Protective effects of the Aeromonas phages pAh1-C and pAh6-C against mass mortality of the cyprinid loach (Misgurnus anguillicaudatus) caused by Aeromonas hydrophila

To investigate methods to control the mass mortality of cyprinid loaches (Misgurnus anguillicaudatus) caused by multiple-antibiotic-resistant Aeromonas hydrophila on a private fish farm in Korea, we isolated bacteriophages (phages), designated pAh1-C and pAh6-C, from the Han River in Seoul. The two isolated phages were morphologically classified as Myoviridae and showed similar infection patterns for A. hydrophila isolates. The two phages possessed approximately 55 kb (pAh1-C) and 58 kb (pAh6-C) of double-stranded genomic DNA, and their gDNAs showed different restriction endonuclease digestion patterns. Both phages showed efficient bacteriolytic activity against fish-pathogenic A. hydrophila from loaches. The latent periods of the phages were estimated to be approximately 30 min (pAh1-C) and 20 min (pAh6-C), while the burst sizes were 60 PFU/cell (pAh1-C) and 10 PFU/cell (pAh6-C). The phages proved to be efficient in the inhibition of bacterial growth, as demonstrated by their in vitro bactericidal effects. Additionally, a single administration of either phage to cyprinid loaches resulted in noticeable protective effects, with increased survival rates against A. hydrophila infection. These results suggest that the phages pAh1-C and pAh6-C constitute potential therapeutic agents for the treatment of A. hydrophila infection in fish.