Use of in vivo induced technology to identify antigens expressed by Photobacterium damselae subsp. piscicida during infection of Senegalese sole (Solea senegalensis)

Virulence Genes and In Vitro Antibiotic Profile of Photobacterium damselae Strains, Isolated from Fish Reared in Greek Aquaculture Facilities

Bacteria belonging to the species Photobacterium damselae are pathogens of cultured marine fish, causing diseases of high importance, such as Pasteurellosis. Thus, they are considered a major threat to the aquaculture sector. Despite the great importance of fish mariculture for the Greek economy, the distribution and abundance of these bacteria are not well documented in aquaculture units in Greece. Keeping this in mind, the scope of the present study was to investigate the presence, antibiotic profile, and virulence of Photobacterium bacteria originating from a representative sample of mariculture units throughout Greece. Samples were collected from diseased fish belonging to three different cultured fish species, namely Sparus aurata, Dicentrarchus labrax, and Pagrus pagrus, from both the Aegean and the Ionian Sea. Tissue samples were cultured in agar media, and bacteria were molecularly identified using both bacterial universal and species-specific primer pairs for Photobacterium spp. Additionally, the identified strains were characterized for the presence of virulence genes as well as antibiotic profiles. According to the results, the aforementioned bacteria are distributed in the Greek aquaculture units and are characterized by high pathogenicity based on the abundance of virulence genes. Furthermore, the majority of the detected strains exhibit some level of antibiotic resistance. In summary, our results indicate the need for systematic surveillance and study of their antibiotic profiles in Greek aquaculture since these bacteria constitute a major threat to the sector.

The moonlighting protein fructose 1,6-bisphosphate aldolase as a potential vaccine candidate against Photobacterium damselae subsp. piscicida in Asian sea bass (Lates calcarifer)

Vaccination is the most effective, safe, and environmentally friendly method to prevent the outbreak of Photobacterium damselae subsp. piscicida (Phdp), a dangerous pathogen in aquaculture worldwide. Here, recombinant proteins of catalase, superoxide dismutase, isocitrate dehydrogenase, fructose 1,6-bisphosphate aldolase (Fba), and a mixture of all four proteins were investigated for their immunoprotective effects against photobacteriosis in Asian sea bass (Lates calcarifer). After immunization, experimental fish showed an increase in specific antibody levels and lysozyme activities, especially the Fba group. After a lethal challenge with Phdp strain AOD105021, the Fba group achieved the highest relative percentage of survival rate (70.21%) and a significantly lower bacterial load in the spleens than other groups 3 days after infection. The results suggest that Fba is a good candidate for subunit vaccine development against photobacteriosis in fish.

Evaluation of an oral DNA nanovaccine against photobacteriosis in Solea senegalensis

Infectious diseases are one of the main causes of social and economical losses in world aquaculture. Senegalese sole (Solea senegalensis) is an important species for aquaculture in southern Europe, whose production is affected by the appearance of bacterial diseases such as photobacteriosis, a septicemia caused by Photobacterium damselae subsp. piscicida (Phdp). The aim of this study was to obtain an oral DNA nanovaccine and to evaluate its efficacy against Phdp in S. senegalensis juveniles. For this purpose, the amplified product corresponding to the protein inosine-5'-monophophate dehydrogenase (IMPDH) from Phdp, was cloned into the expression vector pcDNA™6.2/C-EmGFP-GW obtaining the DNA vaccine named as pPDPimpdh. The correct transcription and protein expression was verified at 48 h post tansfection in HEK293 cells. Chitosan nanoparticles (CS-TPP NPs) were prepared by ionotropic gelation and their features were appropriate for use as oral delivery system. Therefore, pPDPimpdh was protected with chitosan CS-TPP NPs throughout complex coacervation method giving as a result a DNA nanovaccine referred as CS-TPP+pPDPimpdh NPs. Sole juveniles were vaccinated orally with CS-TPP NPs, pPDPimpdh and CS-TPP+pPDPimpdh NPs followed by a challenge with Phdp at 30 days post vaccination (dpv). The relative percentage survival (RPS) for pPDPimpdh vaccinated groups was 6.25%, probably due to its degradation in the digestive tract. RPS value obtained for CS-TPP NPs and CS-TPP+pPDPimpdh NPs was 40% and antibodies were observed in both cases. However, a delay in mortality was observed in sole juveniles vaccinated orally with CS-TPP+pPDPimpdh NPs. In fact, an upregulation of tf, mhcII, cd8a and igm in the posterior gut and c3, hamp1, tf and cd4 in spleen was observed in juveniles vaccinated with CS-TPP+pPDPimpdh NPs. After challenge, a modulation of cd8a and cd4 expression levels in the posterior gut and c3, tf, lyg, cd4, igm and igt expression levels in spleen was observed. Moreover, the concentration of lysozyme in skin mucus significantly increased in fish vaccinated orally with CS-TPP+pPDPimpdh NPs at 11 dpc. These data indicate that oral vaccination with CS-TPP+pPDPimpdh NPs could be acting through the non-specific immune responses as well as the specific humoral and cell mediated immunity and provide the first step toward a development of an oral DNA nanovaccine against Phdp in sole.

Mucosal immune responses in Senegalese sole (Solea senegalensis) juveniles after Tenacibaculum maritimum challenge: A comparative study between ocular and blind sides

Most pathogens start the process of infection at the mucosal surfaces and therefore the mucosal immune response plays an essential role in the course of the infection. Due to the Senegalese sole (Solea senegalensis Kaup) condition of flatfish, the present comparative study aimed to analyse several immune-related enzymes as well as the bactericidal activity in the skin mucus from ocular and blind sides. For this purpose, Senegalese sole juveniles were bath challenged with a sub-lethal dose of Tenacibaculum maritimum for 24 h and sampled at 1, 2 and 3 weeks. The haematological profile and immune-related parameters were also measured in plasma in order to evaluate the systemic immune response after T. maritimum challenge. Results from this study showed that most parameters tested increased in skin mucus of bath challenged fish compared to unchallenged ones. In contrast, the sub-lethal dose tested did not influence the haematological profile including peripheral numbers the different leucocyte types. No variations were observed in plasma lysozyme, peroxidase, protease and haemolytic complement activities between unchallenged and bath challenged fish. This study suggests that the studied innate immune-related molecules are constitutively present in both skin mucus sides but at different levels. Interestingly, the levels of most parameters measured were higher on the ocular side than on the blind side, possibly due to the higher exposure to invasion by waterborne microorganisms on this side. Therefore, the present study brings some insights regarding local immune responses after bacterial challenge in skin mucus from the ocular and blind sides in one of the most valuable flatfish species in southern Europe.

Genotypic diversity, and molecular and pathogenic characterization of Photobacterium damselae subsp. piscicida isolated from different fish species in Taiwan

Photobacteriosis, caused by Photobacterium damselae subsp. piscicida (Phdp), is a serious disease in marine fish species worldwide. To date, the epidemiological characterization of this pathogen in Taiwan remains limited. In this study, we collected 39 Phdp isolates obtained from different farmed fish for phenotypic and genotypic analysis. Phenotype bioassays using API-20E and API-20NE systems showed that the Phdp is a homogeneous group. However, genotyping using the pulsed-field gel electrophoresis (PFGE) technique revealed genetic variability among Phdp isolates when 13 and 11 different PFGE band patterns were obtained with SmaI and NotI as restriction enzymes, respectively. Phylogenetic analysis using 16S rDNA and the Fur gene clustered Taiwanese isolates and other species of P. damselae in the same clade. In contrast, the ToxR phylogenetic tree, a powerful discriminatory marker, separated the two subspecies. Furthermore, the virulence-associated genes, AIP56, P55, PDP_0080, Sod and Irp1, were detected from all isolates. Virulence testing with nine representative isolates in cobia (Rachycentron canadum) and Asian sea bass (Lates calcarifer) showed that some were highly pathogenic with 80%-100% mortality rates. This study provides epidemiological data of Phdp infections in farmed fish in Taiwan, which is necessary to develop comprehensive prevention and control strategies for the disease.

Inclusion of dietary Ulva ohnoi 5% modulates Solea senegalensis immune response during Photobacterium damselae subsp. piscicida infection

Macroalgae represent valuable sources of functional ingredients for fish diets, and the influence of supplemented aquafeeds on growth performance has been studied for some fish and seaweed species. In the present work, the potential immunomodulation exerted by U. ohnoi (5%) as dietary ingredient was investigated in Senegalese sole. After feeding with the experimental diets for 90 d, fish immune response before and after challenge with Photobacterium damselae subsp. piscicida (Phdp) was assessed. In absence of infection, systemic immune response was not modified by 5% U. ohnoi dietary inclusion for 90 d. Thus, no differences in liver and head kidney immune gene transcription or serum lysozyme, peroxidase, antiprotease and complement activities were observed based on the diet received by Senegalese sole specimens. Regarding mucosal immune parameters, no changes in gene transcription were detected in the skin and gills, whilst only tnf, cd4 and cd8 were significantly up-regulated in the intestine of fish fed with U. ohnoi, compared to the values obtained with control diet. On the contrary, when S. senegalensis specimens were challenged with Phdp, modulation of the immune response consisting in increased transcription of genes encoding complement (c1q4, c3, c9), lysozyme g (lysg), tumor necrosis factor alpha (tnfα) as well as those involved in the antioxidant response (gpx, sodmn) and iron metabolism (ferrm, hamp-1) was observed in the liver of fish fed with U. ohnoi. In parallel, decreased inflammatory cytokine and complement encoding gene transcription was displayed by the spleen of fish receiving the algal diet. Though mortality rates due to Phdp challenge were not affected by the diet received, lower pathogen loads were detected in the liver of soles receiving U. ohnoi diet. Further research to investigate the effects of higher inclusion levels of this seaweed in fish diets, feeding during short periods as wells as to assess the response against other pathogens needs to be carried out.

Iron uptake mechanisms as key virulence factors in bacterial fish pathogens

This review summarizes the current knowledge about iron uptake systems in bacterial fish pathogens and their involvement in the infective process. Like most animal pathogens, fish pathogens have evolved sophisticated iron uptake mechanisms some of which are key virulence factors for colonization of the host. Among these systems, siderophore production and heme uptake systems are the best studied in fish pathogenic bacteria. Siderophores like anguibactin or piscibactin, have been described in Vibrio and Photobacterium pathogens as key virulence factors to cause disease in fish. In many other bacterial fish pathogens production of siderophores was demonstrated but the compounds were not yet chemically characterized and their role in virulence was not determined. The role of heme uptake in virulence was not yet clearly elucidated in fish pathogens although there exist evidence that these systems are expressed in fish tissues during infection. The relationship of other systems, like Fe(II) transporters or the use of citrate as iron carrier, with virulence is also unclear. Future trends of research on all these iron uptake mechanisms in bacterial fish pathogens are also discussed.

Outer membrane protein FrpA, the siderophore piscibactin receptor of Photobacterium damselae subsp. piscicida, as a subunit vaccine against photobacteriosis in sole (Solea senegalensis)

Photobacteriosis caused by Photobacterium damselae subsp. piscicida (Pdp) remains one of the main infectious diseases affecting cultured fish in Mediterranean countries. Diverse vaccine formulations based in the use of inactivated bacterial cells have been used with unsatisfactory results, especially in newly cultured species like sole (Solea senegalensis). In this work, we describe the use of the outer membrane receptor (FrpA) of the siderophore piscibactin produced by Pdp as a novel subunit vaccine against photobacteriosis. FrpA has been cloned and expressed in Escherichia coli under an arabinose-inducible promoter. A recombinant protein (rFrpA) containing the pelB localization signal and a His tag was constructed to obtain a pure native form of the protein from E. coli outer membranes. The immunogenicity of rFrpA, and its protective effect against photobacteriosis, was tested by i.p. injection of 30  μg of the protein, mixed with Freund's adjuvant, in sole fingerlings with two immunizations separated by 30 days. Results showed that using either pure rFrpA or whole cells as immobilized antigens in ELISA assays, rFrpA induces the production of specific antibodies in sole. An experimental infection using fish vaccinated with rFrpA or formalin-killed whole cells of Pdp showed that both groups were protected against Pdp infection at similar levels, with no significant differences, reaching RPS values of 73% and 79%, respectively. Thus, FrpA constitutes a promising antigen candidate for the development of novel more effective vaccines against fish photobacteriosis.

Transcription of IVIAT and Virulence Genes in Photobacterium damselae Subsp. piscicida Infecting Solea senegalensis

Photobacterium damselae subsp. piscicida (Phdp) is responsible for disease outbreaks in marine aquaculture worldwide. Solea senegalensis, a valuable fish species for aquaculture in the south of Europe, is frequently affected by this pathogen. It is well established that bacteria respond to environmental signals and, in the case of pathogens, this ability may determine the outcome of their interaction with the host. Determination of gene expression under in vivo conditions constitutes a valuable tool in the assessment of microbial pathogenesis. Considering that different hosts may represent different environments for the pathogen, expression of Phdp virulence and in vivo induced antigen (IVIAT) genes during S. senegalensis infection has been determined in the present work. Increased transcription of genes encoding proteins involved in iron acquisition (Irp1, Irp2, HutB and HutD), oxidative stress defence (AhpC and Sod), adhesion (PDP_0080), toxins (AIP56) and metabolism (Impdh, Shmt and AlaRS) were detected in Phdp infecting S. senegalensis head kidney or liver. The highest increases corresponded to genes involved in survival under iron limiting conditions and oxidative stress, indicating their essential role during infection of sole. Results obtained give insight into Phdp virulence strategies and contribute to the identification of promising targets for the control of photobacteriosis.

A new age in AquaMedicine: unconventional approach in studying aquatic diseases

BACKGROUND

Marine and aquaculture industries are important sectors of the food production and global trade. Unfortunately, the fish food industry is challenged with a plethora of infectious pathogens. The freshwater and marine fish communities are rapidly incorporating novel and most up to date techniques for detection, characterization and treatment strategies. Rapid detection of infectious diseases is important in preventing large disease outbreaks.

MAIN TEXT

One hundred forty-six articles including reviews papers were analyzed and their conclusions evaluated in the present paper. This allowed us to describe the most recent development research regarding the control of diseases in the aquatic environment as well as promising avenues that may result in beneficial developments. For the characterization of diseases, traditional sequencing and histological based methods have been augmented with transcriptional and proteomic studies. Recent studies have demonstrated that transcriptional based approaches using qPCR are often synergistic to expression based studies that rely on proteomic-based techniques to better understand pathogen-host interactions. Preventative therapies that rely on prophylactics such as vaccination with protein antigens or attenuated viruses are not always feasible and therefore, the development of therapies based on small nucleotide based medicine is on the horizon. Of those, RNAi or CRISPR/Cas- based therapies show great promise in combating various types of diseases caused by viral and parasitic agents that effect aquatic and fish medicine.

CONCLUSIONS

In our modern times, when the marine industry has become so vital for feed and economic stability, even the most extreme alternative treatment strategies such as the use of small molecules or even the use of disease to control invasive species populations should be considered.