In vivo imaging to trace the dissemination of Aeromonas hydrophila in common carp (Cyprinus carpio) after intestinal infection.

Aeromonas hydrophila (A. hydrophila) is a pathogenic bacterium that often causes serious economic losses to aquaculture and fisheries. Its infection and pathogenesis of fish are currently not fully understood. In this study, A. hydrophila was labeled with Fluorescein Isothiocyanate (FITC) and administered to common carp via gavage to trace its dissemination in common carp (Cyprinus carpio) after intestinal infection by in vivo imaging system. The optimal FITC concentration for labeling was determined to be 30 μg/mL, with a labeling time of 3 h at a bacterial concentration of 108 CFU/mL, with no significant impact on bacterial viability. The fluorescence imaging results of IVIS showed that FITC-Ah entered the internal organs through the carp intestine, first entering the kidneys and hepatopancreas (0.5 h after oral inoculation), and then the brain (around 10 h). Bacterial clearance occurred first in the hepatopancreas, followed by the brain, with the slowest clearance in the kidneys. Plate culture, 16S rRNA sequencing, and comparison of clinical symptoms confirmed the reliability of the tracing results. At the molecular level, interleukin-6 (il-6) and interleukin-8 (il-8) showed an upregulation trend, while the expression of interleukin-10 (il-10) initially decreased and then increased in the kidneys, hepatopancreas, and spleen of common carp after oral infection. The results revealed that A. hydrophila can not only infect fish through the intestine, but also mainly invade the hepatopancreas, kidneys, and brain after entering the fish's body via the intestine. Furthermore, the clearance kinetics of A. hydrophila in the internal organs of fish show that it is first cleared from the hepatopancreas, followed by the brain, with the slowest clearance occurring in the kidneys.