Antigenic similarities and clinical cross-protection between variant and classic non-viscous strains of Moritella viscosa in Atlantic salmon in Norway

Moritella viscosa (M. viscosa) is one of the major etiological agents of winter-ulcers in Atlantic salmon (Salmo salar) in Norway. Outbreaks of ulcerative disease in farmed fish occur across the North Atlantic region, causing reduced animal welfare and economical challenges, and are of hindrance for sustainable growth within the industry. Commercially available multivalent core vaccines containing inactivated bacterin of M. viscosa reduce mortality and clinical signs related to winter ulcer disease. It has previously been described two major genetic clades within M. viscosa, typical (hereafter referred to as classic) and variant, based on gyrB sequencing. In addition, there are phenotypical traits such as viscosity that may differ between different types of isolates. Western blot using salmon plasma showed that classic non-viscous strains are antigenically different from the classic viscous type included in core vaccines. Further, Western blot also showed that there are similarities in binding patterns between Norwegian variant and classic non-viscous isolates, indicating they may be antigenically related. Vaccination-challenge trials using Norwegian gyrB-classic non-viscous isolates of M. viscosa, demonstrate that the isolates from the classic clade that are included in current commercial multivalent core vaccines, provide limited cross protection against the emerging non-viscous strains. However, a vaccine recently approved for marketing authorization in Norway, containing inactivated antigen of a variant M. viscosa strain, demonstrates reduced mortality as well as clinical signs caused by infections with the classic non-viscous M. viscosa isolated from outbreaks in Norwegian salmon farms. The study shows that there are antigenic similarities between variant and classic non-viscous types of M. viscosa, and these similarities are mirrored in the observed cross-protection in vaccination-challenge trials.

Haemato-Immunological Response of Immunized Atlantic Salmon (Salmo salar) to Moritella viscosa Challenge and Antigens

Winter ulcer disease is a health issue in the Atlantic salmonid aquaculture industry, mainly caused by Moritella viscosa. Although vaccination is one of the effective ways to prevent bacterial outbreaks in the salmon farming industry, ulcer disease related to bacterial infections is being reported on Canada's Atlantic coast. Here, we studied the immune response of farmed immunized Atlantic salmon to bath and intraperitoneal (ip) M. viscosa challenges and evaluated the immunogenicity of M. viscosa cell components. IgM titers were determined after infection, post boost immunization, and post challenge with M. viscosa. IgM+ (B cell) in the spleen and blood cell populations were also identified and quantified by 3,3 dihexyloxacarbocyanine (DiOC6) and IgM-Texas red using confocal microscopy and flow cytometry. At 14 days post challenge, IgM was detected in the serum and spleen. There was a significant increase in circulating neutrophils 3 days after ip and bath challenges in the M. viscosa outer membrane vesicles (OMVs) boosted group compared to non-boosted. Lymphocytes increased in the blood at 7 and 14 days after the ip and bath challenges, respectively, in OMVs boosted group. Furthermore, a rise in IgM titers was detected in the OMVs boosted group. We determined that a commercial vaccine is effective against M. viscosa strain, and OMVs are the most immunogenic component of M. viscosa cells.

Basal and immune-responsive transcript expression of two Atlantic salmon interferon regulatory factor 2 (irf2) paralogues

Atlantic salmon (Salmo salar) is one of the most economically important aquaculture species globally. However, disease has become a prevalent threat to this industry. A thorough understanding of the genes and molecular pathways involved in the immune responses of Atlantic salmon is imperative for selective breeding of disease-resistant broodstock, as well as developing new diets and vaccines to mitigate the impact of disease. Members of the interferon regulatory factor (IRF) family of transcription factors play roles in the induction of interferons and other cytokines involved in host immune responses to intracellular and parasitic pathogens. IRF family members also play diverse roles in other biological processes, such as stress response, reproduction and development. The current study focused on one member of the IRF family: interferon regulatory factor 2 (irf2). As previously shown, due to the genome duplication that occurred ∼80 million years ago in the salmonid lineage, there are two irf2 paralogues in the Atlantic salmon genome. In silico analyses at the cDNA and deduced amino acid levels were conducted followed by phylogenetic tree construction with IRF2 amino acid sequences from various ray-finned fishes, cartilaginous fish and tetrapods. qPCR was then used to analyze paralogue-specific irf2 constitutive expression across 17 adult tissues, as well as responses to the viral mimic pIC (i.e., synthetic double-stranded RNA analog) in cultured macrophage-like cells (in vitro) and to infection with the Gram-negative bacterium Moritella viscosa in skin samples (in vivo). The qPCR studies showed sex- and paralogue-specific differences in expression across tissues. For example, expression of both paralogues was higher in ovary than in testes; expression (considering both sexes together) was highest for irf2-1 in gonad and for irf2-2 in hindgut. Both irf2 paralogues were responsive to pIC stimulation, but varied in their induction level, with irf2-1 having an overall stronger response than irf2-2. Only one paralogue, irf2-2, was significantly responsive to M. viscosa infection. Differences in irf2-1 and irf2-2 transcript expression levels constitutively across tissues, and in response to pIC and M. viscosa, may suggest neo- or subfunctionalization of the duplicated genes. This novel information expands current knowledge and provides insight into how genome duplication events may impact host regulation of important immune markers.

New vaccination strategies are required for effective control of winter ulcer disease caused by emerging variant strains of Moritella viscosa in Atlantic salmon

Moritella viscosa is one on the major etiological agents of winter-ulcers in Atlantic salmon (Salmo salar) in Norway. Outbreaks of ulcerative disease in farmed fish occurs across the North Atlantic region and is an impeding factor for sustainable growth within the industry. Commercially available multivalent core vaccines containing inactivated bacterin of M. viscosa reduce mortality and clinical signs related to winter ulcer disease. Two major genetic clades within M. viscosa have previously been described based on gyrB sequencing, namely typical (hereafter referred to as classic) and variant. Vaccination-challenge trials using vaccines including either variant and or classic isolates of M. viscosa show that classic clade isolates included in current commercial multivalent core vaccines provide poor cross-protection against emerging variant strains, while variant strains confer high level of protection against variant M. viscosa but to a lesser extent to classic clade isolates. This demonstrates that future vaccine regimens should include a combination of strains from both clades.

Atlantic salmon challenged with extracellular products from Moritella viscosa

Skin ulcers in Atlantic salmon Salmo salar in the Canadian east coast salmon aquaculture industry lead to high mortality rates. This condition is clinically similar to winter ulcer disease in Norway with the exception that it occurs at temperatures above 10°C. Moritella viscosa is thought to be the causative agent for winter ulcer disease in Norway, and it is occasionally also isolated from skin ulcer cases in Atlantic Canada. This bacterium is known to produce cytotoxins. The objective of this study was to determine if extracellular products (ECP) from an Atlantic Canadian strain of M. viscosa could induce a tissue response similar to what is observed with M. viscosa infections in Atlantic salmon in eastern Canada. We injected fish subcutaneously with ECP and monitored the development of skin lesions. We sampled fish with early skin lesions and ulcers to describe the pathology associated with the condition. Samples were taken for histopathology, bacterial culture, and quantitative PCR (qPCR). All experimental fish expressed early skin lesions, with 5 fish (8.3%) developing deep skin ulcers after 12 d post-exposure. Our results suggest the ECP of M. viscosa from the east coast of Canada induces a similar tissue response to what is described in ulcer disease in Atlantic salmon. These extracelluar products may partially explain the pathology associated with M. viscosa.

Pan genome and CRISPR analyses of the bacterial fish pathogen Moritella viscosa

BACKGROUND

Winter-ulcer Moritella viscosa infections continue to be a significant burden in Atlantic salmon (Salmo salar L.) farming. M. viscosa comprises two main clusters that differ in genetic variation and phenotypes including virulence. Horizontal gene transfer through acquisition and loss of mobile genetic elements (MGEs) is a major driving force of bacterial diversification. To gain insight into genomic traits that could affect sublineage evolution within this bacterium we examined the genome sequences of twelve M. viscosa strains. Matches between M. viscosa clustered, regularly interspaced, short palindromic, repeats and associated cas genes (CRISPR-Cas) were analysed to correlate CRISPR-Cas with adaptive immunity against MGEs.

RESULTS

The comparative genomic analysis of M. viscosa isolates from across the North Atlantic region and from different fish species support delineation of M. viscosa into four phylogenetic lineages. The results showed that M. viscosa carries two distinct variants of the CRISPR-Cas subtype I-F systems and that CRISPR features follow the phylogenetic lineages. A subset of the spacer content match prophage and plasmid genes dispersed among the M. viscosa strains. Further analysis revealed that prophage and plasmid-like element distribution were reflected in the content of the CRISPR-spacer profiles.

CONCLUSIONS

Our data suggests that CRISPR-Cas mediated interactions with MGEs impact genome properties among M. viscosa, and that patterns in spacer and MGE distributions are linked to strain relationships.

Host specificity and clade dependent distribution of putative virulence genes in Moritella viscosa

Moritella viscosa is the aetiological agent of winter-ulcer disease in farmed salmonids in the North Atlantic. Previously, two major (typical and variant) genetic clades have been demonstrated within this bacterial species, one of which is almost solely related to disease in Atlantic salmon (Salmo salar). In the present study infection trials demonstrated that 'typical' M. viscosa isolated from Norwegian Atlantic salmon was highly virulent in this fish species but resulted in lower levels of mortality in rainbow trout. 'Variant' M. viscosa isolated from rainbow trout resulted in modest mortality levels in both Atlantic salmon and rainbow trout. To investigate the possible genetic background for inter-strain virulence differences, 38 M. viscosa isolates of diverse geographical origin and host species and a number of other Moritella spp. were investigated for the presence/absence of putative virulence related homologs. All isolates were positive for DNA sequences coding for; the Type VI secretion ATPase (clpV), hemolysin co-regulated protein (hcp), bacterioferritins (bfrA and bfrB), lectin (hemG), phospholipase D (pld), multifunctional autoprocessing repeats-in-toxin (martxA), aerolysin (aer), invasin (inv), and cytotoxic necrotizing factor (cnf), with the exception of one isolate in which cnf could not be confirmed. The product of an ABC transporter metal-binding lipoprotein (mat) was consistently detected although 11 isolates, all phylogenetically related, appear to produce a truncated version. A putative insecticidal toxin complex (mitABC) was detected almost exclusively in 'typical' Atlantic salmon isolates, and our data indicate that this complex of genes is expressed and co-transcribed. Transmission electron microscopy investigation revealed pili and flagella surface structures on nine M. viscosa representing both typical and variant isolates. Our results provide strong support for the existence of host specificity/high virulence in 'typical' M. viscosa related to Atlantic salmon. The gene distribution also provides further support for the genetic division within M. viscosa, and constitutes a basis for further study of the importance of the mitABC complex in winter-ulcer pathogenesis.

Co-infection of Atlantic salmon (Salmo salar), by Moritella viscosa and Aliivibrio wodanis, development of disease and host colonization

Two species of bacteria are repeatedly isolated from farmed fish with winter-ulcer disease. Moritella viscosa is the aetiological agent of the disease; the significance of Aliivibrio wodanis is uncertain but has not been related to the primary pathogenesis. A cell culture infection model showed that A. wodanis adhered to, but did not invade the fish cells. Exposure to culture supernatant of A. wodanis caused the fish cells to vacoulate, retract, round up and detach from the surface, and rearrange the actin filaments of the cytoskeleton. These observations suggest that the bacterium secretes toxins into the extracellular environment. Any pathologic effect of A. wodanis and the effect of co-culturing with M. viscosa was studied in Atlantic salmon (Salmo salar) bath challenged with; only M. viscosa or only A. wodanis or both bacteria together. Both M. viscosa and A. wodanis were re-isolated from external surfaces and internal organs from live and deceased co-infected fish. It is further hypothesized that A. wodanis colonization might influence the progression of a M. viscosa infection. This is to our knowledge the first study that reproduces field observations where both bacteria infect Atlantic salmon.