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

This study aims to investigate the differences in microbial community structure between fast-growing (FG) and slow-growing (SG) short-finned eels, <i>Anguilla bicolor pacifica</i>, using high-throughput 16S rDNA sequencing, and to evaluate the potential probiotic properties of <i>Bacillus tropicus</i> 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 <i>Cetobacterium</i>, <i>Clostridium</i>, and <i>Bacteroides</i> were predominant in both groups, with <i>Edwardsiella</i>, <i>Aeromonas</i>, and <i>Fusobacterium</i> 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 <i>Edwardsiella tarda</i> and <i>Aeromonas jandaei</i>. In contrast, FG eels exhibited a greater abundance of the potential probiotic <i>B. tropicus</i>. 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 <i>B. tropicus</i>. A virulence assessment confirmed the non-pathogenic nature of <i>B. tropicus</i> FG2, supporting its probiotic potential. Furthermore, feeding eels a diet supplemented with <i>B. tropicus</i> FG2 significantly enhanced growth performance, as evidenced by increased final weight percentages of weight gain and total production per tank (<i>p</i> < 0.05), while the proximate composition of the dorsal muscle showed an increase in lipid content (<i>p</i> < 0.05). These findings highlight <i>B. tropicus</i> FG2 as a promising probiotic for aquaculture applications.