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

Summary

This protocol outlines the primary steps for obtaining germ-free (GF) fish embryos and maintaining them from larvae until the juvenile stage, including sampling and detecting their sterile status. The use of GF models with infection is important for understanding the role of microbes in host health.

Abstract

Zebrafish serve as valuable models for research on growth, immunity, and gut microbiota due to their genomic similarities with mammals, transparent embryos developed in a relatively clean chorion environment, and extremely rapid development of larvae compared to rodent models. Germ-free (GF) zebrafish (Danio rerio) are crucial for evaluating pollutant toxicity and establishing human-like disease models related to microbial functions. In comparison to conventionally raised (CR) models (fish in common husbandry), GF zebrafish allow for more accurate manipulation of the host microbiota, aiding in determining the causal relationship between microorganisms and hosts. Consequently, they play a critical role in advancing our understanding of these relationships. However, GF zebrafish models are typically generated and researched during the early life stages (from embryos to larvae) due to limitations in immune function and nutrient absorption. This study optimizes the generation, maintenance, and identification of early GF zebrafish models without feeding and with long-term feeding using GF food (such as Artemia sp., brine shrimp). Throughout the process, daily sampling and culture were performed and identified through multiple detections, including plates and 16S rRNA sequencing. The aseptic rate, survival, and developmental indexes of GF zebrafish were recorded to ensure the quality and quantity of the generated models. Importantly, this study provides details on bacterial isolation and infection techniques for GF fish, enabling the efficient creation of GF fish models from larvae to juvenile stages with GF food support. By applying these procedures in biomedical research, scientists can better understand the relationships between intestinal bacterial functions and host health.

Introduction

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 individuals^1,2,3. 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, etc^5,6. 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's, Alzheimer's), aging, and various cancers^7,8,9. 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 models^10,11.

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 activity^12,13. 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-19^th century and primarily developed during the 20^th century. Subsequent refinements, including antibiotic-treated and gnotobiotic models, along with advancements in microbial detection and observation technologies, further perfected these models^14,15,16. 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 subjects^18,19,20,21. This approach has enabled investigations into the potential therapeutic effects of commensal microbiomes on various diseases, including cancer immunotherapy in humans^16,18. 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 diseases^22,23. 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.'s protocol in 2017^16,24,25. 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 embryos^19,26. 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 (>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 experiments^25,30,31. It'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 host^19,25, 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 juveniles^32,33. 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 methods^16,24,30, 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.

Protocol

The fish experiments were conducted in accordance with the guidelines of the Animal Care and Use Committee of Chongqing and the Institutional Animal Care and Use Committee of Chongqing Medical University, China, as well as the standards for experimental animals issued by the State Bureau of Quality and Technical Supervision (Approval ID: GB14922-2001 to GBT14927-2001). Zebrafish (Danio rerio, wild type, AB strain) were sourced from the Institute of Hydrobiology, Chinese Academy of Sciences, and maintained in the…

Representative Results

The GF zebrafish models can be efficiently produced by utilizing the abundant eggs spawned by pairs of zebrafish, with the protocol optimized based on previous GF fish models35. A single 6-well plate can culture approximately 30-48 embryos/larvae, allowing for ample data collection and statistical analysis. After sterile treatment, the GF embryos are cultured in a clean incubator till hatching to larvae at 48-72 h, and changed GZM daily with the detection of collected samples, which is crucial to …

Discussion

Critical steps within the protocols of GF fish and GF food preparation
During the generation of GF fish models, several critical steps were involved, including the preparation of sterile materials, sterilization of embryos, daily renewal of GZM, collection of various samples, and the sterile examination of each sample using multiple methods. Among these steps, the initial treatment of embryos is fundamental and decisive for the success of GF models. Controlling agents, their concentrations, and tre…

Materials

AB-GZM	Amphotericin:Solarbio;  kanamycin:Solarbio; Ampicillin:Solarbio.	Amphotericin:CAS:1397-89-3; kanamycin:CAS: 25380-94-0; Ampicillin:CAS: 69-52-313.	49.6 mL GZM, 50 µL amphotericin stock solution (250 µg/mL), 25 µL kanamycin stock solution (10 mg/mL), and 250 µL ampicillin stock solution (20 mg/mL).
1.5 mL, 15 mL, 50 mL EP tubes	biosharp	BS-15-M	To collect samples, and hold agents
2.4 g/L NaClO	XILONG SCIENTIFIC Co., Ltd.	CAS: 7681-52-9	Diluted with 8% sodium hypochlorite aqueous solution.
6-well plates, 24-, 48- well plates	LABSELECT	11112	To culture fish
Aeronomas	NCBI database	No.MK178499	2019-JPP-ESN
Anaerobic TSA plates	tryptone:Oxoid ; soy peptone:Solarbio ;NaCl:Biosharp; agar powder:BioFroxx.	tryptone:LP0042B; soy peptone:Cat#S9500; NaCl:BS112; agar powder:9002-18-0.	The TSA plates were prepared with 400 mL medium containing 6 g tryptone, 2 g soy peptone, 2 g NaCl, and 6 g agar powder under the anaerobic system.
Anaerobic work station	GENE SCIENCE	E200G	Bacterial isolation, sterile testing
Analysis	GraphPad Prism 5	v6.07	To analysis the data
API 20 E kits	BioMerieux SA, France	No.1005915090	Ref 20100 Kits to detect bacterial metabolism
Artemia (Brine shrimp)	Shangjia Aquarium Co., Ltd.	Aquamaster brand	Artemia cysts, and brine shrimp eggs
Auto cycle system for fish culture	Ningbo Hairui Technology Co., Ltd	No Cat	Maintain the fish
Autoclave	Zeal Way	G154DWS	Prepare the materials
BHI Aerobic	Coolaber	Cat#PM0640	BHI medium was prepared, wherein 100 mL medium included 3.7 g BHI powder.
BHI Anaerobic	Coolaber	Cat#PM0640	BHI medium was prepared and divided into anaerobic tubes under the anaerobic system.
Biochemical incubator	LongYue Co., Ltd	SPX	For fish and plates
Biosafety cabinet	Haier	HR40-IIA2	Sterile treatment and testing
Bleaching agent of 0.02 g/L NaClO	XILONG SCIENTIFIC Co., Ltd.	CAS: 7681-52-9	Working solution with sodium hypochlorite (NaClO) concentration: Diluted with 8% sodium hypochlorite aqueous solution or 166.6 uL 6% sodium hypochlorite with 500 mL distilled water.
Blood plates	sheep blood:Solarbio	Cat. NO. TX0030	Sterile-defibrinated sheep blood was added into TSA to prepare 5% blood plates.
Cell culture flask	Corning	430639	To culture fish
CM-Dil dyes	Molecular Probes	Cat#C7000	To label the bacteria
Constant temperature shaking incubator	Peiving Co., Ltd	HZQ-X100	Bacterial culture
Database	NCBI	Bacteria and Archaea database	Link: Archaea FTP: ftp://ftp.ncbi.nlm.nih.gov/refseq/TargetedLoci/Archaea/ Bacteria FTP: ftp://ftp.ncbi.nlm.nih.gov/refseq/TargetedLoci/Bacteria/
Disposable Pasteur pipette	biosharp	bs-xh-03l	Used to change water, and transfer eggs
Disposable petri dish	biosharp	BS-90-D	To culture fish
DNA kits	Solaribio	Cat#D1600	Bacterial genomic DNA extraction kits
Electric pipette	SCILOGEX	Levo me	Change water
Exiguobacterium	NCBI database	No.MK178504	2019-JPP-ESN
GZM	Sea salt:LANDEBAO Co., Ltd.	No Cat	Composed of 1 L of water and 1.5 mL of sea salt solution (40 g/L), autoclaved. The content of sea salt in the GZM solution was 60 mg/L.
Laboratory pure water system	Hitech Co., Ltd	Prima-S15	Prepare the agents
Microscope	Nikon	SMZ18	With fluorescent light to observe fish larvae
PCR kits	TIANGEN	Cat#ET101	Taq DNA Polymerase kit
Pipette	LABSELECT	sp-013-10	Change water
Povidone iodine (PVP-I)	Aladdin	Lot#H1217005	Aqueous solution povidone iodine 0.4 g/L pure water.
Timing converter	PinYi Co., Ltd	AL-06	To regulate the light
TSA plates	tryptone:Oxoid ; soy peptone:Solarbio ;NaCl:Biosharp; agar powder:BioFroxx.	tryptone:LP0042B; soy peptone:Cat#S9500; NaCl:BS112; agar powder:9002-18-0.	TSA plates were prepared with 400 mL medium containing 6 g tryptone, 2 g soy peptone, 2 g NaCl, 6 g agar powder.
TSB Aerobic	tryptone:Oxoid ; soy peptone:Solarbio ;NaCl:Biosharp;	tryptone:LP0042B; soy peptone:Cat#S9500; NaCl:BS112;	TSB medium was prepared, wherein 400 mL medium included 6 g tryptone, 2 g soy peptone, and 2 g NaCl.
TSB Anaerobic	tryptone:Oxoid ; soy peptone:Solarbio ;NaCl:Biosharp;	tryptone:LP0042B; soy peptone:Cat#S9500; NaCl:BS112;	TSB medium was prepared and divided into the anaerobic tubes under the anaerobic system.
Ultra-clean workbench	Airtech	SW-CJ-2FD	Sterile treatment and testing
Ultra-pure flow system for fish culture	Marine Biological Equipment company	No Cat	Produce water for fish
Vibrio	NCBI database	No.MK178501	2019-JPP-ESN