Ruckeri Isolates Research Articles

Antimicrobial susceptibility of Peruvian Yersinia ruckeri isolates to oxytetracycline and florfenicol using MIC testing.

Yersiniosis, caused by Yersinia ruckeri, has become the most common disease in farmed rainbow trout Oncorhynchus mykiss in Peru, affecting Puno and Junín Regions, important aquaculture areas in the country. Florfenicol (FLO) and oxytetracycline (OXY) are the antimicrobials most frequently used to mitigate losses attributed to this pathogen. This study presents an analysis of the susceptibility patterns of 60 Y. ruckeri isolates (30 isolates each from Puno and Junín), including the type strain CECT 4319T and the strains CECT 955 and CECT 956, against FLO and OXY. Minimum inhibitory concentrations (MICs) were determined following the guideline for standard broth dilution method published by the Clinical and Laboratory Standards Institute. MIC results ranged from 4.0 to 8.0 µg ml-1 for FLO and 0.5 to 4.0 µg ml-1 for OXY. Normalized resistance interpretation (NRI) analysis identified epidemiological cut-off values of ≤16.0 µg ml-1 for FLO and ≤4.0 µg ml-1 for OXY. All Peruvian isolates, including the collection strains, were categorized as wild-type for both antimicrobials. Even though the number of Y. ruckeri isolates with MIC values of 8 µg ml-1 for FLO is more than double in Puno than in Junín (15 vs. 7 isolates), the NRI analysis showed the same epidemiological cutoff of 16 µg ml-1; while for OXY, it was 4.0 µg ml-1 for Puno and 2.0 µg ml-1 for Junín. This study establishes the basis for monitoring susceptibility to FLO and OXY in new Y. ruckeri isolates in Peruvian rainbow trout farming.

Antibacterial effects of peracetic acid disinfection assessed by culturability, enzyme activity and flow cytometry

Aquaculture water disinfection with around 1 mg/L peracetic acid (PAA) was proved in various studies to effectively impair microbial proliferation and reduce pathogen-caused fish mortality. In the present study, we monitored the antibacterial effect of biweekly water disinfection with 1 mg/L PAA at a local flow-through fish farm for 8 weeks. Results from replicate water samples showed inconsistency of antibacterial efficacy assessed with colony forming units (CFU) of total culturable bacteria and a commercial enzyme activity assay. Single PAA disinfection caused up to 90% reduction of CFU in water. In contrast, the microbial hydrolase activity was hardly affected or even enhanced. Both methods have limitations and can't truly represent viable bacteria. We further monitored the temporal response of two Yersinia ruckeri isolates in suspensions to single PAA disinfection via high-throughput flow cytometry. We found out that the bacterial cell membrane damage appeared not instantly but with a lag phase of at least 2 h post disinfection at 4 °C. The abundance of cell membrane damage, considered as cumulative bacterial mortality, kept rising and maximized at 48 h post disinfection. The survived cells were more likely to enter the viable but non-culturable (VBNC) state at higher PAA concentrations. One isolate was more prone to enter the VBNC state than the other. Neither Y. ruckeri isolate showed measurable reaction with the enzyme substrate of the commercial microbial hydrolase activity assay. Our findings refresh knowledge about the dynamics of bacterial cell mortality post PAA disinfection (probably also similar non-thermal disinfection methods). Aquaculture practice should be aware of the delayed disinfection outcomes and pay attention to the dominance of VBNC state.

Co-existence of two Yersinia ruckeri biotypes and serotype O1a retrieved from rainbow trout (Oncorhynchus mykiss) farmed in Puno, Peru.

Yersinia ruckeri causes important economic losses for rainbow trout (Oncorhynchus mykiss) farms worldwide. This bacterial disease is likely the most common among trout in Peru; however, no commercial vaccine is available nationally, which is, in part, due to a lack of information on the bacterium. The aim of the current study was to characterize 29 Y. ruckeri isolates sampled from seven cage-reared farms in the Puno Region, the focal point for aquaculture activities in Peru. For this, samples were taken from fish with clinical signs (i.e. haemorrhages, uni- or bilateral exophthalmia, hyphaemia and/or melanosis). Notable among our findings was the existence of both Y. ruckeri biotype 1 (9 isolates) and biotype 2 (20 isolates; negative for sorbitol and Tween 80). The isolates further differed in API profiles 5307100 (21 isolates), 1307100 (4 isolates), 1305100 (2 isolates), 1307120 (1 isolate) and 5305100 (1 isolate), with the main differences being in the tests for lysine decarboxylase, gelatine hydrolysis and D-saccharose fermentation. Despite these differences, all isolates shared identical ERIC-PCR and REP-PCR profiles and belonged to the O1a serotype. Fingerprints were identical to the reference strain CECT 955 (serotype O1a). The information obtained will be used for epidemiological purposes by health authorities and for the development of a vaccine against Y. ruckeri, a prominent request made by fish farmers in Peru.

Biotyping reveals loss of motility in two distinct Yersinia ruckeri lineages exclusive to Norwegian aquaculture.

Non‐motile strains of Yersinia ruckeri, known as Y. ruckeri biotype 2, now dominate amongst clinical isolates retrieved from rainbow trout internationally. Due to an acute increase in the number of yersiniosis cases in Norway in recent years, followed by introduction of widespread intraperitoneal vaccination against the disease, an investigation on the prevalence of Y. ruckeri biotype 2 in Norwegian aquaculture was conducted. We biotyped 263 Y. ruckeri isolates recovered from diseased salmonids in Norway between 1985 and 2020. A total of seven biotype 2 isolates were identified, four of which were collected between 1985 and 1987, and three of which belong to the current epizootic clone, isolated from two different sea‐farms in 2017. Whole‐genome sequencing revealed single non‐synonymous nucleotide polymorphisms in the flagellar genes flhC in isolates from the 1980s, and in fliP in isolates from 2017. In both variants, motility was restored both by complementation with wild‐type alleles in trans and via spontaneous mutation‐driven reversion following prolonged incubation on motility agar. While biotype 2 strains do not yet seem to have become broadly established in Norwegian aquaculture, the seven isolates described here serve to document a further two independent cases of Y. ruckeri biotype 2 emergence in salmonid aquaculture.

Heterogeneity of Yersinia ruckeri Isolated from Rainbow Trout (Oncorhynchus mykiss) Infected with Enteric Red Mouth Disease

Regarding the increasing prevalence of antimicrobial resistance as a global menace, typing procedures are of great importance in epidemiological surveys. In the current study, interspecies differences of Yersinia ruckeri (Y. ruckeri) isolates were detected by antimicrobial susceptibility profiling and molecular traits to determine the relationship between isolates. The current study was conducted on 27 Y. ruckeri isolates collected from 40 infected rainbow trout (Oncorhynchus mykiss) with clinical enteric red mouth disease in Mashhad, Iran. Interspecies differences of all isolates were detected by antimicrobial susceptibility profiling using disk diffusion method, Repetitive Extragenic Palindromic PCR (REP-PCR), Enterobacterial Repetitive Intergenic Consensus PCR (ERIC-PCR), and Random Amplification of Polymorphic DNA PCR (RAPD-PCR) to determine the relationship between isolates. Simpson’s diversity index was calculated for each typing technique. The most phenotype resistant was against ampicillin, chloramphenicol, and lincomycin. Furthermore, enrofloxacin and ciprofloxacin showed the highest activity (100%) against isolates. According to the results, the highest diversity index was observed in the REP-PCR method (D = 0.91). In conclusion, REP-PCR can be a powerful technique for epidemiological studies.

Fatty Acid Composition in Yersinia ruckeri Strains Isolated from Rainbow Trout Farms

In this study, a total of 33 Yersinia ruckeri isolates were obtained from rainbow trout farms in the five different region of Turkey. We were determined to changes in fatty acids group of the strains. All the isolates were identified as Y. ruckeri based on colonical, cellular morphology and biochemical characters. All Y. ruckeri 11 major fatty acids, including 12:0 (Laurik acid), 14:0 (Miristik acid), 16:0 (Palmitik acid), 16:1n-7 (Palmitoleik acid), 17:1 (Heptadekanoik acid), 18:1n-9c (Oleik acid), 18:2n-6c (Linoleik acid), 18:3n-3c (Alfa-linoleik acid), 22:0 (Behenik acid), 22:1 (Dokosenoik acid) and 24:1 (Tetrakosenoik acid). Compared to isolates biochemical property. There was no different between fatty acids and biochemical characteristics. The results of this study show that the fatty acids composition of Y. ruckeri isolated strains from rainbow trout farms in Turkey is on a large scale homogenous.

Diversification of OmpA and OmpF of Yersinia ruckeri is independent of the underlying species phylogeny and evidence of virulence-related selection

Yersinia ruckeri is the causative agent of enteric redmouth disease (ERM) which causes economically significant losses in farmed salmonids, especially Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss, Walbaum). However, very little is known about the genetic relationships of disease-causing isolates in these two host species or about factors responsible for disease. Phylogenetic analyses of 16 representative isolates based on the nucleotide sequences of 19 housekeeping genes suggests that pathogenic Atlantic salmon and rainbow trout isolates represent distinct host-specific lineages. However, the apparent phylogenies of certain isolates has been influenced by horizontal gene transfer and recombinational exchange. Splits decomposition analysis demonstrated a net-like phylogeny based on the housekeeping genes, characteristic of recombination. Comparative analysis of the distribution of individual housekeeping gene alleles across the isolates demonstrated evidence of genomic mosaicism and recombinational exchange involving certain Atlantic salmon and rainbow trout isolates. Comparative nucleotide sequence analysis of the key outer membrane protein genes ompA and ompF revealed that the corresponding gene trees were both non-congruent with respect to the housekeeping gene phylogenies providing evidence that horizontal gene transfer has influenced the evolution of both these surface protein-encoding genes. Analysis of inferred amino acid sequence variation in OmpA identified a single variant, OmpA.1, that was present in serotype O1 and O8 isolates representing typical pathogenic strains in rainbow trout and Atlantic salmon, respectively. In particular, the sequence of surface-exposed loop 3 differed by seven amino acids to that of other Y. ruckeri isolates. These findings suggest that positive selection has likely influenced the presence of OmpA.1 in these isolates and that loop 3 may play an important role in virulence. Amino acid sequence variation of OmpF was greater than that of OmpA and was similarly restricted mainly to the surface-exposed loops. Two OmpF variants, OmpF.1 and OmpF.2, were associated with pathogenic rainbow trout and Atlantic salmon isolates, respectively. These OmpF proteins had very similar amino acid sequences suggesting that positive evolutionary pressure has also favoured the selection of these variants in pathogenic strains infecting both species.

Investigation of the effects of some phytochemicals on Yersinia ruckeri and antimicrobial resistance.

In this study, it is aimed to investigate the effects of Moringa oleifera and Sorbus domestica plant extracts on bacterial disease agents Yersinia ruckeri in aquaculture. Morphological and biochemical properties of 2 different Y. ruckeri isolates were determined. Then, Real-Time PCR analysis and gene sequencing of the isolates were identified. Phytochemicals (M. oleifera and S. domestica) and antibiotics (Oxytetracycline (OX) and Enrofloxacin (ENR)) were used together in the antibiogram test of antibiotics compared to the effect status of antibiotics. Also, the effects of phytochemicals on Y. ruckeri growth was examined comparatively by spectrophotometrically measuring at 600 nm wavelength every 2 hours according to bacterial growth densities with 10 different groups formed on TSB medium. As a result of the study, it was observed that the isolates formed Gram negative, catalase positive, oxidase negative, mobile and typical Y. ruckeri colonies. After the biochemical tests performed with Microgen ID panel, 99.85% similarity was determined. The isolates overlap with the 16S rRNA gene region after sequence analysis, and 99% of the isolates were similar in phylogenetic analysis. After the antibiogram test, Oxytetracycline and Enrofloxacin antibiotics were resistant to Y. ruckeri but the effects of phytochemicals were less on solid medium (MHA). As a result of the measurements carried out in liquid medium (TSB), it was observed that phytochemicals such as M. oliefera and S. domestica inhibit the growth of bacteria by 40-50%. As the importance of antibiotic resistance is increasing day by day, we believe that these phytochemicals will give positive results in treatment instead of using antibiotics.

Multi-locus Variable-number Tandem-repeat Analysis of the Fish-pathogenic Bacterium Yersinia ruckeri by Multiplex PCR and Capillary Electrophoresis.

Yersinia ruckeri is an important pathogen of farmed salmonids worldwide, but simple tools suitable for epizootiological investigations (infection tracing, etc.) of this bacterium have been lacking. A Multi-Locus Variable-number tandem-repeat Analysis (MLVA) assay was therefore developed as an easily accessible and unambiguous tool for high-resolution genotyping of recovered isolates. For the MLVA assay presented here, DNA is extracted from cultured Y. ruckeri samples by boiling bacterial cells in water, followed by use of supernatant as template for PCR. Primer-pairs targeting ten Variable-number tandem-repeat (VNTR) loci, interspersed throughout the Y. ruckeri genome, are distributed equally amongst two five-plex PCR reactions running under identical cycling conditions. Forward primers are labelled with either of three fluorescent dyes. Following amplicon confirmation by gel electrophoresis, PCR products are diluted and subjected to capillary electrophoresis. From the resulting electropherogram profiles, peaks representing each of the VNTR loci are size-called and employed for calculating VNTR repeat counts in silico. Resulting ten-digit MLVA profiles are thenused to generate Minimum spanning trees enabling epizootiological evaluation by cluster analysis. The highly portable output data, in the form of numerical MLVA profiles, can rapidly be compared across labs and placed in a spatiotemporal context. The entire procedure from cultured colony to epizootiological evaluation may be completed for up to 48 Y. ruckeri isolates within a single working day.

Genotyping and antimicrobial resistance genes of Yersinia ruckeri isolates from rainbow trout farms.

In this study, we compared 142 Yersinia ruckeri isolates collected between 2013 and 2016 from 6 different regions in Turkey. A total of 18 different genogroups were found, though most of the isolates clustered into the same genogroup as serotype O1. As immunization of fish with inactivated Y. ruckeri by injection, immersion, or feeding provide minimal protection against Y. ruckeri infection in Turkey, many fish producers use antimicrobials unrestrictedly, resulting in antimicrobial resistance in aquatic pathogens. Accordingly, we investigated resistance to the antimicrobials most commonly used to treat yersiniosis. More than 80% of the Y. ruckeri isolates were susceptible to sulfamethoxazole-trimethoprim (SXT), florfenicol (FFC), and tetracycline, whereas none were susceptible to sulfamethoxazole. The most commonly used antimicrobials (SXT and FFC) can be effectively administered because the resistance levels to these drugs are the lowest among those reported for agents used to control enteric red mouth disease (12.6 and 14.7%, respectively). In conclusion, to the best of our knowledge, this study is the first characterization of the antimicrobial resistance genes floR, sulI, tetC, tetD, and tetE in Y. ruckeri isolates from aquaculture. Additionally, we detected the sulII gene but not the tetA, tetB, tetM, tetS, or sulIII genes.

Studies on yersiniosis in cultured Mugil seheli for the first time in Suez Governorate

A total of 360 cultured Mugil seheli had clinical signs were collected from private farms in Suez Governorate, Egypt. M. seheli was subjected to clinical, post -mortem and bacteriological examinations for detection of Yersinia ruckeri. Yersinia ruckeri was identified by biochemical reactions and polymerase chain reaction (PCR). One isolate of Y. ruckeri. The results revealed that the presence of Yersinia ruckeri infection was 68.1%. The highest prevalence of Y. ruckeri infection in cultured M. seheli was during autumn season (80%) followed by spring (72.2%), summer (70%) and then winter (50%). PCR amplification of DNA from Y. ruckeri isolates using 16s rRNA (YER8/10) specific primers for Y. ruckeri resulted in PCR products size of 575 bp. All Y. ruckeri isolates were found to be sensitive to Ciprofloxacin, Tobramycin and Trimethoprim. While resisted Erythromycin, Amoxycillin and Novobiocine.

Culture and molecular characterization of phages isolated from rainbow trout farms and sewage treatment plants and investigation of their effects on Yersinia ruckeri

In the present study bacteriophages isolated from rainbow trout farms and sewage treatment plants were genetically identified and their effectiveness on Yersinia ruckeri isolates from clinical cases of red mouth disease was investigated. Fish samples suspected to red mouth disease were collected from rainbow trout farms located in west Azerbaijan. Y. ruckeri, the causative agent of red mouth disease was initially identified using biochemical tests. The biotypes of all Y. ruckeri isolates were determined and their identity was confirmed by employing genus specific primers. Antimicrobial resistance of Y. ruckeri isolates were examined using common antibiotics in use in aquaculture. In order to isolating lytic bacteriophages, environmental samples mainly from rainbow trout farms and sewage treatment plants were collected in a period of six months. Isolated bacteriophages were titrated using two-layer agar method and their bactericidal effects were examined. For molecular characterization of bacteriophages, genomic DNA was extracted. Extracted genomic DNA from bacteriophages was digested using Msp I endonuclease. The results revealed that 4.48% of examined fish were positive for Y. ruckeri. Bacteriophages isolated from urban sewage treatment plants were effective on Y. ruckeri isolates. Maragheh and Urmia sewage treatment plants had the maximum and minimum phage titers, respectively. The genomic DNA of all isolated phages were smaller than genomic DNA of Lambda phage and all examined phages showed similar genomic DNA digestion patterns. It was concluded that sewage treatment plants could be an important source for phages effective on Y. ruckeri and maybe other aquaculture bacterial pathogens.

Analysis of Yersinia ruckeri strains isolated from trout farms in northwest Germany.

Enteric redmouth disease (ERM), caused by Yersinia ruckeri, is among the most important infectious diseases in rainbow trout Oncorhynchus mykiss aquaculture in Europe. Our aim was to analyse the persistence of Y. ruckeri strains in trout farms in northwest Germany and their dissemination between farms based on a detailed molecular and phenotypical characterisation scheme. The data on identification and characterisation of Y. ruckeri strains and examining the distribution of these strains in the field could serve as a basis for preventive disease monitoring plans. During the observation period from June 2011 until June 2012, we collected 48 Y. ruckeri isolates from 12 different rainbow trout hatcheries. In total, 44 (91.7%) of the isolates were non-motile; in particular, all isolates recovered during the sampling period in winter and early spring were non-motile. In several trout farms, characteristic farm-specific Y. ruckeri isolates from particular typing groups were isolated throughout the year, while in other farms, which had a trading relationship between each other, ERM outbreaks were caused by Y. ruckeri from the same typing group. Our data indicate that in some farms, the causative Y. ruckeri strains persisted in the respective trout farm. The presence of Y. ruckeri from the same typing group in farms with a trading relationship indicates a dissemination of the infection between the farms.

Pheno- and genotypic analysis of antimicrobial resistance properties of Yersinia ruckeri from fish

Enteric red-mouth disease, caused by Yersinia ruckeri, is an important disease in rainbow trout aquaculture. Antimicrobial agents are frequently used in aquaculture, thereby causing a selective pressure on bacteria from aquatic organisms under which they may develop resistance to antimicrobial agents. In this study, the distribution of minimal inhibitory concentrations (MICs) of antimicrobial agents for 83 clinical and non-clinical epidemiologically unrelated Y. ruckeri isolates from north west Germany was determined. Antimicrobial susceptibility was conducted by broth microdilution at 22±2°C for 24, 28 and 48h. Incubation for 24h at 22±2°C appeared to be suitable for susceptibility testing of Y. ruckeri. In contrast to other antimicrobial agents tested, enrofloxacin and nalidixic acid showed a bimodal distribution of MICs, with one subpopulation showing lower MICs for enrofloxacin (0.008–0.015μg/mL) and nalidixic acid (0.25–0.5μg/mL) and another subpopulation exhibiting elevated MICs of 0.06–0.25 and 8–64μg/mL, respectively. Isolates showing elevated MICs revealed single amino acid substitutions in the quinolone resistance-determining region (QRDR) of the GyrA protein at positions 83 (Ser83-Arg or -Ile) or 87 (Asn87-Tyr), which raised the MIC values 8- to 32-fold for enrofloxacin or 32- to 128-fold for nalidixic acid. An isolate showing elevated MICs for sulfonamides and trimethoprim harbored a ∼8.9kb plasmid, which carried the genes sul2, strB and a dfrA14 gene cassette integrated into the strA gene. These observations showed that Y. ruckeri isolates were able to develop mutations that reduce their susceptibility to (fluoro)quinolones and to acquire plasmid-borne resistance genes.

Biochemical and molecular heterogeneity among isolates of Yersinia ruckeri from rainbow trout (Oncorhynchus mykiss, Walbaum) in north west Germany

BackgroundEnteric Redmouth Disease (ERM), caused by Yersinia ruckeri, is one of the most important infectious diseases in rainbow trout (Oncorhynchus mykiss) aquaculture in Europe. More recently, non-motile vaccine resistant isolates appear to have evolved and are causing disease problems throughout Europe, including Germany. The aim of this study was to analyse the variation of biochemical and molecular characteristics of Y. ruckeri isolates collected in north west Germany as a basis for strain differentiation. The isolates originated mainly from rainbow trout and were characterised by biochemical profiling, 16S rDNA sequencing, repetitive sequence-based PCRs, including (GTG)5-PCR, BOX-PCR, ERIC-PCR and REP-PCR, and pulsed-field gel electrophoresis (PFGE).ResultsIn total, 83 isolates were characterised, including 48 isolates collected during a field study in north west Germany. All isolates were confirmed as Y. ruckeri by the API 20E system. Five isolates were additionally confirmed as Y. ruckeri by Y. ruckeri-specific PCR and 16S rDNA sequencing. Only 17 isolates hydrolyzed Tween 80/20. Sixty-six isolates (79.5%) were non-motile. Two different patterns were obtained by REP-PCR, five patterns by ERIC-PCR, four patterns by (GTG)5-PCR and three patterns by BOX-PCR. NotI-directed PFGE resulted in 17 patterns that differed from each other by 25–29 fragments. Isolates from the field study clustered together as PFGE type C. According to the results of API 20E, repetitive sequence-based PCRs and PFGE, these isolates could be subdivided into 27 different groups.ConclusionsThe detailed molecular and phenotypic characterisation scheme developed in this study could be used to help trace the dissemination of Y. ruckeri isolates, and thus may represent part of improved disease monitoring plans in the future.

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.

Characterization and expression of the gyrA gene from quinolone resistant Yersinia ruckeri strains isolated from Atlantic salmon (Salmo salar L.) in Norway

Yersinia ruckeri, the causative agent of yersiniosis has been reported in a number of fish species but the most vulnerable are the salmonids including Atlantic salmon (Salmo salar L.). In the present study Y. ruckeri isolates were collected from diseased Atlantic salmon juveniles and characterized for resistance against quinolones. Isolates were screened using disk diffusion assays and MIC determination. The QRDR regions of the gyrA, gyrB, parC and parE genes were sequenced. Quinolone resistant isolates revealed a single bp mutation which replaced serine by arginine at position 83 in the GyrA protein while no mutations were found in gyrB, parC and parE genes. Isolates were also screened for plasmid encoded qnrA, qnrB and qnrS genes but they were found absent. Cloning of gyrA from susceptible and resistant isolates into heterologous Y. ruckeri was not successful. The different gyrA alleles from susceptible and resistant isolates of Y. ruckeri were cloned into Escherichia coli TOPO 10 and E. coli DH5α. While cloning of the resistant allele into the sensitive host had no effect, cloning of the quinolone susceptible gyrA allele into quinolone resistant E. coli DH5α increased the inhibition zone diameter from 25mm to 38mm and decreased the MIC from 4μg/ml to 2μg/ml, suggesting dominance of wild type gyrA over the mutant allele. It is assumed that the wild type GyrA protein has more affinity to form the gyrase–DNA complex than mutant GyrA even in the presence of high levels of the mutated enzyme.

Interactions of Virulent and Avirulent Yersinia ruckeri Strains with Isolated Gill Arches and Intestinal Explants of Rainbow Trout ( Oncorhynchus mykiss, Walbaum)

Yersinia ruckeri is the causative agent of enteric redmouth disease leading to significant losses in salmonid aquaculture worldwide. Little information is available on the pathogenesis of this disease. Basic steps in the establishment of an infection include attachment to the epithelium fol- lowed by invasion at the portal of entry. In this study, the interactions of Y. ruckeri with the gills and the gut of rainbow trout Oncorhynchus mykiss (Walbaum, 1792) were studied using standardized perfusion models. Virulent and avirulent Y. ruckeri isolates appeared to adhere to and invade both tissues without significant differences. For the first time, the gill and gut perfusion models are shown to be suitable to study bacterial invasiveness.

Introduction of Yersinia ruckeri biotype 2 into Finnish fish farms

Number of enteric redmouth disease (ERM) outbreaks, caused by the bacterial fish pathogen Yersinia ruckeri, has increased considerably in Finnish fish farms since 2004. To evaluate if the Y. ruckeri isolates from the recent severe Finnish outbreaks represent a dominant strain and if they differ from older isolates, 44 Finnish isolates from 1986 to 2007 were characterised by biochemical tests, serotyping, pulsed-field gel electrophoresis (PFGE) and outer membrane protein analysis. The Finnish isolates were also compared with isolates from nearby geographical regions (Denmark and Sweden) and with the reference strain NCTC 10476. The results showed that most of the new isolates (2005–2007) from the Archipelago Sea (northern Baltic Sea) in Southwest Finland belong to biotype 2, which had not been previously reported from Finland. PFGE analysis indicated that the new biotype 2 isolates were very similar to isolates from Denmark (pulsotype 35) and that they most likely do not originate from the biotype 1 isolates obtained from fresh water hatcheries in Finland. Our conclusion is that the Y. ruckeri biotype 2 strain was introduced to Finnish fish farms in the Archipelago Sea from elsewhere.

Interactions of virulent and avirulent Yersinia ruckeri strains with isolated gill arches and intestinal explants of rainbow trout Oncorhynchus mykiss

Yersinia ruckeri is the causative agent of enteric redmouth disease leading to significant losses in salmonid aquaculture worldwide. Little information is available on the pathogenesis of this disease. Basic steps in the establishment of an infection include attachment to the epithelium followed by invasion at the portal of entry. In this study, the interactions of Y. ruckeri with the gills and the gut of rainbow trout Oncorhynchus mykiss (Walbaum, 1792) were studied using standardized perfusion models. Virulent and avirulent Y ruckeri isolates appeared to adhere to and invade both tissues without significant differences. For the first time, the gill and gut perfusion models are shown to be suitable to study bacterial invasiveness.