Profiling Branchial Bacteria of Atlantic Salmon (Salmo salar L.) Following Exposure to Antimicrobial Agents

Joel Slinger,Mark B Adams,James W Wynne

doi:10.3389/fanim.2021.756101

Abstract

Microbial gill diseases caused by either opportunistic or specific pathogens are an emerging area of concern for aquaculture producers in part due to their sometimes complex and/or cryptic nature. Many antimicrobial treatments used in aquacultural settings are broad spectrum in nature. The effect of such therapeutics upon reduction and recolonization of commensal or pathogenic microbiota post-treatment has received little attention to date. Commensal bacteria are an integral component of the barrier function of mucosal surfaces in animals. This study evaluated the effect of several commercially relevant antimicrobial treatments upon the diversity and composition of branchial bacteria of Atlantic salmon. Here we exposed Atlantic salmon smolt to a number of commercially relevant antimicrobial treatments including chemotherapeutants (chloramine-t and hydrogen peroxide) and antibiotics (oxytetracycline and florfenicol) in vivo. Subsequently we examined the change in bacterial load, 16S rRNA gene expression, and taxonomic diversity post-treatment upon the gills. Results revealed a decrease in cultivable bacterial colonies after antimicrobial treatment, and a downstream decrease in bacterial richness and abundance post-treatment, with colonization of several prominent pathogenic taxa including Vibrio and Tenacibaculum. Temporal tracing over a 14-day period demonstrated that the bacteriome of gill mucus is sensitive to change, and altered by antimicrobial treatment and handling. This study identified candidate antimicrobial treatments which could be implemented in future studies to illustrate the effect of dysbiosis on microbial gill diseases.

Highlights

Global finfish aquaculture continues to increase rapidly to meet market demands and the need for a sustainable, high yield protein source for a burgeoning global population
Several known antimicrobial treatments administered were in line with FDA recommended delivery and dosage
The bacterial community upon gill surfaces provide protection to the host by maintaining a diverse range of taxa which deter against pathogenic opportunist microbes

Summary

Introduction

Global finfish aquaculture continues to increase rapidly to meet market demands and the need for a sustainable, high yield protein source for a burgeoning global population. Mucosal gill surfaces can be a portal for pathogens to colonize and infiltrate leading to localized or systemic disease, and compromise to the gill can impact upon physiological processes due to the multifunctional nature of the organ. Diseases and disorders of the gill are often multifactorial and complex in nature (Mitchell and Rodger, 2011). Some gill conditions such as complex gill disease (CGD) have multiple known aetiological agents (e.g., Neoparamoeba perurans, Candidatus Piscichlamydia salmonis, Desmozoon lepeophtherii, salmon gill poxvirus and Candidatus Branchiomonas cysticola) (Gjessing et al, 2019) which interact in a co-infection. Other presumed single-agent gill conditions such as columnaris disease (Flavobacterium columnare) and yellow mouth (Tenacibaculum maritimum) have strong environmental influences, including temperature and salinity, which can affect infection severity (Bandilla et al, 2006; Wynne et al, 2020)

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