Capsulated cells of Aeromonas salmonicida grown in vitro have different functional properties than capsulated cells grown in vivo

Influence of external bacterial structures on the efficiency of photodynamic inactivation by a cationic porphyrin

The main targets of photodynamic inactivation (PDI) are the external bacterial structures, cytoplasmic membrane and cell wall. In this work it was evaluated how the external bacterial structures influence the PDI efficiency. To reach this objective 8 bacteria with distinct external structures were selected; 4 Gram-negative bacteria (Escherichia coli, with typical Gram-negative external structures; Aeromonas salmonicida, Aeromonas hydrophila both with an S-layer and Rhodopirellula sp., with a peptidoglycan-less proteinaceous cell wall and with cytoplasm compartmentalization) and 4 Gram-positive bacteria (Staphylococcus aureus, with typical Gram-positive external structures; Truepera radiovictrix, Deinococcus geothermalis and Deinococcus radiodurans, all with thick cell walls that give them Gram-positive stains, but including a second complex multi-layered membrane and structurally analogous to that of Gram-negative bacteria). The studies were performed in the presence of 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin tetraiodide (Tetra-Py(+)-Me) at 5.0 μM with white light (40 W m(-2)). The susceptibility of each bacteria to PDI by Tetra-Py(+)-Me was dependent on bacteria external structures. Although all Gram-positive bacteria were inactivated to the detection limit (reduction of ∼8 log) after 60-180 min of irradiation, the inactivation followed distinct patterns. Among the Gram-negative bacteria, E. coli was the only species to be inactivated to the detection limit (∼8 log after 180 min). The efficiency of inactivation of the two species of Aeromonas was similar (reduction of ∼5-6 log after 270 min). Rhodopirellula was less susceptible (reduction of ∼4 log after 270 min). As previously observed, the Gram-positive bacteria are more easily inactivated than Gram-negative strains, and this is even true for T. radiovictrix, D. geothermalis and D. radiodurans, which have a complex multi-layered cell wall. The results support the theory that the outer cell structures are major bacterial targets for PDI. Moreover, the chemical composition of the external structures has a stronger effect on PDI efficiency than complexity and the number of layers of the external coating, and lipids seem to be an important target of PDI.

Aeromonas flagella and colonisation mechanisms

Aeromonas species are inhabitants of aquatic environments and are able to cause disease in humans and fish among other animals. In aquaculture, they are responsible for the economically important diseases of furunculosis and motile Aeromonas septicaemia (MAS). Whereas gastroenteritis and wound infections are the major human diseases associated with the genus. As they inhabit and survive in diverse environments, aeromonads possess a wide range of colonisation factors. The motile species are able to swim in liquid environments through the action of a single polar flagellum, the flagellin subunits of which are glycosylated; although essential for function the biological role of glycan addition is yet to be determined. Approximately 60% of aeromonads possess a second lateral flagella system that is expressed in viscous environments for swarming over surfaces; both flagellar systems have been shown to be important in the initial colonisation of surfaces. Subsequently, other non-flagellar colonisation factors are employed; these can be both filamentous and non-filamentous. The aeromonads possess a number of fimbrial systems with the bundle-forming MSHA type IV pilus system, having a major role in human cell adherence. Furthermore, a series of outer-membrane proteins have also been implicated in the aeromonad adhesion process. A number of strains are also capable of cell invasion and that maybe linked with the more invasive diseases of bacteraemia or wound infections. These strains employ cell surface factors that allow the colonisation of these niches that protect them from the host's immune system such as S-layers, capsules or particular lipopolysaccharides.

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.

A C1q-binding 40kDa porin from Aeromonas salmonicida: Cloning, sequencing, role in serum susceptibility and fish immunoprotection

A 40 kDa C1q-binding outer membrane protein from Aeromonas salmonicida was identified by direct-binding assay with biotinylated C1q, a subcomponent of the complement classical pathway component C1. The 40 kDa porin structural gene from the A450 A. salmonicida typical strain (A+:O+) was cloned in Escherichia coli and sequenced. The amino acid sequence of the 40 kDa A. salmonicida porin, its ability to bind C1q in an antibody independent process, and its immunological cross-reaction with the A. hydrophila AH-3 porin II, allow us to determine the role of this protein in serum susceptibility. Furthermore, we obtained defined A. salmonicida 40 kDa porin insertion mutants in serum sensitive or resistant strains, and we complemented these mutants with a plasmid harboring only the 40 kDa porin gene from A. salmonicida A450 in order to define its role as an important surface molecule involved in serum susceptibility and C1q binding. Similar complementation was obtained using the A. hydrophila AH-3 porin II gene. The 40 kDa porin gene and/or protein was present in all the A. salmonicida typical or atypical strains tested. Furthermore, the A. hydrophila AH-3 porin II seems to be an important molecule for fish immunoprotection against either A. salmonicida or A. hydrophila strains.

Modulation of the immune response to an Aeromonas hydrophila aroA live vaccine in rainbow trout: effect of culture media on the humoral immune response and complement consumption

The Aeromonas hydrophila aroA is an attenuated strain that has been assessed as a live vaccine in rainbow trout, Oncorhynchus mykiss. In this study the effects of different culture media used to grow the strain on its survival after in vitro exposure to rainbow trout serum, and on its immunogenicity in rainbow trout were compared. Four culture media were tested: Luria broth (LB), Luria broth with 0.25% glucose, trypticase soy broth (TSB), and brain-heart infusion broth (BHIB). Bacteria grown in culture media with glucose (TSB, BHIB and LB with 0.25% glucose) showed reduced complement consumption and a lower serum susceptibility. O. mykiss vaccinated with inocula prepared with BHIB- and LB-grown aroA cells resuspended in phosphate-buffered saline (PBS) showed higher and longer-lasting serum agglutinating antibody titres than those vaccinated with TSB-grown bacteria. Thus, a direct relationship between serum resistance and immunogenicity could not be established, but BHIB and LB culture media were the most effective in increasing the immunogenicity of the A. hydrophila aroA vaccine.

Temperature dependent activation of leucocyte populations of rainbow trout, Oncorhynchus mykiss, after intraperitoneal immunisation with Aeromonas salmonicida

The temperature dependence of in vivo activation of rainbow trout Oncorhynchus mykiss, leucocyte populations after intraperitoneal injection (i.p.) of fish with a T-cell independent antigen Aeromonas salmonicida (strain MT423) was investigated using a proliferation assay and flow cytometric analysis with mab specific for trout leucocyte surface markers. In trout kept at 15-17 degrees C a prominent activation of blood and spleen leucocytes was found. Also, drastic changes of the percentage of the leucocyte populations in blood and spleen occurred: the amount of monocytes in the blood increased between day 2 and day 7 post injection (p.i.), whereas in spleen the amount of monocytes stayed at a high level (approximately 35%) after a depression between day 4 and day 7 p.i. The percentage of B-lymphocytes was increased first in spleen and then in blood. The percentage of granulocytes in blood was elevated during the whole experiment compared to control fish. In trout kept at 10-12 degrees C only blood leucocytes showed a weak activation after i.p. injection of A. salmonicida, whereas spleen leucocytes showed nearly no reaction. Only the percentage of granulocytes in the blood (day 2-14 p.i.) and of monocytes in the spleen (day 2 and day 8 p.i.) was changed compared to phosphate buffered saline (PBS)-injected fish. However, the development of A. salmonicida specific antibodies was contrary to the cellular reaction. Whereas antibodies could first be detected after 16-18 days p.i. in both groups the amount of antibodies was significantly higher in sera of trout kept at 10-12 degrees C at day 22 and day 28 p.i. than in sera of trout kept at 15-17 degrees C. These results indicate stronger A. salmonicida induced activation of monocytes, granulocytes and B-lymphocytes at higher temperature. However, the development of a specific antibody response against A. salmonicida seemed to be more effective at lower temperatures.

Host cell invasion and intracellular residence by Aeromonas salmonicida: role of the S-layer

Virulent strains of the fish pathogen Aeromonas salmonicida, which have surface S-layers (S+), efficiently adhere to, enter, and survive within macrophages. Here we report that S+ bacteria were 10- to 20-fold more adherent to non-phagocytic fish cell lines than S-layer-negative (S-) mutants. When reconstituted with exogenous S-layers, these S- mutants regained adherence. As well, latex beads coated with purified S-layers were more adherent to fish cell lines than uncoated beads, or beads coated with disorganized S-layers, suggesting that purified S-layers were sufficient to mediate high levels of adherence, and that this process relied on S-layer structure. Gentamicin protection assays and electron microscopy indicated that both S+ and S- A. salmonicida invaded non-phagocytic fish cells. In addition, these fish cells were unable to internalize S-layer-coated beads, clearly suggesting that the S-layer is not an invasion factor. Lipopolysaccharide (which is partially exposed in S+ bacteria) appeared to mediate invasion. Surprisingly, A. salmonicida did not show net growth inside fish cells cultured in the presence of gentamicin, as determined by viable bacterial cell counts. On the contrary, bacterial viability sharply decreased after cell infection. We thus concluded that the S-layer is an adhesin that promotes but does not mediate invasion of non-phagocytic fish cell lines. These cell lines should prove useful in studies aimed at characterizing the invasion mechanisms of A. salmonicida, but of limited value in studying the intracellular residence and replication of this invasive bacterium in vitro.

Structural and physiological determinants of resistance ofAeromonas salmonicidato reactive radicals

The facultative intracellular pathogen Aeromonas salmonicida survives and replicates in macrophages, a virulence trait presumed to be associated with its ability to resist reactive radicals. The mechanisms used by A. salmonicida to resist reactive radicals in vitro were shown to have both structural and physiological determinants. The sensitivity of A. salmonicida to exogenous H2O2, superoxide, and nitrogen radicals, as well as endogenous oxygen radicals, differed depending on growth conditions, cell surface structure, and preexposure to sublethal doses of radicals. Whereas sensitivities to exogenous oxygen radicals did not correlate with basal levels of catalase or Fe-superoxide dismutase, under similar culture conditions S-layer positive cells were more resistant to oxygen radicals than S-layer mutants. S-layer mutants recovered resistance when physically reconstituted with S-layer sheets. Hemin-coated S-layers, while protective against nitrogen radicals, sensitized A. salmonicida to H2O2. Sublethal concentrations of H2O2or superoxide induced a highly protective response characterized by de novo synthesis of both catalase and Mn-superoxide dismutase. It is proposed that for A. salmonicida the constitutive S-layer provides a first line of defense and the inducible catalase and Mn-superoxide dismutase provide a powerful second line of defense against macrophage-mediated killing via reactive oxygen species.Key words: Aeromonas salmonicida, oxygen radicals, nitrogen radicals, oxidative stress, S-layers.

Haemophilus somnus immunoglobulin binding proteins and surface fibrils

The high-molecular-weight (HMW) immunoglobulin binding proteins (IgBPs) of Haemophilus somnus and a 76-kDa surface protein (p76) are found in serum-resistant virulent strains but not in several serum-sensitive strains from asymptomatic carriers. For the first time, p76 was shown to be an IgBP also. This was done by competitive inhibition studies with affinity-purified antidinitrophenol (anti-DNP) and DNP to ensure that binding was not antigen specific. The HMW IgBPs, but not the p76 IgBP, were partially purified from concentrated culture supernatant in detergent by fluid-phase liquid chromatography with a gel filtration column. Membrane extraction studies showed that p76 predominated in the Sarkosyl-soluble fraction of the bacterial cell pellet. Since integral outer membrane (OM) proteins are Sarkosyl insoluble, this is consistent with our previous finding that implicated p76 as a peripheral OM protein. The HMW IgBPs were found predominantly in the Sarkosyl-soluble fraction of the culture supernatant. This suggests that they were not integral membrane proteins and that their presence in the supernatant was not due to OM blebbing. We then showed that two IgBP-positive serum-resistant virulent strains have a surface fibrillar network but that two IgBP-negative serum-sensitive H. somnus strains from asymptomatic preputial carriers do not. Fibrils on the surfaces of IgBP+ strains bound gold-labelled bovine immunoglobulin G2 (IgG2) anti-DNP, indicating that these fibrils have IgG2 binding activity. Therefore, this study shows that H. somnus has two IgBPs, including a peripheral membrane protein and a fibrillar surface network.

Complement resistance of capsulated strains of Aeromonas salmonicida

The complement resistance of Aeromonas salmonicida strains grown under conditions promoting capsule formation was investigated using well characterized strains and their isogenic mutants. Complement resistance was previously studied using the same strains growing under non-capsulating conditions. The serum resistant strains were found to activate complement, but rapidly degrade C3b preventing productive formation of the lytic complex C5b-9. Isogenic lipopolysaccharide rough mutants grown under non-capsulating conditions were serum sensitive, binding a large amount of C3b and leading to productive formation of C5b-9. When grown under conditions promoting capsule formation, these mutants were partially resistant to complement because less C3b is bound to them and also partially degraded, with a concomitant reduction in lytic C5b-9.

The role of the capsular polysaccharide of Aeromonas salmonicida in the adherence and invasion of fish cell lines

The ability of several Aeromonas salmonicida strains grown under different conditions (capsulated and non-capsulated) to adhere to and invade two fish cell lines was compared. The level of adherence was slightly higher when the strains were grown under conditions promoting capsule formation than when the same strains were grown under conditions which did not promote capsule formation. However, the most significant difference among the wild-type strains grown under conditions promoting capsule formation was the ability to invade fish cell lines, which was significantly higher than when the same strains were grown under conditions which did not promote capsule formation. From these results we conclude that the capsular polysaccharide, in these strains, is an important factor for intracellular invasion.