The potential novel bacteriophages against multidrug-resistant Edwardsiella tarda isolates from Edwardsiellosis striped catfish

The overuse of antibiotics in the aquaculture sector to prevent and treat the pathogen Edwardsiella tarda in striped catfish Pangasianodon hypophthalmus has led to the rapid emergence of multidrug-resistant (MDR) E. tarda isolates. Alternative strategies, such as phage therapy have been considered to control this MDR bacteria pathogen. We present here three lytic bacteriophages vB_EtA_WAG25P1, vB_EtA_WCT72P1, and vB_EtA_DT115P1 infecting E. tarda isolates, with their geographical origins from Mekong Delta, Vietnam. Morphological analyses combined with their genomic data indicate that the bacteriophages are classified to Autosignataviridae family in Autographivirales order. In the latent period of 35 minutes, the phage WCT72P1 and WAG25P1 had the burst size of about 45 virions and 61 virions per infected cell, respectively, while 51 virions were released in the shorter latent period of 25 minutes for the phage DT115P1. They also owned several prominent biological properties as the high species-specificity of infection, the effective in vitro repression of the MDR E. tarda growth for approximately four hours, pH resistance, and thermal stability. The phage WAG25P1 and DT115P1 exhibited the highest nucleotide identity to Klebsiella phage RCIP0053 with the sequence identity of 67.4% and 68.1%, respectively, while the phage WCT72P1 was closely related to Klebsiella phage RCIP0089 with the similarity of 66.6%. The phages were in a cluster of new species belonging to the new unassigned genus. The genome analysis also showed their safety due to the absence of the recombination, integration and host virulence genes. To the best of our knowledge, this is the first report of novel lytic phages infecting E. tarda isolates in striped catfish Pangasianodon hypophthalmus and it will also pave way for their potential applications for phage biocontrol of the disease in striped catfish.