LAUSR

Abstract The largemouth bass (Micropterus salmoides) is an economically important fish species, but disease caused by Aeromonas hydrophila has led to the high mortality of this fish in farming environments. However, few studies have focused on the differences in the responses of largemouth bass to infection and immunization with A. hydrophila. In the present study, we found high fish mortality after A. hydrophila infection but not after immunization. Then, the immune response difference of largemouth bass in early infection and immunization with A. hydrophila was analyzed using transcriptome profiling and 16S rRNA sequencing. The results from the transcriptome analysis revealed that many upregulated genes were enriched in metabolic pathways after infection and immunization, but the number of upregulated genes associated with metabolic pathways and the magnitude of their upregulated expression in the infection group were much greater than they were in the immunization group, suggesting that A. hydrophila infection might cause metabolic disorders in largemouth bass. Increases in pro-inflammatory cytokine levels and reduced expression of genes associated with adaptive immunity were also observed in the infection group. Moreover, the results of the 16S rRNA analysis revealed that the intestinal microbiota homeostasis of the largemouth bass was severely disrupted by A. hydrophila infection but not by immunization. These results collectively demonstrated that metabolic disorders, higher inflammation, a reduced adaptive immune response and disrupted microbiota homeostasis in early infection might be important factors for the high mortality of largemouth bass caused by A. hydrophila infection.