Evaluating Bacteriophage Impact on <i>Vibrio</i> Composition in the Gut of Broad‐Nosed Pipefish ( <i>Syngnathus typhle</i> )

Bacteriophages play a crucial role in shaping microbial community dynamics in marine systems and have the potential to stimulate surges in pathogenic bacteria, facilitating disease outbreaks. Notwithstanding, bacteriophages also serve as valuable biocontrol agents, underscoring their huge potential for aquaculture therapy treatments. Empirical insights into the intricate tripartite interplay upon exposure to a virulent bacterium, its specific phages, and the host gut microbiome could improve our understanding of how bacteria-phage interactions behave in a natural microbial system. This investigation assessed the influence of a virulent Vibrio alginolyticus (K01M1) infection, in tandem with lytic (фSt2) and filamentous (фK04M1) Vibrio-specific phages, on the broad-nosed pipefish (Syngnathus typhle) gut microbiome using 16S rRNA amplicon sequencing and non-intrusive gastric swabbing. The pipefish microbiome structure was not impacted by Vibrio and phage introductions, while the different infection regimes shaped Vibrio-specific dynamics. In the filamentous phage and Vibrio-only treatments, V. alginolyticus abundances spiked 12 h post-ingestion. In contrast, V. alginolyticus numbers in the lytic phage and control treatment were significantly reduced, suggesting phage activity and specific elimination of the introduced bacteria. Assisted by relative true-gut tissue samples, a newly implemented non-intrusive swabbing method was successful at discerning the activity of two contrasting phages and supports previous work that encourages the use of фSt2 in bacteriophage treatments. Identifying Vibrio-specific phages with similar positive characteristics could be beneficial for the aquaculture trade, which is currently heavily impacted by the antibiotic crisis.