The use of salmonid epithelial cells to characterize the toxicity of Tenacibaculum maritimum soluble extracellular products

AIMS

This study assessed how the etiological agent of mouth rot in farmed Atlantic salmon, Tenacibaculum maritimum, induces toxicity in host salmonid barrier cells, and determined whether environmental changes are relevant for these effects.

METHODS AND RESULTS

Tenacibaculum maritimum soluble extracellular products (ECPs) were collected and used to treat Atlantic salmon and rainbow trout intestinal barrier cell lines as a comparative model of bacterial-salmonid cell interactions. Cellular assays that examine cell membrane integrity, marker expression, and metabolic activity revealed that T. maritimum ECPs induced salmonid epithelial cell death through an apoptosis mechanism. Changes in salinity (25, 29, and 33 ppt) and temperature (12°C, 18°C, and 24°C) within the natural ranges observed in Pacific Northwest aquaculture facilities affected bacterial growth and cytotoxicity of T. maritimum ECPs.

CONCLUSIONS

Our results suggest epithelial barriers as targets of T. maritimum-mediated toxicity in farmed mouth rot-infected Atlantic salmon. The induction of apoptosis by T. maritimum soluble ECPs may also help to explain the absence of overt inflammation typically reported for these fish.

New Zealand rickettsia-like organism and Tenacibaculum maritimum vaccine efficacy study

A cohort of Chinook salmon juveniles was vaccinated, with an autogenous bivalent vaccine against New Zealand RLOs (NZ-RLO1) and Tenacibaculum maritimum. A proportion of the cohort was not vaccinated to act as controls. At smoltification, the fish were challenged with NZ-RLO1, NZ-RLO2, or T. maritimum. We found that challenge with T. maritimum by immersion in (7.5 × 105  cfu/mL of water) did not yield any pathology. Challenge with RLOs produced clinical signs that were more or less severe depending on the challenge route, dose or vaccination status. Survival was significantly higher for vaccinated fish within the groups challenged with NZ-RLO1 by intraperitoneal injection with a relative percent survival (RPS) of 48.84%. Survival was not significantly different between vaccinated and non-vaccinated fish for groups challenged with NZ-RLO2 by intraperitoneal injection or by NZ-RLO1 by immersion. Yet, anecdotally the clinical disease presentation (manifesting as haemorrhagic, ulcerative skin lesions) was more severe for the non-vaccinated fish. This study demonstrates that autogenous vaccine against NZ-RLO is protective against severe disease and death by NZ-RLO1 challenge which warrants implementation and further evaluation under field conditions. Yet, this study also highlights the importance of the route of administration and dose when evaluating pathogenicity and vaccine efficacy.

nov., isolated from a tidal flat sediment

An orange-coloured bacterium, designated as strain GRR-S3-23T, was isolated from a tidal flat sediment collected from Garorim Bay, Chuncheongbuk-do, Republic of Korea. Cells of GRR-S3-23T were aerobic, Gram-stain-negative, rod-shaped and motile. GRR-S3-23T grew at 18-40 °C (optimum, 30 °C), pH 7.0-9.0 (optimum, pH 7.0) and with 2-4 % NaCl (optimum, 2-3 % w/v). Results of 16S rRNA gene sequence analysis indicated that GRR-S3-23T was closely related to Tenacibaculum aiptasiae a4T (97.6 %), followed by Tenacibaculum aestuarii SMK-4T (97.5 %), Tenacibaculum mesophilum MBIC 1140T (97.4 %), Tenacibaculum singaporense TLL-A2T (97.3 %), Tenacibaculum crassostreae JO-1T (97.2 %),and Tenacibaculum sediminilitoris YKTF-3T (97.1 %). The average amino acid identity values between GRR-S3-23T and the related strains were 86.8-72.8 %, the average nucleotide identity values were 83.3-74.1 %, and the digital DNA-DNA hybridization values were 27.0-19.6 %. GRR-S3-23T possessed menaquinone-6 (MK-6) as major respiratory quinone and had summed feature 3 (C16 : 1ω7c and/or C16 : 1ω6c, 20.6 %) and iso-C15 : 1G (10.8 %) as major fatty acids (>10.0 %). The polar lipid profiles of GRR-S3-23T contained phosphatidylethanolamine, one unidentified aminolipid, one unidentified aminophospholipid, three unidentified lipids, one unidentified glycolipid and four unidentified phospholipids. The DNA G+C content of GRR-S3-23T was 33.7%. On the basis of the results of the polyphasic analysis involving phylogenetic, phylogenomic, physiological and chemotaxonomic analyses described in this study, GRR-S3-23T is considered to represent a novel species within the genus Tenacibaculum, for which the name Tenacibaculum tangerinum is proposed. The type strain is GRR-S3-23T (=KCTC 102029T=KACC 23271T=JCM 36353T).

nov., isolated from Atlantic salmon (Salmo salar L.) cultured in Chile

Strain PVT-9aT, a novel Gram-stain-negative, aerobic, non-spore-forming, motile-by-gliding and rod-shaped bacterium, was isolated from a skin lesion of Atlantic salmon (Salmo salar L.) during a tenacibaculosis outbreak that occurred in 2016 at a Chilean fish farm. Phylogenetic analysis based on 16S rRNA gene sequencing confirmed that strain PVT-9aT belonged to the genus Tenacibaculum, being related to the closest type strains Tenacibaculum haliotis KCTC 52419T (98.49 % sequence similarity), Tenacibaculum aestuariivivum JDTF-79T (97.36 %), Tenacibaculum insulae JDTF-31T (97.29 %) and Tenacibaculum ovolyticum IFO 15947T (97.15 %). The genome size of strain PVT-9aT was 2.73 Mb with a DNA G+C content 31.09 mol%. Average nucleotide identity analysis among 30 Tenacibaculum species rendered the most similar strains as follows: T. haliotis KCTC 52419T (87.91 %), T. ovolyticum IFO 15947T (82.47 %), Tenacibaculum dicentrarchi 35/09T (81.08 %), Tenacibaculum finnmarkense gv finnmarkense TNO006T (80.91 %) and T. finnmarkense gv ulcerans TNO010T (80.96 %). Menaquinone MK-6 was the predominant respiratory quinone. The predominant cell fatty acids (>10 %) were iso-C15 : 0, iso-C15 : 1 G and iso-C15 : 0 3-OH. Phenotypic, chemotaxonomic and genomic data supported the assignment of strain PVT-9aT (=DSM 115155T=RGM 3472T) as representing a novel species of Tenacibaculum, for which the name Tenacibaculum bernardetii sp. nov. is proposed.

Early innate immune responses in European sea bass (Dicentrarchus labrax L.) following Tenacibaculum maritimum infection

Introduction

The marine aquaculture industry has been witnessing a worldwide emergence of tenacibaculosis, a poorly understood bacterial disease caused by Tenacibaculum maritimum that affects commercially important fish. So far, knowledge on the T. maritimum virulence mechanisms is scarce and the pathogen-host interaction operating in tenacibaculosis remain to be disclosed. This study aimed at contributing to a better understanding of this disease, by evaluating the early innate immune response triggered in European sea bass (Dicentrarchus labrax) by a bath-challenge with T. maritimum.

Methods

Groups of sea bass were bath-challenged with T. maritimum (challenged fish) or mock-challenged. Undisturbed fish were used as controls (time 0). Samples of blood, liver and mucosal organs (skin, gills and posterior-intestine) were collected at 0 h (control) and at 6, 24, 48 and 72 h post-challenge (n=12). Mucosal organs were used for analyzing the expression of immune-related genes by RT-qPCR, as well as blood samples for assessing haematological and innate humoral parameters and liver for oxidative stress assessment.

Results

An increased expression of il-1β, il8, mmp9 and hamp1 was detected in all mucosal organs of infected fish when compared with control and mock-challenged fish, suggesting a pro-inflammatory response against T. maritimum transversal to all organs. The faster induction of these pro-inflammatory genes was observed in the gills. Regarding the systemic response, challenged fish presented neutrophilia, monocytosis, signs of anemia, and a decrease of bactericidal and lysozyme activities in plasma. Almost no variations were observed regarding hepatic oxidative stress.

Discussion/Conclusions

The present study suggests that T. maritimum induces a local innate immune response upon bath infection not only in the skin of European sea bass, but also in the gills and posterior-intestine, likely triggered by the T. maritimum's capacity to adhere, colonize and damage these organs that can function as entry ways to bacteria, leading ultimately to the seen host's systemic response.

Metagenomic profile of caudal fin morphology of farmed red sea bream Pagrus major

The morphology of farm-reared fish often differs from that of their wild counterparts, impacting their market value. Two caudal fin tip shapes, acutely angled and blunted, are recognized in farmed populations of red sea bream Pagrus major. The angled form is preferred by consumers over the blunt since it resembles that of wild fish. Discovering the cause of the blunted tip is crucial to maximizing the commercial value of farmed red sea bream. We hypothesized that the blunt fin tip is the result of opportunistic bacteria and conducted partial 16S rRNA metagenomic barcoding and generated a clone library of the 16S rRNA gene to compare bacterial communities of the 2 fin forms. Metagenomic barcoding revealed an abundance of 5 bacterial genera, Sulfitobacter, Vibrio, Tenacibaculum, Psychrobacter, and an unknown genus of Rhodobacteraceae, on the caudal fin surface. Sulfitobacter was significantly more common on the angled caudal fin than the blunted. Vibrio is the dominant genus on the blunted caudal fin. The clone library identified these genera to species level, and Sulfitobacter sp., Vibrio harveyi, Tenacibaculum maritimum, and Psychrobacter marincola were frequently observed in blunt caudal fins. Our results suggest that opportunistic pathogenic bacteria such as V. harveyi and T. maritimum are not the primary cause of caudal fin malformation, and multiple factors such as combinations of injury, stress, and pathogenic infection may be involved. The reason for the significantly greater occurrence of Sulfitobacter sp. in the angled caudal fin is unknown, and further investigation is needed.

Healthy and infected Atlantic salmon (Salmo salar) skin-mucus response to Tenacibaculum dicentrarchi under in vitro conditions

Tenacibaculosis caused by Tenacibaculum dicentrarchi is the second most important bacterial disease that affects the Chilean salmon industry. The impacted fish show severe external gross skin lesions on different areas of the body. The external mucus layer that covers fish skin contains numerous immune substances that act as one of the main defense barriers against microbial colonization and invasions by potential pathogens. The present in vitro study aimed to evaluate and elucidate the role of the external mucus layer in the susceptibility of Atlantic salmon (Salmo salar) to three Chilean T. dicentrarchi strains and the type strain. For this, mucus collected from healthy and diseased (i.e., with T. dicentrarchi) Atlantic salmon were used, and various antibacterial and inflammatory parameters were analysed. The T. dicentrarchi strains were attracted to the mucus of Atlantic salmon regardless of health status. All four strains adhered to the skin mucus and very quickly grew using the mucus nutrients. Once infection was established, different mucosal defense components were activated in the fish, but the levels of bactericidal activity and of other enzymes were insufficient to eliminate T. dicentrarchi. Alternatively, this pathogen may be able to neutralize or evade these mechanisms. Therefore, the survival of T. dicentrarchi in fish skin mucus could be relevant to facilitate the colonization and subsequent invasion of hosts. The given in vitro results suggest that greater attention should be given to fish skin mucus as a primary defense against T. dicentrarchi.

Evaluation of the in vitro susceptibility of Tenacibaculum dicentrarchi to tiamulin using minimum inhibitory concentration tests

Tenacibaculosis caused by Tenacibaculum dicentrarchi is the third most important bacterial fish infection affecting the Chilean salmon industry. Losses to this disease are most frequently controlled by treatments with florfenicol and oxytetracycline. However, recent tenacibaculosis outbreaks were controlled through the extra-label, oral administration of tiamulin, resulting in high treatment efficiency. In this study, we present an analysis of susceptibility patterns of 32 T. dicentrarchi isolates and the type strain CECT 7612^T to tiamulin by determining the minimum inhibitory concentrations (MICs) according to the procedures recommended by the Clinical and Laboratory Standard Institute, but fixing incubation temperature to the more appropriate for the growth of T. dicentrarchi (18 ºC). The MICs of the T. dicentrarchi isolates were unimodally distributed (0.06-1.0 µg/ml range), while the CECT 7612^T strain presented an MIC of 0.5 μg/ml. Calculations using Normalized Resistance Interpretation provided epidemiological cut-off values of ≤1.0 µg/ml, with the 33 T. dicentrarchi classified as wild type. In Chile, tiamulin is authorized for use in other livestock species, but application in salmonids is extra-label. Our presented in vitro results suggest that tiamulin is a viable alternative to florfenicol, specifically as tiamulin requires comparatively lower concentrations to inhibit T. dicentrarchi. Considering that tiamulin is also exclusively for veterinary use, is classified as "least important" by the World Health Organization and has not resulted in the development of bacterial resistance, pharmaceutical companies should be requested to register tiamulin and provide alternative antimicrobial treatments for the salmonid industry.

Discovery and Biosynthesis of Antimicrobial Phenethylamine Alkaloids from the Marine Flavobacterium Tenacibaculum discolor sv11

The bacterial genus Tenacibaculum has been associated with various ecological roles in marine environments. Members of this genus can act, for example, as pathogens, predators, or episymbionts. However, natural products produced by these bacteria are still unknown. In the present work, we investigated a Tenacibaculum strain for the production of antimicrobial metabolites. Six new phenethylamine (PEA)-containing alkaloids, discolins A and B (1 and 2), dispyridine (3), dispyrrolopyridine A and B (4 and 5), and dispyrrole (6), were isolated from media produced by the predatory bacterium Tenacibaculum discolor sv11. Chemical structures were elucidated by analysis of spectroscopic data. Alkaloids 4 and 5 exhibited strong activity against Gram-positive Bacillus subtilis DSM10, Mycobacterium smegmatis ATCC607, Listeria monocytogenes DSM20600, and Staphylococcus aureus ATCC25923, with minimum inhibitory concentration (MIC) values ranging from 0.5 to 4 μg/mL, and moderate activity against Candida albicans FH2173 and Aspergillus flavus ATCC9170. Compound 6 displayed moderate antibacterial activities against Gram-positive bacteria. Dispyrrolopyridine A (4) was active against efflux pump deficient Escherichia coli ATCC25922 ΔtolC, with an MIC value of 8 μg/mL, as well as against Caenorhabditis elegans N2 with an MIC value of 32 μg/mL. Other compounds were inactive against these microorganisms. The biosynthetic route toward discolins A and B (1 and 2) was investigated using in vivo and in vitro experiments. It comprises an enzymatic decarboxylation of phenylalanine to PEA catalyzed by DisA, followed by a nonenzymatic condensation to form the central imidazolium ring. This spontaneous formation of the imidazolium core was verified by means of a synthetic one-pot reaction using the respective building blocks. Six additional strains belonging to three Tenacibaculum species were able to produce discolins, and several DisA analogues were identified in various marine flavobacterial genera, suggesting the widespread presence of PEA-derived compounds in marine ecosystems.

The effect of a biofilm-forming bacterium Tenacibaculum mesophilum D-6 on the passive film of stainless steel in the marine environment

The microbiologically influenced corrosion of 304 stainless steel in the presence of a marine biofilm-forming bacterium Tenacibaculum mesophilum D-6 was systematically investigated by means of electrochemical techniques and surface analyses to reveal the effect of the selective attachment and adsorption of the biofilms on the passivity breakdown of the stainless steel. It was found that the T. mesophilum D-6 was electroactive and could oxidize low valent cations and metal, facilitating the local dissolution of the passive film and the substrate in the film defects, nearly doubling the surface roughness. The biofilms of T. mesophilum D-6 with mucopolysaccharide secreta and chloride ions tended to preferentially adsorb at the defects of the passive film on the steel, yielding non-homogeneous microbial aggregates and local Cl^- enrichment there. The adsorption of the bacteria and chloride ions reduced the thickness of passive film by 23.9%, and generate more active sites for pitting corrosion on the passive film and more semiconducting carrier acceptors in the film. The maximum current density of the 304 SS in the presence of T. mesophilum D-6 was over one order of magnitude higher than that in the sterile medium, and the largest pit was deepened 3 times.

Role of mucosal immune response and histopathological study in European eel (Anguilla anguilla L.) intraperitoneal challenged by Vibrio anguillarum or Tenacibaculum soleae

The external mucus layer that covers fish skin contains numerous immune substances scarcely studied that act as the first line of defence against a broad spectrum of pathogens. This study aimed to characterize and describe for the first time several humoral immune defence parameters in the skin mucus of the European eel (Anguilla anguilla) after intraperitoneal injection with Vibrio anguillarum or Tenacibaculum soleae. This study evaluated several immune-related enzymes and bactericidal activity against fish pathogenic bacteria in the skin mucus of European eels at 24, 48, and 72 h post-challenge. The results demonstrated that European eel skin mucus showed significant increments in peroxidase and lysozyme activity at 48 and 72 h after V. anguillarum challenge, compared to other experimental groups. In the case of antiprotease activity, an increase was observed at 24 h in the skin mucus of fish challenged with V. anguillarum compared to unchallenged fish, while this activity was undetected at 48 and 72 h. In contrast, protease activity had decreased at 48 and 72 h in the skin mucus of fish challenged with V. anguillarum compared to the unchallenged group. Regarding bactericidal activity, a high growth capacity of T. soleae was observed in the skin mucus of all experimental groups. Interestingly, the skin mucus from fish challenged with V. anguillarum exhibited increased bactericidal activity against this bacterium at 48 h, compared to unchallenged fish. Finally, severe histopathological alterations were observed in the gills and liver at the end of the trial (72 h), whereas the skin showed only an overspread presence of goblet cells in the challenged fish compared to unchallenged fish. The present results may give new insights into the mucosal immune system of this primitive species with potential applications in aquaculture.