Tissue-specific differences in detection of Yersinia ruckeri carrier status in rainbow trout (Oncorhynchus mykiss)

Effective monitoring for subclinical infections is a cornerstone of proactive disease management in aquaculture. Salmonid fish that survive enteric redmouth disease (ERM) can carry Yersinia ruckeri as a latent infection for several months, potentially facilitating cryptic spread between facilities that exchange fish. In this study, fingerling rainbow trout (Oncorhynchus mykiss) were infected by immersion and sampled for up to 14 weeks post-infection. Yersinia ruckeri was cultured from the posterior kidney of more than 89% of fish up to 4 weeks post-infection, but from 2% or fewer of fish sampled at later time points. In contrast, qPCR-based detection of the Y. ruckeri 16s rRNA gene in intestine and spleen extracts revealed a much higher rate of infection: at 14 weeks post-infection Y. ruckeri was detected in nearly 50% of spleens and 15% of intestines. The difference between spleen and intestine is likely due at least in part to technical limitations of qPCR on intestinal DNA extracts; accordingly, we propose that qPCR of spleen DNA ought to be considered the preferred standard for detection of carriers of Y. ruckeri.

Yersinia ruckeri Isolated from Common Mudpuppy Necturus maculosus

During a routine health inspection of apparently healthy wild-caught common mudpuppies Necturus maculosus, the bacteria Yersinia ruckeri was isolated and the identity confirmed using biochemical and molecular methods. This represents the first isolation of Y. ruckeri from an amphibian. This finding increases the known host range capable of harboring this important fish pathogen and could have serious management implications for aquaculture. Furthermore, addressing wild amphibians in fish hatchery biosecurity plans is discussed.

A highly sensitive, non-invasive qPCR-based strategy for direct quantification of Yersinia ruckeri in fish faeces

Finfish with asymptomatic Yersinia ruckeri infections pose a major risk as they can transmit the pathogen and cause clinical outbreaks in stock populations. Current tools have insufficient quantitative ability for accurately detecting the trace levels of Y. ruckeri typically associated with asymptomatic infection, necessitate invasive or lethal sampling, or require long processing times. This study presents a highly sensitive qPCR-based method, targeting part of the Y. ruckeri 16S rRNA sequence, that is capable of detecting extremely low levels of Y. ruckeri in noninvasively collected faecal samples. Quantitative precision and accuracy of faecal sample analysis was consistent, despite the complexity of the faecal matrix. The assay demonstrated linearity over a six log-wide dynamic range. Its limit of detection (LOD) and limit of quantification (LOQ) were 4 and 10 copies of the target sequence, respectively. Sensitivity of the assay was comparable to other qPCR-based methods without requiring invasive or lethal sampling. Applicability as a screening strategy was tested using passively collected faecal samples. Asymptomatic Y. ruckeri infection was detected in all samples, although none of the fish exhibited overt infection. This method will be beneficial for finfish disease management if developed further as a noninvasive, screening tool against asymptomatic Y. ruckeri infection.

The repeat structure of two paralogous genes, Yersinia ruckeri invasin (yrInv) and a "Y. ruckeri invasin-like molecule", (yrIlm) sheds light on the evolution of adhesive capacities of a fish pathogen

Inverse autotransporters comprise the recently identified type Ve secretion system and are exemplified by intimin from enterohaemorrhagic Escherichia coli and invasin from enteropathogenic Yersiniae. These proteins share a common domain architecture and promote bacterial adhesion to host cells. Here, we identified and characterized two putative inverse autotransporter genes in the fish pathogen Yersinia ruckeri NVH_3758, namely yrInv (for Y. ruckeri invasin) and yrIlm (for Y. ruckeri invasin-like molecule). When trying to clone the highly repetitive genes for structural and functional studies, we experienced problems in obtaining PCR products. PCR failures and the highly repetitive nature of inverse autotransporters prompted us to sequence the genome of Y. ruckeri NVH_3758 using PacBio sequencing, which produces some of the longest average read lengths available in the industry at this moment. According to our sequencing data, YrIlm is composed of 2603 amino acids (7812bp) and has a molecular mass of 256.4kDa. Based on the new genome information, we performed PCR analysis on four non-sequenced Y. ruckeri strains as well as the sequenced. Y. ruckeri type strain. We found that the genes are variably present in the strains, and that the length of yrIlm, when present, also varies. In addition, the length of the gene product for all strains, including the type strain, was much longer than expected based on deposited sequences. The internal repeats of the yrInv gene product are highly diverged, but represent the same bacterial immunoglobulin-like domains as in yrIlm. Using qRT-PCR, we found that yrIlm and yrInv are differentially expressed under conditions relevant for pathogenesis. In addition, we compared the genomic context of both genes in the newly sequenced Y. ruckeri strain to all available PacBio-sequenced Y. ruckeri genomes, and found indications of recent events of horizontal gene transfer. Taken together, this study demonstrates and highlights the power of Single Molecule Real-Time technology for sequencing highly repetitive proteins, and sheds light on the genetic events that gave rise to these highly repetitive genes in a commercially important fish pathogen.

Global 3D imaging of Yersinia ruckeri bacterin uptake in rainbow trout fry

Yersinia ruckeri is the causative agent of enteric redmouth disease (ERM) in rainbow trout, and the first commercially available fish vaccine was an immersion vaccine against ERM consisting of Y. ruckeri bacterin. The ERM immersion vaccine has been successfully used in aquaculture farming of salmonids for more than 35 years. The gills and the gastrointestinal (GI) tract are believed to be the portals of antigen uptake during waterborne vaccination against ERM; however, the actual sites of bacterin uptake are only partly understood. In order to obtain insight into bacterin uptake during waterborne vaccination, optical projection tomography (OPT) together with immunohistochemistry (IHC) was applied to visualize bacterin uptake and processing in whole rainbow trout fry. Visualization by OPT revealed that the bacterin was initially taken up via gill lamellae from within 30 seconds post vaccination. Later, bacterin uptake was detected on other mucosal surfaces such as skin and olfactory bulb from 5 to 30 minutes post vaccination. The GI tract was found to be filled with a complex of bacterin and mucus at 3 hours post vaccination and the bacterin remained in the GI tract for at least 24 hours. Large amounts of bacterin were present in the blood, and an accumulation of bacterin was found in filtering lymphoid organs such as spleen and trunk kidney where the bacterin accumulates 24 hours post vaccination as demonstrated by OPT and IHC. These results suggest that bacterin is taken up via the gill epithelium in the earliest phases of the bath exposure and from the GI tract in the later phase. The bacterin then enters the blood circulatory system, after which it is filtered by spleen and trunk kidney, before finally accumulating in lymphoid organs where adaptive immunity against ERM is likely to develop.

Yersiniosis in Atlantic cod, Gadus morhua (L.), characterization of the infective strain and host reactions

A disease outbreak in farmed Atlantic cod caused by Yersinia ruckeri is reported. Mortality started following vaccination of cod reared in two tanks (A and B). The accumulated mortality reached 1.9% in A and 4.8% in B in the following 30 days when treatment with oxytetracycline was applied. Biochemical and molecular analysis of Y. ruckeri isolates from the cod and other fish species from fresh and marine waters in Iceland revealed a high salinity-tolerant subgroup of Y. ruckeri serotype O1. Infected fish showed clinical signs comparable with those of Y. ruckeri -infected salmonids, with the exception of granuloma formations in infected cod tissues, which is a known response of cod to bacterial infections. Immunohistological examination showed Y. ruckeri antigens in the core of granulomas and the involvement of immune parameters that indicates a strong association between complement and lysozyme killing of bacteria. Experimental infection of cod with a cod isolate induced disease, and the calculated LD50 was 1.7 × 10(4) CFU per fish. The results suggest that yersiniosis can be spread between populations of freshwater and marine fish. Treatment of infected cod with antibiotic did not eliminate the infection, which can be explained by the immune response of cod producing prolonged granulomatous infection.

Yersinia ruckeri Biotypes 1 and 2 in France: presence and antibiotic susceptibility

Yersinia ruckeri is the causative agent of yersiniosis, a disease reported in a number of fish species, especially rainbow trout. This study was undertaken to describe the phenotypes of Y. ruckeri on French rainbow trout farms. More than 100 isolates, collected during recent outbreaks on trout farms, were characterized by phenotypic tests, namely using biochemical tests of the API 20E system, serotyping, biotyping (tests for motility and lipase activity) and by describing the pattern of susceptibility to several antibiotics. The isolates showed a low phenotypic diversity with a prevalent serotype (O1) and API 20E profile 5 1(3)07 100. As in other European countries, Biotype 2 (BT2), which lacks both motility and secreted lipase activity, was found to be present in France. The emergence of 'French' BT2 was different than that observed for other European countries (Finland, Spain, Denmark and the UK). The antibiotic pattern was uniform for all isolates, regardless of the geographical area studied. The results indicate that no resistance has yet emerged, and the efficacy of the antibiotic generally used against yersiniosis in France, trimethoprim/sulfamethoxasol, is not compromised (minimum inhibitory concentrations [MIC] of between 0.016 and 0.128 µg ml-1). Enrofloxacin and doxycycline, not used as a first-line treatment in fish diseases, have reasonably good efficacies (with MICs ≤0.128 and 0.256, respectively).

Isolation of Yersinia ruckeri strain H01 from farm-raised Amur Sturgeon Acipenser schrencki in China

Yersinia ruckeri is the causative agent of enteric redmouth disease or yersiniosis, which affects salmonids and several other species of fish. However, there are no reports on the characteristics and pathogenicity of Y. ruckeri isolated from farm-raised Amur Sturgeon Acipenser schrencki. Here, we isolated and characterized Y. ruckeri strain H01 from the diseased Amur Sturgeon in China. The phenotypic and genotypic characteristics of Y. ruckeri were observed, and its virulence was tested by examining experimentally infected sturgeons. Examination of the flagellar morphology of Y. ruckeri by transmission electron microscopy showed five to eight peritrichous flagella located on the cell body. Actively dividing cells with an obvious cell membrane were approximately 0.64 μm in diameter and between 1.7 and 2.5 μm in length. The dose that was lethal to 50% of the test fish after intraperitoneal injection was determined to be 7.2×10(6) CFU, and Y. ruckeri could be reisolated from the liver and kidneys of infected sturgeon. Antimicrobial susceptibility tests showed that H01 was susceptible to 10 antimicrobial agents. Part of the 16S rRNA sequences (563 base pair) was amplified and sequenced to study the genotypic characterization in Y. ruckeri (GenBank accession number JQ657818). The phylogenetic tree revealed H01 was clustered together with Y. ruckeri strains. Together, this study describes the isolation, characterization, and phenotypic-genotypic analysis of a Y. ruckeri strain isolated from farm-raised Amur Sturgeon. The results discovered may provide some theoretical basis for the prevention and control of yersiniosis in Amur Sturgeon.

In vivo monitoring of Yersinia ruckeri in fish tissues: progression and virulence gene expression

In this study, the utilization of bioluminescence imaging (BLI) allowed us to define the progression of Yersinia ruckeri during the infection of rainbow trout. A luminescent Y. ruckeri 150 strain was engineered using the pCS26-Pac plasmid containing the lux operon from Photorhabdus luminescens. Two different models of infection of rainbow trout were defined depending on the route in which bacteria were administered, being the gut the major organ affected following bath immersion. This indicates that this organ is important for bacterial dissemination inside the fish and the establishment of the infection. Moreover, the expression of three previously selected operons by in vivo expression technology (IVET) was analysed, the yhlBA involved in the production of a haemolysin, the cdsAB related to the uptake of cysteine and the yctCBA implicated in citrate uptake. Apart from these factors, the expression of yrp1 encoding a serralysin metalloprotease involved in pathogenesis was also analysed. The results indicated that all of the assayed promoters were expressed during infection of rainbow trout. In addition to these findings, the methodology described in this work constitutes a useful model for studying the infection process in other fish pathogenic bacteria.

Development and validation of real-time PCR for the detection of Yersinia ruckeri

Yersiniosis (enteric red mouth disease) is a contagious bacterial disease caused by Yersinia ruckeri, which primarily affects salmonids. A real-time PCR assay using a molecular beacon has been developed and validated to improve the detection of the causative biotypes of Y. ruckeri. The assay, which targets the glnA (glutamine synthetase) gene, proved to have 100% analytical specificity and analytical sensitivities of 5 fg and 3 × 10(3) CFU g(-1) for DNA and seeded kidney tissue, respectively. The assay was highly repeatable with low % CV for intra- and inter-run experiments, and the optimized parameters transferred easily between different real-time PCR platforms. Following analytical validation, diagnostic specificity was determined using New Zealand farmed Chinook salmon (n = 750) from 10 farms during 2007/08. The real-time PCR was run in parallel with the bacterial culture detection method, and all fish tested were found to be negative by both methods for Y. ruckeri, resulting in 100% diagnostic specificity (95% confidence interval). The molecular beacon real-time PCR system is specific, sensitive, reproducible and a rapid method for the detection of Y. ruckeri and has the potential to be used for routine diagnostic testing, health certification and active surveillance programmes.

Identification of Yersinia ruckeri from diseased salmonid fish by Fourier transform infrared spectroscopy

Yersinia ruckeri is the causative agent of enteric redmouth disease (ERM), which mainly affects salmonid fish. Isolates of Y. ruckeri from diseased salmonid fish were obtained over a 6-year period from eight fish farms in the State of Baden-Württemberg, Southwest Germany. The strains were characterized by biochemical methods and Fourier transform infrared spectroscopy (FT-IR) combined with artificial neural network analysis. These methods were complemented by 16S rDNA sequencing for several isolates. The set of strains from these fish farms included sorbitol-positive, gelatinase-positive and non-motile Y. ruckeri. These variants were differentiated with an advanced FT-IR module, which is part of a higher-ranking method including more than 200 well-defined Yersinia strains against a background of more than 1000 other Gram-negative isolates. Validation of the newly constructed method yielded 97.4% of Y. ruckeri identified correctly on the species level. Thus, the FT-IR analysis enables distinction of all Y. ruckeri from other Yersinia species (e.g. fish-borne Y. enterocolitica) and other Enterobacteriaceae typically misidentified because of similar biochemical reaction profiles, especially Hafnia alvei. The differentiation of sorbitol-positive variants of Y. ruckeri using FT-IR was demonstrated.

Phenotypic and molecular characterization of Yersinia ruckeri isolates from rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) in Turkey

The aim of the study was the phenotypic and molecular characterization of Yersinia (Y) ruckeri strains, the causative agent of Enteric Redmouth Disease (ERM), by antibiotyping, random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) and sodium dodecylsulphate-polyacrylamide gel electrophoresis (SDS-PAGE) patterns of whole cell proteins. For this aim, a total of 97 Y ruckeri isolates were analyzed. The isolates were distinguished into ten antibiotypes and six phenotypes according to their resistance properties and whole cell protein profiles, respectively. Also, a glycoprotein band of approximately 25.5 kDa was observed in all Y ruckeri strains tested. In all strains, six different RAPD types were observed. In conclusion, Y ruckeri strains isolated from rainbow trout of fish farms in Turkey showed variation according to their phenotypic and genotypic characteristics, and the use of these three typing techniques in double and triple combinations could be more useful for discriminating the strains.

Association between plasma antibody response and protection in rainbow trout Oncorhynchus mykiss immersion vaccinated against Yersinia ruckeri

A key hallmark of the vertebrate adaptive immune system is the generation of antigen-specific antibodies from B cells. Fish are the most primitive gnathostomes (jawed vertebrates) possessing an adaptive immune system. Vaccination of rainbow trout against enteric redmouth disease (ERM) by immersion in Yersinia ruckeri bacterin confers a high degree of protection to the fish. The immune mechanisms responsible for protection may comprise both cellular and humoral elements but the role of specific immunoglobulins in this system has been questioned and not previously described. The present study demonstrates significant increase in plasma antibody titers following immersion vaccination and significantly reduced mortality during Y. ruckeri challenge.

Rainbow trout were immersion-vaccinated, using either a commercial ERM vaccine (AquaVac™ ERM vet) or an experimental Y. ruckeri bacterin. Half of the trout vaccinated with AquaVac™ ERM vet received an oral booster (AquaVac™ ERM Oral vet). Sub-groups of the fish from each group were subsequently exposed to 1x10⁹ CFU Y. ruckeri/ml either eight or twenty-six weeks post vaccination (wpv). All vaccinated groups showed 0% mortality when challenged, which was highly significant compared to the non-vaccinated controls (40 and 28% mortality eight and twenty-six weeks post vaccination (wpv), respectively) (P<0.0001). Plasma samples from all groups of vaccinated fish were taken 0, 4, 8, 12, 16 and 26 wpv. and Y. ruckeri specific IgM antibody levels were measured with ELISA. A significant increase in titers was recorded in vaccinated fish, which also showed a reduced bacteremia during challenge. In vitro plasma studies showed a significantly increased bactericidal effect of fresh plasma from vaccinated fish indicating that plasma proteins may play a role in protection of vaccinated rainbow trout.

Route of entry and tissue distribution of Yersinia ruckeri in experimentally infected rainbow trout Oncorhynchus mykiss

Yersinia ruckeri is the causative agent of enteric redmouth disease, which leads to significant losses in salmonid aquaculture worldwide. Despite the significance of the disease, little information is available on the pathogenesis. In this study, the portal of entry was investigated using a contact-exposure infection method in rainbow trout Oncorhynchus mykiss with 4 different Y. ruckeri strains. Bacteriological and histological examination revealed the presence of high numbers of bacteria in the gills immediately after infection resulting in a rapid spread of Y. ruckeri in the internal organs. However, only a virulent strain was able to survive and multiply in the host, causing septicaemia and death several days after infection. These findings indicate that gills may be an important site of entry and that Y. ruckeri virulence is related to immune evasion.

Loop-mediated isothermal amplification as an emerging technology for detection of Yersinia ruckeri the causative agent of enteric red mouth disease in fish

BACKGROUND

Enteric Redmouth (ERM) disease also known as Yersiniosis is a contagious disease affecting salmonids, mainly rainbow trout. The causative agent is the gram-negative bacterium Yersinia ruckeri. The disease can be diagnosed by isolation and identification of the causative agent, or detection of the Pathogen using fluorescent antibody tests, ELISA and PCR assays. These diagnostic methods are laborious, time consuming and need well trained personnel.

RESULTS

A loop-mediated isothermal amplification (LAMP) assay was developed and evaluated for detection of Y. ruckeri the etiological agent of enteric red mouth (ERM) disease in salmonids. The assay was optimised to amplify the yruI/yruR gene, which encodes Y. ruckeri quorum sensing system, in the presence of a specific primer set and Bst DNA polymerase at an isothermal temperature of 63 degrees C for one hour. Amplification products were detected by visual inspection, agarose gel electrophoresis and by real-time monitoring of turbidity resulted by formation of LAMP amplicons. Digestion with HphI restriction enzyme demonstrated that the amplified product was unique. The specificity of the assay was verified by the absence of amplification products when tested against related bacteria. The assay had 10-fold higher sensitivity compared with conventional PCR and successfully detected Y. ruckeri not only in pure bacterial culture but also in tissue homogenates of infected fish.

CONCLUSION

The ERM-LAMP assay represents a practical alternative to the microbiological approach for rapid, sensitive and specific detection of Y. ruckeri in fish farms. The assay is carried out in one hour and needs only a heating block or water bath as laboratory furniture. The advantages of the ERM-LAMP assay make it a promising tool for molecular detection of enteric red mouth disease in fish farms.

First report of Yersinia ruckeri biotype 2 in the USA

A polyphasic characterization of atypical isolates of Yersinia ruckeri (causative agent of enteric redmouth disease in trout) obtained from hatchery-reared brown trout Salmo trutta in South Carolina was performed. The Y. ruckeri isolates were biochemically and genetically distinct from reference cultures, including the type strain, but were unequivocally ascribed to the species Y. ruckeri, based on API 20E, VITEK, fatty acid methyl ester profiles, and 16S rRNA gene sequencing analysis. These isolates were nonmotile and unable to hydrolyze Tween 20/80 and were therefore classified as Y. ruckeri biotype 2. Genetic fingerprint typing of the isolates via enterobacterial repetitive intergenic consensus (amplified by polymerase chain reaction) and fragment length polymorphism showed biotype 2 as a homogeneous group distinguishable from other Y. ruckeri isolates. This is the first report of Y. ruckeri biotype 2 in the USA.

Strategies for the inclusion of an internal amplification control in conventional and real time PCR detection of Campylobacter spp. in chicken fecal samples

To illustrate important issues in optimization of a PCR assay with an internal control four different primer combinations for conventional PCR, two non-competitive and two competitive set-ups for real time PCR were used for detection of Campylobacter spp. in chicken faecal samples. In the conventional PCR assays the internal control was genomic DNA from Yersinia ruckeri, which is not found in chicken faeces. This internal control was also used in one of the set ups in real time PCR. In the three other set-ups different DNA fragments of 109 bp length prepared from two oligos of each 66 bp by a simple extension reaction was used. All assays were optimized to avoid loss of target sensitivity due to the presence of the internal control by adjusting the amount of internal control primers in the duplex assays and the amount of internal control in all assays. Furthermore, the assays were tested against faecal inhibitors to ensure that the internal control and the target PCR had the same sensitivity towards inhibitors.

Retention and removal of the fish pathogenic bacterium Yersinia ruckeri in biological sand filters

AIMS

To investigate the retention and removal of the fish pathogenic bacterium Yersinia ruckeri in biological sand filters and effects on the microbial community composition.

METHODS AND RESULTS

Sand filter columns were loaded (70 mm day(-1)) with fish farm wastewater and a suspension (10(8) CFU ml(-1)) of Y. ruckeri. Bacterial numbers and protozoan numbers were determined by plate counts and epifluorescence microscopy, respectively, and microbial biomass and community composition were assessed by phospholipid fatty acids (PLFA) analysis. Concentrations of Y. ruckeri in the filter effluent decreased from 10(8) to 10(3)-10(5) CFU ml(-1) during the experiment. Numbers of Y. ruckeri in the sand decreased from 10(6) CFU g(-1) dry weight (DW) sand to 10(4) CFU g(-1) DW sand. In contrast, microbial biomass determined with plate counts and total PLFA increased during the whole experiment. Principal component analysis (PCA) revealed a change in microbial community composition with time, with the most pronounced change in surface layers and towards the end of the experiment. Protozoan numbers increased from ca 0-600 cells g(-1) DW sand, indicating the establishment of a moderate population of bacterial grazers.

CONCLUSIONS

The removal of Y. ruckeri improved during the experiment. Introduction of Y. ruckeri to the sand filter columns stimulated growth of other micro-organisms, which in turn caused a shift in the microbial community composition in the sand.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study increases the understanding of the dynamics of sand filters subjected to a high loading of a pathogenic bacterium and can therefore be used in future work were the overall aim is to provide a more reliable and efficient removal of pathogenic bacteria in biological sand filter systems.