A novel serotype of Flavobacterium psychrophilum detected using antiserum against an isolate from amago, Oncorhynchus masou rhodurus Jordan & Gilbert, in Japan

Host specificity and virulence of Flavobacterium psychrophilum: a comparative study in ayu (Plecoglossus altivelis) and rainbow trout (Oncorhynchus mykiss) hosts

Flavobacterium psychrophilum, the causative agent of bacterial cold-water disease, is a devastating, worldwide distributed, fish pathogen causing significant economic loss in inland fish farms. Previous epidemiological studies showed that prevalent clonal complexes (CC) differ in fish species affected with disease such as rainbow trout, coho salmon and ayu, indicating significant associations between particular F. psychrophilum genotypes and host species. Yet, whether the population structure is driven by the trade of fish and eggs or by host-specific pathogenicity is uncertain. Notably, all F. psychrophilum isolates retrieved from ayu belong to Type-3 O antigen (O-Ag) whereas only very few strains retrieved from other fish species possess this O-Ag, suggesting a role in outbreaks affecting ayu. Thus, we investigated the links between genotype and pathogenicity by conducting comparative bath infection challenges in two fish hosts, ayu and rainbow trout, for a collection of isolates representing different MLST genotypes and O-Ag. Highly virulent strains in one host species exhibited low to no virulence in the other. F. psychrophilum strains associated with ayu and possessing Type-3 O-Ag demonstrated significant variability in pathogenicity in ayu, ranging from avirulent to highly virulent. Strikingly, F. psychrophilum strains retrieved from rainbow trout and possessing the Type-3 O-Ag were virulent for rainbow trout but not for ayu, indicating that Type-3 O-Ag alone is not sufficient for pathogenicity in ayu, nor does it prevent pathogenicity in rainbow trout. This study revealed that the association between a particular CC and host species partly depends on the pathogen's adaptation to specific host species.

Characterization and Pathogenicity of Flavobacterium psychrophilum Isolated from Rainbow Trout (Oncorhynchus mykiss) in Korea

Flavobacterium psychrophilum is the causative agent of bacterial cold-water disease in salmonids and rainbow trout fry syndrome. This pathogen has attained a global presence and can spread both horizontally and vertically. However, it was not documented in Korea before September 2018. In this study, the objectives were to characterize Flavobacterium psychrophilum strain FPRT1, isolated from diseased rainbow trout genotypically and phenotypically. We also conducted various investigations to better understand its impact and assess potential control measures. We acquired fifty rainbow trout (approximately 70 g in weight) and transferred them to a laboratory aquarium. During the initial acclimation period, we observed mortality and examined affected fish for clinical signs. We isolated the bacterium from the spleen of infected rainbow trout using tryptone yeast extract salts agar supplemented with glucose, naming this FPRT1. Antibiotic susceptibility testing was carried out, and from the result, we selected enrofloxacin to administer to the trout orally to reduce mortality. To evaluate pathogenicity, we exposed the trout to FPRT1 at different water temperatures (8, 15, and 22 °C). Genomic analysis was conducted to identify the serotype and relatedness of FPRT1 to European strains. Affected fish displayed clinical signs, such as ulcerative lesions in the mandible, anemia with pale gills, exophthalmia, and increased mucus secretion. Internal symptoms included pale liver and enlarged spleen. FPRT1 was susceptible to erythromycin, enrofloxacin, florfenicol, oxytetracycline, and gentamicin, but resistant to oxolinic acid and sulfamethoxazole/trimethoprim. Oral administration of enrofloxacin resulted in a decrease in mortality from 28% to 6%. Pathogenicity tests revealed varying mortality rates due to FPRT1 at different temperatures. The highest rates were observed at 8 °C (ranging from 43% to 100%) for both intraperitoneal and intramuscular injections, and lower rates occurred at 22 °C (ranging from 0% to 30%), with intramuscular injections displaying higher susceptibility. Genomic analysis identified FPRT1 as serotype 2 and indicated its close genetic relationship with European strains based on the core genome and dispensable genome. The substantial genomic similarity between our strain and European strains suggests the possibility of bacterial spread through the importation of fertilized eggs from Europe. In conclusion, this study highlights the introduction of the previously undocumented pathogen (F. psychrophilum) into Korean rainbow trout populations. The detection of this pathogen and its pathogenicity assessment is not only important for understanding its impact on local aquaculture but also for establishing surveillance and control measures to prevent further transmission and outbreaks in the region.

Host-specific preference of some Flavobacterium psychrophilum multilocus sequence typing genotypes determines their ability to cause bacterial coldwater disease in coho salmon (Oncorhynchus kisutch)

Flavobacterium psychrophilum causes bacterial coldwater disease (BCWD) in salmonids, resulting in significant losses worldwide. Several serotyping and genetic studies of F. psychrophilum have suggested some geno-/serotypes may be either host-specific or generalistic in nature; however, this association has not been adequately explored in vivo using more natural exposure routes. Herein, F. psychrophilum isolate US19-COS, originally recovered from coho salmon (Oncorhynchus kisutch) and belonging to multilocus sequence typing clonal complex (CC) CC-ST9, and isolate US53-RBT, recovered from rainbow trout (Oncorhynchus mykiss) and belonging to CC-ST10, were serotyped via PCR, evaluated for proteolytic activity and utilized to determine their median lethal dose in immersion-challenged coho salmon fingerlings. US19-COS belonged to serotype 0, hydrolysed casein and gelatin but not elastin, led to fulminant multiorgan infections and elicited severe gross and microscopic pathology. In contrast, US53-RBT, belonging to serotype 2, hydrolysed all three substrates, but did not lead to detectable infections, disease signs or mortality in any exposed coho salmon despite proving virulent to rainbow trout in previous experiments. This study provides in vivo evidence for potential host specificity of some F. psychrophilum genotypes that can also be serologically distinct, a matter of importance towards better understanding F. psychrophilum disease ecology and epidemiology.

Current status of fish vaccines in Japan

Aquaculture is an important industry in Japan for the sustainable production of fish. It contributes to the diversity of Japanese traditional food culture, which uses fish such as "sushi" and "sashimi". In the recent aquaculture setting in Japan, infectious diseases have been an unavoidable problem and have caused serious economic losses. Therefore, there is an urgent need to overcome the disease problem to increase the productivity of aquaculture. Although our country has developed various effective vaccines against fish pathogens, which have contributed to disease prevention on fish farms, infectious diseases that cannot be controlled by conventional inactivated vaccines are still a problem. Therefore, other approaches to developing effective vaccines other than inactivated vaccines are required. This review introduces the vaccine used in Japan within the context of the current status of finfish aquacultural production and disease problems. This review also summarizes the current research into vaccine development and discusses the future perspectives of fish vaccines, focusing on the problems associated with vaccine promotion in Japan.

First isolation of Flavobacterium psychrophilum associated with reports of moribund wild European eel (Anguilla anguilla) in Scotland

In late April 2015, the River Dee Trust informed Marine Scotland Science, Fish Health Inspectorate (FHI), that there had been observations of dead and moribund European eels on the River Dee. Later in May, the Spey Fishery Board also reported a number of moribund European eels in a rotary screw smolt trap on the River Spey. In total, 10 cases involving moribund eels were investigated in 2015 and one case in 2016. In addition, a health screen was conducted to investigate the potential presence of Flavobacterium psychrophilum in healthy eels and Atlantic salmon from the River Dee in 2015. Externally, the diseased eels demonstrated white patches in different locations of the body. In all cases, F. psychrophilum was detected by bacterial isolation and/or molecular methods. Three isolates were further characterized by whole-genome sequencing (WGS) as belonging to sequence type 15 (ST15). Histological examination of diseased European eels revealed lesions at the level of the integument. The pathogen screen for F. psychrophilum in wild healthy fish tested negative by PCR. Further investigation is required to understand the pathogenicity of this bacterium on the health of eels and the potential impact on the wild salmonid population.

Transcriptomic response of rainbow trout (Oncorhynchus mykiss) skeletal muscle to Flavobacterium psychrophilum

Flavobacterium psychrophilum is the etiologic agent of rainbow trout fry syndrome (RTFS). This pathogen infects a wide variety of salmonid species during freshwater stages, causing significant losses in the aquaculture industry. Rainbow trout (Oncorhynchus mykiss) infected with F. psychrophilum, presents as the main external clinical sign ulcerative lesions and necrotic myositis in skeletal muscle. We previously reported the in vitro cytotoxic activity of F. psychrophilum on rainbow trout myoblast, however little is known about the molecular mechanisms underlying the in vivo pathogenesis in skeletal muscle. In this study, we examined the transcriptomic profiles of skeletal muscle tissue of rainbow trout intraperitoneally challenged with low infection dose of F. psychrophilum. Using high-throughput RNA-seq, we found that 233 transcripts were up-regulated, mostly associated to ubiquitin mediated proteolysis and apoptosis. Conversely, 189 transcripts were down-regulated, associated to skeletal muscle contraction. This molecular signature was consistent with creatine kinase activity in plasma and oxidative damage in skeletal muscle. Moreover, the increased caspase activity suggests as a whole skeletal muscle atrophy induced by F. psychrophilum. This study offers an integrative analysis of the skeletal muscle response to F. psychrophilum infection and reveals unknown aspects of its pathogenesis in rainbow trout.

Comparative Structural and Antigenic Characterization of Genetically Distinct Flavobacterium psychrophilum O-Polysaccharides

Little is known about the underlying basis of serotype specificity among strains of Flavobacterium psychrophilum, the agent of rainbow trout fry syndrome and bacterial cold-water disease. The identification of different heat-stable O-serotypes among strains of this gram-negative pathogen does, however, suggest structural variations in the O-polysaccharide (O-PS) moiety of cell surface lipopolysaccharide (LPS). A trisaccharide composed of L-rhamnose (L-Rha), 2-acetamido-2-deoxy-L-fucose (L-FucNAc) and 2-acetamido-4-R-2,4-dideoxy-D-quinovose (D-Qui2NAc4NR), where R represents a dihydroxyhexanamido derivative, was previously identified as the repeating unit of Fp CSF259-93 O-PS. Interestingly, the O-PS gene cluster of this strain and that of Fp 950106-1/1, which belongs to a different O-serotype, are identical except for wzy, which encodes the putative polymerase that links trisaccharide repeats into O-PS chains. We have now found from results of glycosyl composition analysis and high-resolution nuclear magnetic resonance, that the linkage of D-Qui2NAc4NR to L-Rha, which is α1-2 for Fp CSF259-93 versus β1-3 for Fp 950106-1/1, is the only structural difference between O-PS from these strains. The corresponding difference in O-serotype specificity was established from the reactions of rabbit and trout anti-F. psychrophilum antibody with purified O-PS and LPS. Moreover, LPS-based differences in antigenicity were noted between strains with O-PS loci identical to those of Fp CSF259-93 or Fp 950106-1/1, except for the genes predicted to direct synthesis of different R-groups in Qui2NAc4NR. The findings provide a framework for defining the genetic basis of O-PS structure and antigenicity and suggest that the repertoire of F. psychrophilum O-serotypes extends beyond what is presently recognized from serological studies of this important fish pathogen.

Local regulation of immune genes in rainbow trout (Oncorhynchus mykiss) naturally infected with Flavobacterium psychrophilum

Flavobacterium psychrophilum is the etiological agent of bacterial cold water disease (BCWD), also referred to as rainbow trout fry syndrome (RTFS), a disease with great economic impact in salmonid aquaculture. Despite this, to date, not many studies have analyzed in depth how the immune system is regulated during the course of the disease. In the current study, we have studied the transcription of several immune genes related to T and B cell activity in the skin of rainbow trout (Oncorhynchus mykiss) naturally infected with F. psychrophilum in a farm located in Lake Titicaca (Peru). The levels of expression of these genes were tested and compared to those obtained in asymptomatic and apparently healthy rainbow trout. In the case of symptomatic fish, skin samples containing characteristic ulcerative lesions were taken, as well as skin samples with no lesions. Our results pointed to a significant local up-regulation of IgD, CD4, CD8, perforin and IFNγ within the ulcerative lesions. On the other hand, no differences between the levels of expression of these genes were visible in the spleen. To confirm these results, the distribution of IgD⁺ and CD3⁺ cells was studied through immunohistochemical techniques in the ulcerative lesions. Our results demonstrate a strong local response to F. psychrophilum in rainbow trout in which IgD and T cells seem to play a major role.

Genomic Characterization of Flavobacterium psychrophilum Serotypes and Development of a Multiplex PCR-Based Serotyping Scheme

Flavobacterium psychrophilum is a devastating bacterial pathogen of salmonids reared in freshwater worldwide. So far, serological diversity between isolates has been described but the underlying molecular factors remain unknown. By combining complete genome sequence analysis and the serotyping method proposed by Lorenzen and Olesen (1997) for a set of 34 strains, we identified key molecular determinants of the serotypes. This knowledge allowed us to develop a robust multiplex PCR-based serotyping scheme, which was applied to 244 bacterial isolates. The results revealed a striking association between PCR-serotype and fish host species and illustrate the use of this approach as a simple and cost-effective method for the determination of F. psychrophilum serogroups. PCR-based serotyping could be a useful tool in a range of applications such as disease surveillance, selection of salmonids for bacterial coldwater disease resistance and future vaccine formulation.

Flavobacterium psychrophilum vaccine development: a difficult task

Bacterial cold water disease (BCWD) is a globally distributed freshwater fish disease caused by the Gram-negative bacterium Flavobacterium psychrophilum. It is a particularly devastating infection in fry salmonids and may lead to high levels of mortality. In spite of its economic impact on fish farms, neither the biology of the bacterium nor the bacterium-host interactions are well understood. This review provides a synopsis of the major problems related to critical remaining questions about research into the use of vaccines against F. psychrophilum and the development of a commercial vaccine against this disease. Studies using sera from convalescent rainbow trout have shown the antigenic properties of different proteins such as OmpH, OmpA and FspA, as well as low and high molecular mass lipopolysaccharide of F. psychrophilum, which are potential candidates for subunit vaccines. Inactivated F. psychrophilum bacterins have been successfully tested as vaccines under laboratory conditions by both immersion and intraperitoneal routes. However, the efficacy and the practical usefulness of these preparations still have to be proved. The use of attenuated and wild-type strains to immunize fish showed that these systems offer high levels of protection. Nevertheless, their application clashes with the regulations for environmental protection in many countries. In conclusion, protective vaccines against BCWD are theoretically possible, but substantial efforts still have to be made in order to permit the development of a commercial vaccine.

Antigenic proteins of Flavobacterium psychrophilum recognized by ayu Plecoglossus altivelis antisera

Flavobacterium psychrophilum is the causative agent of bacterial coldwater disease (BCWD) in ayu Plecoglossus altivelis altivelis and is responsible for substantial economic losses in ayu culture in Japan. To develop effective vaccines for the disease, we identified antigenic proteins of F. psychrophilum using immunoglobulin from ayu that had recovered from BCWD. The whole protein extracted from the bacterium was separated using 2-dimensional polyacrylamide gel electrophoresis and was transferred to a polyvinylidene fluoride membrane. Subsequently, antigenic proteins of the bacterium were detected using western blotting and ayu antisera against F. psychrophilum. Each protein spot showing antigenicity was subjected to tandem mass spectrometry (MS/MS) analysis using a MALDI-QIT-TOF mass spectrometer. Protein identification based on the MS/MS data was performed using the genome database for F. psychrophilum JIP02/86, and the subcellular localization for each identified protein was predicted with web-based tools (LipoP and PSORTb). In total, 62 antigenic proteins were identified: of these, 46 were putative cytoplasmic proteins (e.g. elongation factor Tu and heat shock protein 90). The remaining 21 proteins were identified as putative membrane-bound or secreted proteins and are potential vaccine candidates. These proteins include OmpA, Omp 121, M13 family metallopeptidase, and M48 family metalloprotease.

A novel genotyping technique for distinguishing between Flavobacterium psychrophilum isolates virulent and avirulent to ayu, Plecoglossus altivelis altivelis (Temminck & Schlegel)

We developed a simple genotyping method for Flavobacterium psychrophilum for analysing two single nucleotide polymorphisms (SNPs) in the gyrA gene and to distinguish between isolates that are virulent and avirulent to ayu, Plecoglossus altivelis altivelis (Temminck & Schlegel). The genotyping method is an on/off switch assay and is based on the polymerase chain reaction technique with phosphorothioated primers. We classified 232 isolates from four families of fish (i.e. Plecoglossidae, Osmeridae, Cyprinidae and Salmonidae) into four genotypes (G-C, A-T, A-C and G-T). The G-C type isolates exhibited strong pathogenicity to ayu, whereas the A-T and G-T types did not show any pathogenicity to this species. The A-C type exhibited no or weak pathogenicity to ayu. These results indicate that genotyping F. psychrophilum isolates with two SNPs from gyrA can clearly distinguish between isolates potentially harmful to ayu (G-C type) and those that are potentially not harmful or less harmful (A-C, A-T and G-T type). The on/off switch assay provides a quick, simple, and very powerful DNA genotyping technique for F. psychrophilum isolates.

Development and evaluation of a loop-mediated isothermal amplification assay for rapid and simple detection of Flavobacterium psychrophilum

Flavobacterium psychrophilum is the causative agent of bacterial cold-water disease and rainbow trout fry syndrome of salmonids. The pathogen has been reported from all regions in the world involved in salmonid aquaculture, but also from natural fresh-water environments. We established a quantitative loop-mediated isothermal amplification of DNA (LAMP) method to estimate quantities of F. psychrophilum. LAMP primers were designed based on the sequence of the DNA topoisomerase IV subunit B gene, parE, of F. psychrophilum. parE LAMP exhibited a high specificity for the parE gene of F. psychrophilum but not for other related species. parE LAMP detected the gene in a wide range of concentrations from 2.0 x 10(1) to 2.0 x 10(9) copies/reaction within 70 min and revealed a good correlation between threshold times and gene copy number.

PCR-RFLP genotypes associated with quinolone resistance in isolates of Flavobacterium psychrophilum

A novel genotyping method for epizootiological studies of bacterial cold-water disease caused by Flavobacterium psychrophilum and associated with quinolone resistance was developed. Polymerase chain reaction followed by restriction fragment length polymorphism (PCR-RFLP) was performed on 244 F. psychrophilum isolates from various fish species. PCR was performed with primer pair GYRA-FP1F and GYRA-FP1R amplifying the A subunit of the DNA gyrase (GyrA) gene, which contained the quinolone resistance determining region. Digestion of PCR products with the restriction enzyme Mph1103I showed two genotypes, QR and QS. The difference between these genotypes was amino acid substitutions at position 83 of GyrA (Escherichia coli numbering). The genotype QR indicated an alanine residue at this position associated with quinolone resistance in F. psychrophilum isolates. Of the 244 isolates tested in this study, the number of QR genotype isolates was 153 (62.7%). In isolates from ayu (n=177), 146 (82.5%) were genotype QR. With combination of this technique and previously reported PCR-RFLP genotyping, eight genotypes were observed in F. psychrophilum isolates. Using this genotyping system, the relationships between genotype and host fish species, or locality of isolation, were analysed and are discussed.

Efficacy of injection vaccines against Flavobacterium psychrophilum in rainbow trout, Oncorhynchus mykiss (Walbaum)

Efficacy of mineral oil-based experimental injection vaccines against Flavobacterium psychrophilum were tested in rainbow trout, Oncorhynchus mykiss (Walbaum), under laboratory and field conditions. The vaccines consisted of formalin- or heat-inactivated whole bacterium cell preparations of two different serotypes (Fd and Th) or a combination of serologically different F. psychrophilum (Fd and/or Th and/or Fp(T);Th). Specific antibody responses against the bacterium in plasma and skin mucus were evaluated post-vaccination with enzyme-linked immunosorbent assay. Efficacy of the vaccinations was determined by challenge trials to F. psychrophilum with the vaccinated rainbow trout. Significantly higher antibody levels in plasma were detected in vaccinated fish compared with mock-vaccinated fish. Injection vaccination did not trigger specific antibody production in the skin mucus. Significantly higher survival of i.p. vaccinated fish compared with non-vaccinated fish was observed during the challenge. The results suggest that mineral oil-based injectable vaccines containing formalin- or heat-inactivated virulent cells of F. psychrophilum effectively triggered specific antibody production and protected the fish against bacterial cold water disease.