A simplified synthetic microbial community enhances resistance of crucian carp (Carassius auratus) to Aeromonas hydrophila infection through host immune activation

Bacterial diseases represent a major bottleneck in the sustainable development of aquaculture. The gut microbiota plays a vital role in host growth and health, including the enhancement of disease resistance. Although substantial progress has been made in elucidating the mechanisms of disease resistance in fish, the precise role of the gut microbiota in enhancing pathogen resistance in aquatic animals remains poorly understood. In this study, crucian carp (<i>Carassius auratus</i>) were used as a model to investigate the role of intestinal microbiota in modulating resistance to <i>Aeromonas hydrophila</i>. Individual crucian carp exhibited distinct clinical phenotypes following <i>A</i>. <i>hydrophila</i> infection. Specifically, significant differences were observed in the composition of the intestinal microbiota between fish displaying mild symptoms and those exhibiting severe phenotypic manifestations (α diversity: <i>p</i> < 0.01; β diversity: <i>p</i> = 0.001). Fecal microbiota transplantation (FMT) experiments demonstrated that fish with mild symptoms conferred enhanced resistance to <i>A. hydrophila</i> when their intestinal contents were transplanted into other individuals (<i>p</i> = 0.006). Further microbial analysis identified <i>Cetobacterium</i> (<i>p</i> = 0.013), <i>Paraclostridium</i> (<i>p</i> < 0.01), and <i>Pseudomonas</i> (<i>p</i> < 0.01) as key differential taxa. A simplified microbial community comprising these three strains was subsequently constructed. Feeding experiments confirmed that administration of this community significantly improved host resistance to <i>A. hydrophila</i> (<i>p</i> < 0.05) by activating intestinal immune responses and reinforcing the gut barrier. Overall, our findings underscore the potential of the microbial community as a novel strategy for disease prevention and control in aquaculture, providing a theoretical foundation for the development of microbiome-based therapies in fish health management.