Bacteriophage (phage) is considered as one of the alternatives to antibiotics and an environmentally friendly approach to tackle antimicrobial resistance (AMR) in aquaculture. 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 Chaseviridae 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, temperature 0–40°C and salinity 0–40 ppt. In experimental challenge, pAh6.2TG treatments 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 treatment 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.

Motile Aeromonas septicemia (MAS), a disease caused by Aeromonas spp., is recognized as a major disease in freshwater aquaculture. This study aimed to investigate the distribution and diversity of Aeromonas spp. and their antimicrobial susceptibility patterns. A total of 86 isolates of Aeromonas spp. were recovered from diseased freshwater fishes from 13 farms in Thailand. All isolates were identified using biochemical characteristics, matrix‐assisted laser desorption ionization‐time of flight mass spectrometry (MALDI‐TOF MS), polymerase chain reaction assays, and the gyrB gene sequence analysis. The result of MALDI‐TOF MS showed 100% (86 isolates) accuracy at genus‐level identification, and 88.4% (76 isolates) accuracy at species‐level identification. Six species of Aeromonas were confirmed through nucleotide sequencing and phylogenetic analysis of the gyrB gene Aeromonas veronii (72.1%), Aeromonas jandaei (11.6%), Aeromonas schubertii (9.3%), Aeromonas diversa (3.5%), Aeromonas hydrophila (2.3%), and Aeromonas punctata (1.2%). Antimicrobial susceptibility tests for all isolates revealed resistance against amoxicillin (99%), ampicillin (98%), oxolinic acid (81.4%), oxytetracycline (77%), trimethoprim‐sulfamethoxazole (24%), and enrofloxacin (21%). The multiple antibiotic resistance (MAR) index varied between 0.14 and 0.86, with MAR values more than 0.2 in 99% of isolates. Furthermore, four diverse multidrug‐resistant (MDR) patterns were found among Aeromonas isolates. Our finding show that A. veronii is the most abundant species in Thai cultured freshwater fish with the highest MDR patterns.

Bacteriophage is considered an alternative to antibiotics and environmentally friendly approach to tackle antimicrobial resistance (AMR) in aquaculture. Here, we reported isolation, morphology and genomic characterizations of a newly isolated lytic bacteriophage, 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 family Myoviridae 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, temperature 0 ­ 40 C and salinity 0 ­ 40 ppt. In experimental challenge, pAh6.2TG treatments significantly improved survivability of Nile tilapia exposed to a lethal dose of the pathogenic MDR A. hydrophila, with relative percent survival (RPS) of 73.3% and 50% for phage multiplicity of infection (MOI) 1.0 and 0.1, respectively. Significant reduction of bacterial counts in rearing water at 3 h (6.7 ± 0.5 to 18.1 ± 6.98 folds) and in fish liver at 48 h post-treatment (2.7 ± 0.24 to 34.08 ± 26.4 folds) was observed in phage treatment groups while opposite pattern for bacterial counts was observed in untreated control. Interestingly, the surviving fish provoked specific antibody (IgM) against the challenged A. hydrophila. These results might explain the higher survival in phage treatment groups. In summary, the findings suggested that the lytic bacteriophage pAh6.2TG is an effective alternative to antibiotics to control MDR A. hydrophila in tilapia and possibly other freshwater fish.