The development of the gut microbiota in rainbow trout (<em>Oncorhynchus mykiss</em>) is affected by first feeding and diet type

An influence of the intestinal microbiota in connection to first-feeding of rainbow trout (Oncorhynchus mykiss) fry was demonstrated using Illumina HiSeq sequencing. The period from the end of yolk-sac feeding and until seven weeks post first-feeding was examined after administration of either a marine- or plant based diet with or without the probiont Pediococcus acidilactici. Before first feeding the main part of the sequence reads grouped to the genus Sediminibacterium probably originating from the surrounding water. The microbial abundance and diversity increased after first-feeding and the microbiota then changed towards Firmicutes phylum dominance for plant based fed fish and towards dominance of phylum Proteobacteria for the marine fed fish. After first-feeding, there were significantly higher abundances of Streptococcus,Leuconostoc and Weissella in fish fed the plant-based diet. The microbiota clustered separately according to the diet type, but only minor effects were seen from the probiont when using PCAanalysis. The constitutive transcription level of most examined immune genes increased during the ontogenic shift, but the results could not explain the differences in the composition of the microbiota dependent on diet treatment after first-feeding. The results suggest that the intestine of rainbow trout is colonised at an early state, but is guided in new and different directions dependent on the diet type

Dose‐ and time‐dependent effects of an immune challenge on fish across biological levels

Abstract Due to global changes, fish are increasingly exposed to immune challenges associated with disease outbreaks in aquatic ecosystems. Adjustments in physiology and behavior are generally critical to maintaining homeostasis after an immune challenge, but there is limited knowledge on the specific thresholds and dynamics of responses across levels of biological organization in fish. In this study, we tested how different concentrations of an antigens mixture (phytohemagglutinin and lipopolysaccharide) affected innate immunity with potential consequences on oxidative stress, energy reserves, body condition, and behavior across time, using the common gudgeon ( Gobio sp.) as model species. The immune challenge induced a transitory increase in lytic enzyme activity (i.e., lysozyme) and local immune response (i.e., skin swelling) 2 days after the antigen injection. The available energy stored in muscle was also reduced 4 days after injection, without inducing oxidative stress at the cellular level. Overall, the immune challenge induced limited costs at the molecular and cellular levels but had strong effects at the whole organism level, especially on behavior. Indeed, fish swimming activity and sociability were affected in a dose- and time-dependent manner. These results suggest that immune challenges have dose-dependent effects across levels of biological organization and that behavior is a key response trait to cope with pathogen-induced immune costs in the wild, although fitness consequences remain to be tested. , Highlights The costs induced by immune challenge vary in fish in a dose- and time-dependent manner. Medium and high doses of antigens mostly affected immune and behavioral traits with limited effects on fish condition, likely due to energy reallocation

Red mark syndrome of trout (Oncorhynchus mykiss; Walbaum, 1792): Histopathological scoring and correlation with gross lesions

Red mark syndrome (RMS) is a skin disorder affecting rainbow trout (Oncorhynchus mykiss). The present work aimed to correlate the gross skin lesions affecting 46 fish sampled from farms surveyed for RMS with their microscopic features, identifying histological parameters that may be suggestive of disease progression. Skin lesions were grossly included in one of three categories (types I, II and III) according to the progressive degree of severity. Histological parameters and anti-proliferating cell nuclear antigen (PCNA) tissue immunoreactivity were semi-quantitatively assessed. In the dermis, PCNA-positive lymphocytes, fibroblasts and endothelial cells were indicative of active phlogosis. A significant increase in PCNA-immunoreactive lymphocytes, from gross type I to type III cases, was found only in the hypodermis. The histological parameters significantly associated with the gross lesion severity were progressive loss of the epithelium and scales, recruitment of inflammatory cells in the stratum compactum, loss of architecture of the stratum compactum, perivascular and perineural granulomatous inflammation and increase in lymphocyte infiltration of the muscular layer. In the type II and type III categories, inflammation in the hypodermis and muscle displayed a granulomatous pattern, reinforcing the hypothesis of an immunopathological mechanism. The morphological diagnosis of "deep chronic dermatitis associated to panniculitis and myositis, characterised by lympho-histiocytic and granulomatous reaction" is suggested

Early Immune Responses in Rainbow Trout Liver upon Viral Hemorrhagic Septicemia Virus (VHSV) Infection

<div><p>Among the essential metabolic functions of the liver, in mammals, a role as mediator of systemic and local innate immunity has also been reported. Although the presence of an important leukocyte population in mammalian liver is well documented, the characterization of leukocyte populations in the teleost liver has been only scarcely addressed. In the current work, we have confirmed the presence of IgM⁺, IgD⁺, IgT⁺, CD8α⁺, CD3⁺ cells, and cells expressing major histocompatibility complex (MHC-II) in rainbow trout (<i>Oncorhynchus mykiss</i>) liver by flow cytometry and/or immunohistochemistry analysis. Additionally, the effect of viral hemorrhagic septicemia virus (VHSV) on the liver immune response was assessed. First, we studied the effect of viral intraperitoneal injection on the transcription of a wide selection of immune genes at days 1, 2 and 5 post-infection. These included a group of leukocyte markers genes, pattern recognition receptors (PRRs), chemokines, chemokine receptor genes, and other genes involved in the early immune response and in acute phase reaction. Our results indicate that T lymphocytes play a key role in the initial response to VHSV in the liver, since CD3, CD8, CD4, perforin, Mx and interferon (IFN) transcription levels were up-regulated in response to VHSV. Consequently, flow cytometry analysis of CD8α⁺ cells in liver and spleen at day 5 post-infection revealed a decrease in the number of CD8α⁺ cells in the spleen and an increased population in the liver. No differences were found however in the percentages of B lymphocyte (IgM⁺ or IgD⁺) populations. In addition, a strong up-regulation in the transcription levels of several PRRs and chemokines was observed from the second day of infection, indicating an important role of these factors in the response of the liver to viral infections.</p></div