# Antimicrobial-producing bacteria from fish epidermal mucus alter the fish epidermal bacterial flora and host resistance to infection

## TL;DR

Bacteria from fish mucus that produce antimicrobials can protect fish from infections by altering skin microbiota and suppressing pathogens.

## Contribution

The study identifies Pseudomonas mosselii KH-ZF1 and its antimicrobial compound Fluviol C as a novel mechanism for fish disease control.

## Key findings

- Strain KH-ZF1 alters fish skin microbiota and improves survival against Yersinia ruckeri infection.
- Fluviol C, produced by KH-ZF1, inhibits multiple fish pathogens but is toxic to zebrafish at low concentrations.
- Sub-inhibitory levels of Fluviol C induce bacterial substitution in the epidermal microbiota.

## Abstract

The emergence of antimicrobial-resistant bacteria in aquaculture has raised the need for alternative strategies to control fish infections. Antimicrobial-producing bacteria have been explored as probiotics or biocontrol agents, but their mechanisms of action and impact on host-associated microbiota remain poorly understood. Here, we identified Pseudomonas mosselii KH-ZF1, a bacterium isolated from fish epidermal mucus, as a producer of antimicrobial substances. When applied to zebrafish, strain KH-ZF1 transiently adhered to the epidermal mucus and altered the composition of the skin microbiota. Under an appropriate administration condition, strain KH-ZF1 treatment significantly improved survival in zebrafish infected with Yersinia ruckeri and suppressed pathogen growth on the skin surface. However, in the absence of strain KH-ZF1 or inappropriate conditions, Y. ruckeri dominated the epidermal bacterial community. The antimicrobial compound produced by strain KH-ZF1 was identified as Fluviol C, a pigmented metabolite previously reported from Pseudomonas fluorescens. Fluviol C inhibited the growth of multiple fish pathogens at experimentally determined concentrations (0.5–32 µg/mL) but exhibited toxicity to zebrafish even below its minimum inhibitory concentration. Intriguingly, fluviol C, at sub-inhibitory levels, induced bacterial substitution in the epidermal microbiota, mimicking the effects of strain KH-ZF1. These findings demonstrate that strain KH-ZF1 alters host resistance to infection by promoting bacterial substitution on the fish skin by producing fluviol C. Our study highlights a microbiota-mediated mechanism by which antimicrobial-producing bacteria can control infection through the fish epidermis, suggesting a potential microbiota-mediated approach for disease control in aquaculture.

We show that bacteria producing antibacterial substances, isolated from fish skin mucus, can inhibit percutaneous infections in aquatic environments. These bacteria effectively altered the skin mucus bacterial flora and suppressed pathogen growth. Fish skin acts as a barrier against infections, with its microorganisms being considered to play a crucial role in prevention. Our study highlighted the potential use of these specific microorganisms in the fish skin mucus as a novel fish disease control strategy. By targeting fish skin mucus bacteria that produce antimicrobial substances, we could develop a new approach to managing diseases in aquaculture, such as probiotics for fish skin. This research underscores the importance of studying fish epidermal microorganisms for innovative disease management.