize the intestinal tract. The protocol includes a detailed description of paramecia maintenance, loading with bacteria, determination of bacterial degradation and dose, as well as infection of zebrafish by feeding with paramecia. The advantage of using this method of food-borne infection is that it closely mimics the mode of infection observed in human disease, leads to more robust colonization compared to immersion protocols, and allows the study of a wide range of pathogens. Food-borne infection in the zebrafish model can be used to investigate bacterial gene expression within the host, host-pathogen interactions, and hallmarks of pathogenicity including bacterial burden, localization, dissemination and morbidity.

Glucose Metabolism by Escherichia coli Inhibits Vibrio cholerae Intestinal Colonization of Zebrafish

The Vibrio cholerae O1 serogroup is responsible for pandemic cholera and is divided into the classical and El Tor biotypes. Classical V. cholerae produces acid when using glucose as a carbon source, whereas El Tor V. cholerae produces the neutral product acetoin when using glucose as a carbon source. An earlier study demonstrated that Escherichia coli strains that metabolize glucose to acidic by-products drastically reduced the survival of V. cholerae strains in vitro In the present study, zebrafish were fed 1% glucose and either inoculated with single V. cholerae or E. coli strains or coinfected with both V. cholerae and E. coli A significant decrease in classical biotype colonization was observed after glucose feeding due to acid production in the zebrafish intestine. El Tor colonization was unaffected by glucose alone. However, the El Tor strain exhibited significantly lower colonization of the zebrafish when either of the acid-producing E. coli strains was coinoculated in the presence of glucose. An E. coli sugar transport mutant had no effect on V. cholerae colonization even in presence of glucose. Glucose and E. coli produced a prophylactic effect on El Tor colonization in zebrafish when E. coli was inoculated before V. cholerae infection. Thus, the probiotic feeding of E. coli inhibits V. cholerae colonization in a natural host. This suggests that a similar inhibitory effect could be seen in cholera patients, especially if a glucose-based oral rehydration solution (ORS) is administered in combination with probiotic E. coli during cholera treatment.

Quantifying Vibrio cholerae Colonization and Diarrhea in the Adult Zebrafish Model

Vibrio cholerae is best known as the infectious agent that causes the human disease cholera. Outside the human host, V. cholerae primarily exists in the aquatic environment, where it interacts with a variety of higher aquatic species. Vertebrate fish are known to be an environmental host and are a potential V. cholerae reservoir in nature. Both V. cholerae and the teleost fish species Danio rerio, commonly known as zebrafish, originate from the Indian subcontinent, suggesting a long-standing interaction in aquatic environments. Zebrafish are an ideal model organism for studying many aspects of biology, including infectious diseases. Zebrafish can be easily and rapidly colonized by V. cholerae after exposure in water. Intestinal colonization by V. cholerae leads to the production of diarrhea and the excretion of replicated V. cholerae. These excreted bacteria can then go on to colonize new fish hosts. Here, we demonstrate how to assess V. cholerae-intestinal colonization in zebrafish and how to quantify V. cholerae-induced zebrafish diarrhea. The colonization model should be useful to researchers who are studying whether genes of interest may be important for host colonization and/or for environmental survival. The quantification of zebrafish diarrhea should be useful to researchers studying any intestinal pathogen who are interested in exploring zebrafish as a model system.

Mucus-Pathogen Interactions in the Gastrointestinal Tract of Farmed Animals

Gastrointestinal infections cause significant challenges and economic losses in animal husbandry. As pathogens becoming resistant to antibiotics are a growing concern worldwide, alternative strategies to treat infections in farmed animals are necessary in order to decrease the risk to human health and increase animal health and productivity. Mucosal surfaces are the most common route used by pathogens to enter the body. The mucosal surface that lines the gastrointestinal tract is covered by a continuously secreted mucus layer that protects the epithelial surface. The mucus layer is the first barrier the pathogen must overcome for successful colonization, and is mainly composed of densely glycosylated proteins called mucins. The vast array of carbohydrate structures present on the mucins provide an important setting for host-pathogen interactions. This review summarizes the current knowledge on gastrointestinal mucins and their role during infections in farmed animals. We examine the interactions between mucins and animal pathogens, with a focus on how pathogenic bacteria can modify the mucin environment in the gut, and how this in turn affects pathogen adhesion and growth. Finally, we discuss analytical challenges and complexities of the mucus-based defense, as well as its potential to control infections in farmed animals.

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.

The Vibrio cholerae type VI secretion system can modulate host intestinal mechanics to displace gut bacterial symbionts

Host-associated microbiota help defend against bacterial pathogens; however, the mechanisms by which pathogens overcome this defense remain largely unknown. We developed a zebrafish model and used live imaging to directly study how the human pathogen Vibrio cholerae invades the intestine. The gut microbiota of fish monocolonized by symbiotic strain Aeromonas veronii was displaced by V. cholerae expressing its type VI secretion system (T6SS), a syringe-like apparatus that deploys effector proteins into target cells. Surprisingly, displacement was independent of T6SS-mediated killing of A. veronii, driven instead by T6SS-induced enhancement of zebrafish intestinal movements that led to expulsion of the resident microbiota by the host. Deleting an actin cross-linking domain from the T6SS apparatus returned intestinal motility to normal and thwarted expulsion, without weakening V. cholerae's ability to kill A. veronii in vitro. Our finding that bacteria can manipulate host physiology to influence intermicrobial competition has implications for both pathogenesis and microbiome engineering.