Virulent bacteriophage of Edwardsiella ictaluri isolated from kidney and liver of striped catfish Pangasianodon hypophthalmus in Vietnam

Striped catfish Pangasianodon hypophthalmus farmed in the Mekong Delta, Vietnam, represents an important contribution to Vietnamese aquaculture exports. However, these fish are affected by frequent disease outbreaks across the entire region. One of the most common infections involves white spots in the internal organs, caused by the bacterium Edwardsiella ictaluri. In this study, a virulent phage specific to E. ictaluri, designated MK7, was isolated from striped catfish kidney and liver samples and characterized. Morphological analysis indicates probable placement in the family Myoviridae with a 65 nm icosahedral head and a 147 x 19 nm tail. A double-stranded DNA genome of approximately 34 kb was predicted by restriction fragment analysis following digestion with Smal. The adsorption affinity (k(a)) of the MK7 phage was estimated as 1.6 x 10(-8) ml CFU-1 min(-1), and according to a 1-step growth curve, its latent period and burst size were similar to 45 min and similar to 55 phage particles per infected host cell, respectively. Of the 17 bacterial strains tested, MK7 only infected E. ictaluri, although other species of Edwardsiella were not tested. E. ictaluri was also challenged in vitro, in both broth and water from a striped catfish pond and was inactivated by MK7 for 15 h in broth and 51 h in pond water. Thus, initial characterization of phage MK7 indicates its potential utility as a biotherapeutic agent against E. ictaluri infection in striped catfish. This is the first report of a lytic phage specific to an important striped catfish pathogen.