Purification and properties of an extracellular protease of Aeromonas salmonicida, the causative agent of furunculosis

Inactivated Aeromonas salmonicida impairs adaptive immunity in lumpfish (Cyclopterus lumpus)

Aeromonas salmonicida, a widely distributed aquatic pathogen causing furunculosis in fish, exhibits varied virulence, posing challenges in infectious disease and immunity studies, notably in vaccine efficacy assessment. Lumpfish (Cyclopterus lumpus) has become a valuable model for marine pathogenesis studies. This study evaluated several antigen preparations against A. salmonicida J223, a hypervirulent strain of teleost fish, including lumpfish. The potential immune protective effect of A. salmonicida bacterins in the presence and absence of the A-layer and extracellular products was tested in lumpfish. Also, we evaluated the impact of A. salmonicida outer membrane proteins (OMPs) and iron-regulated outer membrane proteins (IROMPs) on lumpfish immunity. The immunized lumpfish were intraperitoneally (i.p.) challenged with 104 A. salmonicida cells/dose at 8 weeks-post immunization (wpi). Immunized and non-immunized fish died within 2 weeks post-challenge. Our analyses showed that immunization with A. salmonicida J223 bacterins and antigen preparations did not increase IgM titres. In addition, adaptive immunity biomarker genes (e.g., igm, mhc-ii and cd4) were down-regulated. These findings suggest that A. salmonicida J223 antigen preparations hinder lumpfish immunity. Notably, many fish vaccines are bacterin-based, often lacking efficacy evaluation. This study offers crucial insights for finfish vaccine approval and regulations.

INFLUENCE OF PATHOGENS, FISH-RELATED CHARACTERISTICS, AND ENVIRONMENTAL FACTORS ON THE DEVELOPMENT OF SKIN ULCERATIONS IN WILD COMMON DAB (LIMANDA LIMANDA) FROM THE NORTH SEA

Environmental changes or stressors can result in the development of diseases. Through regular fish disease surveys in the Belgian part of the North Sea, attention was drawn to a sudden increase of skin ulceration prevalence between 2011 and 2014 in common dab (Limanda limanda). Information on prevalence, ulceration, bacteriology, fish-related (e.g., length, age, and sex) and (spatial and temporal) environmental factors, and fishing intensity were gathered. This detailed investigation was framed within a long-term monitoring program, executed every spring-autumn from 2000 to present. Ulcerations were observed in 1.3% of fish (n=3,999). Spatial and temporal differences were evident, and highest prevalence was found in summer. Vibrio was the dominant cultivated bacterial genus present in the lesions. Skin ulcerations appeared to be correlated with length and body condition of the fish, as well as with temperature and pH of the seawater and fishing vessel density. Our research suggested the involvement of multiple factors in the development of skin ulcerations in common dab and endorsed the effects of changing environment and human influence on the marine ecosystem through activities such as fishing.

First isolation of Vibrio tapetis and an atypical strain of Aeromonas salmonicida from skin ulcerations in common dab (Limanda limanda) in the North Sea

Skin ulcerations rank amongst the most prevalent lesions affecting wild common dab (Limanda limanda) with an increase in prevalence of up to 3.5% in the Belgian part of the North Sea. A complex aetiology of these ulcerations is suspected, and many questions remain on the exact factors contributing to these lesions. To construct the aetiological spectrum of skin ulcerations in flatfish, a one-day monitoring campaign was undertaken in the North Sea. Fifteen fish presented with one or more ulcerations on the pigmented and/or non-pigmented side. Pathological features revealed various stages of ulcerations with loss of epidermal and dermal tissue, inflammatory infiltrates and degeneration of the myofibers bordering the ulceration, albeit in varying degrees. Upon bacteriological examination, pure cultures of Vibrio tapetis were retrieved in high numbers from five fish and of Aeromonas salmonicida in one fish. The V. tapetis isolates showed cross-reactivity with the sera against the representative strain of serotype O2 originating form a carpet-shell clam (Ruditapes descussatus). Moreover, the A. salmonicida isolates displayed a previously undescribed vapA gene sequence (A-layer type) with possible specificity towards common dab. Further research is necessary to pinpoint the exact role of these agents in the development of skin ulcerations in common dab.

Subunit vaccine candidates against Aeromonas salmonicida in rainbow trout Oncorhynchus mykiss

Aeromonas salmonicida subsp. salmonicida is the etiological agent of furunculosis and a major fish health problem in salmonid aquaculture worldwide. Injection vaccination with commercial mineral oil-adjuvanted bacterin vaccines has been partly successful in preventing the disease but in Danish rainbow trout (Oncorhynchus mykiss, Walbaum) aquaculture furunculosis outbreaks still occur. In this study we tested the efficacy of experimental subunit vaccines against A. salmonicida infection in rainbow trout. We utilized in silico screening of the proteome of A. salmonicida subsp. salmonicida strain A449 and identified potential protective protein antigens that were tested by in vivo challenge trial. A total of 14 proteins were recombinantly expressed in Escherichia coli and prepared in 3 different subunit vaccine combinations to immunize 3 groups of rainbow trout by intraperitoneal (i.p.) injection. The fish were exposed to virulent A. salmonicida 7 weeks after immunization. To assess the efficacy of the subunit vaccines we evaluated the immune response in fish after immunization and challenge infection by measuring the antibody levels and monitoring the survival of fish in different groups. The survival of fish at 3 weeks after challenge infection showed that all 3 groups of fish immunized with 3 different protein combinations exhibited significantly lower mortalities (17–30%) compared to the control groups (48% and 56%). The ELISA results revealed significantly elevated antibody levels in fish against several protein antigens, which in some cases were positively correlated to the survival.

Surface-disorganized, attenuated mutants of Aeromonas salmonicida as furunculosis live vaccines

A slow-growing, aminoglycoside-resistant mutant and a rapidly-growing pseudo-revertant were isolated from Aeromonas salmonicida, the causative agent of salmonid furunculosis. These mutants continued to elicit a variety of classical virulence factors associated with A. salmonicida pathogenesis. They differed morphologically from the wild-type and from one another with respect to A-layer organization, membrane antagonist sensitivity and particularly to aerobic metabolism. Both mutants were drastically altered in the architecture of the 2D crystalline surface array (A-layer), although both were similar to wild-type with respect to cell surface composition. The slow-growing, antibiotic-resistant mutant differed significantly from the wild-type by the apparent loss of virtually all aerobic metabolism; the pseudo-revertant had partially recovered the ability to aerobically metabolize certain carbon sources. Both mutants were avirulent and incapable of tissue persistence. The rapidly-growing, antibiotic-sensitive pseudo-revertant, when administered either intraperitoneally or by immersion, effectively protected salmonid fish from challenge by a heterologous virulent stain suggesting its candidature as a live, attenuated furunculosis vaccine.

Purification and characterization of salmolysin, an extracellular hemolytic toxin from Aeromonas salmonicida

An extracellular hemolytic toxin of Aeromonas salmonicida, termed salmolysin, was purified 945-fold by ammonium sulfate precipitation, anion-exchange chromatography on DEAE-cellulose, and gel filtration chromatography on Sephadex G-100 and Sepharose 2B. Salmolysin appeared homogeneous upon cellulose acetate membrane electrophoresis and immunodiffusion analysis. The molecular weight of the toxin was estimated to be approximately 200,000 by the sedimentation equilibrium method. The UV absorption spectrum showed a maximum at 275 nm and a minimum at 262 nm. The isoelectric point was found to be at pI 5.4. Carbohydrate and protein analyses and other biochemical data indicated that salmolysin is a glycoprotein, containing approximately 62% carbohydrates. The toxin is a heat-labile substance and is stable at a neutral pH value. Ferrous ion inhibited the activity, whereas metal-chelating agents did not affect the activity. Sulfhydryl reagents did not inhibit the toxin, whereas reducing agents, such as L-cysteine and reduced glutathione, inhibited the toxin to a certain extent. Salmolysin was inactivated by a nonionic detergent but was stimulated by an anionic detergent, sodium deoxycholate, at a low concentration. The toxin was also inactivated by subtilisin and trypsin but was not inhibited by papain and pepsin. Salmolysin, with a remarkable hemolytic activity against salmonid erythrocytes, was lethal to rainbow trout when it was injected intramuscularly.

Identification of major common extracellular proteins secreted by Aeromonas salmonicida strains isolated from diseased fish

Ten different strains of Aeromonas salmonicida that were isolated from diseased fish were grown under identical conditions (24 h at 25 degree C) in 3% (wt/vol) tryptone soya broth medium supplemented with vitamins and inorganic ions. In each case the extracellular proteins that were formed were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and it was found that there were two significant common components, one with a molecular weight of 70,000 and the other with a weight of 56,000. Application of enzyme purification techniques to the supernatant fraction proteins of a culture of one of the strains resulted in the isolation of a 70-kilodalton (kDa) component, which was found to be a serine protease, and a 56-kDa component, which was hemolytic to trout erythrocytes. Rocket immunoelectrophoresis with rabbit antibodies to the isolated protease and hemolysin showed the same antigenic components in the supernatant fractions of all the cultures. These activities were assayed, and protease activity was found to vary by a factor of three, from 59 to 195 U/ml, while the range of hemolytic activity was over a narrow band, from 28 to 43 U/ml. There was an inconsistency between the immunoelectrophoretic and direct assay data in only one case. This indicated the presence of additional hemolytic activity, in addition to the 56-kDa component. The detection of large amounts of the same protease and hemolysin, two potent degradative activities, in a random series of strains of A. salmonicida suggests that they may be obligatory virulence factors in the development of furunculosis.

Adhesion of Aeromonas salmonicida strains associated with net electrostatic charges of host tissue cells

The adhesion of Aeromonas salmonicida, the pathogenic bacterium of fish furunculosis in salmon and trout, to the surface of host tissue cells was investigated with two fish tissue culture cell lines (RTG-2 cells from rainbow trout, Salmo gairdneri, and CHSE-214 cells from chinook salmon, Oncorhynchus tshawytscha) and four A. salmonicida strains. Bacterial cells of pathogenic strains were highly adhesive to RTG-2 and CHSE-214 cells and were negatively charged in the net electrostatic charges, as determined by electrophoresis on filter paper strips at pH 7, whereas bacterial cells of nonpathogenic strains were nonadhesive and positively charged. The electrophoresis of RTG-2 and CHSE-214 cells with balanced salt solution (BSS), phosphate-buffered saline, or fish serum diluted with BSS (pH 7) was carried out with an appropriate electrophoretic apparatus that was devised for this study. After electrophoresis with 20 mA of direct current for 15 min at pH 7, the electrophoretic dispositions of these tissue culture cells were determined by the mode of frequency of occurrence of these cells in the partitioned chambers of the device. RTG-2 and CHSE-214 cells with BSS and fish serum were attracted from the central chamber (to which each cell sample was added) to the cathode chambers, but no attraction was detected when these cells were used with phosphate-buffered saline. Noradrenaline- and phosphoenolpyruvate-pretreated RTG-2 cells migrated more to the cathode chambers, whereas succinate- and valine-pretreated RTG-2 cells moved to the anode chambers. These movements to the cathode and anode were alleviated by the use of RTG-2 cells preincubated with pathogenic and nonpathogenic bacterial cells, respectively. The adhesion of the pathogenic bacteria to RTG-2 cells was enhanced by the use of RTG-2 cells pretreated with noradrenaline and phosphoenolpyruvate, whereas the nonpathogenic bacteria were adherent to RTG-2 cells pretreated with succinate and valine. These findings indicate that the adhesion of A. salmonicida strains to host tissue cells is closely associated with mutually converse net electrostatic charges.

Does Aeromonas salmonicida affect the immune system of carp, Cyprinus carpio L.?

Aeromonas salmonicida is a significant bacterial pathogen of cyprinid and salmonid fishes causing the systemic disease furunculosis. Several observations led us to believe that A. salmonicida was able to evade or suppress the immune system of the fish: injection of whole bacteria or surface antigens was unsuccessful at protecting fish against lethal challenges; memory did not develop in survivors of sublethal infections; diseased fish often carried other opportunistic bacterial pathogens in addition to A. salmonicida, and serum protein and particularly immunoglobulin significantly decreased during A. salmonicida infections. We tested the ability of fish sublethally infected with virulent and avirulent A. salmonicida to mount a humoral immune response to sheep erythrocytes and found fewer plaque forming cells in the pronephros and lower serum anti-SRBC antibodies in infected fish as compared to controls. We also monitored the cellular immune response of diseased fish by skin allograft rejection and found an enhancement of the response that increased as the disease progressed. However, the extend of inflammation was reduced in infected fish as compared to non-infected animals. At this moment these preliminary observations are difficult to explain. Our future research will focus more specifically on cell populations that may be affected by A. salmonicida.

A comparison of the distribution of extracellular proteins produced by the protease-secreting organism Aeromonas salmonicida during aerobic and anaerobic growth

Aeromonas salmonicida was grown aerobically and anaerobically in supplemented 3% (w/v) tryptone soya broth medium for 24 h at 25 degrees C. Although the bacterial density achieved was 4.9 times higher in the aerobic culture, the exoprotein produced per unit of bacterial dry weight was only 1.9 times higher than in the anaerobic culture. However, the protease activity of the exoprotein showed a marked reduction anaerobically, being only one-tenth of that of the exoprotein produced aerobically. This finding was consistent with the differing SDS-PAGE patterns of the extracellular proteins from the two cultures, which also showed marked loss and reinforcement of other, as yet unidentified extracellular products.

Significance of Extracellular Protease for Growth of a Heterotrophic Bacterium, Aeromonas salmonicida

The role of protease produced by a heterotrophic bacterium during growth was investigated with Aeromonas salmonicida , the pathogen of fish furunculosis, strain A-7301 and its protease-deficient mutant NTG-1 induced by mutagenesis. Strain A-7301 produced extracellular protease in a mixed amino acid medium (composed of Gly, Ala, Val, Ile, Leu, Thr, Ser, Cys, Met, Phe, Tyr, Lys, Arg, Pro, His, Try, Asp, Asn, Glu, and Gln at equal concentrations of 0.1 g/liter). Its multiplication rate was limited by the amounts of amino acids present, whereas strain NTG-1 showed no protease production despite considerable growth similar to that of A-7301. There was no difference between A-7301 and NTG-1 in amino acid requirements for growth, and seven amino acids (Gly, Ala, Val, Thr, Cys, Met, and His) were found to be indispensable. A defined level of the mixed amino acids (0.4 to 0.5 g/liter) was needed for A-7301 to initiate a large production of protease. Neither of the strains grew well in a casein medium, to which no amino acids were added. However, when a protease fraction obtained from extracellular products of A-7301 by DEAE-cellulose column chromatography was added, NTG-1 successfully reproduced in the casein medium. These results indicate that the extracellular protease plays an important role in supplying A. salmonicida cells with available amino acids as nutrients and that higher growth is closely associated with protease production which stimulates further reproduction.

Purification and characterization of a new proteolytic enzyme produced by Aeromonas salmonicida

A proteolytic enzyme produced by the fish pathogen Aeromonas salmonicida was isolated and purified. It showed the following characteristics: temperature optimum at 48 degrees C, pH optimum at pH 9 and a molecular weight of 87500. The enzyme was inhibited by phenylmethanesulphonylfluoride (PMSF) indicating it to be a serine protease.

Effects of cultural conditions on protease production by Aeromonas hydrophila

Production of extracellular proteolytic activity by Aeromonas hydrophila was influenced by temperature, pH, and aeration. Conditions which produced maximal growth also resulted in maximal protease production. Enzyme production appeared to be modulated by an inducer catabolite repression system whereby NH4+ and glucose repressed enzyme production and complex nitrogen and nonglucose, carbon energy sources promoted it. Under nutritional stress, protease production was high, despite poor growth.