# Valp1, a Newly Identified Temperate Phage Facilitating Coexistence of Lysogenic and Non-Lysogenic Populations of Vibrio anguillarum

## TL;DR

A new temperate phage, Valp1, helps maintain a balance between lysogenic and non-lysogenic populations of the fish pathogen Vibrio anguillarum.

## Contribution

Valp1 is a newly identified phage that enables coexistence of different V. anguillarum populations through a stable equilibrium mechanism.

## Key findings

- Valp1 inhibits the growth of the virulent V. anguillarum strain PF4 but not its lysogenic derivative P1.1.
- Co-cultures of PF4 and P1.1 with Valp1 maintain a stable population balance for 10 hours and across five subcultures.
- Non-lysogenic PF4 strains become resistant to Valp1, while the phage reaches high concentrations (~108 PFU/mL).

## Abstract

Vibrio anguillarum is a pathogen for several fish and shellfish species. Its ecology is influenced by diverse factors, including bacteriophages. Here, we identify and characterize a new temperate bacteriophage (Valp1) of V. anguillarum. Valp1 is a myovirus with a 60 nm head and a 90 nm contractile tail. Its double-stranded DNA genome of 42,988 bp contains 68 genes, including a protelomerase gene, typical of telomeric phages. Valp1 inhibits the growth of the virulent strain of V. anguillarum PF4, while the derived lysogenic strain P1.1 presents a slight reduction in its growth but is not affected by the presence of Valp1. Both strains present similar virulence in a larval zebrafish (Danio rerio) model, and only slight differences have been observed in their biochemical profile. Co-culture assays reveal that PF4 and P1.1 can coexist for 10 h in the presence of naturally induced Valp1, with the proportion of PF4 ranging between 28% and 1.6%. By the end of the assay, the phage reached a concentration of ~108 PFU/mL, and all the non-lysogenic PF4 strains were resistant to Valp1. This equilibrium was maintained even after five successive subcultures, suggesting the existence of a coexistence mechanism between the lysogenic and non-lysogenic populations of V. anguillarum in conjunction with the phage Valp1.