Characterization and protective effects of lytic bacteriophage pAh6.2TG against a pathogenic multidrug-resistant Aeromonas hydrophila in Nile tilapia (Oreochromis niloticus)

Bacteriophage (phage) is considered as one of the alternatives to antibiotics and an environmentally friendly approach to tackle antimicrobial resistance (AMR) in aqua- culture. Here, we reported isolation, morphology and genomic characterizations of a newly isolated lytic phage, designated pAh6.2TG. Host range and stability of pAh6.2TG in different environmental conditions, and protective efficacy against a pathogenic multidrug-resistant (MDR) Aeromonas hydrophila in Nile tilapia were subsequently evaluated. The results showed that pAh6.2TG is a member of the new family Chaseviri- dae which has genome size of 51,780 bp, encoding 65 putative open reading frames (ORFs) and is most closely related to Aeromonas phage PVN02 (99.33% nucleotide identity). The pAh6.2TG was highly specific to A. hydrophila and infected 83.3% tested strains of MDR A. hydrophila (10 out of 12) with relative stability at pH 7–9, tem- perature 0–40◦C and salinity 0–40 ppt. In experimental challenge, pAh6.2TG treat- ments significantly improved survivability of Nile tilapia exposed to a lethal dose of the pathogenic MDR A. hydrophila, with relative per cent survival (RPS) of 73.3% and 50% for phage multiplicity of infection (MOI) 1.0 and 0.1, respectively. Phage treat- ment significantly reduced the concentration of A. hydrophila in both water and fish body. Interestingly, the surviving fish from A. hydrophila challenged groups provoked specific antibody (IgM) against this bacterium. In summary, the findings suggested that the lytic phage pAh6.2TG is an effective alternative to antibiotics to control MDR A. hydrophila in tilapia and possibly other freshwater fish.