Phages infecting hypervirulent Aeromonas hydrophila (vAh) had not been isolated anywhere in the world ; however, phages have been isolated against motile Aeromonas septicemia (MAS) with only few of them having been characterized. This study aimed at developing therapeutic phage against hypervirulent Aeromonas hydrophila infection in aquaculture systems.
A total of 110 water samples were collected aseptically. A novel phage AhFM11 specific to hypervirulent A. hydrophila was isolated and shows lytic activity against reference A. hydrophila (ATCC 35654). Soft agar overlay method was used to determine titer and found to be 1.58 x 10 10 pfu/mL . Host range of the AhFM11 phage was performed for 131 Aeromonas spp. and 10 non Aeromonas. The results indicated that AhFM11 had a broad host range, infecting 65 Aeromonas species. Also, it was found that this phage did not harbor any antibiotic resistance genes. This is the first report of phage against hypervirulent A. hydrophila.
Transmission electron microscopy (TEM) revealed that phage AhFM11 belongs to the family of Myoviridae (Figure 1). O ne-step growth curve of AhFM11 shown that the phage has an average burst size of 276 ± 15 PFU per infected cell, adsorption rate of 97.3% and found to be stable in different environmental conditions. The AhFM11 genome comprised of 176,963 bp with an average G/ C content of 41.5% (NCBI accession No. MZ450807.1). The novel bacteriophage-based strategies therapeutic approach (injection, oral feed and immersion) to prevent and treat A. hydrophila infection were studied. Therapeutic application in injection, immersion and feed impregnated phages showed 100%, 95.11% and 93.11% survival than the challenged untreated fishes. These findings support that phage AhFM11 can be used to treat/control A. hydrophila infection in cultured fishes and has immense value as a potential alternative to antibiotics.