Characterization and application of a novel Aeromonas bacteriophage as treatment for pathogenic Aeromonas hydrophila infection in rainbow trout

Aeromonas hydrophila is an opportunistic pathogen responsible for motile Aeromonas septicemia in several fish species, including rainbow trout. With the emergence of antibiotic-resistant strains and safety concerns on fish products, alternative treatment strategies should be developed. A previously isolated virulent A. hydrophila phage, designated as MJG, meets the main prerequisites for a candidate therapeutic phage based on genomic characterization. In this study, the biological characteristics of MJG were further examined. Results showed that MJG had activity at a broad range of temperature (10 degrees C-60 degrees C) and pH (2 -10), and its latent and rise periods were 30 and 40 min, respectively. However, MJG exhibited a narrow host range and infected only 4 out of 20 A. hydrophila strains. Compared with PBS injection, MJG injection (3.2 x 10(6) PFU/fish) at 2 h post A. hydrophila infection reduced the bacterial numbers from 5.4 x 10(6) to 1.4 x 10(4) CFU/g in liver after 3 days post infection (dpi). The expression levels of pro-inflammatory cytokines (IL-8 and IL-1 beta) in the spleen of MJG-treated fish were significantly higher than those in PBS-treated fish at 1 or 2 dpi but significantly lower than those in PBS-treated fish at 3 dpi. MJG treatment would restore liver tissue damages and abolish the clinical signs of infection. MJG administered through injection or immersion completely protected the fish from death due to A. hydrophila infection. Meanwhile, oral feeding with MJG (3.2 x 10(7) PFU/g) could provide remarkable protection. Our work explores the potential use of phage as an alternative therapeutic agent against A. hydrophila infection in fish.