Bacteriophage adhering to mucus provide protection against Aeromonas veronii infection in scaleless fish

Yellow catfish (Pelteobagrus fulvidraco) is an economically important freshwater cultured fish in China. As a typical scaleless bony fish, the skin mucus plays a critical role in protecting against water-borne microbes. The major challenges in yellow catfish farming are motile Aeromonas septicemia (MAS) and skin ulcerative syndrome, resulting in significant financial losses. In this study, we isolated and identified Aeromonas veronii ZYAH128 as the pathogen responsible for the mass mortality of yellow catfish. To control this pathogen, we explored the use of phages as an alternative approach. Thirty-three phage strains were identified and evaluated for their ability to lyse A. veronii ZYAH128 in vitro. Thereafter, 11 phages were assessed in controlling the infection of A. veronii in yellow catfish via immersion. The two phages, ZPAV-18 and ZPAV-25 provided outstanding prophylactic rates of 85.7% and 71.4%, therapeutic rates of 85.7% and 85.7%, respectively. Phage was able to sustain in yellow catfish mucus for 7 d in flowing water, and the presence of fish mucus increased the susceptibility of A. veronii to phage infection. Our findings supported the symbiotic relationship where phages inhabit mucosal surfaces and provide external immunity against bacterial infection. Overall, this study provides valuable insights into the potential of phage therapy for combating bacterial infections in scaleless fish.