Isolation of rifampicin resistant Flavobacterium psychrophilum strains and their potential as live attenuated vaccine candidates

Characterization of maternal immunity following vaccination of broodstock against IHNV or Flavobacterium psychrophilum in rainbow trout (Oncorhynchusmykiss)

Infectious hematopoietic necrosis (IHN) is a significant viral disease affecting salmonids, whereas Flavobacterium psychrophilum (Fp), the causative agent of bacterial coldwater disease (BCWD), remains one of the most significant bacterial pathogens of salmonids. We explored maternal immunity in the context of IHN and BCWD management in rainbow trout (Oncorhynchus mykiss) aquaculture. Two experimental trials were conducted where different groups of female broodstock were immunized prior to spawning with an IHNV DNA vaccine or a live attenuated F. psychrophilum (Fp B.17-ILM) vaccine alone, or in combination. Progeny were challenged with either a low or high dose of IHNV at 13 days post hatch (dph) and 32 dph or challenged with F. psychrophilum at 13 dph. Mortality following a low-dose IHNV challenge at 13 dph was significantly lower in progeny from vaccinated broodstock vs. unvaccinated broodstock, but no significant differences were observed at 32 dph. Mortality due to BCWD was also significantly reduced in 13 dph fry that originated from broodstock immunized with the Fp B.17-ILM vaccine. After vaccination broodstock developed specific or neutralizing antibodies respectively to F. psychrophilum and IHNV; however, antibody titers in eggs and fry were undetectable. In the eggs and fry mRNA transcripts of the complement components C3 and C5 were detected at much higher levels in progeny from vaccinated broodstock and showed a significantly increased and rapid response post-challenge compared with unvaccinated broodstock. After challenges pro-inflammatory cytokine expression was immediately and considerably elevated in the fry from vaccinated broodstock vs. unvaccinated broodstock, whereas adaptive immune genes were elevated to a lesser degree. Results suggest that maternal transfer of innate and adaptive factors at the transcript level occurred because development of lymphomyeloid organs is not complete in such young fry. In addition to documenting maternally derived immunity in teleosts, this study demonstrates that broodstock vaccination can confer some degree of protection to progeny against viral and bacterial pathogens.

The Attenuation Mechanism and Live Vaccine Potential of a Low-Virulence Edwardsiella ictaluri Strain Obtained by Rifampicin Passaging Culture

The rifampicin-resistant strain E9-302 of Edwardsiella ictaluri strain 669 (WT) was generated by continuous passage on BHI agar plates containing increasing concentrations of rifampicin. E9-302 was attenuated significantly by 119 times to zebrafish Danio rerio compared to WT in terms of the 50% lethal dose (LD₅₀). Zebrafish vaccinated with E9-302 via intraperitoneal (IP) injection at a dose of 1 × 10³ CFU/fish had relative percentage survival (RPS) rates of 85.7% when challenged with wild-type E. ictaluri via IP 14 days post-vaccination (dpv). After 14 days of primary vaccination with E9-302 via immersion (IM) at a dose of 4 × 10⁷ CFU/ml, a booster IM vaccination with E9-302 at a dose of 2 × 10⁷ CFU/ml exhibited 65.2% RPS against challenge with wild-type E. ictaluri via IP 7 days later. These results indicated that the rifampicin-resistant attenuated strain E9-302 had potential as a live vaccine against E. ictaluri infection. A previously unreported amino acid site change at position 142 of the RNA polymerase (RNAP) β subunit encoded by the gene rpoB associated with rifampicin resistance was identified. Analysis of the whole-genome sequencing results revealed multiple missense mutations in the virulence-related genes esrB and sspH2 in E9-302 compared with WT, and a 189 bp mismatch in one gene, whose coding product was highly homologous to glycosyltransferase family 39 protein. This study preliminarily explored the molecular mechanism underlying the virulence attenuation of rifampicin-resistant strain E9-302 and provided a new target for the subsequent study of the pathogenic mechanism of E. ictaluri.

Factors Affecting Post-Challenge Survival of Flavobacterium psychrophilum in Susceptible Rainbow Trout from the Literature

Infectious bacterial pathogens are a concern for aquaculture as estimates suggest that billions of US dollars are lost annually in aquaculture due to disease. One of the most prevalent salmonid pathogens is the bacterium Flavobacterium psychrophilum that causes bacterial coldwater disease. We reviewed the published F. psychrophilum literature and conducted a Bayesian analysis to examine large-scale patterns in rainbow trout (Oncorhynchus mykiss) mortality associated with laboratory challenge. We incorporated factors that were common across a majority of the laboratory exposure studies and these included bacterial dose, culture time, exposure method, bacterial isolate, experimental duration, and fish weight. The comparison showed that injection as the exposure method produced higher mortality than bath immersion, bacterial isolates differed in their effect on mortality, and bacterial dose has an interactive effect with fish weight and exposure method. Our comparison allows for inference on factors affecting rainbow trout mortality due to exposure to F. psychrophilum and suggests avenues to further optimize research protocols to better reach study goals.

A large-scale, multi-year microbial community survey of a freshwater trout aquaculture facility

Aquaculture is an important tool for solving the growing worldwide food demand, but infectious diseases of farmed animals represent a serious roadblock to continued industry growth. Therefore, it is essential to understand the microbial communities that reside within the built environments of aquaculture facilities to identify reservoirs of bacterial pathogens and potential correlations between commensal species and specific disease agents. Here, we present the results from 3 years of sampling a commercial rainbow trout aquaculture facility. We observed that the microbial communities residing on the abiotic surfaces within the hatchery were distinct from those residing on the surfaces at the facility's water source as well as the production raceways, despite similar communities in the water column at each location. Also, a subset of the water community seeds the biofilm communities. Lastly, we detected a common fish pathogen, Flavobacterium columnare, within the hatchery, including at the source water inlet. Importantly, the relative abundance of this pathogen was correlated with clinical disease. Our results characterized the microbial communities in an aquaculture facility, established that the hatchery environment contains a unique community composition and demonstrated that a specific fish pathogen resides within abiotic surface biofilms and is seeded from the natural water source.

Development and efficacy of Streptococcus iniae live-attenuated vaccines in Nile tilapia, Oreochromis niloticus

Streptococcus iniae is a re-emerging bacterial pathogen in freshwater and marine aquaculture worldwide. There are no commercial vaccines available for S. iniae in the United States, and autogenous vaccines are restricted to inactivated whole-cell preparations with limited protection against heterogenous strains. Live-attenuated vaccines (LAV) represent an advantageous alternative to these bacterins, as they induce robust cellular and humoral immunity, and may provide longer lasting protection through less stressful routes of administration. We investigated whether accumulation of mutations in S. iniae by serial passage in the presence of rifampin can generate immunogenic LAV conferring protection against challenge with heterologous wild-type (WT) S. iniae strains in Nile tilapia (Oreochromis niloticus). Three lineages of rifampin-resistant S. iniae strains were generated from three genetically distinct parent strains (n = 9) by multiple passages in increments of Rifamycin SV sodium salt. Growth in liquid media, extent of capsulation, antimicrobial susceptibility, survival in Nile tilapia whole blood, and cytotoxicity in an O. mossambicus endothelial cell line were compared between the passaged and WT strains. Nile tilapia challenges were used to assess strain virulence, generation of anti-S. iniae IgM, and the protection conferred by LAV candidates against virulent S. iniae. Rifampin-resistant strains demonstrated changes in growth rate and cytotoxicity in endothelial cells, as well as significant reductions in whole blood survival (p < 0.05). Selected strains also showed attenuated virulence in the Nile tilapia challenge model, and anti-S. iniae IgM generated against these strains demonstrated cross-reactivity against heterologous bacteria. Immunization by intracoelomic injection induced protection against a virulent WT strain of S. iniae, with relative percent survival up to 95.05%.

Heterogeneous fitness landscape cues, pknG low expression, and phthiocerol dimycocerosate low production of Mycobacterium tuberculosis ATCC25618 rpoB S450L in enriched broth

Multidrug-resistant tuberculosis (isoniazid/rifampin[RIF]-resistant TB) ravages developing countries. Fitness is critical in clinical outcomes. Previous studies on RIF-resistant TB (RR-TB) showed competitive fitness gains and losses, with rpoB-S450L as the most isolated/fit mutation. This study measured virulence/resistance genes, phthiocerol dimycocerosate (PDIM) levels and their relationship with rpoB S450L ATCC25618 RR-TB strain fitness. After obtaining 10 different RR-TB GenoType MTBDRplus 2.0-genotyped isolates (with nontyped, S441, H445 and S450 positions), only one S450L isolate (R9, rpoB-S450L ATCC 25618, RR 1 μg/mL) was observed, with H445Y being the most common. A competitive fitness in vitro assay with wild-type (wt) ATCC 25618: R9 1:1 in 50 mL Middlebrook 7H9/OADC was performed, and generation time (G) in vitro and relative fitness were obtained. mRNA and PDIM were extracted on log and stationary phases. Fitness decreased in rpoB S450L and H445Y strains, with heterogeneous fitness cues in three biological replicas of rpoB-S450L: one high and two low fitness replicas. S450L strain had significant pknG increase. Compared with S450L, wt-rpoB showed increased polyketide synthase ppsA expression and high PDIM peak measured by HPLC-MS in log phase compared to S450L. This contrasts with previously increased PDIM in other RR-TB isolates.

Deciphering the Molecular Basis for Attenuation of Flavobacterium columnare Strain Fc1723 Used as Modified Live Vaccine against Columnaris Disease

Vaccines are widely employed in aquaculture to prevent bacterial infections, but their use by the U.S. catfish industry is very limited. One of the main diseases affecting catfish aquaculture is columnaris disease, caused by the bacterial pathogen Flavobacterium columnare. In 2011, a modified-live vaccine against columnaris disease was developed by selecting mutants that were resistant to rifampin. The previous study has suggested that this vaccine is stable, safe, and effective, but the mechanisms that resulted in attenuation remained uncharacterized. To understand the molecular basis for attenuation, a comparative genomic analysis was conducted to identify specific point mutations. The PacBio RS long-read sequencing platform was used to obtain draft genomes of the mutant attenuated strain (Fc1723) and the parent virulent strain (FcB27). Sequence-based genome comparison identified 16 single nucleotide polymorphisms (SNP) unique to the mutant. Genes that contained mutations were involved in rifampin resistance, gliding motility, DNA transcription, toxin secretion, and extracellular protease synthesis. The results also found that the vaccine strain formed biofilm at a significantly lower rate than the parent strain. These observations suggested that the rifampin-resistant phenotype and the associated attenuation of the vaccine strain result from the altered activity of RNA polymerase (RpoB) and possible disrupted protein secretion systems.

Quantification and comparison of gene expression associated with iron regulation and metabolism in a virulent and attenuated strain of Flavobacterium psychrophilum

Iron is essential for growth and virulence in most pathogenic bacterial strains. In some cases, the hosts for these pathogenic bacteria develop specialized strategies to sequester iron and limit infectivity. This in turn may result in the invading pathogens utilizing high-affinity iron transport mechanisms, such as the use of iron-chelating siderophores, to extend beyond the host defences. Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease (BCWD) in salmonids, relies on iron metabolism for infectivity, and the genome of the model CSF-259-93 strain has recently been made available. Further, this strain serves as a parent strain for a live-attenuated vaccine strain, B.17, which has been shown to provide rainbow trout with protection against BCWD. To elucidate specific gene expression responses to iron metabolism and compare strain differences, both F. psychrophilum strains were grown under iron-limiting conditions and 26 genes related to iron metabolism were mapped for 96 hr in culture via qPCR analyses. Results indicate increased production of the ferrous iron transport protein B (FITB; p =.008), and ferric receptor CfrA (FR 1; p =.012) in the wild-type CSF-259-93 strain at 72 hr and 96 hr post-exposure to iron-limiting media. In the B.17 vaccine strain, siderophore synthase (SS) expression was found to be downregulated at 72 hr, in comparison with 0h (p =.018). When strains were compared, FITB (p =.021), FR1 (p =.009) and SS (p =.016) were also elevated in B.17 at 0 hr and TonB outer protein membrane receptor 1 (TBomr1; p =.005) had a lower expression at 96 hr. Overall, this study demonstrated strain-related gene expression changes in only a fraction of the iron metabolism genes tested; however, results provide insight on potential virulence mechanisms and clarification on iron-related gene expression for F. psychrophilum.

An effective live attenuated vaccine against Aeromonas veronii infection in the loach (Misgurnus anguillicaudatus)

Aeromonas veronii is a major pathogenic bacterium in humans and animals. When it causes outbreaks, there are enormous economic losses to the aquaculture industry. An effective live attenuated vaccine strain, ΔhisJ, was obtained in our previous studies by gene knockout in Aeromonas veronii TH0426 using the suicide vector pRE112. Here, we evaluated whether the live attenuated vaccine ΔhisJ was suitable for prevention of Aeromonas veronii infection by injection and immersion in loaches. Compared with that of the TH0426 wild-type strain, the virulence of the live vaccine was significantly weakened. Vaccine safety assessment results also indicated that 1 × 10⁷ CFU/mL live vaccine was safe and did not induce clinical symptoms or obvious pathological changes. Additionally, after challenging loaches with Aeromonas veronii TH0426, the relative percent survival of the IN3 injection group was 65.66%, and that of the IM group was 50.78%. Our data show that the live attenuated vaccine ΔhisJ can improve the immune protection rate of loaches. Furthermore, increased enzyme activity parameters (SOD, LZM, ACP, and AKP) in the skin mucus, increased enzyme activity parameters (SOD, LZM, ACP, AKP, and GPx) in the serum, increased specific IgM antibodies and cytokine IL-1β contents in the serum, and increased cytokine (IL-15, pIgR, IL-1β, and TNF-α) expression in the liver and spleen were observed. These data are the first to indicate that the live attenuated vaccine ΔhisJ is suitable for the development of a safe and effective vaccine against Aeromonas veronii infection in loach aquaculture.

The Type IX Secretion System Is Required for Virulence of the Fish Pathogen Flavobacterium psychrophilum

Flavobacterium psychrophilum causes bacterial cold-water disease in wild and aquaculture-reared fish and is a major problem for salmonid aquaculture. The mechanisms responsible for cold-water disease are not known. It was recently demonstrated that the related fish pathogen, Flavobacterium columnare, requires a functional type IX protein secretion system (T9SS) to cause disease. T9SSs secrete cell surface adhesins, gliding motility proteins, peptidases, and other enzymes, any of which may be virulence factors. The F. psychrophilum genome has genes predicted to encode components of a T9SS. Here, we used a SacB-mediated gene deletion technique recently adapted for use in the Bacteroidetes to delete a core F. psychrophilum T9SS gene, gldN The ΔgldN mutant cells were deficient for secretion of many proteins in comparison to wild-type cells. Complementation of the mutant with wild-type gldN on a plasmid restored secretion. Compared to wild-type and complemented strains, the ΔgldN mutant was deficient in adhesion, gliding motility, and extracellular proteolytic and hemolytic activities. The ΔgldN mutant exhibited reduced virulence in rainbow trout and complementation restored virulence, suggesting that the T9SS plays an important role in the disease.IMPORTANCE Bacterial cold-water disease, caused by F. psychrophilum, is a major problem for salmonid aquaculture. Little is known regarding the virulence factors involved in this disease, and control measures are inadequate. A targeted gene deletion method was adapted to F. psychrophilum and used to demonstrate the importance of the T9SS in virulence. Proteins secreted by this system are likely virulence factors and targets for the development of control measures.

Understanding the pathogenesis of Flavobacterium psychrophilum using the rainbow trout monocyte/macrophage-like cell line, RTS11, as an infection model

The life cycle of Flavobacterium psychrophilum (Fp), the causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), appears to involve interactions with spleen and head kidney macrophages. To develop an in vitro model for studying this, F. psychrophilum was incubated with a rainbow trout splenic monocyte/macrophage-like cell line (RTS11) and fundamental macrophage functions evaluated. The animal cell basal medium, L15, supplemented with bovine serum (FBS) supports RTS11 maintenance, and surprisingly, L15 with 2% FBS (L15/FBS) also supported F. psychrophilum growth. L15/FBS in which the bacteria had been grown is referred to as F. psychrophilum conditioned medium (FpCM). Adding FpCM to RTS11 cultures caused a small, yet significant, percentage of cells to die, many cells to become more diffuse, and phagocytosis to be temporarily reduced. FpCM also significantly stimulated transcript expression for pro-inflammatory cytokines (IL-1β, TNFα and IL-6) and the anti-inflammatory cytokine (IL-10) after one day of exposure but this upregulation rapidly declined over time. Adding live F. psychrophilum to RTS11 cultures also altered the cellular morphology and stimulated cytokine expression more profoundly than FpCM. Additionally, the phagocytic activity of RTS11 was also significantly impaired by live F. psychrophilum, but not to the same extent as when exposed to FpCM. Adding heat-killed bacteria to RTS11 cultures elicited few changes. These bacteria/RTS11 co-cultures should be useful for gaining a deeper understanding of the pathogenesis of F. psychrophilum and may aid in the development of effective measures to prevent infection and spread of this troublesome disease.

Enhanced efficacy of immersion vaccination in tilapia against columnaris disease by chitosan-coated "pathogen-like" mucoadhesive nanovaccines

Red tilapia (Oreochromis sp.) has become one of the most important fish in aquaculture. Bacterial infection caused by Flavobacterium columnare, the causative agent of columnaris disease, has been now identified as one of the most serious infectious diseases in farmed red tilapia and cause major financial damage to the producers. Among the effective prevention and control strategies, vaccination is one of the most effective approach. As the surface of living fish is covered by mucus and directly associated with the mucosal immunity, we therefore hypothesized that better adsorption on mucosal surfaces and more efficient vaccine efficacy could be enhanced biomimetic nanoparticles mimicking the mucoadhesive characteristic of live F. columnare. In this work, we describe an effective approach to targeted antigen delivery by coating the surface of nanoparticles with mucoadhesive chitosan biopolymer to provide "pathogen-like" properties that ensure nanoparticles binding on fish mucosal membrane. The physiochemical properties of nanovaccines were analyzed, and their mucoadhesive characteristics and immune response against pathogens were also evaluated. The prepared vaccines were nano-sized and spherical as confirmed by scanning electron microscope (SEM). The analysis of hydrodynamic diameter and zeta-potential also suggested the successful modification of nanovaccines by chitosan as indicated by positively charged and the overall increased diameter of chitosan-modified nanovaccines. In vivo mucoadhesive study demonstrated the excellent affinity of the chitosan-modified nanovaccines toward fish gills as confirmed by bioluminescence imaging, fluorescent microscopy, and spectrophotometric quantitative measurement. Following vaccination with the prepared nanovaccines by immersion 30 min, the challenge test was then carried out 30 and 60 days post-vaccination and resulted in high mortalities in the control. The relative percent survival (RPS) of vaccinated fish was greater than 60% for mucoadhesive nanovaccine. Our results also suggested that whole-cell vaccines failed to protect fish from columnaris infection, which is consistent with the mucoadhesive assays showing that whole-cell bacteria were unable to bind to mucosal surfaces. In conclusion, we could use this system to deliver antigen preparation to the mucosal membrane of tilapia and obtained a significant increase in survival compared to controls, suggesting that targeting mucoadhesive nanovaccines to the mucosal surface could be exploited as an effective method for immersion vaccination.

Changes in physiology and virulence during the selection of resistant Yersinia ruckeri mutants under subinhibitory cefotaxime concentrations

Bacterial antibiotic resistance is one of the main healthcare problems currently. Apart from reducing antibiotic efficacy, it has awakened the interest of scientists due to its association with bacterial fitness and virulence. Interestingly, antibiotic resistance can be a source of both increased fitness and decreased fitness, even though the molecular basis of these relationships remains unknown. The aim of this work is to define the effects of sub-MIC concentrations of cefotaxime, an antibiotic extensively used in clinical practice, on the physiology and virulence of Yersinia ruckeri and to determine the importance of these sub-MIC concentrations for the selection of antibiotic-resistant mutants in the aquatic environment. Results indicated that exposure to sub-MIC concentrations of cefotaxime selected Y. ruckeri populations with irreversible alterations in the physiology, such as slow growth, aggregation in liquid cultures and modification of the colony morphology. These bacteria also displayed changes in the OMPs and LPS profiles and a full attenuation of virulence. An overexpression of the envelope stress regulator RpoE was also detected after exposure to the antibiotic. In conclusion, exposure to cefotaxime selected, at high frequency, Y. ruckeri strains that survive the antibiotic stress at the expense of a fitness cost and the loss of virulence.

A Review of Fish Vaccine Development Strategies: Conventional Methods and Modern Biotechnological Approaches

Fish immunization has been carried out for over 50 years and is generally accepted as an effective method for preventing a wide range of bacterial and viral diseases. Vaccination efforts contribute to environmental, social, and economic sustainability in global aquaculture. Most licensed fish vaccines have traditionally been inactivated microorganisms that were formulated with adjuvants and delivered through immersion or injection routes. Live vaccines are more efficacious, as they mimic natural pathogen infection and generate a strong antibody response, thus having a greater potential to be administered via oral or immersion routes. Modern vaccine technology has targeted specific pathogen components, and vaccines developed using such approaches may include subunit, or recombinant, DNA/RNA particle vaccines. These advanced technologies have been developed globally and appear to induce greater levels of immunity than traditional fish vaccines. Advanced technologies have shown great promise for the future of aquaculture vaccines and will provide health benefits and enhanced economic potential for producers. This review describes the use of conventional aquaculture vaccines and provides an overview of current molecular approaches and strategies that are promising for new aquaculture vaccine development.

Efficacy of live attenuated vaccine derived from the Streptococcus agalactiae on the immune responses of Oreochromis niloticus

Streptococcus agalactiae species have been recognized as the main pathogen causing high mortality in fish leading to significant worldwide economical losses to the aquaculture industries. Vaccine development has become a priority in combating multidrug resistance in bacteria; however, there is a lack of commercial live attenuated vaccine (LAV) against S. agalactiae in Malaysia. The aim of this study is to compare two methods using attenuated bacteria as live vaccine and to evaluate the efficacy of selected LAV on the immune responses and resistance of Oreochromis niloticus (tilapia) against S. agalactiae. The LAV derived from S. agalactiae had been weakened using the chemical agent Acriflavine dye (LAV1), whereas the second vaccine was weakened using serial passages of bacteria on broth media (LAV2). Initial immunization was carried out only on day one, given twice-in the morning and evening, for the 42 day period. Serum samples were collected to determine the systemic antibody (IgM) responses and lysozymal (LSZ) activity using ELISA. On day 43 after immunization, the fish were injected intraperitoneally (i.p) with 0.1 mL of S. agalactiae at LD₅₀ = 1.5 × 10⁵ (CFU)/fish. Fish were monitored daily for 10 days. Clinical signs, mortality and the relative percent of survival (RPS) were recorded. Trial 1 results showed a significant increased (P < 0.05) in serum IgM titers and LSZ activity as compared to LAV2 and the control group (unvaccinated fish). The efficacy of LAV1 was proven effective as determined by the RPS values, LAV1 at 81.58% as compared to LAV2 at 65.79%. Trial 2 of LAV1 and control group were further determined by administering primary and booster doses revealed a RPS value for LAV1 of 82.05%, with the significant enhancement on the immune responses of tilapia as compared to control group. In conclusion, LAV revealed to elevate antibody IgM levels, LSZ activity and provide long-term protection when added to feed. LAV is a low-cost vaccine shown to rapidly increase the immune response of fish and increase survival rates of fish against S. agalactiae infection.

Assessment of cross-protection to heterologous strains of Flavobacterium psychrophilum following vaccination with a live-attenuated coldwater disease immersion vaccine

Bacterial coldwater disease, caused by Flavobacterium psychrophilum, remains one of the most significant bacterial diseases of salmonids worldwide. A previously developed and reported live-attenuated immersion vaccine (F. psychrophilum; B.17-ILM) has been shown to confer significant protection to salmonids. To further characterize this vaccine, a series of experiments were carried out to determine the cross-protective efficacy of this B.17-ILM vaccine against 9 F. psychrophilum isolates (representing seven sequence types/three clonal complexes as determined by multilocus sequence typing) in comparison with a wild-type virulent strain, CSF-259-93. To assess protection, 28-day experimental challenges of rainbow trout (Oncorhynchus mykiss) fry were conducted following immersion vaccinations with the B.17-ILM vaccine. F. psychrophilum strains used in challenge trials were isolated from several fish species across the globe; however, all were found to be virulent in rainbow trout. The B.17-ILM vaccine provided significant protection against all strains, with relative percent survival values ranging from 51% to 72%. All vaccinated fish developed an adaptive immune response (as measured by F. psychrophilum-specific antibodies) that increased out to the time of challenge (8 weeks postimmunization). Previous studies have confirmed that antibody plays an important role in protection against F. psychrophilum challenge; therefore, specific antibodies to the B.17-ILM vaccine strain appear to contribute to the cross-protection observed to heterologous strain. The ability of such antibodies to bind to similar antigenic regions for all strains was confirmed by western blot analyses. Results presented here support the practical application of this live-attenuated vaccine, and suggest that it will be efficacious even in aquaculture operations affected by diverse strains of F. psychrophilum.

Enhanced degradation of chlorpyrifos in rice (Oryza sativa L.) by five strains of endophytic bacteria and their plant growth promotional ability

Endophytic bacteria reside in plant tissues, such as roots, stems, leaves and seeds. Most of them can stimulate plant growth or alleviate phytotoxicity of pollutants. There are handful species with dual functions stimulating plant growth and degrading pollutants have been reported. Five endophytic bacteria were isolated from chlorpyrifos (CP) treated rice plants and identified as Pseudomonas aeruginosa strain RRA, Bacillus megaterium strain RRB, Sphingobacterium siyangensis strain RSA, Stenotrophomonas pavanii strain RSB and Curtobacterium plantarum strain RSC according to morphological characteristics, physiological and biochemical tests, and 16S rDNA phylogeny. All of them possessed some plant growth promotional traits, including indole acetic acid and siderophore production, secretion of phosphate solubilization and 1-aminocyclopropane-1-carboxylate deaminase. The bacteria were marked with the green fluorescent protein (gfp) gene and successfully colonized into rice plants. All isolates were able to degrade CP in vitro and in vivo. The five isolates degraded more than 90% of CP in 24 h when the initial concentration was lower than 5 mg/L. CP degradation was significantly enhanced in the infested rice plants and rice grains. The final CP residual was reduced up to 80% in the infested rice grains compared to the controls. The results indicate that these isolates are promising bio-inoculants for the removal or detoxification of CP residues in rice plants and grains.

Efficacy of a polyvalent immersion vaccine against Flavobacterium psychrophilum and evaluation of immune response to vaccination in rainbow trout fry (Onchorynchus mykiss L.)

Rainbow trout fry syndrome (RTFS) is a disease caused by the Gram-negative bacterium Flavobacterium psychrophilum, responsible for significant economic losses in salmonid aquaculture worldwide. The diversity of F. psychrophilum isolates and the inherent difficulties in vaccinating juvenile fish has hampered the development of a vaccine for RTFS. Disease episodes tend to occur between 10–14 °C with necrotic lesions often seen on the skin surrounding the dorsal fin and tail. At present no commercial vaccines are available for RTFS in the UK, leaving antibiotics as the only course of action to control disease outbreaks. The current work was performed as a pilot study to assess the efficacy of a polyvalent, whole cell vaccine containing formalin-inactivated F. psychrophilum, to induce protective immunity in rainbow trout fry. Duplicate groups of 30 trout (5 g) were immersed in 1 L of the vaccine for 30 s. Samples were taken 4 h, day 2 and 7 post-vaccination (pv) of skin mucus, tissues for histology and gene expression analysis; serum and histology samples were taken 6 weeks pv. A booster vaccination was given at 315 degree days (dd) also by immersion. Challenge was by immersion with a heterologous isolate of F. psychrophilum 630 dd post primary vaccination. The vaccine provided significant protection to the trout fry with a RPS of 84% (p < 0.0001). Detection of increased numbers of IgT positive cells in systemic organs, up-regulation of IgT expression in hind-gut and an increase in total IgT in serum was observed in vaccinated fish; however a functional role of IgT in the observed protection remains to be demonstrated.

Isolation and Characterization of the Lytic Cold-Active Bacteriophage MYSP06 from the Mingyong Glacier in China

As unique ecological systems, glaciers are characterized by low temperatures and low nutrient levels, which allow them to be considered as “living fossils” for the purpose of researching the evolution of life and the environmental evolution of the earth. Glaciers are also natural microbial “reservoirs”. In this work, a lytic cold-active bacteriophage designated MYSP06 was isolated from Janthinobacterium sp. MYB06 from the Mingyong Glacier in China, and its major characteristics were determined. Electron microscopy revealed that bacteriophage MYSP06 had an isometric head (74 nm) and a long tail (10 nm in width, 210 nm in length). It was classified as a Siphoviridae with an approximate genome size of 65–70 kb. A one-step growth curve revealed that the latent and burst periods were 95 and 65 min, respectively, with an average burst size of 16 bacteriophage particles per infected cell. The bacteriophage particles (100 %) adsorbed to the host cells within 10 min after infection. Moreover, the pH value and thermal stability of bacteriophage MYSP06 were also investigated. The maximum stability of the bacteriophage was observed at the optimal pH 7.0, and the bacteriophage became completely unstable at the extremely alkaline pH 11.0; however, it was comparatively stable at the acidic alkaline pH 6.0. As MYSP06 is a cold-active bacteriophage with a lower production temperature, its characterization and its relationship with its host Janthinobacterium sp. MYB06 deserve further study.

Optimization of efficacy of a live attenuated Flavobacterium psychrophilum immersion vaccine

This study was aimed at optimizing the efficacy of a recently developed live attenuated immersion vaccine (B.17-ILM) as a promising vaccine against bacterial coldwater disease (BCWD) caused by Flavobacterium psychrophilum in salmonids. Rainbow trout (RBT) fry were vaccinated by immersion, and different parameters affecting vaccination such as fish size, vaccine delivery time, dose, duration of protection, booster regimes and vaccine growth incubation time were optimized. Specific anti-F. psychrophilum immune response was determined by ELISA. Protective efficacy was determined by challenging with a virulent strain of F. psychrophilum (CSF-259-93) and calculating cumulative percent mortality (CPM) and relative percent survival (RPS). All vaccinated fish developed significantly higher levels of serum antibody titers by week 8 when compared to their respective controls. Immersion vaccination for 3, 6 and 30 min produced significant protection with comparable RPS values of 47%, 53% and 52%, respectively. This vaccine provided significant protection for fish as small as 0.5 g with an RPS of 55%; larger fish of 1 g and 2 g yielded slightly higher RPS values of 59% and 60%, respectively. Fish vaccinated with higher vaccine doses of ∼10(10) and 10(8) colony forming units mL(-1) (cfu ml(-1)) were strongly protected out to at least 24 weeks with RPS values up to 70%. Fish vaccinated with lower doses (∼10(6) and 10(5) cfu mL(-1)) had good protection out to 12 weeks, but RPS values dropped to 36% and 34%, respectively by 24 weeks. Vaccine efficacy was optimum when the primary vaccination was followed by a single booster (week 12 challenge RPS = 61%) rather than two boosters (week 12 challenge RPS = 48%). Vaccination without a booster resulted in a lower RPS (13%) indicating the necessity of a single booster vaccination to maximize efficacy. This study presents key findings that demonstrate the efficacy and commercial potential for this live attenuated BCWD vaccine.

Microencapsulation of a putative probiotic Enterobacter species, C6-6, to protect rainbow trout, Oncorhynchus mykiss (Walbaum), against bacterial coldwater disease

Flavobacterium psychrophilum is the causative agent of bacterial coldwater disease (BCWD), which has a major impact on salmonid aquaculture globally. An Enterobacter species, C6-6, isolated from the gut of rainbow trout, Oncorhynchus mykiss (Walbaum), has been identified as a potential probiotic species providing protection against BCWD. This study examined the effects of alginate microencapsulation on the protective efficacy of C6-6 against BCWD in vivo when administered to rainbow trout fry orally or by intraperitoneal (IP) injection. Viable C6-6 bacteria were microencapsulated successfully, and this process (microencapsulation) did not significantly deteriorate its protective properties as compared to the administration of non-microencapsulated C6-6 bacteria. Both oral and IP delivery of C6-6 achieved significantly better protection than control treatments that did not contain C6-6 bacteria. The highest relative percent survival (RPS) resulted from IP delivery (71.4%) and was significantly greater than the highest oral RPS (38.6%). Successful intestinal colonization was not critical to protective effects of C6-6. The study showed that C6-6 administration, with or without encapsulation, was a viable choice for protecting fry from BCWD especially when administered intraperitoneally.

Potential mechanisms of attenuation for rifampicin-passaged strains of Flavobacterium psychrophilum

Background

Flavobacterium psychrophilum is the etiologic agent of bacterial coldwater disease in salmonids. Earlier research showed that a rifampicin-passaged strain of F. psychrophilum (CSF 259-93B.17) caused no disease in rainbow trout (Oncorhynchus mykiss, Walbaum) while inducing a protective immune response against challenge with the virulent CSF 259–93 strain. We hypothesized that rifampicin passage leads to an accumulation of genomic mutations that, by chance, reduce virulence. To assess the pattern of phenotypic and genotypic changes associated with passage, we examined proteomic, LPS and single-nucleotide polymorphism (SNP) differences for two F. psychrophilum strains (CSF 259–93 and THC 02–90) that were passaged with and without rifampicin selection.

Results

Rifampicin resistance was conveyed by expected mutations in rpoB, although affecting different DNA bases depending on the strain. One rifampicin-passaged CSF 259–93 strain (CR) was attenuated (4 % mortality) in challenged fish, but only accumulated eight nonsynonymous SNPs compared to the parent strain. A CSF 259–93 strain passaged without rifampicin (CN) accumulated five nonsynonymous SNPs and was partially attenuated (28 % mortality) compared to the parent strain (54.5 % mortality). In contrast, there were no significant change in fish mortalities among THC 02–90 wild-type and passaged strains, despite numerous SNPs accumulated during passage with (n = 174) and without rifampicin (n = 126). While only three missense SNPs were associated with attenuation, a Ser492Phe rpoB mutation in the CR strain may contribute to further attenuation. All strains except CR retained a gliding motility phenotype. Few proteomic differences were observed by 2D SDS-PAGE and there were no apparent changes in LPS between strains. Comparative methylome analysis of two strains (CR and TR) identified no shared methylation motifs for these two strains.

Conclusion

Multiple genomic changes arose during passage experiments with rifampicin selection pressure. Consistent with our hypothesis, unique strain-specific mutations were detected for the fully attenuated (CR), partially attenuated (CN) and another fully attenuated strain (B17).

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0518-1) contains supplementary material, which is available to authorized users.

Systemic and mucosal immune response of rainbow trout to immunization with an attenuated Flavobacterium psychrophilum vaccine strain by different routes

Teleosts possess three immunoglobulin (Ig) heavy chain isotypes viz., IgM, IgT and IgD and all three isotypes are reported in rainbow trout. The expression of these Ig isotypes in response to different immunization routes was investigated and results provide a better understanding of the role these Igs in different tissues. Rainbow trout (Oncorhynchus mykiss) were immunized with an attenuated Flavobacterium psychrophilum strain, 259-93-B.17 grown under iron limiting conditions, by intraperitoneal, anal intubation and immersion routes. Serum, gill mucus, skin mucus and intestinal mucus samples were collected at 0, 3, 7, 14, 28, 42 and 56 days post immunization by sacrificing four fish from each treatment group and the unimmunized control group, and the IgM levels were estimated by an enzyme linked immunosorbent assay (ELISA). In addition, blood, gill, skin and intestinal tissue samples were collected for Ig gene expression studies. The secretory IgM, IgD and IgT gene expression levels in these tissues were estimated by reverse transcription quantitative real time PCR (RT-qPCR). Levels of IgM in serum, gill and skin mucus increased significantly by 28 days after immunization in the intraperitoneally immunized group, while no significant increase in IgM level was observed in fish groups immunized by other routes. Secretory IgD and IgT expression levels were significantly upregulated in gills of fish immunized by the immersion route. Similarly, secretory IgT and IgD were upregulated in intestines of fish immunized by anal intubation route. The results confirm mucosal association of IgT and suggest that IgD may also be specialized in mucosal immunity and contribute to immediate protection to the fish at mucosal surfaces.

Use of Penicillin and Streptomycin to Reduce Spread of Bacterial Coldwater Disease II: Efficacy of Using Antibiotics in Diluents and During Water Hardening

Bacterial coldwater disease, caused by Flavobacterium psychrophilum, has lead to the loss of significant numbers of hatchery-reared salmonids. The bacteria can be spread from parent to progeny within contaminated sperm and ovarian fluid and can enter the egg during fertilization. The addition of antibiotics to diluents and water-hardening solutions could prevent the spread of the disease. In separate trials, a mixture of 0.197 mg/mL penicillin plus 0.313 mg/mL streptomycin was added to both a 0.5% sodium chloride fertilization diluent and hatchery well water during hardening. Tests showed that the addition of the antibiotics to the diluent and during up to 60 min of water hardening had no effect on the eye-up, hatch and deformity rates of Rainbow Trout Oncorhynchus mykiss eggs compared with the nonantibiotic-treated controls. Also, significant reductions in the prevalence of F. psychrophilum on the surface and inside eggs were observed when compared with controls. These results indicate that the addition of penicillin and streptomycin to diluents and during water hardening can prevent the vertical transmission of bacterial coldwater disease.

Entericidin is required for a probiotic treatment (Enterobacter sp. strain C6-6) to protect trout from cold-water disease challenge

Flavobacterium psychrophilum causes bacterial cold-water disease in multiple fish species, including salmonids. An autochthonous Enterobacter strain (C6-6) inhibits the in vitro growth of F. psychrophilum, and when ingested as a putative probiotic, it provides protection against injection challenge with F. psychrophilum in rainbow trout. In this study, low-molecular-mass (≤3 kDa) fractions from both Enterobacter C6-6 and Escherichia coli K-12 culture supernatants inhibited the growth of F. psychrophilum. The ≤3-kDa fraction from Enterobacter C6-6 was analyzed by SDS-PAGE, and subsequent tandem mass spectroscopy identified EcnB, which is a small membrane lipoprotein that is a putative pore-forming toxin. Agar plate diffusion assays demonstrated that ecnAB knockout strains of both Enterobacter C6-6 and E. coli K-12 no longer inhibited F. psychrophilum (P < 0.001), while ecnAB-complemented knockout strains recovered the inhibitory phenotype (P < 0.001). In fish experiments, the engineered strains (C6-6 ΔecnAB and C6-6 ΔecnAB<pET101::ecnAB>) and the wild-type strain (C6-6) were added to the fish diet every day for 38 days. On day 11, the fish were challenged by injection with a virulent strain of F. psychrophilum (CSF 259-93). Fish that were fed C6-6 had significantly longer survival than fish fed the ecnAB knockout strain (P < 0.0001), while fish fed the complemented knockout strain recovered the probiotic phenotype (P = 0.61). This entericidin is responsible for the probiotic activity of Enterobacter C6-6, and it may present new opportunities for therapeutic and prophylactic treatments against similarly susceptible pathogens.

Complete Genome Sequence of Flavobacterium psychrophilum Strain CSF259-93, Used To Select Rainbow Trout for Increased Genetic Resistance against Bacterial Cold Water Disease

The genome sequence of Flavobacterium psychrophilum strain CSF259-93, isolated from rainbow trout (Oncorhynchus mykiss), consists of a single circular genome of 2,900,735 bp and 2,701 predicted open reading frames (ORFs). Strain CSF259-93 has been used to select a line of rainbow trout with increased genetic resistance against bacterial cold water disease.

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.

Enhanced efficacy of an attenuated Flavobacterium psychrophilum strain cultured under iron-limited conditions

An attenuated strain of Flavobacterium psychrophilum (CSF259-93B.17) has shown potential as a vaccine for prevention of bacterial coldwater disease (BCWD) in rainbow trout, Oncorhynchus mykiss (Walbaum). Because BCWD outbreaks can result in high mortality in other salmonid species, specifically coho salmon, Oncorhynchus kisutch (Walbaum), the live-attenuated strain was tested as a vaccine in this species. Additionally, we hypothesized that culture of the vaccine strain under iron-limited conditions would lead to improved protection against BCWD. To test this hypothesis, coho salmon were either injection or immersion immunized with CSF259-93B.17 cultured in iron-replete or iron-limited medium. Resultant antibody titers were low and not significantly different between the two treatments regardless of vaccine delivery method (P > 0.05). Following injection challenge with a virulent F. psychrophilum strain, mortality for injection vaccinated fish was significantly reduced compared to the control but did not differ by treatment (P > 0.05). Relative percent survival (RPS) was high in both treatments (90% in iron-replete, 98% in iron-limited medium). Fish immunized by immersion with CSF259-93B.17 grown in iron-replete medium exhibited lower mortality (29.3%; RPS 46%) when compared to mock immunized fish, but this was not significant. However, mortality was significantly lower in fish immunized with CSF259-93B.17 grown in iron-limited medium (14.7%; RPS 73%) when compared to mock immunized fish. The results demonstrate that the live-attenuated F. psychrophilum strain can confer protection to coho salmon and vaccine efficacy is enhanced by culturing the strain under iron-limited conditions.

Comparative proteomic analysis of virulent and rifampicin-attenuated Flavobacterium psychrophilum

Flavobacterium psychrophilum is the aetiologic agent of bacterial coldwater disease and rainbow trout fry syndrome. In this study, we compared a wild-type strain (CSF 259-93) with a rifampicin-resistant strain and virulence-attenuated strain of F. psychrophilum (CSF 259-93B.17). The attenuated strain harboured a mutation in the rpoB gene consistent with resistance to rifampicin. Two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) and mass spectrometry demonstrated an altered proteome with eight proteins characteristic for the parent strain and six that were unique to the attenuated strain. Immunoblotting with a diagnostic monoclonal antibody (FL-43) identified a putative antigen (FP1493) that was subsequently cloned, expressed as a recombinant protein and confirmed as recognized by FL-43. 2D-PAGE, immunoblotting with rainbow trout, Oncorhynchus mykiss (Walbaum), convalescent antisera and mass spectrometry of bacterial whole-cell lysates revealed several uniquely expressed immunoreactive proteins including FP1493. An FP1493 recombinant subunit vaccine was tested, but did not provide protection against challenge with the CSF259-93 strain. While the exact mechanism responsible for altered protein synthesis and attenuation of CSF 259-93B.17 is still unknown, the differentially expressed immunoreactive proteins are a valuable resource to develop subunit vaccines and to identify proteins that are potentially involved in disease.

Development and efficacy of an attenuated Vibrio harveyi vaccine candidate with cross protectivity against Vibrio alginolyticus

Vibrio harveyi is a Gram-negative bacterial pathogen that can infect a wide range of marine animals. In previous studies, we have reported a virulent V. harveyi strain, T4D. In the present study, an attenuated mutant of T4D, T4DM, was obtained by selection of rifampicin resistance. Compared to the wild type, T4DM was different in whole-cell protein profile and much slower in growth rate when cultured in stress conditions caused by iron depletion. Virulence analysis showed that compared to T4D, T4DM exhibited a dramatically increased median lethal dose, impaired tissue dissemination capacity, defective hemolytic activity, and significantly reduced resistance against the killing effect of host serum. To examine the potential of T4DM as a live attenuated vaccine, Japanese flounder (Paralichthys olivaceus) were vaccinated with T4DM via intraperitoneal injection or immersion. The results showed that at one and two months post-vaccination, fish administered with T4DM via both approaches, in particular that of immersion, were effectively protected against not only V. harveyi but also Vibrio alginolyticus, another important fish pathogen. Microbiological analysis showed that following immersion vaccination, T4DM was recovered from the internal organs of the vaccinated fish in a time-dependent manner within the first 6 days post-vaccination. Serum antibodies against V. harveyi and V. alginolyticus were detected in T4DM-vaccinated fish, and, compared to serum from control fish, serum from T4DM-vaccinated fish was significantly enhanced in bactericidal activity. These results indicate that T4DM is an attenuated strain with residual infectivity and that T4DM can induce effective cross-species protection against both V. harveyi and V. alginolyticus when used as a live immersion vaccine.

The use of warmed water treatment to induce protective immunity against the bacterial cold-water disease pathogen Flavobacterium psychrophilum in ayu (Plecoglossus altivelis)

We investigated the induction of protective immunity against bacterial cold-water disease (BCWD) caused by Flavobacterium psychrophilum by warmed water treatment in ayu (Plecoglossus altivelis). Fish were immersed in a live bacterial suspension (10⁷ CFU mL⁻¹) for 30 min and placed in 700 L concrete tanks. The 28 °C warmed water treatment lasted 3 days and began 1, 6, and 24 h after immersion in the live bacterial suspension. A naïve control fish group was immersed in a sterilized modified Cytophaga (MCY) broth instead of the bacterial suspension. Fourteen days after the immersion, agglutination antibody titers against F. psychrophilum were measured by using micro-titer methods. Fish were then exposed to a bacterial bath to infect them with live F. psychrophilum, and cumulative mortality was monitored. Fish treated with warmed water at 1, 6, and 24 h after immersion in the live bacterial suspension had cumulative mortalities of 36%, 30%, and 18%, respectively, all of which were significantly lower than the cumulative mortality of the naïve control fish (90%). Treated fish also showed high antibody titers against F. psychrophilum in agglutination tests. These results demonstrate that warmed water treatment could not only cure BCWD but also immunize the fish against the causative agent F. psychrophilum.

Isolation and analysis of the vaccine potential of an attenuated Edwardsiella tarda strain

Edwardsiella tarda is an important aquaculture pathogen that can infect a wide range of marine and freshwater fish worldwide. In this study, a modified E. tarda strain, TX5RM, was selected by multiple passages of the pathogenic E. tarda strain TX5 on growth medium containing the antibiotic rifampicin. Compared to the wild type strain, the rifampicin-resistant mutant TX5RM (i) shows drastically increased median lethal dose and reduced capacity to disseminate in and colonize fish tissues and blood; (ii) exhibits slower growth rates when cultured in rich medium or under conditions of iron depletion; and (iii) differs in the production profile of whole-cell proteins. The immunoprotective potential of TX5RM was examined in a Japanese flounder (Paralichthys olivaceus) model as a vaccine delivered via intraperitoneal injection, oral feeding, bath immersion, and oral feeding plus immersion. All the vaccination trials, except those of injection, were performed with a booster at 3-week after the first vaccination. The results showed that TX5RM administered via all four approaches produced significant protection, with the highest protection levels observed with TX5RM administered via oral feeding plus immersion, which were, in terms of relative percent of survival (RPS), 80.6% and 69.4% at 5- and 8-week post-vaccination, respectively. Comparable levels of specific serum antibody production were induced by TX5RM-vaccinated via different routes. Microbiological analyses showed that TX5RM was recovered from the gut, liver, and spleen of the fish at 1-10 days post-oral vaccination and from the spleen, liver, kidney, and blood of the fish at 1-14 days post-immersion vaccination. Taken together, these results indicate that TX5RM is an attenuated E. tarda strain with good vaccine potential and that a combination of oral and immersion vaccinations may be a good choice for the administration of live attenuated vaccines.

Characterization of susceptibility and carrier status of burbot, Lota lota (L.), to IHNV, IPNV, Flavobacterium psychrophilum, Aeromonas salmonicida and Renibacterium salmoninarum

In this study, susceptibility and potential carrier status of burbot, Lota lota, were assessed for five important fish pathogens. Burbot demonstrated susceptibility and elevated mortality following challenge with infectious haematopoietic necrosis virus (IHNV) by immersion and to Aeromonas salmonicida by intraperitoneal (i.p.) injection. IHNV persisted in fish for at least 28 days, whereas A. salmonicida was not re-isolated beyond 17 days post-challenge. In contrast, burbot appeared refractory to Flavobacterium psychrophilum following intramuscular (i.m.) injection and to infectious pancreatic necrosis virus (IPNV) by immersion. However, i.p injection of IPNV resulted in re-isolation of virus from fish for the duration of the 28 day challenge. Renibacterium salmoninarum appeared to induce an asymptomatic carrier state in burbot following i.p. injection, but overt manifestation of disease was not apparent. Viable bacteria persisted in fish for at least 41 days, and bacterial DNA isolated by diagnostic polymerase chain reaction was detected from burbot kidney tissue 90 days after initial exposure. This study is the first to investigate susceptibility of burbot to selected fish pathogens, and this information will aid in efforts to culture and manage this species.

Isolation and identification of bacteriophages infecting ayu Plecoglossus altivelis altivelis specific Flavobacterium psychrophilum

In order to investigate methods for controlling systemic bacterial coldwater disease (CWD), bacteriophages that infect Flavobacterium psychrophilum were isolated by the enrichment method from pond water collected from Japanese ayu farms. The five phages isolated were classified as members of Myoviridae (PFpW-3, PFpC-Y), Podoviridae (PFpW-6, PFpW-7), and Siphoviridae (PFpW-8) and had highly variable patterns of infectivity for different F. psychrophilum isolates (n=128). The stability tests of the phages in different waters, pHs and temperatures were assessed, and the results indicated that none of the phages were affected by ayu farm conditions. Among the phages, PFpW-3 had high infectivity for F. psychrophilum isolated from ayu and other fish and demonstrated sufficient survivability in the stability tests. Thus, PFpW-3 and its indicator strain N2-3 were inoculated into cytophaga broth at different doses of multiplicity of infection (MOI) and proved to be efficient for the reduction of bacterial growth. This study may be the basis for a further evaluation of phage therapy in the treatment of CWD in Japanese ayu farms.