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

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.

Signatures of dysbiosis in fish microbiomes in the context of aquaculture

“…A similar pattern was observed on gut microbiota when antimicrobials were delivered through bath ( n = 13), although the number of case studies reporting a decrease in alpha diversity ( n = 8) exceeded those that did not ( n = 5, Table 1). Most studies on external fish mucosae did not report a decrease in alpha diversity ( n = 9), regardless of delivery mode 53,54 (Figure 1a). Complex responses to antibiotics by mice‐associated microbiota, which do not necessarily entail a linear decrease in alpha diversity, demonstrated that microbial changes may be drug specific and also related to diet 119 .…”

“…Notably, Rosado et al 53 reported that neither diversity nor structure of microbiota on external mucosa of the seabass were fully recovered after an 8‐day OTC treatment and a 3‐week recovery period, although the skin microbiota of the seabass was more resilient to disturbance caused by OTC than the gill. Microbial function was not investigated in studies on external fish mucosae, and changes to P:B and P:F ratios only clearly occurred in one study 54 …”

“…In seven of those studies, the disinfectants were delivered by bath and only three studies focused on gut microbiota (Table 1). The effects of triclosan and chloramine‐T were evaluated in two case studies each, 54,55,69,70 and the effects of hydrogen peroxide, potassium permanganate, Praziquantel and copper sulphate were also the focus of one study each 54,71–73 …”

“…Florfenicol was the second most studied antibiotic in fish microbiome literature following tetracyclines, with a total of six studies. However, only one study evaluated its effects on external fish mucosae (salmon gill, 54 ) (Table 1). Most studies focused on commercially valuable and aquaculture‐relevant species (salmon, channel catfish ( Ictalurus punctatus [Rafinesque, 1818]) and the small‐scaled pacu ( Piaractus mesopotamicus [Holmberg, 1887]) and antibiotics predominantly delivered through feed.…”

“…Few studies examined the effects of tetracyclines on fish external mucosae ( n = 4; Table 1). Specifically, the effects of OTC were investigated on the skin and gill of diseased European seabass 53 and the gill of the Atlantic salmon ( Salmo salar Linnaeus, 1758) 54,55 . In these studies, therapeutic doses of OTC were delivered through feed for 8 days in the case of the European seabass and for 10 and 21 days in the case of salmon.…”