Aims This study describes the physicochemical and genomic characterization of phage vB_Vc_SrVc9 and its potential for phage therapy application against a pathogenic Vibrio campbellii strain. Methods and Results A lytic phage vB_Vc_SrVc9 against V. campbellii was isolated from shrimp farm sediment, and characterized physicochemical and genomically. The use of vB_Vc_SrVc9 phage increased the survival in brine shrimp Artemia franciscana and reduced presumptive V. campbellii to nondetectable numbers. Genomic analysis showed a genome with a single contig of 43·15 kb, with 49 predicted genes and no tRNAs, capable of recognizing and generating complete inhibition zones of three Vibrio sp. Conclusions To our knowledge vB_Vc_SrVc9 is a lytic phage that could be used against Vibrio infections, reducing vibrio presence without any apparent impact over the natural microbiota at the family level in 28 libraries tested. Significance and Impact of the Study vB_Vc_SrVC9 is a novel phage and ecofriendly alternative for therapeutic applications and biotechnological purposes because is stable at different environmental conditions, has the potential to eliminate several strains, and has a short latent period with a good burst size. Therefore, the use of phages, which are natural killers of bacteria, represents a promising strategy to reduce the mortality of farmed organisms caused by pathogenic bacteria.

Aims This study describes the effect of phage therapy on hatching of longfin yellowtail (Seriola rivoliana) eggs challenged with Photobacterium damselae subsp. damselae. Methods and results A lytic phage (vB_Pd_PDCC‐1) against P. damselae subsp. damselae was isolated and characterized. The use of phage vB_Pd_PDCC‐1 increased the hatching rate of eggs, and reduced presumptive Vibrio species to non‐detectable numbers, even in non‐disinfected eggs. High‐throughput 16S rRNA gene sequencing analysis revealed that phage vB_Pd_PDCC‐1 caused significant changes in the composition and structure of the associated microbiota, allowing that members (e.g. those belonging to the family Vibrionaceae) of the class Gammaproteobacteria to be displaced by members of the class Alphaproteobacteria. Conclusions To the best of our knowledge, this represents the first study evaluating phage therapy to control potential negative effects of P. damselae subsp. damselae during hatching of longfin yellowtail eggs. Significance and Impact of the Study The Seriola genus includes several important commercial fish species due to its rapid growth and easy adaptability to confinement conditions. However, bacterial infections (especially those caused by Vibrio and Photobacterium species) are among the main limiting factors for the intensification of marine fish aquaculture, particularly during early development stages. Therefore, the use of phages, which are natural killers of bacteria, represents a promising strategy to reduce the mortality of farmed organisms caused by pathogenic bacteria.

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