The effect of in vivo growth on the cellular and extracellular components of the marine bacterial pathogen Photobacterium damsela subsp. piscicida

Immune response of European sea bass (Dicentrarchus labrax L.) against combination of antigens from three different pathogens

Three of the most important diseases of Mediterranean intensive European sea bass farming are, viral nervous necrosis (VNN) caused by the red grouper nervous necrosis virus (RGNNV) genotype of b-nodavirus, photobacteriosis caused by Photobacterium damselae subsp. piscicida (Phdp) and vibriosis caused mainly by the O1 serotype of Vibrio anguillarum (VaO1). Prevention against these diseases is performed through vaccination with a monovalent vaccine against the viral disease and, usually, with bivalent vaccines against the bacterial diseases. However, it is very difficult to program two vaccinations during the same season for the same fish stock and producers are forced to either vaccinate for the viral or the bacterial diseases or to perform double vaccination with both vaccines, without any prior knowledge on any interactions that may occur due to the plethora of antigens (Ag) injected. Ideally, therefore, a trivalent vaccine should be developed against all three diseases. The objective of this work was to analyse the immune response of sea bass against combinations of Ags from all three pathogens, namely viral particles, Phdp whole cells (WC), lipopolysaccharide (LPS), capsular polysaccharide (CPS) and extracellular products (ECPs) and VaO1 WC and ECPs in respect to the identification of any phenomena of immunodominance/immunosuppression between Ags with a view to select candidate Ags for inclusion in a trivalent vaccine formulation. Eight triplicate groups of fish were immunized with different combinations of the aforementioned Ags and another triplicate group served as negative control. Blood serum was isolated at various time-points post-immunization for the measurement of specific antibodies against each Ag and, in addition, leucocytes were isolated at day 29 post-immunization for analysis of various cellular activities. Results indicated that best levels of specific a-NNV virus antibodies (Abs) were produced when VaO1 ECPs were not included in the Ag combinations, in contrast to the leucocytes proliferation assay where best stimulation against NNV Ags was measured when VaO1 ECPs were present in Ag combinations. VaO1 ECPs apparently is a strong immunogen for both humoral and cellular responses but suppresses immunological reactions against the other Ags.VaO1 WC, Phdp LPS and ECPs raised good humoral immune responses in the groups with best responses against VNN Ags, but only VaO1 WC and Phdp ECPs provided good stimulation of leucocytes, with Phdp WC and CPS effecting either similar stimulation with untrained leucocytes (control groups) or down-stimulation. Results are discussed with a view to select Ags from all three pathogens for inclusion in trivalent vaccine against all three pathogens.

The apoptogenic toxin AIP56 is a metalloprotease A-B toxin that cleaves NF-κb P65

AIP56 (apoptosis-inducing protein of 56 kDa) is a major virulence factor of Photobacterium damselae piscicida (Phdp), a Gram-negative pathogen that causes septicemic infections, which are among the most threatening diseases in mariculture. The toxin triggers apoptosis of host macrophages and neutrophils through a process that, in vivo, culminates with secondary necrosis of the apoptotic cells contributing to the necrotic lesions observed in the diseased animals. Here, we show that AIP56 is a NF-κB p65-cleaving zinc-metalloprotease whose catalytic activity is required for the apoptogenic effect. Most of the bacterial effectors known to target NF-κB are type III secreted effectors. In contrast, we demonstrate that AIP56 is an A-B toxin capable of acting at distance, without requiring contact of the bacteria with the target cell. We also show that the N-terminal domain cleaves NF-κB at the Cys³⁹-Glu⁴⁰ peptide bond and that the C-terminal domain is involved in binding and internalization into the cytosol.

The apoptosis inducing protein of 56 kDa (AIP56) is a key virulence factor secreted by Photobacterium damselae piscicida (Phdp), a Gram-negative bacterium that causes septicaemic infections in economically important marine fish species. It is known that AIP56 induces massive destruction of the phagocytic cells of the infected host, allowing the extracellular multiplication of the bacteria and contributing to the genesis of the pathology. Here we show that AIP56 acts by cleaving NF-κB p65. The NF-κB family of transcription factors is evolutionarily conserved and plays a central role in the host responses to microbial pathogen invasion, regulating the expression of inflammatory and anti-apoptotic genes. Pathogenic bacteria have evolved complex strategies to interfere with NF-κB signalling, usually by injecting protein effectors directly into the cell's cytosol through bacterial secretion machineries that require contact with host cells. In contrast, AIP56 acts at distance and has an intrinsic ability to reach the cytosol due to the presence of a C-terminal domain that functions as “delivery module.”

AIP56: a novel bacterial apoptogenic toxin

Photobacterium damselae subsp. piscicida (Phdp) is a Gram-negative pathogen agent of an important fish septicemia. The key virulence factor of Phdp is the plasmid-encoded exotoxin AIP56, which is secreted by exponentially growing pathogenic strains. AIP56 has 520 amino acids including an N-terminal cleavable signal peptide of 23 amino acid residues, two cysteine residues and a zinc-binding region signature HEXXH that is typical of most zinc metallopeptidases. AIP56 induces in vitro and in vivo selective apoptosis of fish macrophages and neutrophils through a caspase-3 dependent mechanism that also involves caspase-8 and -9. In vivo, the AIP56-induced phagocyte apoptosis progresses to secondary necrosis with release of cytotoxic phagocyte molecules including neutrophil elastase. Fish injected with recombinant AIP56 die with a pathology similar to that seen in the natural infection.

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.

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.

Amoebic gill disease (AGD)-affected Atlantic salmon, Salmo salar L., are resistant to subsequent AGD challenge

There is inconsistent evidence of resistance of Atlantic salmon, Salmo salar L., to amoebic gill disease (AGD). Here, evidence is presented that demonstrates that Atlantic salmon exposed and subsequently challenged with AGD are more resistant than naïve control fish. Seventy-three per cent of Atlantic salmon previously exposed to AGD survived to day 35 post-challenge compared with 26% exposed to Neoparamoeba sp. for the first time, yet the gill pathology of surviving naïve control or previously exposed fish was not significantly different. Development of resistance to AGD is associated with anti-Neoparamoeba sp. antibodies that were detectable in serum of 50% of surviving Atlantic salmon previously exposed to AGD. However, anti-Neoparamoeba sp. antibodies were not detectable in cutaneous mucus of resistant fish. Increased resistance of Atlantic salmon after secondary Neoparamoeba sp. infection and detection of specific serum antibodies provides support for the development of a vaccine for AGD.

Cardiovascular responses of three salmonid species affected with amoebic gill disease (AGD)

The cardiovascular effects of amoebic gill disease (AGD) were investigated immediately following surgery in three salmonid species; Atlantic salmon (Salmo salar L.), brown trout (Salmo trutta L.) and rainbow trout (Oncorhynchus mykiss Walbaum). Fish, both naïve (control) and infected (AGD-affected) of each species, were fitted with dorsal aorta catheters and cardiac flow probes. Cardiac output and dorsal aortic pressures were then continuously measured over a 6-h period following surgery. Results showed that Atlantic salmon, brown trout and rainbow trout displayed similar dorsal aortic pressure, cardiac output, and systemic vascular resistance (mean dorsal aotic pressure divided by cardiac output) values. However, the only significant differences relating to disease status i.e. infected or control, were found in Atlantic salmon. Although no significant differences were seen in dorsal aortic pressure values, AGD-affected salmon displayed significantly elevated systemic vascular resistance at 4 and 6 h post surgery. Cardiac output was also approximately 35% lower in AGD-affected salmon compared to the non-affected control counterparts. These results comparatively examine cardiac function in response to AGD across three salmonid species and highlight species-specific cardiovascular responses that occur in association with disease. It is suggested that the apparent cardiac dysfunction seen in AGD-affected Atlantic salmon could, under stressful conditions, become exacerbated. Cardiac failure is therefore suggested to be a possible physiological mechanism by which AGD causes or contributes to mortality in Atlantic salmon.

The viscosity and glycoprotein biochemistry of salmonid mucus varies with species, salinity and the presence of amoebic gill disease

Fish mucus has previously been reported to change in appearance and composition among species and in response to changes in salinity and disease status. This study reports on the mucus viscosity and glycoprotein biochemistry of Atlantic salmon (Salmo salar L.), brown trout (Salmo trutta L.) and rainbow trout (Oncorhynchus mykiss Walbaum) in freshwater and seawater, both naive to and affected by amoebic gill disease (AGD). Cutaneous mucus viscosity was measured over a range of shear rates (11.5, 23, 46 and 115 s(-1)), and non-Newtonian behaviour was demonstrated for all three species. Mucus viscosity was significantly greater in seawater than in freshwater for all species, and significantly lower in AGD-affected Atlantic salmon and brown trout. Mucus glucose, total protein and osmolality data indicated that differences in viscosity due to salinity were mostly attributed to changes in mucus hydration, while differences due to disease were mostly attributed to changes in mucus composition. Trends in gill mucus cell histochemistry included shifts in glycoproteins from neutral mucins in freshwater to acidic mucins in seawater, and shifts towards neutral mucins, with an increase in mucus cell numbers, in response to AGD. Results suggested that Atlantic salmon and brown trout are more similar to one another in their mucus profile than to rainbow trout. Atlantic salmon and brown trout both exhibited a whole-body mucus response to AGD, whereas rainbow trout exhibited only a local gill response. Findings hold implications for fish physiology and pathology, and indicate that future fish-disease management strategies should be species and condition specific.