Abstract
Cunner (Tautogolabrus adspersus) is a cleaner fish being considered for utilized in the North Atlantic salmon (Salmo salar) aquaculture industry to biocontrol sea lice infestations. However, bacterial diseases due to natural infections in wild cunners have yet to be described. This study reports the isolation of Pseudomonas sp. J380 from infected wild cunners and its phenotypic, genomic, and transcriptomic characterization. This Gram-negative motile rod-shaped bacterium showed a mesophilic (4–28 °C) and halotolerant growth. Under iron-limited conditions, Pseudomonas sp. J380 produced pyoverdine-type fluorescent siderophore. Koch’s postulates were verified in wild cunners by intraperitoneally (i.p.) injecting Pseudomonas sp. J380 at 4 × 103, 4 × 105, and 4 × 107 colony forming units (CFU)/dose. Host-range and comparative virulence were also investigated in lumpfish and Atlantic salmon i.p. injected with ~106 CFU/dose. Lumpfish were more susceptible compared to cunners, and Atlantic salmon was resistant to Pseudomonas sp. J380 infection. Cunner tissues were heavily colonized by Pseudomonas sp. J380 compared to lumpfish and Atlantic salmon suggesting that it might be an opportunistic pathogen in cunners. The genome of Pseudomonas sp. J380 was 6.26 megabases (Mb) with a guanine-cytosine (GC) content of 59.7%. Biochemical profiles, as well as comparative and phylogenomic analyses, suggested that Pseudomonas sp. J380 belongs to the P. fluorescens species complex. Transcriptome profiling under iron-limited vs. iron-enriched conditions identified 1159 differentially expressed genes (DEGs). Cellular metabolic processes, such as ribosomal and energy production, and protein synthesis, were impeded by iron limitation. In contrast, genes involved in environmental adaptation mechanisms including two-component systems, histidine catabolism, and redox balance were transcriptionally up-regulated. Furthermore, iron limitation triggered the differential expression of genes encoding proteins associated with iron homeostasis. As the first report on a bacterial infection in cunners, the current study provides an overview of a new marine pathogen, Pseudomonas sp. J380.
Highlights
The ectoparasitic infestation by sea louse (e.g., Lepeophtheirus salmonis) is the most serious threat to both wild and cage-cultured salmonids (e.g., Salmo salar) in the Northern hemisphere [1]
Pseudomonas sp. strain J380 was isolated from the head kidney, liver, and spleen of infected wild cunners captured at Conception Bay, Newfoundland and Labrador (NL), Canada (Figure S1A,B)
J380 was isolated from the head kidney, liver, and spleen of infected wild cunners, and we found that all the strains isolated showed identical characteristics (Table 1)
Summary
The ectoparasitic infestation by sea louse (e.g., Lepeophtheirus salmonis) is the most serious threat to both wild and cage-cultured salmonids (e.g., Salmo salar) in the Northern hemisphere [1]. Sea lice feed on salmonid tissues leading to skin erosion, osmoregulatory failure, immunosuppression, increased disease susceptibility and, 4.0/). Microorganisms 2021, 9, 812 death [1,2]. Sea lice escalate the economic burden on salmon farmers both by causing mortalities and demanding expensive control regimes [3]. The Atlantic salmon industry has used several methods to combat sea lice infestation. Chemotherapeutic treatment was the dominant sea lice control method for decades until the recent emergence of resistance in sea lice to several active components of drugs [4] and increasing negative public opinion about the impacts of anti-lice drugs on ecological equilibrium. Mechanical abrasion and thermal treatment are routinely used as physical delousing methods, but have adverse effects on salmon health and welfare [5]
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