generation of germ-free zebrafish models from embryos

Optimizing GF Zebrafish Models: Generation, Maintenance, and Microbial Studies

The microbiota (i.e., Archaea, Bacteria, Eukarya, and viruses) play crucial roles in maintaining host health and contributing to the development of various diseases by influencing physiological and pathological processes through symbiotic interactions within the intestinal barrier, epithelial surface, and mucin functions in individuals1,2,3. The composition of the microbiota across different life stages, from infancy to juvenility, adulthood, and aging, as well as its presence in various locations such as nares, oral, skin, and gut sites, is dynamically shaped by diverse habitats and environments4. The intestinal microbiota in organisms is involved in nutrient absorption, immune response, pathogen invasion, metabolic regulation, etc5,6. Studies on patients have demonstrated that disruptions in gut microbiota are related to human obesity, sleep disorders, depression, inflammatory bowel disease (IBD), neurodegenerative diseases (Parkinson&#39;s, Alzheimer&#39;s), aging, and various cancers7,8,9. Furthermore, interactive pathways between gut microbiota and hosts involve inflammatory factors, neurotransmitters, metabolites, intestinal barrier, and oxidative stress, as observed in previous research using mice and fish models10,11.
Recently, multiple bacteria-related approaches or therapies, including potential probiotics and fecal microbiota transplantation (FMT), have been explored for these disorders in clinical and animal models. These explorations are based on discoveries related to the microbiota-gut-brain/liver/kidney axis, microbiota-derived products, and altered receptor activity12,13. However, the development, various functions, and mechanisms of the microbiota-host system are still incompletely understood and identified due to the complexity of the microbial community and the challenge of generating powerful human-like disease models.
To address these issues, germ-free (GF) animal models were urgently proposed in the mid-19th century and primarily developed during the 20th century. Subsequent refinements, including antibiotic-treated and gnotobiotic models, along with advancements in microbial detection and observation technologies, further perfected these models14,15,16. GF animals, created by erasing their own background and avoiding environmental microbes, offer an excellent strategy for exploring the interactions between microorganisms and their hosts17. Through the application of animal models and refined protocols, researchers have successfully replicated similar microbial compositions found in patients in GF mice and fish. Additionally, other GF animal models, such as dogs, chickens, and pigs, provide diverse options as research subjects18,19,20,21. This approach has enabled investigations into the potential therapeutic effects of commensal microbiomes on various diseases, including cancer immunotherapy in humans16,18. GF models offer more accurate insights into the characteristics and mechanisms of specific bacterial colonization, migration, multiplication, and interaction within hosts. This provides crucial novel insights into the occurrence and development of microbiota-related diseases22,23. The history of establishing and applying GF zebrafish in microbial research has evolved from the reports of Rawls et al. in 2004 and Bates et al. in 2006 to Melancon et al.&#39;s protocol in 201716,24,25. However, the feasibility of adult or breeding GF models is still a prolonged process, accompanied by variable longevity, success rates, and health challenges.
Among various animal models, zebrafish (Danio rerio) stands out as a critical tool for both basic and biomedical research due to its advantageous similarity to human organs and genomics, short developmental cycle, high fecundity, and transparent embryos19,26. Zebrafish, serving as reliable human disease models, offer a visual representation of physiological and pathological processes in vivo, providing insights into the attractive features of host-microbe interactions. Notably, zebrafish exhibit distinct cell lineages, allowing imaging of intestinal physiology, microbial dynamics, gonads and reproductive development, maturation of the host immune system, behavior, and metabolism27. Zebrafish embryos develop within protective chorions until hatching, becoming larvae at 3 days post-fertilization (dpf). They actively hunt for food at 5 dpf and reach sexual maturity around 3 months post-fertilization (mpf)28. The first successful germ-free (GF) zebrafish, reported by Rawls et al.24, showed that larvae fed with autoclaved feed after yolk absorption exhibited tissue necrosis from 8 dpf and total death at 20 dpf. This indicated the effects of diet or the importance of considering exogenous nutrient supply in experiments involving long-term (&#62;7 dpf) GF fish29. Subsequent studies improved the generation protocol of GF fish, employing sterile food and methods perfected in different fish models16.
However, most research on GF zebrafish models has focused on early life stages, involving bacterial infection at 5 dpf for 24 h to 48 h, with samples collected before 7 dpf at the conclusion of the experiments25,30,31. It&#39;s widely acknowledged that the microbiota in organisms, including humans and zebrafish, is colonized at the beginning of life and shaped during growth and development. The composition remains stable at adult stages, with the roles of microbiota in the host being crucial throughout life, especially in aging, neurodegenerative, metabolic-related obesity, and intestinal disease aspects3. Thus, perspectives from GF animals with longer survival can provide insights into the mechanisms of microbial roles in host organ development and functions, considering the immature immune and reproductive systems of fish larvae in early life. While bacterial strains in zebrafish intestines have been isolated and identified in previous studies, offering the potential for infecting GF animal models to select probiotics or research bacterial functions in the host19,25, the generation and application of GF fish models have primarily been restricted to early life stages. This limitation, attributed to the complex production process, high maintenance costs, and associated issues with food and immunity, hinders research efforts aimed at investigating the developmental and chronic effects of microbiota in the host.
The survival rate, behavior, growth, maturation, and overall health of fish, especially in germ-free (GF) models, are significantly influenced by feeding practices, encompassing nutrition intake and absorption during the mouth-open period from early larvae to juveniles32,33. However, one of the challenges in GF fish husbandry is the scarcity of suitable sterile diets, limiting the effectiveness of nutritional support for sustaining the growth and survival of larvae. Resolving this issue is crucial to restoring the life of GF fish, considering their developmental defense mechanisms and weak digestion abilities due to the absence of an intestinal microbiome. In terms of food, live brine shrimp (Artemia sp.) emerges as the most suitable diet for mouth-open larvae to juvenile fish. It has been observed that fish fed with live brine shrimp exhibit higher growth and survival rates compared to those fed with cooked egg yolk or other natural and synthetic baits34. While early life models of GF fish can survive with yolk support and GF larvae models can be maintained with sterile feeding, generating long-term models from larvae to juveniles and reaching sexual maturity remains challenging. Additionally, flake or powder food is limited by unequal nutritional composition and can impact water quality. In contrast, live Artemia has advantages such as survival in both salt and freshwater, small size suitable for larvae to adults, ease of batching, and higher hatching quality35. Building upon previous methods16,24,30, we have simplified the complex treatment process and addressed the diet challenge by establishing easily incubated GF live Artemia sp. as sterile food for longer durations than early-life GF fish.
This study presents an optimized protocol covering (1) generation, (2) maintenance, (3) identification of sterile rate, and (4) maintenance and feeding to ensure the growth of germ-free (GF) zebrafish from embryos to larvae and juvenile stages. The results offer preliminary evidence on the hatching, survival, growth, and sterility of GF zebrafish, along with essential indices for GF Artemia sp. as sterile food. The detailed steps in model generation and preparation of sterile live foods provide crucial technical support for constructing and applying long-term GF fish models, as well as GF Artemia sp. in microbiota-host interaction research. The protocol addresses bacterial isolation, identification, and infection on GF fish models, outlining methods for bacterial fluorescence labeling and observing their colonization in fish intestines under a microscope. GF fish, gnotobiotic fish with bacterial infection, or transferred human microbiota models will undergo various detections to elucidate their functions and effects on host immunity, digestion, behavior, transcriptomic regulation, and metabolic aspects. In the long term, this protocol can be extended to different wild-type fish species, such as marine medaka, and potentially to other selected transgenic zebrafish lines correlated to specific tissues or diseases.

Author Spotlight: Establishing Germ-Free Zebrafish for Microbiota-Host Relationship Studies

Generation, Maintenance, and Identification of Germ-Free Zebrafish Models from Larvae to Juvenile Stages

The scope of this research is generation of novel, germ-free animal models, which will be helpful and powerful tools in investigation of various microbiome functions. In this field, the early germ-free fish models have been used recently, but the long-term models are still limited due to various culture conditions and the staff developmental defense. The current GF fish models are normally early life ones.Thus, the technologies including the sterile scale, the sample detection methods, and providing sterile food are very important challenges to overcome. The germ-free zebrafish model is crucial for studying host-microbiota interaction and their impacts on development and the immunity and disease. This model allows researchers to precisely manipulate the single gut bacteria and observe its effect on the transparent zebrafish embryos.It provides valuable insights into the roles of gut microbes inheriting the disease. In the future, our research will mainly focused on establishing a germ-free zebrafish model for entire life cycle to better understanding the function of the gut microbiome.

Generation of Germ-Free Zebrafish Models from Embryos

generation-of-germ-free-zebrafish-models-from-embryos

Research

JoVE Journal

Developmental Biology

106 Views.  Chongqing Medical University. Generation of Germ-Free Zebrafish Models from Embryos