Role of germ-free animal models in understanding interactions of gut microbiota to host and environmental health: A special reference to zebrafish

The interactions between the host immunity and intestinal microorganisms in fish

There is a huge quantity of microorganisms in the gut of fish, which exert pivotal roles in maintaining host intestinal and general health. The fish immunity can sense and shape the intestinal microbiota and maintain the intestinal homeostasis. In the meantime, the intestinal commensal microbes regulate the fish immunity, control the extravagant proliferation of pathogenic microorganisms, and ensure the intestinal health of the host. This review summarizes developments and progress on the known interactions between host immunity and intestinal microorganisms in fish, focusing on the recent advances in zebrafish (Danio rerio) showing the host immunity senses and shapes intestinal microbiota, and intestinal microorganisms tune host immunity. This review will offer theoretical references for the development, application, and commercialization of intestinal functional microorganisms in fish. KEY POINTS: • The interactions between the intestinal microorganisms and host immunity in zebrafish • Fish immunity senses and shapes the microbiota • Intestinal microbes tune host immunity in fish.

Assessment of Probiotics' Impact on Neurodevelopmental and Behavioral Responses in Zebrafish Models: Implications for Autism Spectrum Disorder Therapy

Autism spectrum disorder (ASD) is a neurodevelopmental disorder; the prevalence of which has been on the rise with unknown causes. Alterations in the gut-brain axis have been widely recognized in ASD patients, and probiotics are considered to potentially benefit the rescuing of autism-like behaviors. However, the effectiveness and mechanisms of multiple probiotics on zebrafish models are still not clearly revealed. This study aims to use the germ-free (GF) and conventionally raised (CR) AB wild-type zebrafish and the mutant Tbr1b-/- and Katnal2-/- lines as human-linked ASD animal models to evaluate the effects of multiple probiotics on mitigating developmental and behavioral defects. Results showed that the addition of probiotics increased the basic important developmental indexes, such as body length, weight, and survival rate of treated zebrafish. Moreover, the Lactobacillus plantarum and Lactobacillus rhamnosus affected the behavior of CR zebrafish by increasing their mobility, lowering the GF zebrafish manic, and mitigating transgenic zebrafish abnormal behavior. Moreover, the expression levels of key genes related to gamma-aminobutyric acid (GABA), dopamine (DA), and serotonin (5-HT) as important neuropathways to influence the appearance and development of autism-related disorders, including gad1b, tph1a, htr3a, th, and slc6a3, were significantly activated by some of the probiotics' treatment at some extent. Taken together, this study indicates the beneficial effects of different probiotics, which may provide a novel understanding of probiotic function in related diseases' therapy.

Evaluating the probiotic effects of spraying lactiplantibacillus plantarum P-8 in neonatal piglets

BACKGROUND

Gut microbes play an important role in the growth and health of neonatal piglets. Probiotics can promote the healthy growth of neonatal piglets by regulating their gut microbes. The study investigated the effects of spraying Lactiplantibacillus plantarum P-8 (L. plantarum P-8) fermentation broth on the growth performance and gut microbes of neonatal piglets.

RESULTS

The animals were randomly divided into probiotics groups (109 neonatal piglets) and control groups (113 neonatal piglets). The probiotics group was sprayed with L. plantarum P-8 fermented liquid from 3 day before the expected date of the sow to the 7-day-old of piglets, while the control group was sprayed with equal dose of PBS. Average daily gain (ADG), immune and antioxidant status and metagenome sequencing were used to assess the changes in growth performance and gut microbiota of neonatal piglets. The results showed that L. plantarum P-8 treatment significantly improved the average daily gain (P < 0.05) of neonatal piglets. L. plantarum P-8 increased the activities of CAT and SOD but reduced the levels of IL-2 and IL-6, effectively regulating the antioxidant capacity and immunity in neonatal piglets. L. plantarum P-8 adjusted the overall structure of gut microflora improving gut homeostasis to a certain extent, and significantly increased the relative abundance of gut beneficial bacteria such as L. mucosae and L. plantarum.

CONCLUSION

Spraying L. plantarum P-8 can be a feasible and effective probiotic intervention not only improving the growth of neonatal piglets, regulating the antioxidant capacity and immunity of neonatal piglets, but also improving the gut homeostasis to a certain extent.

Investigating the impact of chlorine dioxide in shrimp-rearing water on the stomach microbiome, gill transcriptome, and infection-related mortality in shrimp

AIMS

This study aimed to assess the effects of chlorine dioxide (ClO2) in water on whiteleg shrimp Penaeus vannamei, evaluating its impact on the stomach microbiota, gill transcriptome, and pathogens.

METHODS AND RESULTS

ClO2 was added to the aquarium tanks containing the shrimp. The application of ClO2 to rearing water was lethal to shrimp at concentrations above 1.2 ppm. On the other hand, most of the shrimp survived at 1.0 ppm of ClO2. Microbiome analysis showed that ClO2 administration at 1.0 ppm significantly reduced the α-diversity of bacterial community composition in the shrimp stomach, and this condition persisted for at least 7 days. Transcriptome analysis of shrimp gill revealed that ClO2 treatment caused massive change of the gene expression profile, including stress response genes. However, after 7 days of the treatment, the gene expression profile was similar to that of shrimp in the untreated control group, suggesting a recovery to the normal state. This 1.0-ppm ClO2 significantly reduced shrimp mortality in artificial challenges with an acute hepatopancreatic necrosis disease-causing Vibrio parahaemolyticus and white spot syndrome virus, which were added to rearing water.

CONCLUSIONS

The use of ClO2 at appropriate concentrations effectively eliminates a significant portion of the bacteria in the shrimp stomach and pathogens in the water. The results of this study provide fundamental knowledge on the disinfection of pathogens in water using ClO2 and the creation of semi germ-free shrimp, which has significantly decreased microbiome in the stomach.

Effects of the plastic additive 2,4-di-tert-butylphenol on intestinal microbiota of zebrafish

Plastic additives such as the antioxidant 2,4-di-tert-butylphenol (2,4-DTBP) have been widely detected in aquatic environments, over a wide range of concentrations reaching 300 μg/L in surface water, potentially threatening the health of aquatic organisms and ecosystems. However, knowledge of the specific effects of 2,4-DTBP on aquatic vertebrates is still limited. In this study, adult zebrafish were exposed to different concentrations of 2,4-DTBP (0, 0.01, 0.1 and 1.0 mg/L) for 21 days in the laboratory. The amplicon sequencing results indicated that the diversity and composition of the zebrafish gut microbiota were significantly changed by 2,4-DTBP, with a shift in the dominant flora to more pathogenic genera. Exposure to 2,4-DTBP at 0.1 and 1.0 mg/L significantly increased the body weight and length of zebrafish, suggesting a biological stress response. Structural assembly defects were also observed in the intestinal tissues of zebrafish exposed to 2,4-DTBP, including autolysis of intestinal villi, adhesions and epithelial detachment of intestinal villi, as well as inflammation. The transcriptional expression of some genes showed that 2,4-DTBP adversely affected protein digestion and absorption, glucose metabolism and lipid metabolism. These results are consistent with the PICRUSt2 functional prediction analysis of intestinal microbiota of zebrafish exposed to 2,4-DTBP. This study improves our understanding of the effects of 2,4-DTBP on the health of aquatic vertebrates and ecosystems.

Zebrafish: A trending model for gut-brain axis investigation

Zebrafish (Danio rerio) has ascended as a pivotal model organism in the realm of gut-brain axis research, principally owing to its high-throughput experimental capabilities and evolutionary alignment with mammals. The inherent transparency of zebrafish embryos facilitates unprecedented real-time imaging, affording unparalleled insights into the intricate dynamics of bidirectional communication between the gut and the brain. Noteworthy are the structural and functional parallels shared between the zebrafish and mammalian gut-brain axis components, rendering zebrafish an invaluable model for probing the molecular and cellular intricacies inherent in this critical physiological interaction. Recent investigations in zebrafish have systematically explored the impact of gut microbiota on neurodevelopment, behaviour, and disease susceptibility, underscoring the model's prowess in unravelling the multifaceted influence of microbial communities in shaping gut-brain interactions. Leveraging the genetic manipulability inherent in zebrafish, researchers have embarked on targeted explorations of specific pathways and molecular mechanisms, providing nuanced insights into the fundamental functioning of the gut-brain axis. This comprehensive review synthesizes pivotal findings and methodological advancements derived from zebrafish-based gut-brain axis research, accentuating the model's potential to significantly advance our understanding of this complex interplay. Furthermore, it underscores the translational significance of these insights, offering promising avenues for the identification of therapeutic targets in neuro-gastroenterological disorders and psychiatric conditions intricately linked with gut-brain interactions.

Lactobacillus rhamnosus GG triggers intestinal epithelium injury in zebrafish revealing host dependent beneficial effects

Lactobacillus rhamnosus GG (LGG), the well-characterized human-derived probiotic strain, possesses excellent properties in the maintenance of intestinal homeostasis, immunoregulation and defense against gastrointestinal pathogens in mammals. Here, we demonstrate that the SpaC pilin of LGG causes intestinal epithelium injury by inducing cell pyroptosis and gut microbial dysbiosis in zebrafish. Dietary SpaC activates Caspase-3-GSDMEa pathways in the intestinal epithelium, promotes intestinal pyroptosis and increases lipopolysaccharide (LPS)-producing gut microbes in zebrafish. The increased LPS subsequently activates Gaspy2-GSDMEb pyroptosis pathway. Further analysis reveals the Caspase-3-GSDMEa pyroptosis is initiated by the species-specific recognition of SpaC by TLR4ba, which accounts for the species-specificity of the SpaC-inducing intestinal pyroptosis in zebrafish. The observed pyroptosis-driven gut injury and microbial dysbiosis by LGG in zebrafish suggest that host-specific beneficial/harmful mechanisms are critical safety issues when applying probiotics derived from other host species and need more attention.

An azole fungicide climbazole damages the gut-brain axis in the grass carp

Azole antifungal climbazole has frequently been detected in aquatic environments and shows various effects in fish. However, the underlying mechanism of toxicity through the gut-brain axis of climbazole is unclear. Here, we investigated the effects of climbazole at environmental concentrations on the microbiota-intestine-brain axis in grass carp via histopathological observation, gene expression and biochemical analyses, and high-throughput sequencing of the 16 S rRNA. Results showed that exposure to 0.2 to 20 μg/L climbazole for 42 days significantly disrupted gut microbiota and caused brain neurotoxicity in grass carp. In this study, there was an alteration in the phylum and genus compositions in the gut microbiota following climbazole treatment, including reducing Fusobacteria (e.g., Cetobacterium) and increasing Actinobacteria (e.g., Nocardia). Climbazole disrupted intestinal microbial abundance, leading to increased levels of lipopolysaccharide and tumor necrosis factor-alpha in the gut, serum, and brain. They passed through the impaired intestinal barrier into the circulation and caused the destruction of the blood-brain barrier through the gut-brain axis, allowing them into the brain. In the brain, climbazole activated the nuclear factor kappaB pathway to increase inflammation, and suppressed the E2-related factor 2 pathway to produce oxidative damage, resulting in apoptosis, which promoted neuroinflammation and neuronal death. Besides, our results suggested that this neurotoxicity was caused by the breakdown of the microbiota-gut-brain axis, mediated by reduced concentrations of dopamine, short chain fatty acids, and intestinal microbial activity induced by climbazole.

Exploring the impact of antibiotics, microplastics, nanoparticles, and pesticides on zebrafish gut microbiomes: Insights into composition, interactions, and health implications

This review addresses the impact of various chemical entities like pesticides, antibiotics, nanoparticles and microplastic on gut microbiota of zebrafish. Gut microbiota plays a vital role in metabolic regulation in every organism. As majority of metabolic pathways coordinated by microbiota, small alterations associated with mild to serious outcomes. Because of their unstoppable usage in day-to-day life, the present-day research on gut microbiota is mostly comprising aforementioned chemicals. It is better to understand how gut microbiome is dysbiosed by various environmental factors, to keep our microbiota safe. We tried to delineate the natural flora of zebrafish gut microbiome and the metabolic and other pathways associated and what are the common flora that was dysbiosed during the treatment. Based on the existing literature, we reviewed pesticides like Imazalil, Difenoconazole, Chlorpyrifos, Metamifop, Carbendazim, Imidacloprid, Phoxim, Niclosamide, Dieldrin, and antibiotics like Oxytetracycline, Enrofloxacin, Florfenicol, Sulfamethoxazole, Tetracycline, Streptomycin, Doxycycline, and in the category of nanoparticles, Titanium dioxide nanoparticles (nTiO2), Abalone viscera hydrolysates decorated silver nanoparticles (AVH-AgNPs), Lead-halide perovskite nanoparticles (LHP NPs), Copper nanoparticles (Cu-NPs), silver nanoparticles (Ag-NPs) and microplastic types like polyethylene and polystyrene microplastic. Other studies with miscellaneous chemical entities on zebrafish gut microbiome include Ferulic acid, Polychlorinated biphenyls, Cadmium, Disinfection by-products, Triclosan, microcystin-LR, Fluoride, and Amitriptyline.

Synbiotic Agents and Their Active Components for Sustainable Aquaculture: Concepts, Action Mechanisms, and Applications

Aquaculture is a fast-emerging food-producing sector in which fishery production plays an imperative socio-economic role, providing ample resources and tremendous potential worldwide. However, aquatic animals are exposed to the deterioration of the ecological environment and infection outbreaks, which represent significant issues nowadays. One of the reasons for these threats is the excessive use of antibiotics and synthetic drugs that have harmful impacts on the aquatic atmosphere. It is not surprising that functional and nature-based feed ingredients such as probiotics, prebiotics, postbiotics, and synbiotics have been developed as natural alternatives to sustain a healthy microbial environment in aquaculture. These functional feed additives possess several beneficial characteristics, including gut microbiota modulation, immune response reinforcement, resistance to pathogenic organisms, improved growth performance, and enhanced feed utilization in aquatic animals. Nevertheless, their mechanisms in modulating the immune system and gut microbiota in aquatic animals are largely unclear. This review discusses basic and current research advancements to fill research gaps and promote effective and healthy aquaculture production.

Disruption of intestinal structure, tight junction complex, immune response and microbiota after chronic exposure to copper in swamp eel (Monopterus albus)

As an essential micronutrient, copper is crucial in aquatic organisms' growth and development. Numerous studies have consistently reported that excessive intake of copper can have harmful effects on organisms. However, there are limited studies on the impact of copper on the intestine of the swamp eel (Monopterus albus). This study aimed to investigate the changes of intestinal histopathology, tight junction complex, immune response, and microbiota in swamp eel treated with 0 mg/L Cu2+, 0.05 mg/L Cu2+, and 0.10 mg/L Cu2+ for 56 d. Intestinal histopathology showed major changes such as the increased number of erythrocytes and goblet cells in the lamina propria, and separation of the lamina propria. The expression of genes involved in tight junction complex (ZO-1, Claudin-3, Claudin-12 and Claudin-15) was significantly changed. In addition, copper exposure significantly increased the mRNA levels of TLR3, TLR7, TLR8, NF-κB, I-κB, TNF-α and IL-8, especially in 0.10 mg/L Cu2+ group. In contrast, the relative expression level of anti-inflammatory cytokine TGF-β was significantly decreased after exposure to copper. Analysis of the intestinal microbiome showed the intestinal microbiota of swamp eels in the control and copper exposure groups were dominated by Firmicutes and Proteobacteria at the phylum level. Notably, copper exposure changed the diversity of the intestinal microbiota and decreased the relative abundance of Firmicutes and Proteobacteria in the intestine of swamp eel. Collectively, this study demonstrates that chronic copper exposure induces intestinal pathologic changes and inflammatory response, disrupts the intestinal microbial diversity and microbiota composition, and decreases intestinal barrier function in swamp eel, which enhances our understanding of copper-induced intestinal toxicity in fish.

Deciphering the dysbiosis caused in the fish microbiota by emerging contaminants and its mitigation strategies-A review

The primary barrier to nutrient absorption in fish is the intestinal epithelium, followed by a community of microorganisms known as the gut microbiota, which can be thought of as a hidden organ. The gastrointestinal microbiota of fish plays a key role in the upholding of overall health by maintaining the homeostasis and disease resistance of the host. However, emerging contaminants as the result of anthropogenic activities have significantly led to disruptions and intestinal dysbiosis in fish. Which probably results in fish mortalities and disrupts the balance of an ecosystem. Therefore, we comprehensively seek to compile the effects and consequences of emerging contaminations on fish intestinal microbiota. Additionally, the mitigation strategies including prebiotics, probiotics, plant-based diet, and Biofloc technology are being outlined. Biofloc technology (BFT) can treat toxic materials, i.e., nitrogen components, and convert them into a useful product such as proteins and demonstrated promising elevating technique for the fish intestinal bacterial composition. However, it remains unclear whether the bacterial isolate is primarily responsible for the BFT's removal of nitrate and ammonia and the corresponding removal mechanism. To answer this, real time polymerase chain reaction (RT-PCR) with metagenomics, transcriptomics, and proteomics techniques probably provides a possible solution.

Food and drug design for gut microbiota-directed regulation: Current experimental landscape and future innovation

Most diets and medications enhance host health via microbiota-dependent ways, but it is in the present situation of untargeted regulation. Non-targeted regulation may lead to the ineffectiveness of dietary supplements or drug treatment. Microbiota-directed food, aiming to improve diseases by targeting specific microbes without affecting other bacteria, have been proposed to deal with this problem. However, there is currently no universally applicable method to explore such foods or drugs. In this review, thirty studies on recent efforts in microbiota directed diets and medications are summarized from various databases. The methods used to find new foods and medications are primarily divided into four groups depending on the experimental models: in vivo and in vitro, as well as predictions based on bioinformatics. We also discuss their implementation, interpretation, and respective limitations, and describe the present situation. We further put forward a framework for microbiota-directed foods and medicine according to above methods and other microbiome manipulation, which will spur precision medicine.

Effects of fermented soybean meal substitution for fish meal on intestinal flora and intestinal health in pearl gentian grouper

This study explored the role of replacing fish meal protein with fermented soybean meal (FSBM) protein on the growth performance and intestinal morphology, immunity, and microbiota of the pearl gentian grouper (Epinephelus fuscoguttatus♀ × E. lanceolatus♂). Three isonitrogenous and isolipidic diets with increasing levels of FSBM (0%, 20% and 40%; referred to as FM, FSBM20 and FSBM40 diets, respectively) as a replacement for fish meal were selected for this study. The pearl gentian grouper were fed these diets for 10 weeks. The findings revealed that the growth of fish fed the FSBM diets (FSBM20 and FSBM40) were remarkably lower than the fish fed the FM diet. Pathological manifestations of intestinal inflammation, such as shortened intestinal mucosal folds and thickened lamina propria, were observed in the fish fed the FSBM diets. Moreover, the gene expression levels of IL1β, IL12, IL17, and TNFα were remarkably upregulated in fish fed the FSBM40 diet, in contrast to the gene expression levels of IL4, IL5, IL10, and TGFβ1, which were remarkably downregulated (p < 0.05). The FSBM diets significantly affected the stability of the fish gut microbiota. Photobacterium was the dominant phylum in all experimental groups, and the proportion of these bacteria gradually decreased with increasing FSBM substitution. The composition of intestinal flora at the genus level was not the same in the three experimental groups, with a richer composition of intestinal bacteria detected in the FSBM20 and FSBM40 groups (p < 0.05). The correlation between intestinal flora balance and immune gene expression revealed that only Photobacterium was negatively correlated with the above upregulated genes, while other bacteria were positively correlated with these pro-inflammatory factors (p < 0.05). Photobacterium was positively correlated with the above downregulated genes, while other bacteria were negatively correlated with these anti-inflammatory factors (p < 0.05). In conclusion, high levels of substitution of FSBM for fish meal causes intestinal inflammation in pearl gentian grouper. This is likely associated with changes to the intestinal flora. More attention should be paid to the negative role of dietary FSBM on intestinal flora.

Small fish, big discoveries: zebrafish shed light on microbial biomarkers for neuro-immune-cardiovascular health

Zebrafish (Danio rerio) have emerged as a powerful model to study the gut microbiome in the context of human conditions, including hypertension, cardiovascular disease, neurological disorders, and immune dysfunction. Here, we highlight zebrafish as a tool to bridge the gap in knowledge in linking the gut microbiome and physiological homeostasis of cardiovascular, neural, and immune systems, both independently and as an integrated axis. Drawing on zebrafish studies to date, we discuss challenges in microbiota transplant techniques and gnotobiotic husbandry practices. We present advantages and current limitations in zebrafish microbiome research and discuss the use of zebrafish in identification of microbial enterotypes in health and disease. We also highlight the versatility of zebrafish studies to further explore the function of human conditions relevant to gut dysbiosis and reveal novel therapeutic targets.

Bacteriophage-based techniques for elucidating the function of zebrafish gut microbiota

Bacteriophages (or phages) are unique viruses that can specifically infect bacteria. Since their discovery by Twort and d'Herelle, phages with bacterial specificity have played important roles in microbial regulation. The intestinal microbiota and host health are intimately linked with nutrient, metabolism, development, and immunity aspects. However, the mechanism of interactions between the composition of the microbiota and their functions in maintaining host health still needs to be further explored. To address the lack of methodology and functions of intestinal microbiota in the host, we first proposed that, with the regulations of special intestinal microbiota and applications of germ-free (GF) zebrafish model, phages would be used to infect and reduce/eliminate the defined gut bacteria in the conventionally raised (CR) zebrafish and compared with the GF zebrafish colonized with defined bacterial strains. Thus, this review highlighted the background and roles of phages and their functional characteristics, and we also summarized the phage-specific infection of target microorganisms, methods to improve the phage specificity, and their regulation within the zebrafish model and gut microbial functional study. Moreover, the primary protocol of phage therapy to control the intestinal microbiota in zebrafish models from larvae to adults was recommended including phage screening from natural sources, identification of host ranges, and experimental design in the animal. A well understanding of the interaction and mechanism between phages and gut bacteria in the host can potentially provide powerful strategies or techniques for preventing bacteria-related human diseases by precisely regulating in vitro and in vivo, which will provide novel insights for phages' application and combined research in the future. KEY POINTS: • Zebrafish models for clarifying the microbial and phages' functions were discussed • Phages infect host bacteria with exquisite specificity and efficacy • Phages can reduce/eliminate the defined gut bacteria to clarify their function.

Effect of long-term exposure to dyeing wastewater treatment plant effluent on growth and gut microbiota of adult zebrafish (Danio rerio)

Strict standards have been put forward for the treatment and discharge of dyeing wastewater worldwide. However, there are still traces amount of pollutants, especially emerging pollutants in dyeing wastewater treatment plant (DWTP) effluent. Few studies have focused on the chronic biological toxicity effect and mechanism of DWTP effluent. In this study, 3-month chronic compound toxic effects were investigated by the exposure of DWTP effluent using adult zebrafish. Significantly higher mortality and fatness and significantly lower body weight and body length were found in the treatment group. In addition, long-term exposure to DWTP effluent also obviously reduced liver-body weight ratio of zebrafish, causing abnormal liver development of zebrafish. Moreover, DWTP effluent led to obvious changes in the gut microbiota and microbial diversity of zebrafish. At phylum level, significantly higher of Verrucomicrobia but lower Tenericutes, Actinobacteria, and Chloroflexi were found in the control group. At genus level, the treatment group had significantly higher abundance of Lactobacillus, but significantly lower abundance of Akkermansia, Prevotella, Bacteroides, and Sutterella. These results suggested that long-term exposure to DWTP effluent led to imbalance of gut microbiota in zebrafish. In general, this research indicated that DWTP effluent pollutants could result in negative health outcomes to aquatic organisms.

Associated long-term effects of decabromodiphenyl ethane on the gut microbial profiles and metabolic homeostasis in Sprague-Dawley rat offspring

Decabromodiphenyl ethane (DBDPE) as a widely used brominated flame retardant is harmful to human health due to its toxicity, including cardiovascular toxicity, reproductive toxicity, and hepatotoxicity. However, the knowledge of the long-term effects and structural and metabolic function influence on gut microbiota from DBDPE exposure remains limited. This study was mainly aimed at the gut microbiome and fecal metabolome of female rats and their offspring exposed to DBDPE in early life. 16S rRNA gene sequencing demonstrated that maternal DBDPE exposure could increase the α-diversity of gut microbiota in immature offspring while decreasing the abundance of Bifidobacterium, Clostridium, Muribaculum, Escherichia, and Lactobacillus in adult offspring. The nonmetric multidimensional scaling showed a consistency in the alternation of β-diversity between pregnant rats and their adult offspring. Furthermore, the short-chain fatty acids produced by gut microbiota dramatically increased in adult offspring after maternal DBDPE exposure, revealing that DBDPE treatment disrupted the gut microbial compositions and altered the gut community's metabolic functions. Untargeted metabolomics identified 41 differential metabolites and seven metabolic pathways between adult offspring from various groups. Targeted metabolomic showed that maternal high dose DBDPE exposure obviously decreased the level of glutathione, taurine, and l-carnitine in their adult offspring, which verified the correlation between weight loss and amino acid metabolites. An interesting link between some gut bacteria (especially the Firmicutes) and fecal metabolites demonstrated the shifts in gut microbiota may drive the metabolic process of fecal metabolites. The current findings provide new insight into long-term effects on human health.

Zebrafish: an efficient vertebrate model for understanding role of gut microbiota

Gut microbiota plays a critical role in the maintenance of host health. As a low-cost and genetically tractable vertebrate model, zebrafish have been widely used for biological research. Zebrafish and humans share some similarities in intestinal physiology and function, and this allows zebrafish to be a surrogate model for investigating the crosstalk between the gut microbiota and host. Especially, zebrafish have features such as high fecundity, external fertilization, and early optical transparency. These enable the researchers to employ the fish to address questions not easily addressed in other animal models. In this review, we described the intestine structure of zebrafish. Also, we summarized the methods of generating a gnotobiotic zebrafish model, the factors affecting its intestinal flora, and the study progress of gut microbiota functions in zebrafish. Finally, we discussed the limitations and challenges of the zebrafish model for gut microbiota studies. In summary, this review established that zebrafish is an attractive research tool to understand mechanistic insights into host-microbe interaction.

Humanized Germ-Free Mice for Investigating the Intervention Effect of Commensal Microbiome on Cancer Immunotherapy

Significance: A growing body of evidence has demonstrated that the commensal microbiome is deeply involved in the host immune response, accounting for significantly divergent clinical outcomes among cancer patients receiving immunotherapy. Therefore, precise screening and evaluating of functional bacterial strains as novel targets for cancer immunotherapy have attracted great enthusiasm from both academia and industry, which calls for the construction and application of advanced animal models to support translational research in this field. Recent Advances: Significant progress has been made to elucidate the intervention effect of commensal microbiome on immunotherapy based on animal experiments. Especially, correlation between gut microbiota and host response to immunotherapy has been continuously discovered in a variety of cancer types, laying the foundation for causality establishment and mechanism research. Critical Issues: In oncology research, it is particularly not uncommon to see that a promising preclinical result fails to translate into clinical success. The use of conventional murine models in immunotherapy-associated microbiome research is very likely to bring discredit on the preclinical findings. We emphasize the value of germ-free (GF) mice and humanized mice as advanced models in this field. Future Directions: Integrating rederivation and humanization to generate humanized GF mice as preclinical models would make it possible to clarify the role of specific bacterial strains in immunotherapy as well as obtain preclinical findings that are more predictive for humans, leading to novel microbiome-based strategies for cancer immunotherapy. Antioxid. Redox Signal. 37, 1291-1302.

The Function and the Affecting Factors of the Zebrafish Gut Microbiota

Gut microbiota has become a topical issue in unraveling the research mechanisms underlying disease onset and progression. As an important and potential “organ,” gut microbiota plays an important role in regulating intestinal epithelial cell differentiation, proliferation, metabolic function and immune response, angiogenesis and host growth. More recently, zebrafish models have been used to study the interactions between gut microbiota and hosts. It has several advantages, such as short reproductive cycle, low rearing cost, transparent larvae, high genomic similarity to humans, and easy construction of germ-free (GF) and transgenic zebrafish. In our review, we reviewed a large amount of data focusing on the close relationship between gut microbiota and host health. Moreover, we outlined the functions of gut microbiota in regulating intestinal epithelial cell differentiation, intestinal epithelial cell proliferation, metabolic function, and immune response. More, we summarized major factors that can influence the composition, abundance, and diversity of gut microbiota, which will help us to understand the significance of gut microbiota in regulating host biological functions and provide options for maintaining the balance of host health.

Enrofloxacin Induces Intestinal Microbiota-Mediated Immunosuppression in Zebrafish

The immunosuppressive effects of antibiotics and the potential associations with the intestinal microbiota of the host have been increasingly recognized in recent years. However, the detailed underlying mechanisms of immune interference of antibiotics in environmental organisms remain unclear, particularly at the early life stage of high sensitivity. To better understand the gut microbiome and immune function interactions, the vertebrate model, zebrafish, was treated with environmentally relevant concentrations of a frequently detected antibiotic, enrofloxacin (ENR), ranging from 0.01 to 100 μg/L. 16S ribosomal RNA sequencing indicated diminished diversity, richness, and evenness of intestinal flora following ENR treatment. Twenty-two taxa of gut bacteria including Rickettsiales, Pseudomonadales, and Flavobacteriales were significantly correlated with immunosuppressive biomarkers, including a significant decrease in the abundance of macrophages and neutrophils. To validate the immunomodulatory effects due to altered intestinal microbial populations, zebrafish reared under sterile and non-sterile husbandry conditions were compared after ENR treatment. A significant inhibitory effect was induced by ENR treatment under non-sterile conditions, while the number of macrophages and neutrophils, as well as biomarkers of immunosuppressive effects, were significantly salved in zebrafish under sterile conditions, confirming for the first time that immunosuppression by ENR was closely mediated through alterations of the intestinal microbiome in fish.

Acute effects of antimony exposure on adult zebrafish (Danio rerio): From an oxidative stress and intestinal microbiota perspective

The rapid development of the textile industry has resulted in a large influx of wastewater production. The "national discharge standards of water pollutants for dyeing and finishing of textile industry (GB4287-2012)" stipulates that the discharge of total Sb from textile industry effluent must be < 0.10 mg/L, but it is difficult to meet the standard at present. Antimony is potentially carcinogenic, and the pathogenic mechanism of antimony is poorly understood. In this study, the acute toxic effects of various concentrations of antimony on adult zebrafish (Danio rerio) were investigated, including effects on oxidative stress, neurotransmitters and intestinal microbiota. The activities of catalase (CAT), glutathione peroxidase (GSH-Px), malondialdehyde (MDA), superoxide dismutase (SOD), total antioxidant capacity (T-AOC) and acetylcholinesterase (AChE) were measured in zebrafish muscle and intestine tissue samples. In addition, intestinal microbial community composition and diversity of zebrafish were also analyzed. The results demonstrated that SOD, CAT and GSH-Px activities in the zebrafish gut showed a decreasing and then increasing trend with antimony concentration increasing. SOD, CAT and MDA in zebrafish muscle decreased with increasing exposure time. GSH-Px activities increased with increasing exposure time. T-AOC increased and then decreased. In addition, antimony exposure was neurotoxic to zebrafish, and a significant decrease in AChE activity was found in the intestine with increased exposure time. The neurotoxicity caused by antimony in the high concentration group (40 mg/L) was stronger than that in low concentration groups (10 mg/L and 20 mg/L). Notably, antimony exposure caused increases in the relative abundance of phyla Fusobacteriota and Actinomycetes, but decreases in the relative abundance of the phyla Firmicutes and Proteobacteria in zebrafish intestine. These outcomes will advance our understanding of antimony-induced biotoxicity, environmental problems, and health hazards. In conclusion, this study shows that acute exposure of antimony to zebrafish induces host oxidative stress and neurotoxicity, dysregulates the intestinal microbiota, showing adverse effects on the health and gut microbiota of zebrafish.

Effect of Chronic Exposure to Textile Wastewater Treatment Plant Effluents on Growth Performance, Oxidative Stress, and Intestinal Microbiota in Adult Zebrafish (Danio rerio)

The ever-increasing production and processing of textiles will lead to greater risks of releasing pollutants into the environment. Textile wastewater treatment plants (TWTPs) effluent are an important source of persistent toxic pollutants in receiving water bodies. The effects of specific pollutants on organisms are usually studied under laboratory conditions, and therefore, comprehensive results are not obtained regarding the chronic combined effects of pollutants under aquatic environmental conditions. Thus, this study aimed to determine the combined effects of TWTP effluents on the growth performance, oxidative stress, inflammatory response, and intestinal microbiota of adult zebrafish (Danio rerio). Exposure to TWTP effluents significantly inhibited growth, exacerbated the condition factor, and increased the mortality of adult zebrafish. Moreover, markedly decreases were observed in the activities of antioxidant enzymes, such as CAT, GSH, GSH-Px, MDA, SOD, and T-AOC, mostly in the intestine and muscle tissues of zebrafish after 1 and 4 months of exposure. In addition, the results demonstrated that TWTP effluent exposure affected the intestinal microbial community composition and decreased community diversity. Slight changes were found in the relative abundance of probiotic Lactobacillus, Akkermansia, and Lactococcus in zebrafish guts after chronic TWTP effluent exposure. The chronic toxic effects of slight increases in opportunistic pathogens, such as Mycoplasma, Stenotrophomonas, and Vibrio, deserve further attention. Our results reveal that TWTP effluent exposure poses potential health risks to aquatic organisms through growth inhibition, oxidative stress impairment of the intestine and muscles, and intestinal microbial community alterations.

Are microorganisms everywhere they can be?

Baas-Becking is famously attributed with the conjecture that 'everything is everywhere, but the environment selects'. Although this aphorism is largely challenged by microbial biogeographical data, even weak versions of the claim leave unanswered the question about whether all environments that could theoretically support life contain life. In the last decade, the discovery of thermally sterilized habitable environments disconnected from inhabited regions, and habitats within organisms such as the sterile, but habitable human fetal gut, suggest the existence of a diversity of macroscopic habitable environments apparently devoid of actively metabolizing or reproducing life. Less clear is the status of such environments at the micron scale, for example, between colonies of organisms within rock interstices or on and within other substrates. I discuss recent evidence for these types of environments. These environments have practical uses in: (i) being negative controls for understanding the role of microbial processes in geochemical cycles and geological processes, (ii) yielding insights into the extent to which the biosphere extends into all spaces it theoretically can, (iii) suggesting caution in interpreting the results of life detection instrumentation, and (iv) being useful for establishing the conditions for the origin of life and its prevalence on other planetary bodies.

Disruption of Intestinal Homeostasis Through Altered Responses of the Microbial Community, Energy Metabolites, and Immune System in Zebrafish After Chronic Exposure to DEHP

Di-(2-ethylhexyl) phthalate (DEHP) is ubiquitously reported in global water bodies and exhibits various environmental and human health risks. However, the effects of DEHP chronic exposure on the intestinal microbiota and associated host health concerns in aquatic species are still largely unexplored. In this study, chronic exposure to DEHP at environmental levels significantly increased the body weight, length, and body mass index (BMI), especially in male fish. The microbial community was disrupted with the relative abundance of phylum Firmicutes and genera diversity for Prevotella-7, Deefgea, PeM15, Halomonas, Akkermansia, Chitinibacter, and Roseomonas, which are significantly activated in zebrafish after exposure to DEHP. The height of the gut villus, the thickness of muscularis layer, and the number of goblet cells per villus were significantly decreased, as well as showed differences between female and male zebrafish. Further, the levels of energy-related metabolites in gut tissues were increased, compared to the control group. The expression levels of immune-related genes (interleukin 8, il-8, also referred to as cxcl8a), microbial defense-related genes (lysozyme, lyz, interleukin 10, and il-10), and obesity-related genes (aquaporin 8a, aqp8, mucin 2.1, muc2.1, fibroblast growth factor 2, fgf2, and proopiomelanocortin a, pomca) were significantly up-regulated in zebrafish, except the down-regulated expressions of toll-like receptor-5 (tlr-5) and interleukin 1β (il-1β) in the females and pomca in the males, respectively. Importantly, Spearman's correlation analyses revealed that the levels of metabolites and gene expressions in the gut were closely related to the dominant microbial genera, such as Aeromonas, Deefgea, Akkermansia, PeM15, Mycobacterium, and Rhodobacter. Taken together, chronic exposure to DEHP at environmental levels disturbed bacterial composition accompanied by the altered expressions of intestinal metabolites and the critical immune and intestinal function-related genes, which provided novel insights into DEHP effects on perturbation of gut microbiota and metabolic homeostasis in zebrafish.