Lipopolysaccharides isolated from Aeromonas salmonicida and Vibrio anguillarum show quantitative but not qualitative differences in inflammatory outcome in Sparus aurata (Gilthead seabream)

In fish, the defence system recognises pathogenic microorganisms via pathogen recognition receptors (PRRs) that sense particular structures of the pathogens; the so-called pathogen associated molecular patterns (PAMPs) such as bacterial lipopolysaccharides (LPSs). The result of the PAMP-PRR interactions leads to complex and orchestrated immune responses. In this study, Sparus aurata (Gilthead seabream) were intraperitoneally injected with purified lipopolysaccharide (LPS) from Aeromonas salmonicida (As)- and Vibrio anguillarum (Va) (1 mg*Kgfish(-1)), both Gram negative bacteria responsible for vibriosis and furunculosis respectively, therefore causing an impact upon marine fish cultures. Head-kidney, intestine, spleen, liver and blood samples were collected at 3, 6, 12 and 24 h post-injection. Plasma levels of cortisol, prostaglandins and lactate were measured and were significantly increased after As-LPS and Va-LPS treatment. Furthermore, tissue-specific differences of the gene regulatory patterns were evident for each LPS. When monocyte/macrophage cell cultures were challenged with As-LPS and Va-LPS, the pro-inflammatory cytokine mRNA abundances present a similar pattern of response. However, As-LPS always triggered a stronger response concerning TNFα, IL1β and cyclooxygenase-2 (COX2) mRNA abundance as well as PGE2 levels in the supernatant. Overall, the results indicate that specific LPSs do not activate different pro-inflammatory responses and that the observed gene expression pattern is tissue and concentration dependent.

Enhanced Aeromonas salmonicida bacterin uptake and side effects caused by low frequency sonophoresis in rainbow trout (Oncorhynchus mykiss)

Low frequency sonophoresis (LFS) has been recognized as one of the most advanced technologies in transdermal delivery of substances, due to the modification of the stratum corneum lipid bilayer, in focal skin applications in mammals. Based on these findings, LFS has been suggested as a potential technology to be used for enhancement in immersion fish vaccination. In contrast to mammals where LFS is applied to discrete regions of the skin, in fish the whole individual needs to be exposed for practical purposes. The current study evaluated the impact of LFS at 37 kHz on the uptake of an Aeromonas salmonicida bacterin and side effects of the treatment in rainbow trout. Quantitative real time PCR (qPCR) and immunohistochemistry were used to examine the bacterin uptake into skin and gill tissue. Side effects were assessed by behavioural examination, histology and blood serum analysis. The sonication intensity of 171 mW/cm² was enough for increasing skin permeability, but caused heavy erratic swimming and gill haemorrhages. Sonication intensities as low as 105 mW/cm² did not modify skin permeability and enhanced the bacterin uptake into the gill tissue by factor 15 compared to conventional immersion. Following sonication, the gill permeability for the bacterin decreased after 20 min and 120 min by factor 3 and 2, respectively. However, during sonication, erratic swimming of the fish raised some concerns. Further reduction of the sonication intensity to 57 mW/cm² did not induce erratic swimming, and the bacterin uptake into the gill tissue was still increased by factor 3. In addition, a decreasing albumin-globulin ratio in the serum of the rainbow trout within 40 min revealed that LFS leads to an inflammatory response. Consequently, based on both increased bacterin uptake and the inflammatory response, low intensity LFS has the potential to enhance vaccine immunity without significant side effects.

Carbohydrate analysis and serological classification of typical and atypical isolates of Aeromonas salmonicida: a rationale for the lipopolysaccharide-based classification of A. salmonicida

The cell envelope of Aeromonas salmonicida contains a lipopolysaccharide (LPS) essential for the physical integrity and functioning of bacterial cell membrane. Using a recently developed in-source fragmentation technique, we screened 39 typical and atypical isolates of A. salmonicida and established their O-chain polysaccharide structure by capillary electrophoresis-mass spectrometry (CE-MS), compositional and linkage analyses and comparison to the previously determined O-chain polysaccharide structure of A. salmonicida strain A449. These studies have demonstrated that A. salmonicida isolates fall into three distinct structural types, types A-C, based on chemical structures of their respective O-chain polysaccharide components. Subsequent immunoblotting and serological studies with salmon polyclonal antisera produced to formalin-fixed cells of A. salmonicida strains A449, N4705 and 33659 representing three structural types A-C revealed that variations in the O-chain polysaccharide structure have led to significant serological differences between strains belonging to type A and non-type A, where non-type A species include chemically separated structural types B and C. Due to the presence of common antigenic determinants shared by their respective O-chain polysaccharide components, serological cross-reactions were observed between A. salmonicida strains belonging to structural types B and C. These findings suggest the possibility of developing LPS-based classification system of A. salmonicida sub-species consisting of two serologically distinct types, type A and non-type A.

Recombinant prostate-specific antigen proaerolysin shows selective protease sensitivity and cell cytotoxicity

Native proaerolysin is a channel-forming bacterial protoxin that binds to cell-surface receptors and then is activated by furin or furin-like proteases. We genetically engineered proaerolysin by replacing the furin-cleavage sequence with a prostate-specific antigen-selective sequence. The recombinant modified proaerolysin was expressed and purified from Aeromonas salmonicida in good yields and purity. Recombinant modified proaerolysin had no furin sensitivity and markedly increased prostate-specific antigen sensitivity relative to wild-type proaerolysin. Human prostate cancer cells were significantly more sensitive to recombinant modified proaerolysin in the presence of active prostate-specific antigen when compared with the absence of prostate-specific antigen or the presence of potent prostate-specific antigen inhibitors. Most normal human cells with the exception of prostate and renal epithelial cells showed very low sensitivity to recombinant modified proaerolysin. Our results suggest that recombinant modified proaerolysin is a potent prostate-specific antigen-sensitive protoxin that deserves further development for regional therapy of benign and malignant prostate growths.

Structural characterization of the lipid A region of Aeromonas salmonicida subsp. salmonicida lipopolysaccharide

The lipid A components of Aeromonas salmonicida subsp. salmonicida from strains A449, 80204-1 and an in vivo rough isolate were isolated by mild acid hydrolysis of the lipopolysaccharide. Structural studies carried out by a combination of fatty acid, electrospray ionization-mass spectrometry and nuclear magnetic resonance analyses confirmed that the structure of lipid A was conserved among different isolates of A. salmonicida subsp. salmonicida. All analyzed strains contained three major lipid A molecules differing in acylation patterns corresponding to tetra-, penta- and hexaacylated lipid A species and comprising 4'-monophosphorylated beta-2-amino-2-deoxy-d-glucopyranose-(1-->6)-2-amino-2-deoxy-d-glucopyranose disaccharide, where the reducing end 2-amino-2-deoxy-d-glucose was present primarily in the alpha-pyranose form. Electrospray ionization-tandem mass spectrometry fragment pattern analysis, including investigation of the inner-ring fragmentation, allowed the localization of fatty acyl residues on the disaccharide backbone of lipid A. The tetraacylated lipid A structure containing 3-(dodecanoyloxy)tetradecanoic acid at N-2',3-hydroxytetradecanoic acid at N-2 and 3-hydroxytetradecanoic acid at O-3, respectively, was found. The pentaacyl lipid A molecule had a similar fatty acid distribution pattern and, additionally, carried 3-hydroxytetradecanoic acid at O-3'. In the hexaacylated lipid A structure, 3-hydroxytetradecanoic acid at O-3' was esterified with a secondary 9-hexadecenoic acid. Interestingly, lipid A of the in vivo rough isolate contained predominantly tetra- and pentaacylated lipid A species suggesting that the presence of the hexaacyl lipid A was associated with the smooth-form lipopolysaccharide.

AopP, a type III effector protein of Aeromonas salmonicida, inhibits the NF-κB signalling pathway

Aeromonas salmonicida subsp. salmonicida contains a functional type III secretion system that is responsible for the secretion of the ADP-ribosylating toxin AexT. In this study, the authors identified AopP as a second effector protein secreted by this system. The aopP gene was detected in both typical and atypical A. salmonicida isolates and was found to be encoded on a small plasmid of approximately 6.4 kb. Sequence analysis indicates that AopP is a member of the YopJ family of effector proteins, a group of proteins that interfere with mitogen-activated protein kinase (MAPK) and/or nuclear factor kappa B (NF-κB) signalling pathways. AopP inhibits the NF-κB pathway downstream of IκB kinase (IKK) activation, while a catalytically inactivated mutant, AopPC177A, does not possess this inhibitory effect. Unlike other effectors of the YopJ family, such as YopJ and VopA, AopP does not inhibit the MAPK signalling pathway.

The effect of oral immuno-stimulation in juvenile carp (Cyprinus carpio L.)

The effect of a 2-week period of oral immuno-stimulation from the age of 2 or 6 weeks post-fertilisation (wpf; before and after reaching the ability to produce antibodies) onwards was investigated on various immune functions of the common carp, Cyprinus carpio. The immuno-stimulants Aeromonas salmonicida lipopolysaccharide, Yeast DNA (containing unmethylated CpG motifs) or high-M alginate (an extract of algae containing poly-mannuronic acid) were used. The effect of this treatment was studied on the kinetics of B cells in head kidney and peripheral blood leucocytes using flow cytometry, on the total plasma IgM level using ELISA, on cytokine and inducible nitric oxide synthase (iNOS) expression in the intestine, and acute phase protein expression in the liver, using real time quantitative PCR, and on exposure to Vibrio anguillarum. Oral administration of immuno-stimulants from 6 wpf resulted in decreased WCI12(+) (B) cell percentages in PBL (only after administration of LPS) and head kidney (all test groups), and a decreased total IgM level in plasma, suggesting that suppressive effects are strongly indicative of oral or juvenile tolerance. After administration from 2 wpf, the effects on WCI12(+) (B) cell percentages were less pronounced: the group fed with Yeast DNA showed higher percentages compared to the control group at 6 wpf, but lower percentages at 8 wpf. No changes were observed in the cytokine or iNOS expression levels in the intestine or acute phase protein expression in the liver. A challenge with V. anguillarum resulted in an initially higher cumulative mortality in the group fed with LPS, but lower mortality in the groups fed with Yeast DNA or high-M alginate compared to the control group, providing a provisional warning especially for the use of pathogen-derived immuno-stimulants, such as A. salmonicida LPS, in larval and juvenile fish.

Immunostimulation of larvae and juveniles of cod, Gadus morhua L

Cod larval culture is currently hampered by high mortalities in the first 2-3 weeks after hatching, often due to infectious diseases. The immune system of cod is not fully competent until 2-3 months after hatching. Conventional vaccination is, therefore, not of value before this time, and the larvae are wholly reliant on non-specific parameters for their defence against infection. A range of substances, generally derived from bacterial, fungal or plant origin, can activate these non-specific parameters. During three hatching seasons, 2001-2003, at the Marine Institute's Experimental Station, Stadur, Grindavik, Iceland, the effects of several immunostimulants on survival and disease resistance of cod larvae and juveniles were examined. Both bathing treatments and administration in the feed were used. One of these substances, lipopolysaccharide (LPS), isolated from the bacterium Aeromonas salmonicida (ssp. salmonicida or achromogenes), appeared in some instances to improve survival and have a beneficial effect on disease resistance. Other substances tested had limited effects. The results emphasize the need for further work in this field.

Identification of the major outer membrane proteins of Aeromonas salmonicida

Aeromonas salmonicida subsp. salmonicida is a Gram-negative bacterium that is the etiological agent of furunculosis, a serious infectious disease of salmonids. Aeromonas spp. are ubiquitous waterborne bacteria responsible for a wide spectrum of diseases among aquatic organisms and humans. Bacterial outer membrane proteins (OMPs) play a significant role in virulence as they comprise the outermost surface in contact with host cells and immune defense factors. To identify the major OMPs of A. salmonicida a proteomic analysis was undertaken using a carbonate OMP-enrichment protocol. The enriched OMP-extracts were separated by 2-dimensional electrophoresis (2-DE) and the spots identified using liquid chromatography/mass spectrometry/mass spectrometry (LC/MS/MS) via an electrospray ionization source. In total, 76 unique proteins were identified from the 125 spots observed on the 2-D gel. The surface layer (S-layer) VapA protein dominated the A. salmonicida OMP 2-D profile, accounting for 60% of the protein on the 2-D gels. Among the other outer membrane proteins identified were at least 10 porins and various receptors involved in nutrient acquisition. Also identified in the carbonate insoluble fraction were phosphoglycerate kinase, enolase and others that lacked classical export sorting signals. The putative association of these proteins with the cell surface might provide new insights concerning the biological and pathogenic roles of these molecules in A. salmonicida infection. This work represents the first systematic attempt to characterize the cell surface of A. salmonicida.

Structural studies of the core region of Aeromonas salmonicida subsp. salmonicida lipopolysaccharide

The core oligosaccharide structure of the in vivo derived rough phenotype of Aeromonas salmonicida subsp. salmonicida was investigated by a combination of compositional, methylation, CE-MS and one- and two-dimensional NMR analyses and established as the following: [carbohydrate: see text] where R=alpha-D-Galp-(1-->4)-beta-D-GalpNAc-(1--> or alpha-D-Galp-(1--> (approx. ratio 4:3). Comparative CE-MS analysis of A. salmonicida subsp. salmonicida core oligosaccharides from strains A449, 80204-1 and an in vivo rough isolate confirmed that the structure of the core oligosaccharide was conserved among different isolates of A. salmonicida.

Structural and serological characterization of the O-chain polysaccharide of Aeromonas salmonicida strains A449, 80204 and 80204-1

The O-chain polysaccharide (O-PS) of Aeromonas salmonicida was studied by a combination of compositional, methylation, CE-ESMS and one- and two-dimensional NMR analyses. It was found to be a branched polymer of trisaccharide-repeating units composed of L-rhamnose (Rha), D-glucose (Glc), 2-acetamido-2-deoxy-D-mannose (ManNAc) and O-acetyl group (OAc) and having the following structure: CE-ESMS analysis of A. salmonicida cells from strains A449, 80204 and 80204-1 grown under different conditions confirmed that the O-PS structure was conserved. ELISA-based serological study with native LPS-specific antisera performed on the native O-PS and its O-deacetylated and periodate-oxidized derivatives confirmed the importance of the O-PS backbone structure as an immunodominant determinant.

Structural studies of the capsular polysaccharide and lipopolysaccharide O-antigen of Aeromonas salmonicida strain 80204-1 produced under in vitro and in vivo growth conditions

Aeromonas salmonicida is a pathogenic aquatic bacterium and the causal agent of furunculosis in salmon. In the course of this study, it was found that when grown in vitro on tryptic soy agar, A. salmonicida strain 80204-1 produced a capsular polysaccharide with the identical structure to that of the lipopolysaccharide O-chain polysaccharide. A combination of 1D and 2D NMR methods, including a series of 1D analogues of 3D experiments, together with capillary electrophoresis-electrospray MS (CE-ES-MS), compositional and methylation analyses and specific modifications was used to determine the structure of these polysaccharides. Both polymers were shown to be composed of linear trisaccharide repeating units consisting of 2-acetamido-2-deoxy-D-galacturonic acid (GalNAcA), 3-[(N-acetyl-L-alanyl)amido]-3,6-dideoxy-D-glucose[3-[(N-acetyl-L-alanyl)amido]-3-deoxy-D-quinovose, Qui3NAlaNAc] and 2-acetamido-2,6-dideoxy-D-glucose (2-acetamido-2-deoxy-D-quinovose, QuiNAc) and having the following structure: [-->3)-alpha-D-GalpNAcA-(1-->3)-beta-D-QuipNAc-(1-->4)-beta-D-Quip3NAlaNAc-(1-]n, where GalNAcA is partly presented as an amide and AlaNAc represents N-acetyl-L-alanyl group. CE-ES-MS analysis of CPS and O-chain polysaccharide confirmed that 40% of GalNAcA was present in the amide form. Direct CE-ES-MS/MS analysis of in vivo cultured cells confirmed the formation of a novel polysaccharide, a structure also formed in vitro, which was previously undetectable in bacterial cells grown within implants in fish, and in which GalNAcA was fully amidated.

Elucidation of the molecular structure of lipid A isolated from both a rough mutant and a wild strain of Aeromonas salmonicida lipopolysaccharides using electrospray ionization quadrupole time-of-flight tandem mass spectrometry

The chemical structure of lipid A, isolated by mild acid hydrolysis from a rough mutant and a wild strain of Aeromonas salmonicida lipopolysaccharide, was investigated using electrospray ionization quadrupole time-of-flight (QqToF) hybrid tandem mass spectrometry and showed a great degree of microheterogeneity. The chemical structure of the main constituent of this heterogeneous mixture was identified as a beta-D-(1 --> 6) linked D-glucosamine disaccharide substituted by two phosphate groups, one being bound to the non-reducing end at position O-4' and the other to the position O-1 of the reducing end of the D-glucosamine disaccharide. The location of the fatty acids linked to the disaccharide backbone was established by identifying diagnostic ions in the conventional QqToF-MS scan. Low-energy collision tandem mass spectrometry analysis of the selected precursor diagnostic ions confirmed, unambiguously, their proposed molecular structures. We have established that myristyloxylauric (C14:0(3-O(12:0))) acid residues were both N-2' and O-3' linked to the non-reducing end of the D-GlcN residue, and that two 3-hydroxymyristic (C14:0(3-OH)) acid chains acylated the remaining positions of the reducing end. The MS and MS/MS data obtained allowed us to determine the complex molecular structure of lipid A. The QqToF-MS/MS instrument has shown excellent superiority over a conventional quadrupole-hexapole-quadrupole tandem instrument which failed to fragment the selected precursor ion.