# Comparative Analysis of Gut Microbiota Between Fast-Growing and Slow-Growing Short-Finned Eels, Anguilla bicolor pacifica, and the Application of Bacillus tropicus FG2 as a Probiotic to Enhance Growth Performance of Eels

## TL;DR

This study compares gut bacteria in fast- and slow-growing eels and finds that a specific probiotic, Bacillus tropicus FG2, improves eel growth and health.

## Contribution

The study identifies Bacillus tropicus FG2 as a novel probiotic strain that enhances eel growth and is non-pathogenic.

## Key findings

- Fast-growing eels had higher abundance of B. tropicus, while slow-growing eels had more potential pathogens.
- Bacillus tropicus FG2 improved eel growth performance and increased lipid content in muscle.
- FG2 showed strong digestive enzyme activity and was confirmed non-pathogenic.

## Abstract

The gut microbiome strongly affects animal growth, physiology, and overall health. A balanced and diverse intestinal microbial community improves feed digestion, nutrient absorption, and gut barrier function, making microbiome stability a key factor in aquaculture performance. This study compares the intestinal microbiota of fast- and slow-growing eels to identify bacterial taxa linked to superior growth. Certain gut commensals may promote growth by enhancing digestion, producing enzymes, or regulating metabolic compounds. By isolating these beneficial strains, we aim to develop physiologically based probiotics for eel culture. The findings provide microbiome insights and functional strains that support evidence-based probiotic applications, offering practical value for improving eel health and farming efficiency.

This study aims to investigate the differences in microbial community structure between fast-growing (FG) and slow-growing (SG) short-finned eels, Anguilla bicolor pacifica, using high-throughput 16S rDNA sequencing, and to evaluate the potential probiotic properties of Bacillus tropicus isolated from eel intestinal microbiota to enhance growth performance. High-throughput 16S rDNA sequencing revealed no significant differences in the α-diversity between FG and SG eels. Bacterial genera such as Cetobacterium, Clostridium, and Bacteroides were predominant in both groups, with Edwardsiella, Aeromonas, and Fusobacterium being more abundant in SG eels, suggesting a higher presence of potential pathogens. The analysis of the relative abundance of gut microorganisms revealed that SG eels harbored higher levels of potentially pathogenic bacteria, including Edwardsiella tarda and Aeromonas jandaei. In contrast, FG eels exhibited a greater abundance of the potential probiotic B. tropicus. Six strains of bacteria with relative abundance were isolated from the FG group, displaying superior digestive enzyme activity, including protease, lipase, amylase, cellulase, xylanase, and phytase, particularly strain FG2. Phylogenetic analysis confirmed that FG2 was closely related to B. tropicus. A virulence assessment confirmed the non-pathogenic nature of B. tropicus FG2, supporting its probiotic potential. Furthermore, feeding eels a diet supplemented with B. tropicus FG2 significantly enhanced growth performance, as evidenced by increased final weight percentages of weight gain and total production per tank (p < 0.05), while the proximate composition of the dorsal muscle showed an increase in lipid content (p < 0.05). These findings highlight B. tropicus FG2 as a promising probiotic for aquaculture applications.