Polyphasic characterization of Aeromonas salmonicida isolates recovered from salmonid and non-salmonid fish

An Outbreak of Aeromonas salmonicida in Juvenile Siberian Sturgeons (Acipenser baerii)

Aeromonas salmonicida is one of the major threats to world aquaculture, causing fish furunculosis and high mortality rates in cultured fish, particularly salmonids. Although Aeromonas spp. is a thoroughly studied pathogen, little is known regarding aeromoniasis in sturgeons. After a mortality outbreak, four juvenile sturgeons (Acipenser baerii) were submitted for autopsy and tissue samples were collected for histopathological and microbiological studies. The external examination revealed size heterogenicity, skin hyperpigmentation and reduced body condition of sturgeons. Within the abdominal cavity, mild hepatomegaly and splenomegaly were observed, as well as generalized organic congestion. Histology revealed severe multifocal haemorrhagic and ulcerative dermatitis, mainly localized in the dorsal and latero-ventral areas of fish. The histological study also showed moderate to severe inflammation of gills and organic lesions compatible with septicaemia. Bacterial isolates were identified as Aeromonas salmonicida subsp. salmonicida using MALDI-TOF MS and PCR. Overall, the lesions first described here are consistent with those previously reported in other cultured fish species and contribute to a better understanding of the pathogenesis of Aeromonas salmonicida subsp. salmonicida in the Siberian sturgeon, aside from providing new diagnostic tools for bacterial diseases impacting the fast-growing industry of caviar.

Early Molecular Immune Responses of Turbot (Scophthalmus maximus L.) Following Infection with Aeromonas salmonicida subsp. salmonicida

Turbot aquaculture production is an important economic activity in several countries around the world; nonetheless, the incidence of diseases, such furunculosis, caused by the etiological agent A. salmonicida subsp. salmonicida, is responsible for important losses to this industry worldwide. Given this perspective, this study aimed to evaluate early immune responses in turbot (S. maximus L.) following infection with A. salmonicida subsp. salmonicida. For this, 72 fish were individually weighed and randomly distributed into 6 tanks in a circulating seawater system. For the bacterial challenge, half of the individuals (3 tanks with 36 individuals) were infected using a peritoneal injection with the bacterial suspension, while the other half of individuals were injected with PBS and kept as a control group. Several factors linked to the innate immune response were studied, including not only haematological (white blood cells, red blood cells, haematocrit, haemoglobin, mean corpuscular volume, mean cell haemoglobin, mean corpuscular haemoglobin concentration, neutrophils, monocytes, lymphocytes, thrombocytes) and oxidative stress parameters, but also the analyses of the expression of 13 key immune-related genes (tnf-α, il-1β, il-8, pparα-1, acox1, tgf-β1, nf-kB p65, srebp-1, il-10, c3, cpt1a, pcna, il-22). No significant differences were recorded in blood or innate humoral parameters (lysozyme, anti-protease, peroxidase) at the selected sampling points. There was neither any evidence of significant changes in the activity levels of the oxidative stress indicators (catalase, glutathione S-transferase, lipid peroxidation, superoxide dismutase). In contrast, pro-inflammatory (tnf-α, il-1β), anti-inflammatory (il-10), and innate immune-related genes (c3) were up-regulated, while another gene linked with the lipid metabolism (acox1) was down-regulated. The results showed new insights about early responses of turbot following infection with A. salmonicida subsp. salmonicida.

Relative expression and validation of Aeromonas salmonicida subsp. salmonicida reference genes during ex vivo and in vivo fish infection

The genus Aeromonas is found worldwide in freshwater and marine environments and has been implicated in the etiology of human and animal diseases. In fish, among Aeromonas species, A. salmonicida causes massive mortality and great economic losses in marine and continental aquaculture species. Currently, several aspects of the clinical signs and pathogenesis of this Gram-negative bacterium have been described; however, determination of an appropriate reference gene is essential to normalize cellular mRNA data remain unknown. Here we evaluate the stability of seven candidate reference genes to be used for data normalization during ex vivo and in vivo experiments conducted in Atlantic cod, Atlantic salmon, and lumpfish. To assess this, raw Ct values obtained were evaluated by using geNorm, NormFinder, BestKeeper, Delta Ct comparison, and the comprehensive ranking, through the bioinformatic open-access portal RefFinder. We determined that fabD and era were most suitable reference genes in Atlantic cod primary macrophages, hfq and era in Atlantic salmon primary macrophages, rpoB and fabD in lumpfish head kidney samples, and hfq and era in lumpfish spleen. Our study demonstrates that use of multiple reference genes and its validation before measurements helps to minimize variability arising in qPCR studies that evaluate A. salmonicida gene expression in fish tissues. Overall, this study provided with an expanded list of reliable reference genes for A. salmonicida gene expression using qPCR during fish infection studies.

Isolation and characterization of Flavobacterium columnare strains infecting fishes inhabiting the Laurentian Great Lakes basin

Flavobacterium columnare, the aetiological agent of columnaris disease, causes significant losses in fish worldwide. In this study, the prevalence of F. columnare infection was assessed in representative Great Lakes fish species. Over 2000 wild, feral and hatchery-propagated salmonids, percids, centrarchids, esocids and cyprinids were examined for systemic F. columnare infections. Logistic regression analyses showed that the prevalence of F. columnare infection varied temporally and by the sex of the fish, whereby females had significantly higher prevalence of infection. A total of 305 isolates of F. columnare were recovered. Amplification of the near complete 16S rRNA gene from 34 representative isolates and subsequent restriction fragment length polymorphism analyses demonstrated that all belonged to F. columnare genomovar I. Phylogenetic analysis of near complete 16S rRNA gene sequences also placed the isolates in genomovar I, but revealed some intragenomovar heterogeneity. Together, these results suggest that F. columnare genomovar I is widespread in the Great Lakes Basin, where its presence may lead to mortality.

Aeromonas salmonicida: updates on an old acquaintance

Aeromonas salmonicida is the oldest known infectious agent to be linked to fish disease and constitutes a major bacterial pathogen of fish, in particular of salmonids. This bacterium can be found almost worldwide in both marine and freshwater environments and has been divided into several sub-species. In this review, we present the most recent developments concerning our understanding of this pathogen, including how the characterization of new isolates from non-salmonid hosts suggests a more nuanced picture of the importance of the so‑called 'atypical isolates'. We also describe the clinical presentation regarding the infection across several fish species and discuss what is known about the virulence of A. salmonicida and, in particular, the role that the type 3 secretion system might play in suppressing the immune response of its hosts. Finally, isolates have displayed varied levels of antibiotic resistance. Hence, we review a number of solutions that have been developed both to prevent outbreaks and to treat them once they occur, including the application of pre- and probiotic supplements.

Flavobacterium psychrophilum Infections in Salmonid Broodstock and Hatchery-Propagated Stocks of the Great Lakes Basin

Bacterial coldwater disease (BCWD), caused by Flavobacterium psychrophilum, threatens wild and propagated salmonids worldwide and leads to substantial economic losses. In addition to being horizontally transmitted, F. psychrophilum can be passed from infected parents to their progeny, furthering the negative impacts of this pathogen. In Michigan, both feral and captive salmonid broodstocks are the gamete sources used in fishery propagation efforts. A 5-year study was initiated to follow the prevalence of systemic F. psychrophilum infections in feral broodstocks of four species (steelhead Oncorhynchus mykiss [potadromous Rainbow Trout]; Coho Salmon O. kisutch; Chinook Salmon O. tshawytscha; and Atlantic Salmon Salmo salar) residing in three Great Lakes watersheds. Additionally, captive broodstocks of four species (Rainbow Trout, Brown Trout Salmo trutta, Lake Trout Salvelinus namaycush, and Brook Trout Salvelinus fontinalis) maintained at two facilities were assessed for the presence of F. psychrophilum. The resultant offspring from each broodstock population were sampled for F. psychrophilum infections multiple times throughout hatchery residency. Using selective flavobacterial culture and PCR confirmation, F. psychrophilum was detected in all broodstocks except the captive Lake Trout and Brook Trout. Logistic regression analysis demonstrated that among the infected feral broodstocks, Chinook Salmon from the Lake Michigan watershed had the highest prevalence of systemic F. psychrophilum infection (mean = 63.2%). Among the captive broodstocks, the Gilchrist Creek strain of Brown Trout had the highest infection prevalence (mean = 5%). Collectively, the captive broodstocks were found to have significantly lower infection prevalence than the feral broodstocks. Despite the high prevalence of systemic F. psychrophilum infections in many broodstock populations, the bacterium was rarely detected in their progeny during hatchery rearing. However, heavy losses associated with clinical BCWD outbreaks did occur. Collectively, our results reinforce that BCWD continues to threaten Great Lakes basin salmonids.