Infection routes of Aeromonas salmonicida in rainbow trout monitored in vivo by real-time bioluminescence imaging

Characterization of the rainbow trout (Oncorhynchus mykiss) mucosal glycosphingolipid repertoire and Aeromonas salmonicida binding to neutral glycosphingolipids

Infections pose a challenge for the fast growing aquaculture sector. Glycosphingolipids are cell membrane components that pathogens utilize for attachment to the host to initiate infection. Here, we characterized rainbow trout glycosphingolipids from five mucosal tissues using mass spectrometry and nuclear magnetic resonance and investigated binding of radiolabeled Aeromonas salmonicida to the glycosphingolipids on thin-layer chromatograms. 12 neutral and 14 acidic glycosphingolipids were identified. The glycosphingolipids isolated from the stomach and intestine were mainly neutral, whereas glycosphingolipids isolated from the skin, gills and pyloric caeca were largely acidic. Many of the acidic structures were poly-sialylated with shorter glycan structures in the skin compared to the other tissues. The sialic acids found were Neu5Ac and Neu5Gc. Most of the glycosphingolipids had isoglobo and ganglio core chains, or a combination of these. The epitopes on the rainbow trout glycosphingolipid glycans differed between epithelial sites leading to differences in pathogen binding. A major terminal epitope was fucose, that occurred attached to GalNAc in a α1-3 linkage but also in the form of HexNAc-(Fuc-)HexNAc-R. A. salmonicida were shown to bind to neutral glycosphingolipids from the gill and intestine. This study is the first to do a comprehensive investigation of the rainbow trout glycosphingolipids and analyze binding of A. salmonicida to glycosphingolipids. The structural information paves the way for identification of ways of interfering in pathogen colonization processes to protect against infections in aquaculture and contributes towards understanding A. salmonicida infection mechanisms.

Advances in the Development of Bacterial Bioluminescence Imaging

Bioluminescence imaging (BLI) is a powerful method for visualizing biological processes and tracking cells. Engineered bioluminescent bacteria that utilize luciferase-catalyzed biochemical reactions to generate luminescence have become useful analytical tools for in vitro and in vivo bacterial imaging. Accordingly, this review initially introduces the development of engineered bioluminescent bacteria that use different luciferase-luciferin pairs as analytical tools and their applications for in vivo BLI, including real-time bacterial tracking of infection, probiotic investigation, tumor-targeted therapy, and drug screening. Applications of engineered bioluminescent bacteria as whole-cell biosensors for sensing biological changes in vitro and in vivo are then discussed. Finally, we review the optimizations and future directions of bioluminescent bacteria for imaging. This review aims to provide fundamental insights into bacterial BLI and highlight the potential development of this technique in the future.

Characterization of caspase gene family in Sebastes schlegelii and their expression profiles under Aeromonas salmonicida and Vibrio anguillarum infection

The caspase, functioning as a proteinase, plays a crucial role in eukaryotic cell apoptosis, regulation of apoptosis, cellular growth, differentiation, and immunity. The identification of caspase gene family in Sebastes schlegelii is of great help to understand its antimicrobial research. In S. schlegelii, we totally identified nine caspase genes, including four apoptosis initiator caspases (caspase 2, caspase 8, caspase 9 and caspase 10), four apoptosis executioners (caspase 3a, caspase 3b, caspase 6, and caspase 7) and one inflammatory executioner (caspase 1). The duplication of caspase 3 genes on chr3 and chr8 may have been facilitated by whole genome duplication (WGD) events or other complex evolutionary processes. In general, the number of caspase genes relatively conserved in high vertebrates, while exhibiting variation in teleosts. Furthermore, syntenic analysis and phylogenetic relationships analysis supported the correct classification of these caspase gene family in S. schlegelii, especially for genes with duplicated copies. Additionally, the expression patterns of these caspase genes in different tissues of S. schlegelii under healthy conditions were assessed. The results revealed that the expression levels of most caspase genes were significantly elevated in the intestine, spleen, and liver. To further investigate the potential immune functions of these caspase genes in S. schlegelii, we challenged individuals with A. salmonicida and V. anguillarum, respectively. After infection with A. salmonicida, the expression levels of caspase 1 in the liver and spleen of S. schlegelii remained consistently elevated throughout the infection time points. The expression levels of most caspase family members in the intestine exhibited significant divergence following V. anguillarum infection. This study provides a comprehensive understanding of the caspase gene families in S. schlegelii, thereby establishing a solid foundation for further investigations into the functional roles of these caspase genes.

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.

Development of Bioluminescent Virulent Aeromonas hydrophila for Understanding Pathogenicity

Virulent Aeromonas hydrophila (vAh) strains that cause motile Aeromonas septicemia (MAS) in farmed channel catfish (Ictalurus punctatus) have been an important problem for more than a decade. However, the routes of infection of vAh in catfish are not well understood. Therefore, it is critical to study the pathogenicity of vAh in catfish. To this goal, a new bioluminescence expression plasmid (pAKgfplux3) with the chloramphenicol acetyltransferase (cat) gene was constructed and mobilized into vAh strain ML09-119, yielding bioluminescent vAh (BvAh). After determining optimal chloramphenicol concentration, plasmid stability, bacteria number-bioluminescence relationship, and growth kinetics, the catfish were challenged with BvAh, and bioluminescent imaging (BLI) was conducted. Results showed that 5 to 10 µg/mL chloramphenicol was suitable for stable bioluminescence expression in vAh, with some growth reduction. In the absence of chloramphenicol, vAh could not maintain pAKgfplux3 stably, with the half-life being 16 h. Intraperitoneal injection, immersion, and modified immersion (adipose fin clipping) challenges of catfish with BvAh and BLI showed that MAS progressed faster in the injection group, followed by the modified immersion and immersion groups. BvAh was detected around the anterior mouth, barbels, fin bases, fin epithelia, injured skin areas, and gills after experimental challenges. BLI revealed that skin breaks and gills are potential attachment and entry portals for vAh. Once vAh breaches the skin or epithelial surfaces, it can cause a systemic infection rapidly, spreading to all internal organs. To our best knowledge, this is the first study that reports the development of a bioluminescent vAh and provides visual evidence for catfish-vAh interactions. Findings are expected to provide a better understanding of vAh pathogenicity in catfish.

Time effect of experimental infection on Rainbow trout (Oncorhynchus mykiss) by immersion with Aeromonas salmonicida subsp. salmonicida

Furunculosis caused by Aeromonas salmonicida subsp salmonicida (Ass) is a medically and economically important bacterial disease in salmonid farms that requires therapeutic measures to prevent and control the disease. Evaluation of the effectiveness of traditional measures such as antibiotics or vaccines usually requires infecting fish experimentally. The objective of this study is to develop a method of infectious challenge of large (250-g) Rainbow trout by immersion close to natural infection conditions. We compare mortality, morbidity and anti-Ass antibody production of Rainbow trout following different bathing times (2, 4, 8 and 24 h) at a final bacterial concentration of 10⁶ CFU/mL. One hundred sixty fish divided in five groups corresponding to the 4 bathing times and the non-challenged group were studied. The 24 h contact duration resulted in the infection of all fish, with a mortality rate of 53.25%. The challenged fish developed acute infection with symptoms and lesions (inappetance, altering of swimming behaviour, presence of boils) similar to those observed in furunculosis, and produced antibodies against the bacterium at 4 weeks after challenging, in contrast with the non-challenged group.

First Record of the Rare Species Aeromonas lusitana from Rainbow Trout (Oncorhynchus mykiss, Walbaum): Comparative Analysis with the Existing Strains

The species Aeromonas lusitana was first described in 2016 with five strains recovered from untreated water and vegetables from Portugal. Since then, no further records exist of this species. During a surveillance study on the presence of Aeromonas in fish farms in Mexico, a new strain (ESV-351) of the mentioned species isolated from a rainbow trout was recovered. It was identified because it clustered phylogenetically with the type strain of A. lusitana based on the analysis of the rpoD gene sequences. In the present study, phenotypic characteristics, antimicrobial resistance profiles, and the presence of putative virulence genes of this novel strain (ESV-351) were determined in parallel to the five isolates from the original species description. Phenotypic differential characteristics exhibited by A. lusitana ESV-351 depicted an evident similarity to the characteristics exhibited by the other evaluated strains. However, the novel strain was positive for the production of indole using conventional methods, while the rest of the strains, including the type strain, were negative for its production. Furthermore, intermediate resistance to ampicillin, amoxicillin-clavulanic acid and cephalothin was detected in both the novel and the type strain. Five different virulence-related genes were detected in the novel strain and in the previously described strains, with the type strain exhibiting the highest number of virulence-related genes. In addition to this, the genome of the novel strain (ESV-351) was sequenced and compared with the genomes from the type strain (A. lusitana CECT 7828T) and other Aeromonas spp. The genomic analysis defined Aeromonas tecta as the closest species to A. lusitana with a highly similar number of predicted proteins. The genomic size, the number of protein-encoding genes and the number of different tRNAs, among other characteristics, make it possible to propose that the ESV-351 strain could potentially have the capacity to adapt to different environments. Genome comparison of the ESV-351 strain with the type strain revealed that both possess a similar sequence of the citrate synthase gene. In addition to this finding, the chromosomal region containing the citrate synthase locus of the novel strain exhibits some similarity to the chromosomal region in the genome of the A. hydrophila type strain and other known human pathogens, such as Vibrio cholerae. This could suggest a possible virulence role for the citrate synthase gene in A. lusitana (ESV-351).

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

Isolation and Characterization of Fish-Gut Bacillus spp. as Source of Natural Antimicrobial Compounds to Fight Aquaculture Bacterial Diseases

Aquaculture is responsible for more than 50% of global seafood consumption. Bacterial diseases are a major constraint to this sector and associated with misuse of antibiotics, pose serious threats to public health. Fish-symbionts, co-inhabitants of fish pathogens, might be a promising source of natural antimicrobial compounds (NACs) alternative to antibiotics, limiting bacterial diseases occurrence in aquafarms. In particular, sporeforming Bacillus spp. are known for their probiotic potential and production of NACs antagonistic of bacterial pathogens and are abundant in aquaculture fish guts. Harnessing the fish-gut microbial community potential, 172 sporeforming strains producing NACs were isolated from economically important aquaculture fish species, namely European seabass, gilthead seabream, and white seabream. We demonstrated that they possess anti-growth, anti-biofilm, or anti-quorum-sensing activities, to control bacterial infections and 52% of these isolates effectively antagonized important fish pathogens, including Aeromonas hydrophila, A. salmonicida, A. bivalvium, A. veronii, Vibrio anguillarum, V. harveyi, V. parahaemolyticus, V. vulnificus, Photobacterium damselae, Tenacibaculum maritimum, Edwardsiela tarda, and Shigella sonnei. By in vitro quantification of sporeformers' capacity to suppress growth and biofilm formation of fish pathogens, and by assessing their potential to interfere with pathogens communication, we identified three promising candidates to become probiotics or source of bioactive molecules to be used in aquaculture against bacterial aquaculture diseases.

Effective isolation of GALT cells: Insights into the intestine immune response of rainbow trout (Oncorhynchus mykiss) to different bacterin vaccine preparations

The teleost gut is a multifunction complex structure that plays a pivotal immunological role in homeostasis and the maintenance of health, in addition to digestion of food and/or nutrient absorption. In vitro examination of the intestine leucocyte repertoire has the potential to aid our understanding of gut immune competence and allows a rapid screen of host-microorganism interactions in different immunological contexts. To explore this possibility, in the present study we investigated the response of isolated gut leucocytes to 4 bacterins of Aeromonas salmonicida, prepared from different strains, combinations and strains grown in different environments, in comparison to a Yersinia ruckeri bacterin for which a commercial/effective oral booster vaccine has been developed. To aid this study we also optimized further our method of GALT cell isolation from rainbow trout, so as to avoid mechanical clearance of the intestine contents. This drastically increased the cell yield from ~12 × 10⁶ to ~210 × 10⁶/fish with no change in the percent cell viability over time or presence of transcripts typical of the key leucocyte types needed for the study of immune modulation (i.e. T- and B-cells, dendritic cells and macrophages). A wide array of immune transcripts were modulated by the bacterins, demonstrating the diversity of GALT cell responses to bacterial stimulation. Indeed, the GALT leucocyte responses were sensitive enough to distinguish the different bacterial species, strains and membrane proteins, as seen by distinct kinetics of immune gene expression. However, the response of the GALT cells was often relatively slow and of a low magnitude compared to those of PBL. These results enhance our knowledge of the gut biocapacity and help validate the use of this model for screening of oral vaccine candidates.

Aeromonas salmonicida infection kinetics and protective immune response to vaccination in sablefish (Anoplopoma fimbria)

Effective vaccine programs against Aeromonas salmonicida have been identified as a high priority area for the sablefish (Anoplopoma fimbria) aquaculture. In this study, we established an A. salmonicida infection model in sablefish to evaluate the efficacy of commercial vaccines and an autogenous vaccine preparation. Groups of 40 fish were intraperitoneally (ip) injected with different doses of A. salmonicida J410 isolated from infected sablefish to calculate the median lethal dose (LD₅₀). Samples of blood, head kidney, spleen, brain, and liver were also collected at different time points to determine the infection kinetics. The LD₅₀ was estimated as ~3 × 10⁵ CFU/dose. To evaluate the immune protection provided by an autogenous vaccine and two commercial vaccines in a common garden experimental design, 140 fish were PIT-tagged, vaccinated and distributed equally into 4 tanks (35 fish for each group, including a control group). Blood samples were taken every 2 weeks to evaluate IgM titers. At 10 weeks post-immunization, all groups were ip challenged with 100 times the calculated LD₅₀ for A. salmonicida J410. A. salmonicida was detected after 5 days post-infection (dpi) in all collected tissues. At 30 days post-challenge the relative percentage survival (RPS) with respect to the control group was calculated for each vaccine. The RPS for the bacterin mix was 65.22%, for Forte Micro 4® vaccine was 56.52% and for Alpha Ject Micro 4® was 30.43%, and these RPS values were reflected by A. salmonicida tissue colonization levels at 10 days post-challenge. Total IgM titers peaked at 6-8 weeks post-immunization, where the autogenous vaccine group showed the highest IgM titers and these values were consistent with the RPS data. Also, we determined that the A. salmonicida A-layer binds to immunoglobulins F(ab)' in a non-specific fashion, interfering with immune assays and potentially vaccine efficacy. Our results indicate that vaccine design influences sablefish immunity and provide a guide for future sablefish vaccine programs.

Pinpointing the role of Aeromonas salmonicida in the development of skin ulcerations in common dab (Limanda limanda)

Aeromonas salmonicida was isolated from ulcerations in common dab (Limanda limanda). An experiment was performed to pinpoint its role in ulceration development, considering the importance of the skin barrier and the pigmented and non-pigmented sides. The skin of dab was treated in three zones, one where scales and epidermis were removed, one where mucus was discarded and one non-treated zone. Fish were tagged to allow individual identification and challenged with A. salmonicida. Mortality and severity of the developing lesions were recorded for 21 days post-inoculation. Starting 12 days post-inoculation, mortality occurred gradually in challenged fish; however, no direct cause could be established. Both control fish and challenged fish developed ulcerations containing A. salmonicida. Sequencing of vapA gene revealed that isolates retrieved from both groups were distinct, suggesting the presence of A. salmonicida prior to the trial. Most ulcerations developed in zones where skin was removed, suggesting that abrasion might be a predisposing factor in ulceration development. Ulcerations were also observed at the insertion site of the tag, where exposed muscle tissue might have favoured the development of ulcerations. In conclusion, A. salmonicida seems to be involved in the development of skin ulcerations in dab, although the exact pathogenesis needs to be elucidated.

Vaccine-Induced Protection Against Furunculosis Involves Pre-emptive Priming of Humoral Immunity in Arctic Charr

With respect to salmonid aquaculture, one of the most important bacterial pathogens due to high mortality and antibiotic usage is the causative agent of typical furunculosis, Aeromonas salmonicida spp. salmonicida (Asal). In Atlantic salmon, Salmo salar, the host response during infections with Asal is well-documented, with furunculosis outbreaks resulting in significant mortality in commercial settings. However, less is known about the host-pathogen interactions in the emerging aquaculture species, Arctic charr Salvelinus alpinus. Furthermore, there is no data on the efficacy or response of this species after vaccination with commonly administered vaccines against furunculosis. To this end, we examined the immunological response of S. alpinus during infection with Asal, with or without administration of vaccines (Forte Micro®, Forte Micro® + Renogen®, Elanco Animal Health). Artic charr (vaccinated or unvaccinated) were i.p.-injected with a virulent strain of Asal (10⁶ CFUs/mL) and tissues were collected pre-infection/post-vaccination, 8, and 29 days post-infection. Unvaccinated Arctic charr were susceptible to Asal with 72% mortalities observed after 31 days. However, there was 72–82% protection in fish vaccinated with either the single or dual-vaccine, respectively. Protection in vaccinated fish was concordant with significantly higher serum IgM concentrations, and following RNA sequencing and transcriptome assembly, differential expression analysis revealed several patterns and pathways associated with the improved survival of vaccinated fish. Most striking was the dramatically higher basal expression of complement/coagulation factors, acute phase-proteins, and iron hemostasis proteins in pre-challenged, vaccinated fish. Remarkably, following Asal infection, this response was abrogated and instead the transcriptome was characterized by a lack of immune-stimulation compared to that of unvaccinated fish. Furthermore, where pathways of actin assembly and FcγR-mediated phagocytosis were significantly differentially regulated in unvaccinated fish, vaccinated fish showed either the opposite regulation (ForteMicro®), or no impact at all (ForteMicro®Renogen®). The present data indicates that vaccine-induced protection against Asal relies on the pre-activation and immediate control of humoral immune parameters that is coincident with reduced activation of apoptotic (e.g., NF-κB) and actin-associated pathways.

Zebrafish (Danio rerio) as a model to visualize infection dynamics of Vibrio anguillarum following intraperitoneal injection and bath exposure

Vaccine development is important for sustainable fish farming and novel vaccines need to be efficacy tested before release to the market. Challenge of fish with the pathogen towards which the vaccine has been produced can be conducted either by external exposure though bathing or cohabitation, or by bypassing the mucosa through injection. The latter approach is often preferred since it is easier to control than the former. However, injection is not a very natural route of infection, and the bypass of the mucosa may result in a different efficacy profile of experimental fish compared to farmed fish, for which the vaccines are targeted. The zebrafish is by now a well established practical vertebrate model species due in part to its size and ease of maintenance and genetic manipulation. Here we use zebrafish as a model to visualize and compare the development of infection of Vibrio anguillarum on and in the fish following injection or bathing. Injection of 10³ bacteria per fish resulted in approximately 50% mortality by day 4 post-injection. Similar mortality levels were reached in the other group by bathing in 1.25 × 10⁹ bacteria for 1 min. The spreading of bacteria was followed for the first 24 h after injection/bathing by immunohistochemistry and optical projection tomography. The tissues and organs where bacteria were detected differed significantly as a result of time as well as treatment. In the bath group, bacteria were initially found on external surfaces including gut. After 24 h V. anguillarum still persisted in gut but had now also spread to the blood. In the injection group bacteria were found in the blood throughout all sampling times, as well as in the hypodermis and body cavity at most sampling times.