Survival against exogenous hydrogen peroxide of Photobacterium damselae subsp. piscicida under different culture conditions

Evaluation of the Differential Postbiotic Potential of Shewanella putrefaciens Pdp11 Cultured in Several Growing Conditions

The increased knowledge of functional foods has led to the development of a new generation of health products, including those containing probiotics and products derived from them. Shewanella putrefaciens Pdp11 (SpPdp11) is a strain described as a probiotic that exerts important beneficial effects on several farmed fish. However, the use of live probiotic cells in aquaculture has limitations such as uncertain survival and shelf life, which can limit their efficacy. In addition, its efficacy can vary across species and hosts. When probiotics are administered orally, their activity can be affected by the environment present in the host and by interactions with the intestinal microbiota. Furthermore, live cells can also produce undesired substances that may negatively impact the host as well as the risk of potential virulence reversion acquired such as antibiotic resistance. Therefore, new alternatives emerged such as postbiotics. Currently, there is no knowledge about the postbiotic potential of SpPdp11 in the aquaculture industry. Postbiotic refers to the use of bacterial metabolites, including extracellular products (ECPs), to improve host physiology. However, the production of postbiotic metabolites can be affected by various factors such as cultivation conditions, which can affect bacterial metabolism. Thus, the objective of this study was to evaluate the postbiotic potential of ECPs from SpPdp11 under different cultivation conditions, including culture media, temperature, growth phase, and salinity. We analyzed their hydrolytic, antibacterial, antiviral, and cytotoxic capacity on several fish cell lines. The results obtained have demonstrated how each ECP condition can exert a different hydrolytic profile, reduce the biofilm formation by bacterial pathogens relevant to fish, lower the titer of nervous necrosis virus (NNV), and exert a cytotoxic effect on different fish cell lines. In conclusion, the ECPs obtained from SpPdp11 have different capacities depending on the cultivation conditions used. These conditions must be considered in order to recover the maximum number of beneficial capacities or to choose the appropriate conditions for specific activities.

Dietary ARA, DHA, and Carbohydrate Ratios Affect the Immune Status of Gilthead Sea Bream Juveniles upon Bacterial Challenge

This study aims to assess the effects of different dietary n-6/n-3 long-chain polyunsaturated fatty acid ratios and CHO content in the immune response of gilthead seabream. For that purpose, gilthead sea bream juveniles (initial body weight = 47.5 g) were fed for 84 days with four isoproteic (47% crude protein) and isolipidic (18% crude lipids) diets with high (20%) or low (5%) level of gelatinized starch (HS or LS diets, respectively) and included approximately 2.4% ARA or DHA. At the end of the trial, the DHA-enriched groups presented increased red blood cell (RBC) count, hemoglobin, plasmatic nitric oxide (NO) content, and antiprotease and alternative complement activities. The ARA groups had increased thrombocyte count, and plasmatic bactericidal activity against Vibrio anguillarum was lower in the fish fed the ARA/LS diet. After the feeding trial, the fish were challenged with an intraperitoneal injection (i.p.) of killed Photobacterium damselae subsp. piscicida (Phdp) and sampled at 4 and 24 h after the challenge. At 4 h after i.p., the ARA groups presented increased plasma total immunoglobulins (Ig) and bactericidal activity against V. anguillarum. In addition, the fish fed the ARA/LS diet presented lower white blood cell (WBC) and alternative complement activity. At 24 h after i.p., the ARA groups presented increased RBC, WBC, and thrombocyte numbers, total IG, plasma peroxidase activity, and casp3 expression in the distal intestine. The HS groups presented increased plasma NO content and bactericidal activity against Phdp and decreased protease, antiprotease activity, and bactericidal activity against V. anguillarum. In conclusion, high dietary DHA levels seemed to improve the immune status of unchallenged gilthead sea bream juveniles, while high dietary ARA levels improved the fish immune response to a bacterial challenge. The energy provided by dietary starch seems to be important to promote a fast response by the fish immune system after a challenge.

Comprehensive transcriptome profiling and functional analysis of the meagre (Argyrosomus regius) immune system

Meagre (Argyrosomus regius) belongs to the family Sciaenidae and is a promising candidate for Mediterranean aquaculture diversification. As a relatively recent species in aquaculture, the physiological consequences of the immune system activation in meagre are understudied. Spleen, as a primary lymphoid organ has an essential role in meagre immune and inflammatory responses. In this study, we have evaluated the in vivo effects of lipopolysaccharide (LPS) on the spleen transcriptome of meagre by RNA-seq analysis at 4 and 24 h after injection.

Bacillus spp. Inhibit Edwardsiella tarda Quorum-Sensing and Fish Infection

The disruption of pathogen communication or quorum-sensing (QS) via quorum-quenching (QQ) molecules has been proposed as a promising strategy to fight bacterial infections. Bacillus spp. have recognizable biotechnology applications, namely as probiotic health-promoting agents or as a source of natural antimicrobial molecules, including QQ molecules. This study characterized the QQ potential of 200 Bacillus spp., isolated from the gut of different aquaculture fish species, to suppress fish pathogens QS. Approximately 12% of the tested Bacillus spp. fish isolates (FI). were able to interfere with synthetic QS molecules. Ten isolates were further selected as producers of extracellular QQ-molecules and their QQ capacity was evaluated against the QS of important aquaculture bacterial pathogens, namely Aeromonas spp., Vibrio spp., Photobacterium damselae, Edwardsiela tarda, and Shigella sonnei. The results revealed that A. veronii and E. tarda produce QS molecules that are detectable by the Chr. violaceum biosensor, and which were degraded when exposed to the extracellular extracts of three FI isolates. Moreover, the same isolates, identified as B. subtilis, B. vezelensis, and B. pumilus, significantly reduced the pathogenicity of E. tarda in zebrafish larvae, increasing its survival by 50%. Taken together, these results identified three Bacillus spp. capable of extracellularly quenching aquaculture pathogen communication, and thus become a promising source of bioactive molecules for use in the biocontrol of aquaculture bacterial diseases.

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.

Evaluation of the Potential of Marine Algae Extracts as a Source of Functional Ingredients Using Zebrafish as Animal Model for Aquaculture

Research on immunotherapeutic agents has become a focus for the treatment of fish diseases. The ability of algae to produce secondary metabolites of potential interest as immunotherapeutics has been documented. The present research intended to assess antiviral and antibacterial activities of macro- and microalgae extracts against viral and bacterial pathogens and explore their immunomodulatory potential using zebrafish (Danio rerio) larvae as a model organism. The cytotoxicity and antiviral activity of eight methanolic and ethanolic extracts from two macroalgae (Fucus vesiculosus, Ulva rigida) and two microalgae (Nannochloropsis gaditana, Chlorella sp.) were analyzed in established fish cell lines. Six extracts were selected to evaluate antibacterial activity by disk diffusion and growth inhibition assays. The three most promising extracts were characterized in terms of fatty acid composition, incorporated at 1% into a plant-based diet, and evaluated their effect on zebrafish immune response and intestinal morphology in a short-term feeding trial. All extracts exhibited in vitro antiviral activity against viral hemorrhagic septicemia and/or infectious pancreatic necrosis viruses. Methanolic extracts from F. vesiculosus and U. rigida were richer in saturated fatty acids and exhibited in vitro antibacterial action against several bacteria. Most promising results were obtained in vivo with F. vesiculosus methanol extract, which exerted an anti-inflammatory action when incorporated alone into diets and induced pro-inflammatory cytokine expression, when combined with the other extracts. Moreover, dietary inclusion of the extracts improved intestinal morphology. In summary, the results obtained in this study support the potential of algae as natural sources of bioactive compounds for the aquaculture industry.

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.

Effects of the sulfated polysaccharide ulvan from Ulva ohnoi on the modulation of the immune response in Senegalese sole (Solea senegalensis)

Sulfated polysaccharides derived from green seaweeds exhibit many beneficial biological activities and have great potential to be used as nutraceutical in aquaculture. In this work, we evaluated the effects of the sulfated polysaccharide ulvan from Ulva ohnoi on Senegalese sole (Solea senegalensis) juveniles at the transcriptomic level. Cytotoxicity assay performed in liver primary cell cultures from sole determined that the different ulvan concentrations assayed did not impair cell viability. Juveniles were intraperitoneally (IP) injected with ulvan (0.5 mg/fish) followed by a challenge with Photobacterium damselae subsp. piscicida (Phdp) at 7 days. RNASeq analyses at 2 days post injection (dpi) revealed that 402 transcripts were differentially expressed in liver between ulvan IP injected and control groups before the challenge. Genes related to bacterial and antiviral defence, complement system, chemokines, proteasomes and antigen presentation were upregulated in ulvan treated groups. A detailed expression analysis of sixteen genes related to innate and adaptive immune system was performed in two systemic tissues: liver and spleen. Ulvan injection provoked the upregulation of tlr22 and a transient inflammatory response was initiated in both liver and spleen at 2 dpi. As consequence, expression of acute phase proteins, antimicrobial peptides and complement genes was induced. Moreover, expression of mhcI, mhcII, psmb10 and bcl6 was also induced 2 dpi. At 2 dpi with Phdp, inflammatory cytokines and genes related to bacterial and antiviral defense, iron metabolism, complement system and antigen presentation were differentially modulated in survival juveniles previously IP injected with ulvan. Moreover, mortality was retarded in ulvan treated juveniles. These results provide new evidence about the role of ulvan as a bioactive compound with immunomodulatory activity in Senegalese sole as well as its possible use as vaccine adjuvant against Phdp. This is the first published study that evaluates the transcriptomic response of Senegalese sole IP injected with ulvan.

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.

Transcription of immune related genes in Solea senegalensis vaccinated against Photobacterium damselae subsp. piscicida. Identification of surrogates of protection

Solea senegalensis is a flatfish with a great potential for aquaculture, but infectious diseases restrict its production, being this fish species highly susceptible to Photobacterium damselae subsp. piscicida (Phdp) infections. A better understanding of the mechanisms related to fish immune response is crucial for the development of effective approaches in disease management. In the present work, transcriptional changes of immune related genes have been evaluated in farmed S. senegalensis specimens vaccinated against Phdp by intraperitoneal injection (IP) and immersion (IM). IP fish showed higher antibody levels and increased transcription of genes encoding lysozyme C1, complement factors involved in the classical pathway and components involved in the opsonization and the limitation of free iron availability, all of them facilitating the faster elimination of the pathogen and promoting higher RPS after the infection with Phdp. The results of this study seem to support a different intensity of the specimens immune response in the head kidney. Analysis of the immune response in 15 day post-challenged fish showed up-regulation of genes involved in all stages of S. senegalensis immune response, but especially those genes encoding proteins related to the innate response such as complement, lysozyme and iron homeostasis in the head kidney. On the other hand, liver transcription was higher for genes related to inflammation, apoptosis and cell mediated cytotoxicity (CMC). Furthermore, comparison of the differential response of S. senegalensis genes in vaccinated and unvaccinated fish to Phdp infection allowed the identification of a potential biosignature, consisting in 10 genes, as a surrogate of protection and therefore, as indicator of vaccine success against fotobacteriosis after IP vaccination. These results provide important insights into the S. senegalensis protection against Phdp induced by vaccination.

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

Photobacterium damselae subsp. piscicida (Phdp), the causative agent of photobacteriosis, is an important pathogen in marine aquaculture that affects many different fish species worldwide, including Solea senegalensis, an important fish species for aquaculture in the south of Europe. Bacteria express different repertoires of proteins in response to environmental conditions and when invading a host, sense in vivo environment and adapt by changing the expression of specific proteins. In the case of pathogens, identification of genes with up-regulated expression in vivo compared to in vitro conditions might give an insight into the genes relevant to the bacterial virulence. In the present work, in vivo induced antigen technology (IVIAT) has been used to search for Phdp genes only expressed or up-regulated in infected S. senegalensis. An expression library from Phdp was assayed against pooled sera from convalescent S. senegalensis specimens and 18 clones were positive, indicating that proteins encoded are expressed by Phdp during S. senegalensis infection and are immunogenic for this fish species. In addition, five proteins were reactive against adsorbed sera, indicating their in vivo induced character. Inosine-5'-monophosphate dehydrogenase, serine hydroxy methyltransferase and alanyl-tRNA synthethase, involved in aminoacid and nucleotide metabolism, the protein with antioxidant activity alkyl hydroperoxide reductase and a non-ribosomal peptide synthetase responsible for the synthesis of the siderophore piscibactin have been identified as antigens induced in Phdp during S. senegalensis infection. Proteins induced during in vivo growth of Phdp represent promising targets for the development of novel antimicrobial or prophylactic agents in the treatment and prevention of photobacteriosis.

Terminal carbohydrates abundance, immune related enzymes, bactericidal activity and physico-chemical parameters of the Senegalese sole (Solea senegalensis, Kaup) skin mucus

Recently, interest in mucosal surfaces, more specifically fish skin and its secreted mucus, has greatly increased among immunologists. The abundance of terminal carbohydrates, several enzymes (proteases, lysozyme, peroxidase, alkaline phosphatase, esterases and ceruloplasmin), bactericidal activity against fish pathogenic and non-pathogenic bacteria and several physico-chemical parameters (protein concentration, pH, conductivity, redox potential, osmolarity, density and viscosity) in the skin mucus of Senegalese sole (Solea senegalensis, Kaup) have been evaluated. Present results evidence the abundance of N-acetylneuraminic acid, mannose, glucose and N-acetyl-galactosamine in skin mucus. The levels of lysozyme, proteases, esterases and alkaline phosphatase were very similar (from 20 to 30 Units mg^-1 protein). However, 93 Units mg^-1 protein were detected of ceruloplasmin and only 4'88 Units mg^-1 protein of peroxidase. Skin mucus of S. senegalensis showed high bactericidal activity against the tested pathogen bacteria but weak activity against non-pathogenic bacteria. Finally, a clear relationship between mucus density and temperature was detected, while viscosity showed a direct shear- and temperature-dependent behaviour. These results could be useful for better understanding the role of the skin mucus as a key component of the innate immune system, as well as, for elucidating possible relationships between biological and physico-chemical parameters and disease susceptibility.

Two routes of infection with Photobacterium damselae subsp. piscicida are effective in the modulation of the transcription of immune related genes in Solea senegalensis

The marine fish pathogen Photobacterium damselae subsp. piscicida (Phdp) is responsible for important disease outbreaks affecting cultured fish species including the flatfish Solea senegalensis. In the present work, transcription of iron metabolism related genes (TF, FERR-M, HP-1 and HAMP-1) as well as innate immune system components such as complement proteins (C3 and C7), lysozyme (LYS-G), TNF family (TNFα, TRAF-3), NCCRP-1 and heat shock protein encoding genes (HSP70, HSP90AA, HSP90AB and GP96) has been determined in the liver and kidney of S. senegalensis specimens after Phdp infection. Intraperitoneal injection (IP) and immersion (IM) routes have been used for infection. Fish developed specific antibodies in both cases, higher levels being detected in IP infected specimens. Both infection routes resulted in increased relative transcript levels of FERR-M, HP-1 and HAMP-1 genes and TF decreased relative transcription, conducting to lower iron availability for the pathogen. This response can be considered as a strategy to limit iron availability for Phdp, a pathogen capable to obtain iron from transferrin. Relative transcription of genes encoding lysozyme and complement factors C3 and C7 were also increased regardless the infection route; the liver was the main organ involved in the initial stages and the kidney in later stages of the infection. TNFα and TRAF-3 relative gene transcription increased 24h post-infection. TRAF-3 gene induction was detected 30 d post-infection, whilst no changes in TNFα were observed 72h or 30 d post-infection. NCCRP-1 changes were observed after IP infection in the liver and kidney; however, IM infection resulted only in slight changes in the kidney of infected fish. This different response observed maybe related to a lower number of invaded cells by the pathogen. Finally, changes in HSP90AB and GP96 have been detected after infection by both routes. Different late modulation has been observed in assayed genes depending on the route of infection. Thus, only LYS-G, TF, NCCRP-1, GP96 and HSP90AB gene transcription was modulated 30 d post-infection in the kidney of IM infected specimens; however, IP infected fish showed modulation in a higher number of genes both in liver and kidney tissues. The implications of these responses in resistance to infection by Phdp need to be elucidated.

Effects of dietary Bacillus subtilis, Tetraselmis chuii, and Phaeodactylum tricornutum, singularly or in combination, on the immune response and disease resistance of sea bream (Sparus aurata L.)

Combined or individual effects of two microalgae (Phaeodactylum tricornutum and Tetraselmis chuii) and Bacillus subtilis on immune response, gene expression, and survival to challenge with Photobacterium damselae subsp. piscicida of gilthead sea bream were investigated. To test the capacity of B. subtilis to grow employing the microalgae polysaccharides as energy and carbon source, an in vitro assay was defined, and demonstrated that the digestion product of microalgae, mainly P. tricornutum, supported the growth of B. subtilis much better than glucose. For the in vivo study, fish were distributed in six equal groups (each of two replicates) and received one of the following experimental diets: C) control, non-supplemented diet; T) T. chuii 100 g kg(-1); P) P. tricornutum 100 g kg(-1); B) B. subtilis (10(7) cfu g(-1)); BT) B. subtilis (10(7) cfu g(-1))+T. chuii (100 g kg(-1)); and BP) B. subtilis (10(7) cfu g(-1))+P. tricornutum (100 g kg(-1)). The complement activity, serum IgM level, respiratory burst, phagocytic activity, and expression of seven selected immune-related genes in head-kidney were evaluated following two and four weeks of treatment. At the end of the feeding trial, fish were challenged by intraperitoneal injection of LD(50) concentration of P. damselae subsp. piscicida and mortality was recorded. This is the first study testing the immunomodulatory capacity of the microalgae used in the present work. The dietary applications of B. subtilis, T. chuii, and P. tricornutum, singly or in combination, may exhibit up-regulating effects on gilthead sea bream immune parameters. P. tricornutum demonstrated the highest immunostimulant activity. There were no significant differences between combination feeding and feeding ingredients separately. Our results demonstrated the potential of microalgae as immunostimulants for fish, although further studies regarding the implications and effects of a stimulated immune system against pathogens, especially the protective capacity against specific diseases, are necessary.

Increases in immune parameters by inulin and Bacillus subtilis dietary administration to gilthead seabream (Sparus aurata L.) did not correlate with disease resistance to Photobacterium damselae

The present work evaluates the effects of inulin and Bacillus subtilis, single or combined, on immune parameters, immune-related gene expression and protection against Photobacterium damselae subsp. piscicida in gilthead seabream (Sparus aurata). Three trials were conducted. In the first trial, different concentrations of inulin (10, 15 and 30 g kg(-1)) (as a prebiotic) were administered to determine the optimal concentration for stimulating the seabream's immune system. In the second trial, the optimum concentration of inulin (10 g kg(-1)) was combined with B. subtilis (as a probiotic). Following two and four weeks of the treatment, the main immune parameters, as well as the expression of seven immune-related genes, were measured. In the final trial, fish fed the same diet as in the second trial were challenged intraperitoneally with P. damselae subsp. piscicida (10(9) cfu g(-1)). Treatment groups for the second and third trial were control (non-supplemented diet), inulin (10 g kg(-1)), B. subtilis (10(7) cfu g(-1)) and inulin + B. subtilis (10 g kg(-1) and 10(7) cfu g(-1) respectively). Dietary administration of inulin or B. subtilis for two weeks stimulated the serum complement activity and the IgM level, as well as leucocyte phagocytic activity; furthermore, inulin stimulated leucocyte respiratory burst activity. When inulin and B. subtilis were administered together (as a synbiotic), only the serum complement activity and the IgM level increased in a statistically significant manner. Furthermore, the complement activity showed a significant increase in fish fed the three experimental diets for four weeks. The challenge experiment showed that the fish fed inulin or the synbiotic diet had non-significantly lower or significantly higher cumulative mortality, respectively, compared with the control group (non-supplemented diet). These results suggest that inulin and B. subtilis modulate the immune response of the gilthead seabream, although the combined administration increases susceptibility to infection by P. damselae subsp. piscicida.

An in vitro screening method to evaluate chemicals as potential chemotherapeutants to control Aeromonas hydrophila infection in channel catfish

AIMS

To develop an in vitro screening method to be used for identifying potential effective chemotherapeutants to control Aeromonas hydrophila infections.

METHODS AND RESULTS

Using catfish gill cells G1B and four chemicals (hydrogen peroxide, sodium chloride, potassium permanganate and D-mannose), the feasibility of using an in vitro screening method to identify potential effective chemotherapeutants was evaluated in this study. In vitro screening results revealed that, at concentration of 100 mg l⁻¹, H₂O₂ was the only chemical tested that was able to completely abolish the attachment and invasion of Aer. hydrophila to catfish gill cells. In vivo virulence studies using live channel catfish through bath immersion confirmed that H₂O₂ was the only chemical tested that was able to significantly (P < 0·001) reduce the mortality (from 90 or 100% to 0 or 20%) caused by Aer. hydrophila infections.

CONCLUSIONS

The in vitro screening method using catfish gill cells G1B could be used to initially identify potential effective chemotherapeutants to control Aer. hydrophila.

SIGNIFICANCE AND IMPACT OF THE STUDY

An in vitro screening method using catfish gill cells to identify potential effective chemotherapeutants described here will cut cost in research compared with the method of using live fish to screen lead compounds for fish disease control.

In vivo morphological and antigenic characteristics of Photobacterium damselae subsp. piscicida

The present study was conducted to examine the morphology and antigenicity of Photobacterium damselae subsp. piscicida by culturing the bacterium in vivo in the peritoneal cavity of sea bass (Dicentrarchus labrax) within dialysis bags with either a low molecular weight (LMW) cut-off of 25 kDa or a high molecular weight (HMW) cut-off of 300 kDa. Differences were observed in the growth rate between the bacteria cultured in vivo or in vitro. Bacteria cultured in vivo were smaller and produced a capsular layer, which was more prominent in bacteria cultured in the HMW bag. Antigenicity was examined by Western blot analysis using sera from sea bass injected with live Ph. d. subsp. piscicida. The sera recognised bands at 45 and 20 kDa in bacteria cultured in vivo in the LMW bag. Bacteria cultured in vivo in the HMW bag did not express the 45 kDa band when whole cell extracts were examined, although the antigen was present in their extracellular products. In addition, these bacteria had a band at 18 kDa rather than 20 kDa. Differences in glycoprotein were also evident between bacteria cultured in vitro and in vivo. Bacteria cultured in vitro in LMW and HMW bags displayed a single 26 kDa band. Bacteria cultured in the LMW bag in vivo displayed bands at 26 and 27 kDa, while bacteria cultured in vivo in the HMW bag possessed only the 27 kDa band. These bands may represent sialic acid. The significance of the changes observed in the bacterium's structure and antigenicity when cultured in vivo is discussed.

Effect of dietary administration of Porphyridium cruentum on the respiratory burst activity of sole, Solea senegalensis (Kaup), phagocytes

The stimulatory effect of the red microalga Porphyridium cruentum on respiratory burst activity of sole phagocytes was evaluated in vivo. Oral administration of a diet supplemented with lyophilized P. cruentum cells (10 g kg(-1)) stimulated respiratory burst activity after 4 weeks feeding in sole vaccinated with Photobacterium damselae subsp. piscicida bacterin.

Variation in the molecular weight of Photobacterium damselae subsp. piscicida antigens when cultured under different conditions in vitro

The antigenicity of Photobacterium damselae (Ph. d.) subsp. piscicida, cultured in four different growth media [tryptone soya broth (TSB), glucose-rich medium (GRM), iron-depleted TSB (TSB + IR^-), and iron-depleted GRM (GRM + IR^-)] was compared by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis using sera obtained from sea bass (Dicentrarchus labrax) raised against live or heat-killed Ph. d. subsp. piscicida. The antigenic expression of Ph. d. subsp. piscicida was found to differ depending on the culture medium used. A significantly higher antibody response was obtained with iron-depleted bacteria by ELISA compared with non-iron depleted bacteria obtained from the sera of sea bass raised against live Ph. d. subsp. piscicida. The sera from sea bass raised against live bacteria showed a band at 22 kDa in bacteria cultured in TSB + IR^- or GRM+ IR^- when bacteria that had been freshly isolated from fish were used for the screening, while bands at 24 and 47 kDa were observed with bacteria cultured in TSB or GRM. When bacteria were passaged several times on tryptic soya agar prior to culturing in the four different media, only bands at 24 and 47 kDa were recognized, regardless of the medium used to culture the bacteria. It would appear that the molecular weight of Ph. d. subsp. piscicida antigens change in the presence of iron restriction, and sera from sea bass infected with live bacteria are able to detect epitopes on the antigens after this shift in molecular weight.

Superoxide dismutase and catalase activities in Photobacterium damselae ssp. piscicida

The ability of a set of Photobacterium damselae ssp. piscicida strains isolated from different fish species to produce different superoxide dismutase (SOD) and catalase enzymes was determined. Unlike other bacterial pathogens, P. damselae ssp. piscicida is not able to produce different isoforms of SOD or catalase containing different metal cofactors when cultured under oxidative stress induced by hydrogen peroxide or methyl viologen, or under iron depleted conditions. However, iron content of the growth medium influenced the levels of SOD and catalase activity in cells, these levels decreasing with iron availability of the medium. Comparison of virulent and non-virulent strains of P. damselae ssp. piscicida showed similar contents of SOD, but higher levels of catalase were detected in cells of the virulent strain. Incubation of bacteria with sole, Solea senegalensis (Kaup), phagocytes has shown that survival rates range from 19% to 62%, these rates being higher for the virulent strain. The increased levels of catalase activity detected in the virulent strain indicates a possible role for this enzyme in bacterial survival.

Adhesion to sole, Solea senegalensis Kaup, mucus of microorganisms isolated from farmed fish, and their interaction with Photobacterium damselae subsp. piscicida

Abstract Most studies carried out to select microorganisms as candidate probiotics have focused on in vitro antagonism tests, such as the production of inhibitory compounds against pathogenic microorganisms. However, attachment to mucous surfaces could be another criterion to be considered when selecting potential probiotics for aquaculture. Nineteen isolates obtained from farmed Senegalese sole, Solea senegalensis Kaup, and gilthead sea bream, Sparus aurata L., have been evaluated for their capacity to adhere to skin and intestinal mucus of Senegalese sole, and their antagonistic effect against Photobacterium damselae subsp. piscicida, an important pathogen for farmed sole. The isolates from gilthead sea bream showed the highest percentage of adhesion to sole mucus, whilst the pathogenic microorganisms assayed and the isolates from sole showed, in general, a lower ability to adhere to sole mucus. The results suggest that the adhesion to fish mucus was more dependent on the isolate tested than on the host mucus. The isolates from gilthead sea bream also showed a higher antagonistic activity against P. damselae subsp. piscicida than those from Senegalese sole. Four isolates were selected, on the basis of their adhesive ability and antagonistic effect on P. damselae subsp. piscicida, to study their interactions with the pathogen in respect of adhesion to skin and intestinal mucus under exclusion, competition and displacement conditions. The results obtained show the ability of three isolates to reduce the adhesion of P. damselae subsp. piscicida to sole mucus under displacement and competition conditions. The adhesion of the pathogen to sole intestinal mucus was also significantly reduced when three isolates were assayed under exclusion conditions.

Effectiveness of a divalent vaccine for sole, Solea senegalensis (Kaup), against Vibrio harveyi and Photobacterium damselae subsp. piscicida

The protection of cultured sole, Solea senegalensis, against Vibrio harveyi and Photobacterium damselae subsp. piscicida was evaluated following the use of a divalent vaccine prepared with formalized whole cells and extracellular products of virulent strains of both pathogenic microorganisms and administered by the immersion route. Two prolonged immersions of 5-10 g fish in the divalent bacterin at a 1-month interval gave high levels of protection similar to those obtained when the respective monovalent vaccines were administered by the intraperitoneal route [relative percentage of survival (RPS) values >70%], which indicates that the former procedure can be a useful strategy with small fish. The high protection afforded by the divalent vaccine in sole lasted for 4 months after which the RPS values against both pathogens decreased significantly.