Isolation and Complete Genome Sequencing of NS-I, a Lytic Bacteriophage Infecting Fish Pathogenic Strains of Nocardia seriolae.

Background: Nocardiosis, an infection caused by bacterium Nocardia seriolae, contributes to severe economic losses in the aqua-cultured fish species Seriola quinqueradiata and Seriola dumerili in Japan. Phage therapy is a potential solution for combating this infection.

Materials and methods: A lytic bacteriophage infecting N. seriolae (JCM3359) was isolated from water samples collected at various fish farms. The bacteriophage was characterized by electron microscopy, host range analysis, and genomic profiling.

Results: The isolated lytic bacteriophage, NS-I (accession no. OP429095.1), had a siphovirus morphotype with a broad lytic spectrum, as it infected all the tested N. seriolae strains (15/15). Its genome was a 43,361 bp double-stranded DNA molecule, exhibiting a GC content of 67% with 94% similarity to Gordonia bacteriophage Gustav at the genome level.

Conclusion: Lytic bacteriophage NS-I shows promise as a biocontrol agent for nocardiosis infection in fish farms and warrants further investigation.