RESUMEN
Bacteriophages infecting Edwardsiella ictaluri have been less investigated, although the host bacterium is one of the most important fish pathogens causing enteric septicemia of catfish (ESC). We present here two distinctly novel bacteriophages vB_EiM_PVN06 and vB_EiA_PVN09 infecting Edwardsiella ictaluri E1, with their geographical origins from the Mekong Delta, Vietnam. Bacteriophage vB_EiM_PVN06 native to a mud sample reveals complete differences of biological properties with the phage vB_EiA_PVN09 originated from a viscus of a healthy catfish (Pangasianodon hypophthalmus) cultured in the same area. Morphological analyses combined with genomic data indicate that phage vB_EiM_PVN06 is classified to Myoviridae family and shares high similarity with E. ictaluri phage PEi21 genome, while vB_EiA_PVN09 is a member of Teseptimavirus genus, Autographiviridae family, and mostly closes to phage vB_EcoP_IME390. The vB_EiA_PVN09 is a T7-like bacteriophage, which has been firstly found infecting to E. ictaluri, and host range analysis also evidences for the cross-infection of this phage to Escherichia coli K12 and Escherichia coli DH5α. Together, our research highlights the diversity of bacteriophages infecting the pathogen E. ictaluri and suggests further explorations of lytic phages in environmental niches, to be exploited in feasible strategies of phage therapy in ESC disease control.
RESUMEN
Haemorrhagic septicaemia caused by Aeromonas hydrophila in striped catfish (Pangasianodon hypophthalmus) is one of the most important aquatic diseases in the Mekong Delta, Vietnam. However, antibiotic-resistant A. hydrophila strains have become popular and resulted in inadequate control of the disease in striped catfish farms. This study investigates the protective efficacy of bacteriophage PVN02 against haemorrhagic septicaemia in striped catfish via oral administration. The phage-containing pellets were prepared by spraying the phage solution on food pellets at 20 ml/kg. The rate of phage desorption from the food pellets into the water was very low; the phage titres in the water were approximately log 1.0 PFU/ml or undetectable. The in vivo experiment evaluating the protective efficacy of PVN02 against haemorrhagic septicaemia in striped catfish was conducted using 21 groups of 1,260 fish in 50-L plastic tanks in triplicate. The catfish were fed twice daily with phage-sprayed pellets. Different densities of bacterial suspensions were added into the tanks for 24 hr. Without the existence of the phage, the highest mortality rate was 68.3 ± 2.9% at the highest density of bacterial suspension. In contrast, the mortality rate at the highest density of bacterial suspension was significantly reduced to 8.33 ± 2.9% or 16.67 ± 2.9% at the phage dose of log 6.2 ± 0.09 or log 4.2 ± 0.09 PFU/g. This study provides a very practical manner of applying phage therapy to prevent disease in large-scale striped catfish farms.
