Epidemiological cut-off values for Yersina ruckeri disc diffusion data generated by a standardised method

In order to establish the meaning of data generated in antimicrobial agent susceptibility tests, it is necessary to develop internationally harmonised interpretive criteria. Currently, such criteria have not been developed for data generated in studies of the susceptibility of the fish pathogen Yersinia ruckeri. This work generated the data that would be required to set epidemiological cut-off values for the susceptibility data of this species that had been generated using a standardised disc diffusion method that specified the use of Mueller Hinton agar and incubation at 22°C for 24-28 h. Using this method, sets of inhibition zones data for 4 antimicrobial agents were generated by 3 independent laboratories. The data from these laboratories were aggregated and analysed using the statistically based normalised resistance interpretation. For ampicillin, florfenicol, oxytetracycline and trimethoprim-sulfamethoxazole the cut-off values calculated by this analysis were ≥16, ≥23, ≥24 and ≥30 mm, respectively. Evidence is presented demonstrating that the data for these 4 agents was of sufficient quantity and quality that they could be used by the relevant authorities to set internationally harmonised, consensus epidemiological cut-off values for Y. ruckeri.

Yersinia enterocolitica biovar 1A: An underappreciated potential pathogen in the food chain

Yersinia enterocolitica is an underreported cause of foodborne gastroenteritis. Little is known of the diversity of Y. enterocolitica isolated from food and which food commodities contribute to human disease. In this study, Y. enterocolitica was isolated from 37/50 raw chicken, 8/10 pork, 8/10 salmon and 1/10 leafy green samples collected at retail in the UK. Up to 10 presumptive Y. enterocolitica isolates per positive sample underwent whole genome sequencing (WGS) and were compared with publicly available genomes. In total, 207 Y. enterocolitica isolates were analyzed and belonged to 38 sequence types (STs). Up to five STs of Y. enterocolitica were isolated from individual food samples and isolates belonging to the same sample and ST differed by 0-74 single nucleotide polymorphisms (SNPs). Biotype was predicted for 205 (99 %) genomes that all belonged to biotype 1A, previously described as non-pathogenic. However, around half (51 %) of food samples contained isolates belonging to the same ST as previously isolated from UK human cases. The closest human-derived isolates shared between 17 and 7978 single nucleotide polymorphisms (SNPs) with the food isolates. Extensive food surveillance is required to determine what food sources are responsible for Y. enterocolitica infections and to re-examine the role of biotype 1A as a human pathogen.

Yersinia ruckeri infection activates local skin and gill B cell responses in rainbow trout

Teleost fish lack organized structures in mucosal tissues such as those of mammals, but instead contain dispersed B and T cells with the capacity to respond to external stimuli. Nonetheless, there is still a great lack of knowledge regarding how B cells differentiate to plasmablasts/plasma cells in these mucosal surfaces. To contribute to a further understanding of the mechanisms through which fish mucosal B cells are activated, in the current study, we have studied the B cell responses in the skin and gills of rainbow trout (Oncorhynchus mykiss) exposed to Yersinia ruckeri. We have first analyzed the transcription levels of genes related to B cell function in both mucosal surfaces, and in spleen and kidney for comparative purposes. In a second experiment, we have evaluated how the infection affects the presence and size of B cells in both skin and gills, as well as the presence of plasmablasts secreting total or specific IgMs. The results obtained in both experiments support the local differentiation of B cells to plasmablasts/plasma cells in the skin and gills of rainbow trout in response to Y. ruckeri. Interestingly, these plasmablasts/plasma cells were shown to secrete specific IgMs as soon as 5 days after the exposure. These findings contribute to a further understanding of how B cells in the periphery respond to immune stimulation in teleost fish.

Efficacy of polyvalent vaccine on immune response and disease resistance against streptococcosis/lactococcosis and yersiniosis in rainbow trout (Oncorhynchus mykiss)

Diseases are the most significant challenge in the development and stability of aquaculture. In this study, the immunogenic efficiency of polyvalent streptococcosis/lactococcosis and yersiniosis vaccines was evaluated by injection and immersion methods in rainbow trout.. The 450 fish with an average weight of 50 ± 5 g were divided into three treatments and three replications as follows: injection vaccine treatment, immersion vaccine treatment and control group without vaccine administration. Fish were kept for 74 days and sampling was done on days 20, 40 and 60. Then, from the 60th to the 74th day, the immunized groups were challenged with three bacteria Streptococcus iniae (S. iniae), Lactococcus garvieae (L. garvieae) and Yersinia ruckeri (Y. ruckeri) separately. A significant difference was observed in the weight gained (WG) in the immunized groups compared to the control group (P < 0.05). The relative survival percentage (RPS) after 14 days of challenge with S. iniae, L. garvieae and Y. ruckeri in the injection group compared to the control group increased respectively (60%, 60% and 70%), (P < 0.05). Also, RPS in the immersion group had an increase respectively (30%, 40% and 50%) after the challenge with S. iniae, L.garvieae and Y. ruckeri compared to the control group. Immune indicators such as antibody titer, complement and lysozyme activity significantly increased in comparison to the control group (P < 0.05). In general, it can be concluded that applying three vaccines by injection and immersion method has significant effects on immune protection and survival rate. However, the injection method is more effective and more suitable than the immersion method.

Effect of Dietary Origanum onites on Growth, Non Specific Immunity and Resistance against Yersinia ruckeri of Rainbow Trout ( Oncorhynchus mykiss)

Natural substances has been identified to maintain health and improve growth performance in the aquaculture. The effect of Origanum onites on growth and immune response of rainbow trout was investigated. Experimental groups (A and B) of 70 fish were separated into 10 different treatments. A groups were fed with dietary administration of O. onites essential oil (0.5 mL kg-1 and 3.0 mL kg-1) and crude powder (1.0 g kg-1 and 10.0 g kg-1) for a period of 8 weeks. Other groups (B) were vaccinated against Yersinia ruckeri at the beginning of experiment and then fed the same diets described above. Results showed that feed conversion ratio in fish fed a combination of O. onites and vaccine was statistically better than the control. NBT-positive cells, phagocytic activity, serum lysozyme activity and immunoglobulin M level were stimulated in both non vaccinated and vaccinated fish (p<0.05). Cumulative mortality in fish fed O. onites was lower than controls following challenge with Y. ruckeri. No mortality was observed in vaccinated fish fed with 0.5 mL kg-1 of O. onites. These results indicated that dietary administration of O. onites could act as an enhanced non specific immune response, growth performance and resistance to Y. ruckeri.

Screening and activity of potential gastrointestinal probiotic lactic acid bacteria against Yersinia ruckeri O1b

Yersiniosis of cultured Atlantic salmon is a recurrent fish health management challenge in many continents. The causative organism, Yersinia ruckeri, can reside latently in the gut and lead to acute infection and disease during hatchery and sea-transfer stages. One potential prevention approach is the administration of probiotic bacteria to suppress gut colonization of Y. ruckeri. Our study aimed to isolate and identify anti-Yersinia activity among lactic acid bacteria (LAB) isolated from the gastrointestinal tract (GIT) of aquatic animals. Of the 186 aquatic GIT isolates examined, three strains showed diffusible antimicrobial activity towards Y. ruckeri O1b. Analysis of 16 s rRNA gene sequences indicated the three bacterial strains were Enterococci, related to Enterococcus sp. (99%), Enterococcus thailandicus (99%), and Enterococcus durans (99%). Anti-Yersinia activity was maintained at neutral pH (~6.5-7.0), and in-vitro environmental tolerance assays showed the three strains could withstand simulated salmonids gastrointestinal tract conditions of: low pH (3.4) and 3% bile salt content. All three Enterococci strains showed higher adhesion to the intestinal mucus of Atlantic salmon than Y. ruckeri O1b (E. durans 24%, E. enterococcus sp. 25% and E. thailandicus 98%, compared to Y. ruckeri O1b 5%). However, only Enterococcus sp. and E. thailandicus were able to grow in the salmon intestinal mucus broth while E. durans showed no growth. Anti-Yersinia activity was completely inactivated by proteinase-K treatment, suggesting that the active compound/s are proteinaceous and may be bacteriocin-like inhibitory substances (BLIS). Our data indicate that Enterococcus sp. MA176 and E. thailandicus MA122 are potential probionts for the prevention of yersiniosis in salmonids. Further in-vivo studies are required to determine whether these bacteria reduce the incidence of yersiniosis in Atlantic salmon.

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.

Effects of white mustard (Sinapis alba) oil on growth performance, immune response, blood parameters, digestive and antioxidant enzyme activities in rainbow trout (Oncorhynchusmykiss)

A study was conducted to evaluate the effects of dietary supplementation of white mustard (Sinapis alba) oil (WMO) on growth performance, immune responses, digestive and antioxidant enzyme activities in juvenile rainbow trout (Oncorhynchus mykiss). For this purpose, fish (initial weight: 25.77 ± 0.13 g) were divided into four experimental groups in triplicate and fed ad libitum twice a day with diets containing WMO at 0 (control), 0.5, 1, and 1.5% of diet for 9 weeks. Three fish from each tank (n:9 per treatment) were sampled on 21st, 42nd, and 63rd days for further analyses. At the end of the feeding period, fish were challenged with Aeromonas hydrophila and Yersinia ruckeri in two separate experimental setups. Results showed that final weight, weight gain, and specific growth rate were significantly increased in all experimental groups compared to the control. Feed conversion ratio was similar among treatments. Respiratory burst and potential killing activity decreased in all experimental groups compared to the control (P < 0.05). Lysozyme and myeloperoxidase activities were elevated in all experimental groups at the end of the experiment compared to the control (P < 0.05). Cytokine gene expressions in the head kidney and intestine were elevated in all experimental groups compared to that of the control in general (P < 0.05). Hematological responses of the experimental fish groups were similar to that of the control (P > 0.05). Pepsin and trypsin levels decreased in all experimental groups (P < 0.05). In terms of antioxidant enzyme activities, significant improvement in liver superoxide dismutase, catalase, and glutathione s-transferase activities in all treatment groups were determined (P < 0.05). In addition, a significant decline in liver lipid peroxidation levels was recorded in all treated groups at all sampling times compared to the control (P < 0.05). At the end of this feeding trial, no significant differences (P > 0.05) were observed in survival against A. hydrophila among experimental groups compared to the control (P > 0.05). However, increased survival against Y. ruckeri was determined in experimental fish groups (P < 0.05). This study suggests that white mustard oil had a favorable effect on the overall health and growth of rainbow trout.

Yersinia enterocolitica in wild and peridomestic rodents within Great Britain, a prevalence study

Yersinia enterocolitica is a human pathogen transmitted via the faecal-oral route among animals and humans and is a major foodborne public health hazard. This study explores the role of Y. enterocolitica transmission at the livestock-wildlife interface and investigates the potential role wild and peridomestic rodents play as a source of this zoonotic pathogen. The total of 342 faecal samples collected from the seven rodent species and one insectivore was examined using an optimized protocol to culture and identify Y. enterocolitica. Positive samples were also bioserotyped for grouping and determination of sample pathogenicity. Wildlife species sampled in this study were separated into two sample groups: randomly sampled (brown rats, house mice, wood mice, bank voles, field voles and the common shrew), as well as targeted sampling (red and grey squirrels). The overall prevalence of Y. enterocolitica in the randomly sampled population was 3.73%. Brown rats were chosen as sentinel species and tested to determine if location (pig farm vs non-pig farm) was a significant factor affecting Y. enterocolitica prevalence. In this study, location was not significant. All positive samples were found to be of biotype 1A, deemed non-pathogenic. Three of the samples were serotype 09, six were serotype 27 and five had an unidentifiable serotype. This study represents the first time Y. enterocolitica has been identified in these species of wildlife within mainland Britain. In addition, this study's findings are entirely novel and overall with regard to field voles and common shrews. However, the role of wild and peridomestic rodents in the transmission of pathogenic Y. enterocolitica remains unknown, as this study was unable to detect the presence of pathogenic Y. enterocolitica strains in these species.

qPCR screening for Yersinia ruckeri clonal complex 1 against a background of putatively avirulent strains in Norwegian aquaculture

Although a number of genetically diverse Yersinia ruckeri strains are present in Norwegian aquaculture environments, most if not all outbreaks of yersiniosis in Atlantic salmon in Norway are associated with a single specific genetic lineage of serotype O1, termed clonal complex 1. To investigate the presence and spread of virulent and putatively avirulent strains in Norwegian salmon farms, PCR assays specific for Y. ruckeri (species level) and Y. ruckeri clonal complex 1 were developed. Following extensive screening of water and biofilm, the widespread prevalence of putatively avirulent Y. ruckeri strains was confirmed in freshwater salmon hatcheries, while Y. ruckeri clonal complex 1 was found in fewer farms. The formalin-killed bacterin yersiniosis vaccine was detected in environmental samples by both PCR assays for several weeks post-vaccination. It is thus important to interpret results from recently vaccinated fish with great care. Moreover, field studies and laboratory trials confirmed that stressful management procedures may result in increased shedding of Y. ruckeri by sub-clinically infected fish. Analysis of sea water sampled throughout thermal delousing procedures proved effective for detection of Y. ruckeri in sub-clinically infected populations.

Isolation and characterization of Yersinia enterocolitica from foods in Apulia and Basilicata regions (Italy) by conventional and modern methods

Yersiniosis is the third most reported food-borne zoonosis in Europe. The aim of the present study was to perform the search for Yersinia enterocolitica in food samples collected from Apulia and Basilicata regions (Southern Italy) and to characterize any isolates by classical and modern analytical methods. A total of 130 samples were analyzed between July 2018 and July 2019: most of them were raw milk and dairy products made from it. Furthermore, 8 out of 130 samples were individual milk samples collected from bovines reared in a Brucella-free farm which showed false positive serological reaction for brucellosis due to the presence of pathogenic Y. enterocolitica O:9 biotype 2 in faeces. The Real Time PCR targeting the ail gene and the culture method were performed to detect pathogenic Y. enterocolitica. Isolates were subjected to API 20E (Biomerieux) and MALDI-TOF MS (Matrix Assisted Laser Desorption Ionization Time-of-Flight) for species identification. All samples were negative for the ail gene. The culture method allowed to isolate suspicious colonies from 28 samples. The API 20E system and the MALDI-TOF MS technique identified 20 Y. enterocolitica and 1 Y. intermedia in a concordant way. The remaining 7 strains were all identified as Y. enterocolitica by the API 20E system, while the MALDI-TOF MS recognized 4 Y. intermedia, 1 Y. bercovieri and 2 Y. massiliensis. Genotypic characterization of the discordant strains was performed by rMLST and it confirmed the MALDI-TOF MS’ results. Only non-pathogenic Y. enterocolitica biotype 1A strains were found, although with a non-negligible prevalence (P = 0.15 with CI _95% = ± 0.06). This study indicates a poor circulation of pathogenic Y. enterocolitica in food products made and marketed in the investigated areas. However, the small number of samples, insufficient for some food categories such as meat and vegetable, does not allow to exclude the presence of pathogenic strains at all.

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.

Modulatory effect of dietary copper nanoparticles and vitamin C supplementations on growth performance, hematological and immune parameters, oxidative status, histology, and disease resistance against Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss)

Copper and vitamin C are micronutrients needed for the living organism's functions. Vitamin C has a great effect on the immune system of fish. The present study aimed to evaluate the effects of dietary copper nanoparticles (Cu-NPs) and vitamin C (VC) supplementations on rainbow trout (Oncorhynchus mykiss) juveniles. So, 216 rainbow trout juveniles were randomly assigned to six groups with trial diets supplemented with Cu-NPs and VC including 0/0 (T1, control diet), 0/250 (T2), 0/500 (T3), 2/250 (T4), 2/500 (T5), and 2/0 (T6) mg Cu-NPs/VC per kg diet. After the feeding trial for 60 days, the fish were challenged with Yersinia ruckeri, and the survival rate was calculated for 15 days. Based on the data analysis, weight gain (WG), specific growth rate (SGR), protein efficiency ratio (PER), lysozyme, alternative complement activity (ACH50), hematocrit (Hct), hemoglobin (Hb), and mean corpuscular volume (MCV) were significantly (p < 0.05) increased in the fish fed on T4 and T5 diets compared with the control group. Catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPX) were significantly (p < 0.05) decreased in the fish fed with diets contain Cu-NPs and VC (T4 and T5). The expressions of TNF-α, IL-1ß, IL-10, SOD, CAT, and GPX genes were significantly (p < 0.05) decreased in the fish fed on T3, T4, and T5 diets versus the control. In addition, the dietary Cu-NPs and VC supplementations significantly enhanced resistance against pathogens and led to the control of infection in rainbow trout. In conclusion, Cu-NPs and VC administered as feed additives at 2/250-500 mg/kg elevated the growth performance, antioxidant capacity, and health of rainbow trout.

Effect of microplastics on Yersinia ruckeri infection in rainbow trout (Oncorhynchus mykiss)

Exposure to microorganisms such as Yersinia ruckeri can significantly affect bacterial infections in fish. Microplastics (MPs) may predispose fish to infection and act as carriers in pathogen transmission. Therefore, this study is designed to evaluate MPs' effect on damage caused by exposure to Y. ruckeri in rainbow trout. In this study, blood biochemical parameters and hepatic oxidative biomarkers as clinical signs were measured in the fish co-exposed to Y. ruckeri (5 and 10% the median lethal dose (LD₅₀)) and MPs (500 and 1000 mg Kg^-1) for 30 days. There were no significant changes in the creatinine, triglyceride, cholesterol levels, and glutamic-pyruvic transaminase activity in the blood of fish infected with Y. ruckeri. In contrast, exposure to MPs had a significant effect on most clinical parameters. The total protein, albumin, globulin, total immunoglobulins, high-density lipoprotein, low-density lipoprotein, cholesterol levels, and γ-glutamyltransferase activity decreased, whereas glucose, triglyceride, and creatinine levels, and glutamic-oxaloacetic transaminase, glutamic-pyruvic transaminase, alkaline phosphatase, and lactate dehydrogenase activities increased in the plasma of fish after co-exposure to MPs and Y. ruckeri. Dietary MPs combined with a Y. ruckeri challenge decreased catalase and glutathione peroxidase activities, and total antioxidant levels. However, superoxide dismutase activity and malondialdehyde contents increased in the hepatocyte of fish co-exposed to MPs and Y. ruckeri. This study suggests that fish exposure to MPs and simultaneous challenge with Y. ruckeri could synergistically affect clinical parameters.

Dual RNA-Seq of Trunk Kidneys Extracted From Channel Catfish Infected With Yersinia ruckeri Reveals Novel Insights Into Host-Pathogen Interactions

Host-pathogen intectarions are complex, involving large dynamic changes in gene expression through the process of infection. These interactions are essential for understanding anti-infective immunity as well as pathogenesis. In this study, the host-pathogen interaction was analyzed using a model of acute infection where channel catfish were infected with Yersinia ruckeri. The infected fish showed signs of body surface hyperemia as well as hyperemia and swelling in the trunk kidney. Double RNA sequencing was performed on trunk kidneys extracted from infected channel catfish and transcriptome data was compared with data from uninfected trunk kidneys. Results revealed that the host-pathogen interaction was dynamically regulated and that the host-pathogen transcriptome fluctuated during infection. More specifically, these data revealed that the expression levels of immune genes involved in Cytokine-cytokine receptor interactions, the NF-kappa B signaling pathway, the JAK-STAT signaling pathway, Toll-like receptor signaling and other immune-related pathways were significantly upregulated. Y. ruckeri mainly promote pathogenesis through the flagellum gene fliC in channel catfish. The weighted gene co-expression network analysis (WGCNA) R package was used to reveal that the infection of catfish is closely related to metabolic pathways. This study contributes to the understanding of the host-pathogen interaction between channel catfish and Y. ruckeri, more specifically how catfish respond to infection through a transcriptional perspective and how this infection leads to enteric red mouth disease (ERM) in these fish.

Effect of autochthonous lactobacilli on immunologically important molecules of rainbow trout after bacterial infection studied on intestinal primoculture

Nowadays, the aquaculture industry is one of the fastest growing industries. Intensive aquaculture has a negative impact on fish health. Probiotic bacteria are often used due to beneficial effect to health of host, e.i. decrease of diseases outbreaks, immunomodulatory effect or better utilization of feed. The aim of this work was to study the influence of probiotic bacteria on the immune response of trout intestinal cells in primoculture infected with pathogenic bacteria. In the experiment, we tested the effect of pre-treatment of intestinal cells with an autochthonous strain of Lactobacillus plantarum R2 Biocenol™ (CCM 8674) following infection with the most serious salmonid pathogens - Aeromonas salmonicida subsp. salmonicida (CCM 1307) and Yersinia ruckeri (CCM 6093). Tested probiotic strain reduced inflammation after A. salmonicida infection through decreased expression of pro-inflammatory cytokines and increased expression of anti-inflammatory cytokines. In contrast, after infection with Y. ruckeri, which causes immunosuppression, the probiotic strain stimulated immunity by up-regulation of expression of proinflammatory cytokines and suppressed the expression of anti-inflammatory cytokines. These results are a prerequisite for the immunomodulatory potential of the strain, but its action must be confirmed in subsequent in vivo experiments.

Immersion vaccines against Yersinia ruckeri infection in rainbow trout: Comparative effects of strain differences

The protective effects of autogenous and commercial ERM immersion vaccines (bacterins based on Yersinia ruckeri, serotype O1, biotypes 1 and 2) for rainbow trout (Oncorhynchus mykiss) were compared in order to evaluate whether the use of local pathogen strains for immunization can improve protection. In addition, the effect of the bacterin concentration was established for the commercial product. Following sublethal challenge of vaccinated and non-vaccinated control fish with live bacteria, we followed the bacterial count in the fish (gills, liver and spleen). The expression of genes encoding immune factors (IL-1β, IL-6, IL-8, IL-10, IFN-γ, MHCI, MHCII, CD4, CD8, TCRβ, IgM, IgT, IgD, cathelicidins 1 and 2, SAA and C3) and densities of immune cells in organs were recorded. Both vaccines conferred protection as judged from the reduced bacterial load in exposed fish. Innate immune genes were upregulated in all groups following bacterial challenge but significantly more in non-vaccinated naive fish in which densities of SAA-positive immune cells increased. Immunoglobulin genes were upregulated on day 5 post-challenge, and fish vaccinated with the high commercial bacterin dosage showed increased IgM levels by ELISA on day 14 post-challenge, reflecting that the vaccine dosage was correlated to protection. In conclusion, both vaccine types offered protection to rainbow trout when exposed to live Y. ruckeri and no significant difference between commercial and autogenous vaccines was established.

Supplementation of zebrafish (Danio rerio) diet using a short antimicrobial peptide: Evaluation of growth performance, immunomodulatory function, antioxidant activity, and disease resistance

Short-chain bioactive peptides are new and promising antimicrobial, immune moderating, and antioxidant agents. Therefore, the present study was conducted to evaluate in vitro antibacterial activity of CM11, a short antimicrobial peptide (AMP), against Streptococcus iniae and Yersinia ruckeri as fish pathogenic bacteria using standard disk diffusion and microdilution assays. In addition, in vivo effects of CM11 on fish growth, immunity, antioxidant activity, and disease resistance were evaluated using zebrafish (Danio rerio) as an animal model. For in vivo study, based on in vitro susceptibility results, four diets were designed to include zero (as control), 10, 20, and 50 μg of CM11 per g diet referred to as control, P1, P2, and P3 treatments, respectively. After eight weeks of dietary trial, fish were challenged with Streptococcus iniae, and the survival rate was calculated for a period of two weeks. Results showed that CM11 effectively inhibited the growth of S. iniae and Y. ruckeri on agar plates at a concentration of eight μg/ml. Minimum inhibitory and minimum bactericidal concentrations of CM11 were measured at 8 and 32 μg/ml for S. iniae and 16 and 64 μg/ml Y. ruckeri, respectively. In vivo results showed no noticeable effects on fish growth parameters, however, feed conversion ratio (FCR) was found lower in P3 and P2 compared to control (P < 0.05). Immunological and antioxidant responses were found strongly affected by CM11 in all treatment groups in which the highest values were found in the P3 treated group. Key immune and antioxidant genes were up-regulated particularly in fish receiving the highest level of CM11 (P3). Fish receiving the CM11 peptide showed better survival when challenged with S. iniae. These findings suggest the potential of CM11 for use in aquaculture as an antibacterial and immunostimulant agent.

A long-term observation for ecology of pathogenic Yersinia in wild rodents living in Fukushima Prefecture, Japan

From 2012 to 2021, prevalence of pathogenic Yersinia in wild rodents captured in Fukushima Prefecture, Japan was investigated twice a year to clarify the ecology of this pathogen in wild rodent populations. Pathogenic Yersinia enterocolitica O8 was isolated from 13 (1.7%) of 755 wild rodents. The Y. enterocolitica O8 isolates harbored three virulent genes (ail, fyuA, and virF). This pathogen was isolated repeatedly from wild rodents in April 2015, 2016, and 2017, in June and November 2020, and in April 2021, which was 6 of 19 times of observations. All Y. enterocolitica O8 isolates showed the same PFGE patterns. These results indicated that the same clone of pathogenic Y. enterocolitica O8 has been maintained in wild rodent populations in Fukushima Prefecture. Therefore, wild rodent populations contribute substantially to the continuous transmission of Y. enterocolitica O8 and its persistence in the ecosystem. This is the first report on the isolation of pathogenic Y. enterocolitica O8 in wild rodents in Fukushima Prefecture, Japan.

Effects of Greek juniper (Juniperus excelsa) extract on immune responses and disease resistance against Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss)

This study investigated the effects of Greek juniper extract on immune responses of rainbow trout. In this experiment, 4 doses [0 (Control), 1 (J1), 4 (J4) and 8 (J8) mg/kg] of the extract were administered orally using an oral gavage twice a day for 14 days. Immune responses were measured on 7th and 14th days. On 14th day, Yersinia ruckeri was injected intraperitoneally to all fish of all groups. On 14th day, ORP in fish of J1 group increased significantly. Lysozyme activity (LA) was increased in J8 group on 7th day (p < .05). On 14th day, a significant decrease was determined in J1 and J4 treatments in LA. Myeloperoxidase activity was significantly decreased in all groups irrespective of sampling times (p < .05). Interleukin (IL)-1β was significantly elevated in fish of J8 group on 7th day. IL-8 increased in fish of J8 and J4 groups on 7th day of the study. IL-12 gene expression was significantly up-regulated in J8 fish group on 7th day, and in J4 fish group on 14th day. Survival rate was higher in J8 treatment compared to the control and other treatments (p < .05). The results suggest that Juniperus excelsa provides protection against Y. ruckeri in rainbow trout.

A novel herbal immunostimulant for rainbow trout (Oncorhynchus mykiss) against Yersinia ruckeri

In this 21-day study, we examined the effects of the aqueous methanolic extract of thin-skinned plum (Prunus domestica) on growth, immune response and resistance to a pathogenic bacterium, Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss). Fish were fed with diets containing thin-skinned plum extract doses as 0 (Control) 0.1 (PD01), 0.5 (PD05) and 1% (PD1) ad libitum twice in a day. At the end of the study, growth was affected positively but not significantly. Feed conversion ratio (FCR) was decreased in the PD01 group (P < 0.05). There were elevated respiratory burst and potential bacterial killing activities on the 7th day in the PD1 fish group. No differences were observed in lysozyme activity (P > 0.05). An increased myeloperoxidase activity was recorded on the 14th day of study. Expression of interleukin and COX-2 genes was elevated on the 7th day of study in the kidney and intestine of treated fish. Histological results indicated no marked changes in organs (gill, kidney, liver and spleen) of PD treated fish groups. Challenge results of fish in all plum extract-treated groups showed an increased survival rate against Y. ruckeri (P < 0.05). This study indicated that the thin-skinned plum aqueous methanolic extract could improve innate immunity, survival against Y. ruckeri and decrease the FCR level.

Immune gene expression and genome-wide association analysis in rainbow trout with different resistance to Yersinia ruckeri infection

Selective breeding programmes involving marker assisted selection of innately pathogen resistant strains of rainbow trout rely on reliable controlled infection studies, extensive DNA typing of individual fish and recording of expression of relevant genes. We exposed juvenile rainbow trout (6 h bath to 2.6 × 10⁵ CFU mL^-1) to the fish pathogen Yersinia ruckeri serotype O1, biotype 2, eliciting Enteric Red Mouth Disease ERM, and followed the disease progression over 21 days. Cumulative mortality reached 42% at 12 days post challenge (dpc) after which no disease signs were recorded. All fish were sampled for DNA-typing (50 k SNP chip, Affymetrix®) throughout the course of infection when they showed clinical signs of disease (susceptible fish) or at day 21 when fish showed no clinical signs of disease (survivors - resistant fish). Genome-wide association analyses of 1027 trout applying single nucleotide polymorphisms (SNPs) as markers revealed an association between traits (susceptible/resistant) and certain regions of the trout genome. It was indicated that multiple genes are involved in rainbow trout resistance towards ERM whereby it is considered a polygenic trait. A corresponding trout group was kept as non-exposed controls and a comparative expression analysis of central innate and adaptive immune genes in gills, spleen and liver was performed for three fish groups: 1) moribund trout exhibiting clinical signs 7 dpc (CS), 2) exposed fish without clinical signs at the same sampling point (NCS) and 3) surviving fish at 21 dpc (survivors). Immune genes encoding inflammatory cytokines (IL-1β, IL-2A, IL-6A, IL-8, IL-10A, IL-12, IL-17A/F2A, IL-17C1, IL-17C2, IL-22, IFNγ, TNFα), acute phase reactants (SAA, C3, cathelicidins, lysozyme) were expressed differently in CS and NCS fish. Correlation (negative or positive) between expression of genes and bacterial load suggested involvement of immune genes in protection. Down-regulation of adaptive immune genes including IgDm, IgDs, IgT and TCR-β was seen primarily in CS and NCS fish whereas survivors showed up-regulation of effector molecule genes such as cathelicidins, complement and lysozyme suggesting their role in clearing the infection. In conclusion, SNP analyses indicated that ERM resistance in rainbow trout is a multi-locus trait. The gene expression in surviving fish suggested that several immune genes are associated with the trait conferring resistance.

Primary immunization using low antigen dosages and immunological tolerance in rainbow trout

Enteric redmouth disease (ERM), caused by the Gram negative enterobacterium Yersinia ruckeri, affects farming of salmonids, but vaccination against ERM confers a certain degree of protection dependent on the administration route. Recent studies on oral vaccination of rainbow trout suggest that immunological tolerance may be induced by primary immunization using a low antigen dosage. We have examined if low dosages of Y. ruckeri antigens, applied in feed or bath exposure over a prolonged period of time, leave rainbow trout more susceptible to infection. Groups of rainbow trout were immunized, either by immersion or feeding using different vaccine dosages, and subsequently challenged by live Y. ruckeri. Survival was recorded and immune reactions in surviving fish were evaluated (ELISA and qPCR). Trout, bath-vaccinated in a highly diluted vaccine or fed the same amount of bacterin in feed over 10 days, were not protected against Y. ruckeri challenge infection and in some cases these sub-optimally immunized fish experienced lower survival compared to non-primed controls. Genes encoding FoxP3 and immune-suppressive cytokines were down-regulated in fish vaccinated with a high antigen dosage when compared to groups exposed to low antigen dosages, suggesting a higher regulatory T cell activity in the latter fish groups. The study suggests that repeated exposure to low antigen concentrations induces some degree of immune tolerance in rainbow trout and we recommend application of high antigen dosages for primary immunization of trout.

Immunogenicity and protective efficacy of Yersinia ruckeri lipopolysaccharide (LPS), encapsulated by alginate-chitosan micro/nanoparticles in rainbow trout (Oncorhyncus mykiss)

Considering the many advantages of oral vaccines in aquaculture, several studies have been conducted in this area recently. In this study, immunization and protective power of the oral vaccine of Yersinia ruckeri encapsulated with Alginate-Chitosan micro/nanoparticles were evaluated in rainbow trout. For this purpose, 720 juvenile rainbow trout (9 ± 1.8 g) were divided into 8 groups in three replications (30 fish each) as follows: Groups A, B and C, were immunized with Yersinia ruckeri lipopolysaccharide (LPS), LPS+Formalin Killed Cells (FKC) and FKC alone, groups D, E, and F were immunized with encapsulated LPS, LPS+FKC and FKC, respectively. The G and H groups considered as encapsulated and non-encapsulated control, respectively. Micro/nanoencapsulation with alginate-chitosan was performed by internal emulsification method and vaccination were conductrd in the first and third weeks via oral route. Sampling was performed on days 0, 30, and 60 of experiment. Anti Y. ruckeri antibody titer in serum, intestine and skin mucus were measured via ELISA method. Non-specific immune response including: serum lysozyme, complement, bactericidal and respiratory burst activity, serum protein and globulin level, as well as white blood cell count were compared among the groups. The expression of IgT gene in the intestine and TCR gene in the anterior kidney were also investigated. At the end of the study, the fish were challenged with Y. ruckeri through immerssion and intraperitoneal routs and the relative survival rate was evaluated. Result showed that the antibody level in serum, skin and intestine was significantly higher in group E and F than control groups (P < 0.05), meanwhile serum, skin and intestine antibody level in all vaccinated groups were significantly (P < 0.01) higher in day 30 and 60 compare to zero day. Non-specific immunity factors including: serum lysozyme, complement, and respiratory burst activity as well as WBC, protein and Globulin level were significantly higher in E and F groups not only in day 30 but also in day 60 of experiment (P < 0.05). Cumulative mortality following injection and bath challenge were significantly (P = 0.004) lower (35%-45%) in groups E and F compare to control group (80%). The IgT and TCR gene expression in groups D, E and F were significantly higher (P < 0.05) than control group. Highest upregulation of IgT and TCR gene expression in vaccinated groups were seen at day 30 and 60 respectively which were significantly (P < 0.001) higher than day zero. Generally, it can be concluded that nano/micronanoencapsulation of Y. ruckeri FKC+LPS with chitosan-alginate, not only increases protective efficacy of oral vaccine, but improves specific and non-specific immune responses in rainbow trout.

Is Yersinia bercovieri Surpassing Yersinia enterocolitica in Wild Boars (Sus scrofa)?

Yersiniosis was the fourth reported zoonosis in the European Union in 2018. As well-known, pigs are recognized important reservoirs of Yersinia enterocolitica. The study was focused on Y. enterocolitica detection in mesenteric lymph nodes and faeces of 305 wild boars, but Yersinia bercovieri was more common, being isolated from 108 animals (35.4%). Cold season (p = 1.17 × 10^-5) and young age (p = 0.004) significantly increased Y. bercovieri detection. Y. enterocolitica 1A belonging to six serotypes (O:4.32-4.33; O:5; O:6.30-6.31; O:7.8-8-8.19; O:10-34; O:12.25-12.26) was isolated from 8.2% (25/305) of the animals. Cold season significantly affected (p = 0.037) Y. enterocolitica detection.

Toll/IL-1 receptor-containing proteins STIR-1, STIR-2 and STIR-3 synergistically assist Yersinia ruckeri SC09 immune escape

Immune escape is a common feature of bacteria, viruses, parasites and even cancer cells. Our earlier work on an integrative and conjugative element (ICEr2) of Yersinia ruckeri SC09 demonstrated contributory roles of stir-1, stir-2 and stir-3 in bacterial toxicity and ability to code for immune evasion. Here, we further examined the ability of stir-4 in ICE (r2) and its encoded STIR-4 protein to mediate immune evasion using comparative genomic analysis. Additionally, the mechanisms underlying the synergistic activities of STIR-1, STIR-2, STIR-3 and STIR-4 in immune evasion were examined. Our results showed that STIR-4 did not contribute to bacterial toxicity, either in vivo nor in vitro, or show the ability to assist in bacterial immune escape. STIR-1, STIR-2, and STIR-3 formed heterotrimers in bacteria while facilitating immune evasion, which we speculate may be essential to maintain their stability. This discovery also partially explains the previous finding that a single gene can mediate immune evasion. Our data provide further knowledge on the distribution of ICE (r2)-like elements in bacteria, validating the prevalence of large-scale gene transfer in pathogens and its potential for enhancing virulence levels. Further studies are necessary to establish the biological significance of the ICE (r2) component.

Advancing the fathead minnow (Pimephales promelas) as a model for immunotoxicity testing: Characterization of the renal transcriptome following Yersinia ruckeri infection

Recent studies have utilized the fathead minnow (Pimephales promelas) to explore the immunotoxic effects associated with a variety of environmental contaminants in the absence of immunological stimuli. Though this approach allows for alterations in the resting immune system to be detected, previous evidence suggests that many immunotoxic effects may only manifest in the activated immune system. However, basic immune responses to pathogens have not been well described in this species. To expand the utility of the fathead minnow as a model for immunotoxicity testing, a more comprehensive understanding of the activated immune system is required. As such, the main goal of this study was to characterize the transcriptomic response to pathogen infection in the fathead minnow using RNA sequencing. To achieve this goal, female fathead minnows were intraperitoneally injected with either Hank's Balanced Salt Solution (sham-injected) or Yersinia ruckeri (pathogen-injected). Eight hours following injection, fish were sacrificed for the assessment of general morphological (i.e., mass, length, condition factor, hepatic index) and immunological (i.e., leukocyte counts, spleen index) endpoints. To assess the molecular immune response to Y. ruckeri, kidney tissue was collected for transcriptomic analysis. A comparison of sham- and pathogen-injected fish revealed that >1800 genes and >500 gene networks were differentially expressed.Gene networks associated with inflammation, innate immunity, complement, hemorrhaging and iron absorption are highlighted and their utility within the context of immunotoxicity is discussed. These data reveal pathogen-related molecular endpoints to improve data interpretation of future studies utilizing the fathead minnow as a model for immunotoxicity.

The Prevalence of Yersinia enterocolitica and Yersinia pseudotuberculosis in Small Wild Rodents in Poland

Rodents are a large group of mammals that can be carriers of zoonotic pathogens such as Yersinia strains that cause yersiniosis. The prevalence of Yersinia enterocolitica and Yersinia pseudotuberculosis was determined in 214 small wild rodents from south-eastern Poland. Samples were analyzed by precultivation and PCR. Nine (4.2%) Y. enterocolitica and one (0.5%) Y. pseudotuberculosis isolates were received. Most of them (n = 5) were obtained from the common vole (Microtus arvalis). All Y. enterocolitica strains were classified as biotype (BT) 1A. A PCR analysis of virulence markers revealed that all Y. enterocolitica isolates contained the ystB gene and five isolates harbored a rare genetic combination of ail/ystB. Three of the four ail/ystB-positive isolates belonged to serotype O:5.27. The Y. pseudotuberculosis inv-positive isolate was classified as BT 1. A genetic analysis of Y. enterocolitica harboring the ystB gene revealed 100% similarity between the analyzed sequences and the sequences from diarrhea patients in India and the United Kingdom as well as high similarity with the sequences from different species of wild animals from Poland. The Y. pseudotuberculosis inv sequence was 100% identical to the sequence isolated from fully virulent clinical strain from France and Australia. The results of our study suggest that small wild rodents, especially voles and yellow-necked mice, may act as carriers of Yersinia strains. The high similarity of the tested gene sequences between our isolates and the isolates from other free-living animals indicates that small wild rodents can play a role in the epidemiology of yersiniosis and can shed Yersinia spp. into the environment.

Yersinia ruckeri strain SC09 disrupts proinflammatory activation via Toll/IL-1 receptor-containing protein STIR-3

Virulent pathogenic microorganisms often enhance their infectivity through immune evasion mechanisms. Our research on the integrative and conjugative element (ICE(r2)) of the virulent fish pathogen Yersinia ruckeri SC09 led to the identification of genes related to immune evasion (designated stir-1, stir-2, stir-3 and stir-4), among which stir-1 and stir-2 were determined as the key contributors to bacterial toxicity and immune evasion. Here, we further examined the ability of stir-3 to mediate immune evasion based on detailed bioinformatic analysis of ICE(r2) from Y. ruckeri SC09. Interactions among the translated STIR-1, STIR-2, STIR-3 and STIR-4 proteins in the secretory process were additionally explored. STIR-3 was positively correlated with bacterial toxicity and inhibited host toll-like receptor (TLR) signaling by interacting with MyD88, thereby facilitating bacterial survival in host cells. Importantly, our data showed co-secretion of STIR-1, STIR-2 and STIR-3 as a complex, with secretion failure occurring in the absence of any one of these proteins. While stir-1, stir-2, stir-3 and stir-4 genes werespecific to Y. ruckeri SC09, the ICE(r2) region where these genes were located is a mobile component widely distributed in bacteria. Therefore, the potential transmission risk of these immune evasion genes requires further research attention.

Characterization of a novel Yersinia ruckeri serotype O1-specific bacteriophage with virulence-neutralizing activity

A lytic bacteriophage (φNC10) specific to serotype O1 Yersinia ruckeri has been identified and evaluated as a model to assess the potential use of bacteriophages and their products for disease control in aquaculture. Electron microscopy of purified φNC10 revealed a virion particle with a small (70 nm) polyhedral head and short tail. φNC10 infected only serotype O1 strains of Y. ruckeri and failed to bind a defined Y. ruckeri mutant strain lacking O1 lipopolysaccharides (O1-LPS), suggesting that φNC10 uses O1-LPS as its receptor. In addition, spontaneous φNC10-resistant mutants of Y. ruckeri exhibited defects in O1-LPS production and were sensitive to rainbow trout serum. Purified φNC10 displayed a polysaccharide depolymerase activity capable of degrading Y. ruckeri O1-LPS and thereby sensitizing Y. ruckeri to the bactericidal effects of rainbow trout serum. The φNC10-associated polysaccharide depolymerase activity also reduced the ability of Y. ruckeri cells to cause mortality following intraperitoneal injection into rainbow trout. These data demonstrate a potential utility of φNC10 and its associated polysaccharide depolymerase activity for Y. ruckeri disease prevention.

Extract of common mallow (Malvae sylvestris) enhances growth, immunity, and resistance of rainbow trout (Oncorhynchus mykiss) fingerlings against Yersinia ruckeri infection

The dietary effects of a native medicinal plant from Iran, common mallow (Malvae sylvestris), was evaluated on growth performance, innate immune parameters, mucosal immune parameters, and resistance of rainbow trout (Oncorhynchus mykiss) against Yersinia ruckeri. Therefore, 360 fish (initial weight 10.42 ± 0.09 g) were randomly distributed into 12 fiberglass tanks. Experimental diets supplemented with 0 (as control- C), 1% (M1), 3% (M2) and 5% (M3) levels of M. sylvestris flowers extract were fed to the fish based on 3% of body weight for 8 weeks. At the terminal sampling, growth performance, liver and digestive enzymes activities, blood and mucosal immune responses were determined. Results showed that M2 and M3 had greater final weight, weight gain, SGR, survival rate and lower FCR; higher levels of total protein, albumin, globulin, and lower cortisol levels in comparison to control; 5% extract also lowered cholesterol and glucose levels as well as Lactate Dehydrogenase (LDH) activity. We reported higher values of hematocrit, hemoglobin, Mean Corpuscular Hemoglobin (MCH), Mean Corpuscular Volume (MCV), White Blood Cell (WBC), Red Blood Cell (RBC) and lymphocytes for treated groups. Innate immune responses (Alternative complement activity (ACH50) in M2 and M3 group, total Immunoglobulin (Ig) and lysozyme in M3), mucosal immune parameters (ACH50, total Ig for M2 and M3 group and lysozyme in all treated groups) were enhanced. Activities of digestive enzymes (protease in all treated groups, amylase for 3 and 5%, while lipase only for 5%) and lower activity of liver ALT enzyme in individuals treated with highest dose was observed. Overall results indicated that the extract can positively affect growth performance and immune responses of rainbow trout.

Growth performance, haematological changes, immune response, antioxidant activity and disease resistance in rainbow trout (Oncorhynchus mykiss) fed diet supplemented with ellagic acid

The present study was performed to investigate the effects of various levels of dietary ellagic acid (EA) on growth performance, haematological values, immune response, protection against Yersinia ruckeri infection, and oxidant/antioxidant status in rainbow trout, Oncorhynchus mykiss. Fish were fed with the control diet and three different experimental diets containing three graded levels of EA (50, 100 and 200 mg kg^-1 diet) for 8 weeks. At the end of the experiment, the growth performance [weight gain (WG), specific growth rate (SGR) and feed conversion ratio (FCR)], haematological values [the red blood cell (RBC) count, haemoglobin (Hb) concentration, haematocrit (Ht) level and erythrocyte indices: mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC)], immune response [white blood cell (WBC) count, oxidative radical production (nitroblue tetrazolium (NBT) assay), phagocytic activity (PA) and phagocytic index (PI), total protein (TP) and immunoglobulin M (IgM) levels, serum bactericidal activity (BA), lysozyme (LYZ) and myeloperoxidase (MPO) activities] and oxidant/antioxidant status [tissue malondialdehyde (MDA) levels and superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) activities] were analysed. In addition, fish were challenged by Y. ruckeri and survival rate was recorded for 14 days. In the groups fed with EA the growth parameters such as WG, SGR, and FCR did not vary significantly. The RBC count, Hb concentration, and Ht level increased in the groups fed with EA when compared with the control group. However, there were no significant differences in the MCV, MCH and MCHC values among the groups. The results demonstrated enhancement in all the immunological parameters in the groups fed with EA compared to the control group. The results obtained from challenge with Y. ruckeri revealed reduction in the mortalities in the groups fed with EA. The dietary EA stimulated the SOD, CAT and GSH-Px activities in liver, head kidney and spleen as compared to the control group; however, a reverse trend was observed in the MDA levels of tissues. The present study suggest that EA can effectively enhance the haematological values, immune response, antioxidant capacity, and disease resistance in rainbow trout.

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.

TcpA, a novel Yersinia ruckeri TIR-containing virulent protein mediates immune evasion by targeting MyD88 adaptors

TIR domain-containing protein is an important member for some bacterial pathogens to subvert host defenses. Here we described a fish virulent Yersinia ruckeri SC09 strain that interfered directly with Toll-like receptor (TLR) function by a TIR-containing protein. Firstly, the novel TIR-containing protein was identified by bioinformatics analysis and named as TcpA. Secondly, the toxic effects of TcpA in fish was demonstrated in vivo challenge experiments through knockout mutant and complement mutant of tcpA gene. Thirdly, The study in vitro revealed that TcpA could down-regulate the expression and secretion of IL-6, IL-1β and TNF-α. Finally, we demonstrated that TcpA could inhibit the TLR signaling pathway through interaction with myeloid differentiation factor 88 (MyD88) in experiments such as NF-κB dependent luciferase reporter system, co-immunoprecipitation, GST pull-down and yeast two-hybrid. The study revealed that TcpA was essential for virulence and was able to interact with the TIR adaptor protein MyD88 and inhibit the pre-inflammatory signal of immune cells and promote the intracellular survival of pathogenic Yersinia ruckeri SC09 strain. In conclusion, our results showed that TcpA acted as a new virulence factor in Y. ruckeri could suppress innate immune response and increase virulence by inhibiting TLR and MyD88-mediated specific signaling, highlighting a novel strategy for innate immune evasion in bacteria.

An overview of Yersinia enterocolitica and related species in samples of different origin from San Luis, Argentina

This study is aimed at offering an overview of the prevalence of Yersinia enterocolitica and related species in San Luis, Argentina, from samples of diverse origin received in our laboratory between 1984 and 2014, and providing an analysis of the distribution of Yersinia isolates according to their isolation sources, highlighting bioserotypes and potential reservoirs and vehicles of transmission to humans. From a total of 4572 samples of human, animal, food and environmental origins analyzed by traditional culture methods and molecular techniques, 229 (5%) samples were Yersinia positive. The highest frequency of Yersinia isolates was observed in environmental specimens (14.3%), followed by animal (9.2%), food (5%) and human (0.6%) samples. A total of 255 Yersinia isolates were characterized, including 183 Y. enterocolitica and 72 isolates of other Yersinia species. Biotype 1A associated to several serotypes was identified in Y. enterocolitica isolates from environment (100%), animals (95.5%), foods (71.7%) and human samples (40%); bioserotype 2/O:9 was identified in isolates from foods (25.5%), and biotype 3 was associated with strains from humans (60%), animals (4.5%) and foods (2.8%). This biotype included three strains O:3 and six strains O:5. The data highlight animals and foods as the main Y. enterocolitica sources in our region.

Protective effect of in-feed specific IgM towards Yersinia ruckeri in rainbow trout

Tightened regulations and an environmentally friendly approaches in fish production have greatly reduced the use of antibiotics but green solutions are continuously being explored. The use of functional feed may have a potential in the aquaculture sector in securing biomass and minimizing the loss from disease. In the present study, we tested the concept that blood from the fish slaughterhouse can be used for mass purification of specific antibodies which subsequently can be used for feeding fish and thereby confer protection against diseases. IgM was purified from serum from Yersinia ruckeri vaccinated rainbow trout and an IgM sandwich ELISA was developed for quantification of rainbow trout IgM. The purified IgM was encapsulated in alginate microparticles and top-coated in fish feed. IgM re-extracted from the alginate microparticles was shown to retain high reactivity towards Y. ruckeri antigens indicating that its bioactivity remained intact after encapsulation. IgM release from the alginate microparticles was only observed at high pH (pH 8.2) and minimal at low pH, indicating protection of IgM at low pH in the fish stomach during passage. In a feeding - challenge experiment (feeding 1 week before Y. ruckeri challenge and for two weeks following challenge), a statistically non-significant 10% lower mortality was observed in the high dose (400 μg IgM/fish/day fed over 3 weeks) group.

Comparative susceptibilities and immune-related gene expressions of brown trout strains and their hybrids infected with Lactococcus garvieae and Yersinia ruckeri

Brown trout are polymorphic salmonid species, and it is of importance to investigate whether hybridization affects disease resistance. In this study, susceptibility of brown trout (Salmo trutta Abant, Anatolian, Black Sea, and Caspius) strains and their hybrids to Lactococcus garvieae and Yersinia ruckeri as well as their immune-related gene expression profiles were studied. Results indicated that reciprocal hybridization did not affect disease resistance in brown trout strains. Purebred Black Sea strain of brown trout was the most resistant group against Y. ruckeri, followed by other Black Sea strain hybrids. On the other hand, purebred Anatolian strain was the most resistant group to L. garvieae, followed by other Anatolian strain hybrids. Expression pattern of target genes differed in families, but the overall gene expression was comparatively high in Y. ruckeri infected families. Upregulations were mainly significant at 7 and 28 d post infection while marginal regulations were observed 8 h after infection. Disease resistance status of strains was supported by high expression of immune-related genes such as major histocompatibility complex class I (MHC-I), immunoglobulin light chain (IgL), and antioxidant- and hemoglobin-related gene expression. Therefore, our findings suggest that Black Sea and Anatolian strains could be used to develop fish stock that are resistant for yersiniosis and lactocaccosis, respectively.

A pentavalent vaccine for rainbow trout in Danish aquaculture

Mariculture in Denmark is based on production of rainbow trout grown two years in fresh water followed by one growth season in sea cages. Although the majority of rainbow trout are vaccinated against the most serious bacterial pathogens - Aeromonas salmonicida subsp. salmonicida, Vibrio anguillarum and Yersinia ruckeri, by the use of commercially available vaccines, disease outbreaks requiring treatment with antibiotics still occur. The present study tested the potential of a new experimental multicomponent vaccine that is based on local bacterial strains, isolated from rainbow trout in Danish waters, and thus custom-designed for Danish rainbow trout mariculture. The vaccination with the multicomponent vaccine resulted in protection against three relevant bacterial diseases (yersiniosis, furunculosis, vibriosis) under experimental conditions. We showed that i.p. injection of the vaccine induced specific antibody responses in trout against the different bacterial antigens and regulated expression of genes encoding SAA, C3, IL-1β, IL-6, IL-8, IgD and MHCII.

Possible effect of hala extract (Pandanus tectorius) on immune status, anti-tumour and resistance to Yersinia ruckeri infection in rainbow trout (Oncorhynchus mykiss)

The possible effect of dietary administration of hala extract (Pandanus tectorius) on rainbow trout (Oncorhynchus mykiss) immune status as well as its effect as an anti-tumour agent was studied. Fish were divided into 4 groups before feeding with commercial diet (0%, control; 0.5%, 1% and 2% of hala extract) for 2 weeks. The effect of diet on the humoral immune parameters, ie total protein, myeloperoxidase content, antiproteases, lysozyme and bactericidal activities were studied. Also, the effect of the diets on the expression of some immune-related genes in rainbow trout head-kidney (TNF, LYZ2, IL-8 and CD-4) as well as tumour suppressor gene (WT-1a) was investigated. At the end of the feeding trial fish groups were challenged with Yersinia ruckeri. The results demonstrated enhancement in all the immune parameters in fish fed hala extract diets compared to control fish especially with the highest dose (2%) which recorded the highest significant increase (p < 0.05) in some parameters (total protein, myeloperoxidase content, antiproteases, and bactericidal activities) compared to the control. The results obtained from challenge with Y. ruckeri revealed reduction in the mortalities in fish groups fed with 1% and 2% doses of hala extract. Feeding with hala extract provoked upregulation in all immune- related genes. Again, the highest dose of hala extract showed a significant upregulation in WT1a expression (p < 0.05). The current study suggest that the hala extract, especially the highest dose, could be considered a good food additive to improve the immune status, resist tumour formation and to resist or control infectious diseases of rainbow trout.

Effect of immunostimulatory feed supplements on the development of acquired immunity in rainbow trout (Oncorhynchus mykiss)

Immunostimulatory feed supplements are an increasingly common feature of aquaculture management and their benefit has been confirmed for a wide area of products. However, these investigations have often focused on the benefit of these supplements on the innate immune system. In the current project, we investigated a mixture of two commercial feed supplements (Biotronic^® Top 3 and Levabon^® Aquagrow E) with a known protective effect against bacterial infections. The effect of the supplemented diet on antibody titters of Oncorhynchus mykiss vaccinated against Yersinia ruckeri was determined by ELISA. Furthermore, an infection trial was performed to confirm the effect of the supplements on the survival of the fish. Finally, their effects on the growth parameters of the fish were also determined. The results from this study found no significant effect on the general antibody titters. However, when considering only the titters of specific anti-Y ruckeri antibodies, the supplemented feed was associated with an improved response to the vaccine, significantly better than in the fish that had received the control feed.

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.

Dissecting the immune pathways stimulated following injection vaccination of rainbow trout (Oncorhynchus mykiss) against enteric redmouth disease (ERM)

Enteric redmouth disease (ERM or yersiniosis) is one of the most important diseases of salmonids and leads to significant economic losses. It is caused by the Gram-negative bacterium Yersinia ruckeri but can be controlled by bacterin vaccination. The first commercial ERM vaccine was licenced in 1976 and is one of the most significant and successful health practices within the aquaculture industry. Although ERM vaccination provides complete protection, knowledge of the host immune response to the vaccine and the molecular mechanisms that underpin the protection elicited is limited. In this report, we analysed the expression in spleen and gills of a large set of genes encoding for cytokines, acute phase proteins (APPs) and antimicrobial peptides (AMPs) in response to ERM vaccination in rainbow trout, Oncorhynchus mykiss. Many immune genes in teleost fish are known to have multiple paralogues that can show differential responses to ERM vaccination, highlighting the necessity to determine whether all of the genes present react in a similar manner. ERM vaccination immediately activated a balanced inflammatory response with correlated expression of both pro- and anti-inflammatory cytokines (eg IL-1β1-2, TNF-α1-3, IL-6, IL-8 and IL-10A etc.) in the spleen. The increase of pro-inflammatory cytokines may explain the systemic upregulation of APPs (eg serum amyloid A protein and serum amyloid protein P) and AMPs (eg cathelicidins and hepcidin) seen in both spleen and gills. We also observed an upregulation of all the α-chains but only one β-chain (p40B2) of the IL-12 family cytokines, that suggests specific IL-12 and IL-23 isoforms with distinct functions might be produced in the spleen of vaccinated fish. Notably the expression of Th1 cytokines (IFN-γ1-2) and a Th17 cytokine (IL-17A/F1a) was also up-regulated and correlated with enhanced expression of the IL-12 family α-chains, and the majority of pro- and anti-inflammatory cytokines, APPs and AMPs. These expression profiles may suggest that ERM vaccination activates host innate immunity and expression of specific IL-12 and IL-23 isoforms leading to a Th1 and Th17 biased immune response. A late induction of Th2 cytokines (IL-4/13B1-2) was also observed, that may have a homeostatic role and/or involvement in antibody production. This study has increased our understanding of the host immune response to ERM vaccination and the adaptive pathways involved. The early responses of a set of genes established in this study may provide essential information and function as biomarkers in future vaccine development in aquaculture.

Effect of oral booster vaccination of rainbow trout against Yersinia ruckeri depends on type of primary immunization

Vaccination of rainbow trout against Enteric Redmouth Disease (ERM) caused by Yersinia ruckeri can be successfully performed by administering vaccine (a bacterin consisting of formalin killed bacteria) by immersion, bath or injection. Booster immunization is known to increase the protection of fish already primed by one of these vaccination methods. Oral vaccination of trout (administering vaccine in feed) is an even more convenient way of presenting antigen to the fish but the effect of an oral booster has not previously been described in detail. The present work describes to what extent protection may be enhanced by oral boostering following priming with different administration methods. The study confirms that vaccination by 30 s dip into a bacterin (diluted 1:10) may confer a significant protection compared to non-vaccinated fish. The immunity may be optimized by booster immunization either provided as dip (most effective), bath (less effective) or orally (least effective). Oral immunization may be used as booster after dip but applied as a single oral application it induced merely a slight and statistically non-significant response. It is noteworthy that primary oral immunization followed by an oral booster vaccination showed a trend for an even weaker response. It should be investigated if continued exposure to a low antigen concentration - as performed by two oral immunizations - may induce tolerance to the pathogen and thereby leave the fish more vulnerable.

Assessment of the feasibility of serological monitoring and on-farm information about health status for the future meat inspection of fattening pigs

Current macroscopic meat inspection cannot detect the most common pork-borne pathogens (Salmonella spp., Yersinia enterocolitica and Toxoplasma gondii). Furthermore, food chain information (FCI) may not provide sufficient data for visual-only inspection, which is supposed to be the common way of inspection of pigs in the European Union. Our observational study aimed to evaluate the serological monitoring and the clinical evaluation of on-farm health status of pigs and assess the feasibility of these data as part of the FCI in meat inspection. We studied the serological status of Salmonella spp., Yersinia spp. and T. gondii in pigs during the fattening period. Additionally, we evaluated the association between on-farm health status and meat inspection findings. On 57 indoor fattening pig farms in Finland, we collected blood samples (mean of 20 pigs/farm) and assessed the on-farm health (coughing, tail biting, lameness) at the end of the fattening period. We visited 34 of these farms also at the beginning of the fattening for sampling and on-farm health evaluation of the same pigs. Meat inspection results were obtained after slaughter for all 57 farms. Salmonella seroprevalence was low at the end of the fattening period: it was 17.6%, 10.6% or 1.9%, with the cut-off values of OD15% (recommended by the test manufacturer), OD20% (used by Danish monitoring programme) and OD40% (used by German monitoring programme), respectively. The overall seroprevalence of Salmonella spp. and Yersinia spp. increased significantly (P < 0.001) during the fattening period (from 8.1% to 17.2% and from 30.3% to 72.3%, respectively), while the seroprevalence of T. gondii remained low (<1%). The within-farm seroprevalences of Salmonella spp. and Yersinia spp. differed significantly between the farms and this farm-level serological data could be used as FCI for risk-based decisions to improve food safety. Such potentially feasible decisions could include additional carcass testing, carcass decontamination, carcass processing, slaughtering arrangements and improved biosecurity measures at the farm. However, risk mitigation targets and procedures must be carefully adjusted for each pathogen regarding also economic aspects. Tail biting observed on farm was associated with partial carcass condemnations and arthritis at slaughter. This information could be included in the FCI and used when making decisions regarding meat inspection procedure: visual-only or additional inspections.

Adjuvant efficacy of G2 (buffalo spleen extraction) against Yersinia septicemia in rainbow trout (Onchorhynchus mykiss)

Adjuvant effect of G2 (extraction of buffalo spleen) was assessed in intraperitoneally immunized rainbow trout (100-150 g) with killed- Y. ruckeri bacterin biotype I [0.1 mL (1 × 10⁷ cells/fish) of vaccine diluted with the adjuvant in a ratio of 1:1 (V/V)] at 12.4 ± 1.3 °C for 10 weeks. Leukocyte and lymphocyte counts, anti-Y. ruckeri antibody titer and relative percent survival (RPS) in fish vaccinated with vaccine containing the adjuvant were significantly higher than the immunized fish with Y. ruckeri antigen alone throughout the experiment (P < 0.05), but neutrophils count and lysozyme activity were mostly significantly higher in the latter group (P < 0.05). No difference was seen in the complement activity between vaccinated fish containing the adjuvant and vaccine alone (P > 0.05). The results of this work for the first time demonstrated that inclusion of G2 as an adjuvant in Y. urckeri vaccine can improve the efficacy of the vaccine against Yersinia septicemia in rainbow trout.

Embryo-larval BDE-47 exposure causes decreased pathogen resistance in adult male fathead minnows (Pimephales promelas)

Exposures to polybrominated diphenyl ethers (PBDEs) have been shown to alter immune function in adult organisms across a variety of taxa. However, few if any studies have investigated the long-term consequences of early life stage PBDE exposures on immune function in fish. This study sought to determine the effects of early life stage BDE-47 exposure on pathogen resistance in the fathead minnow (Pimephales promelas) following an extended depuration period (≥180 d). Minnows were exposed to BDE-47 via a combination of maternal transfer and diet through 34 days post fertilization (dpf), raised to adulthood (>215 dpf) on a clean diet, then subjected to pathogen resistance trials. Early life stage exposures to BDE-47 did not affect the ability of females to survive from Yersinia ruckeri infection. However, the survival of BDE-47 exposed males was significantly reduced relative to controls, indicating that developmental exposures to BDE-47 altered male immunity. Because BDE-47 is a known thyroid hormone disruptor and thyroid hormone disruptors have the potential to adversely impact immune development and function, metrics indicative of thyroid disruption were evaluated, as were immune parameters known to be altered in response to thyroid disruption. BDE-47 exposed minnows exhibited signs of thyroid disruption (i.e., reduced growth); however, no alterations were observed in immune parameters known to be influenced by thyroid hormones (i.e., thymus size, expression of genes associated with lymphoid development) suggesting that the observed alterations in immunocompetence may occur through alternative mechanisms. Regardless of the mechanisms responsible, the results of this study demonstrate the potential for early life stage PBDE exposures to adversely impact immunity and illustrate that the immunological consequences of PBDE exposures are sex dependent.

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.

Dietary olive leaf (Olea europea L.) extract alters some immune gene expression levels and disease resistance to Yersinia ruckeri infection in rainbow trout Oncorhynchus mykiss

The aim for the present study was to investigate the effects of olive leaf (Olea europea L.) extract (OLE) on the control of Yersinia ruckeri infection in rainbow (Oncorhynchus mykiss) trout and to assess the impact on the expression of immune-related genes in the spleen and serum biochemical parameters of rainbow trout. Five experimental diets were prepared by adding 0.0%, 0.1%, 0.25%, 0.50% and 1.0% of OLE. Each diet was fed to triplicate groups of fish (mean body weight 51.22 ± 3.04 g) twice a day (at 09:00 and 17:00 h) for 60 days. The dietary supplementation of OLE did not affect growth performance and feed utilization (P > 0.05). Major changes due to graded levels of OLE in the diets were observed in blood biochemical parameters (P < 0.05). TNFα, IL1-β and IL-8 gene expressions were significanlty up-regulated in OLE 0.1% group compared with others (P < 0.05). Also, diet supplemented with OLE reduced mortality in rainbow trout fed with OLE 0.1% added diet. Present study suggests that OLE especially at 0.1% added feed may effectivelly enhance the serum biochemical parameters, survival rate and immune gene expression in rainbow trout.

Distribution of enteropathogenic Yersinia spp. and Salmonella spp. in the Swedish wild boar population, and assessment of risk factors that may affect their prevalence

BACKGROUND

Pure Eurasian wild boars and/or hybrids with domestic pigs are present in the wild on most continents. These wild pigs have been demonstrated to carry a large number of zoonotic and epizootic pathogens such as Salmonella spp., Yersinia enterocolitica and Y. pseudotuberculosis. Wild boar populations throughout Europe are growing and more and more wild boar meat is being consumed, the majority within the homes of hunters without having passed a veterinary inspection. The aim of this study was to investigate if factors such as population density, level of artificial feeding, time since establishment of a given population, and the handling of animal by-products from slaughtered animals could influence the presence of these pathogens in the wild boar.

RESULTS

In total, 90 wild boars from 30 different populations in Sweden were sampled and analysed using a protocol combining pre-cultivation and PCR-detection. The results showed that 27% of the sampled wild boars were positive for Salmonella spp., 31% were positive for Y. enterocolitica and 22% were positive for Y. pseudotuberculosis. In 80% of the sampled populations, at least one wild boar was positive for one of these enteropathogens and in total, 60% of the animals carried at least one of the investigated enteropathogens. The presumptive risk factors were analysed using a case-control approach, however, no significant associations were found.

CONCLUSION

Human enteropathogens are commonly carried by wild boars, mainly in the tonsils, and can thus constitute a risk for contamination of the carcass and meat during slaughter. Based on the present results, the effect of reducing population densities and number of artificial feeding places might be limited.