Flagellin A is essential for the virulence of Vibrio anguillarum

A flagellin gene from the fish pathogen Vibrio anguillarum was cloned, sequenced, and mutagenized. The DNA sequence suggests that the flaA gene encodes a 40.1-kDa protein and is a single transcriptional unit. A polar mutation and four in-frame deletion mutations (180 bp deleted from the 5' end of the gene, 153 bp deleted from the 3' end of the gene, a double deletion of both the 180- and 153-bp deletions, and 942 bp deleted from the entire gene) were made. Compared with the wild type, all mutants were partially motile, and a shortening of the flagellum was seen by electron microscopy. Wild-type phenotypes were regained when the mutations were transcomplemented with the flaA gene. Protein analysis indicated that the flaA gene corresponds to a 40-kDa protein and that the flagellum consists of three additional flagellin proteins with molecular masses of 41, 42, and 45 kDa. N-terminal sequence analysis confirmed that the additional proteins were flagellins with N termini that are 82 to 88% identical to the N terminus of FlaA. Virulence studies showed that the N terminal deletion, the double deletion, and the 942-bp deletion increased the 50% lethal dose between 70- and 700-fold via immersion infection, whereas infection via intraperitoneal injection showed no loss in virulence. In contrast, the polar mutant and the carboxy-terminal deletion mutant showed approximately a 10(4)-fold increase in the 50% lethal dose by both immersion and intraperitoneal infection. In summary, FlaA is needed for crossing the fish integument and may play a role in virulence after invasion of the host.

Vibrio anguillarum as a fish pathogen: virulence factors, diagnosis and prevention

Vibrio anguillarum, also known as Listonella anguillarum, is the causative agent of vibriosis, a deadly haemorrhagic septicaemic disease affecting various marine and fresh/brackish water fish, bivalves and crustaceans. In both aquaculture and larviculture, this disease is responsible for severe economic losses worldwide. Because of its high morbidity and mortality rates, substantial research has been carried out to elucidate the virulence mechanisms of this pathogen and to develop rapid detection techniques and effective disease-prevention strategies. This review summarizes the current state of knowledge pertaining to V. anguillarum, focusing on pathogenesis, known virulence factors, diagnosis, prevention and treatment.

Morphological characterization of small cells resulting from nutrient starvation of a psychrophilic marine vibrio

Upon starvation, Ant-300, a psychrophilic marine vibrio, was observed to decrease in size and change in shape from a rod to a coccus. After 3 weeks of starvation 50% of the starved population was able to pass through a filter with a pore size of 0.4 mum. Electron microscopy of thin sections of the small cells revealed normal cell structure except for an enlarged periplasmic space. When inoculated into a fresh medium, starved cells growth without a significant lag and regained "normal" size and shape within 48 h.

Dual flagellar systems enable motility under different circumstances

Flagella are extremely effective organelles of locomotion used by a variety of bacteria and archaea. Some bacteria, including Aeromonas, Azospirillum, Rhodospirillum, and Vibrio species, possess dual flagellar systems that are suited for movement under different circumstances. Swimming in liquid is promoted by a single polar flagellum. Swarming over surfaces or in viscous environments is enabled by the production of numerous peritrichous, or lateral, flagella. The polar flagellum is produced continuously, while the lateral flagella are produced under conditions that disable polar flagellar function. Thus at times, two types of flagellar organelles are assembled simultaneously. This review focuses on the polar and lateral flagellar systems of Vibrio parahaemolyticus. Approximately 50 polar and 40 lateral flagellar genes have been identified encoding distinct structural, motor, export/assembly, and regulatory elements. The sodium motive force drives polar flagellar rotation, and the proton motive force powers lateral translocation. Polar genes are found exclusively on the large chromosome, and lateral genes reside entirely on the small chromosome of the organism. The timing of gene expression corresponds to the temporal demand for components during assembly of the organelle: RpoN and lateral- and polar-specific sigma(54)-dependent transcription factors control early/intermediate gene transcription; lateral- and polar-specific sigma(28) factors direct late flagellar gene expression. Although a different gene set encodes each flagellar system, the constituents of a central navigation system (i.e., chemotaxis signal transduction) are shared.

Effect of transposon-induced motility mutations on colonization of the host light organ by Vibrio fischeri

Vibrio fischeri is found both as a free-living bacterium in seawater and as the specific, mutualistic light organ symbiont of several fish and squid species. To identify those characteristics of symbiosis-competent strains that are required for successful colonization of the nascent light organ of juvenile Euprymna scolopes squids, we generated a mutant pool by using the transposon Mu dI 1681 and screened this pool for strains that were no longer motile. Eighteen independently isolated nonmotile mutants that were either flagellated or nonflagellated were obtained. In contrast to the parent strain, none of these nonmotile mutants was able to colonize the juvenile squid light organ. The flagellated nonmotile mutant strain NM200 possessed a bundle of sheathed polar flagella indistinguishable from that of the wild-type strain, indicating that the presence of flagella alone is not sufficient for colonization and that it is motility itself that is required for successful light organ colonization. This study identifies motility as the first required symbiotic phenotype of V. fischeri.

Phenotypic evaluation of acapsular transposon mutants of Vibrio vulnificus

Translucent, avirulent spontaneous phase variants of Vibrio vulnificus MO6-24 reverted back to the original opaque, encapsulated phenotype under both in vivo and in vitro conditions. Two translucent, acapsular mutants, which did not show phase variation, were constructed by using the transposon Tn5 IS50L::phoA (TnphoA). Loss of capsule was accompanied by decreases in virulence, hydrophilicity, and serum resistance. The ability to utilize transferrin-bound iron for growth was lost in only one of the two unencapsulated mutants. Our data emphasize the apparent importance of capsule in the virulence of V. vulnificus and indicate that utilization of transferrin-bound iron is independent of encapsulation.

Relation of capsular materials and colony opacity to virulence of Vibrio vulnificus

Colonies which varied in opacity were isolated from the four strains of Vibrio vulnificus. Opaque and translucent colonial types of the strains were distinguished from the corresponding parent strains. Variation in the opacity of colonies formed by each strain was accompanied by variation of capsular material formation, which was clarified by electron microscopy of the organisms stained with ruthenium red. The opaque-type colonies of the strains had capsular materials. On the other hand, three translucent-type colonies had no observable capsular materials, and one had incomplete capsular materials, in contrast to the corresponding opaque type. The corresponding opaque and translucent types of the strains were compared for points of virulence in mice and guinea pigs. By having capsular materials, the bacterial strains acquired resistance to serum bactericidal action, antiphagocytic activity, high lethality for mice, and strong invasiveness in the subcutaneous tissue of guinea pigs. Capsular materials of V. vulnificus were considered to be important for the expression of virulence.

Chemotactic motility is required for invasion of the host by the fish pathogen Vibrio anguillarum

The role of the flagellum and motility in the virulence of the marine fish pathogen Vibrio anguillarum was examined. Non-motile mutants were generated by transposon mutagenesis. Infectivity studies revealed that disruption of the flagellum and subsequent loss of motility correlated with an approximate 500-fold decrease in virulence when fish were inoculated by immersion in bacteria-containing water. However, the flagellar filament and motility were not required for pathogenicity following intraperitoneal injection of fish. The transposon-insertion site for six mutants was determined by cloning and sequencing of the Vibrio DNA flanking the transposon. V. anguillarum genes whose products showed strong homology to proteins with an established role in flagellum biosynthesis were identified. One of the aflagellate mutants had a transposon insertion in the rpoN gene of V. anguillarum. This rpoN mutant failed to grow at low concentrations of available iron and was avirulent by both the immersion and intraperitoneal modes of inoculation. A chemotaxis gene, cheR, was located upstream of one transposon insertion and an in-frame deletion was constructed in the coding region of this gene. The resulting non-chemotactic mutant exhibited wild-type pathogenicity when injected intra-peritoneally into fish but showed a decrease in virulence similar to that seen for the non-motile aflagellate mutants following immersion infection. Hence, chemotactic motility is a required function of the flagellum for the virulence of V. anguillarum.

Membrane fatty acid and virulence changes in the viable but nonculturable state of Vibrio vulnificus

The nonculturable state of Vibrio vulnificus and, for comparison, that of Escherichia coli were studied in artificial-seawater microcosms at 5 degrees C. Total cell counts were monitored by acridine orange epifluorescence, metabolic activity by direct viable counts, and culturability by plate counts on selective and nonselective media. Whereas total counts remained constant, plate counts of V. vulnificus suggested nonculturability by day 24. In contrast, direct viable counts indicated significant cell viability throughout 32 days of incubation. As an indication of the metabolic changes that occurred as cells entered the state of nonrecoverability, membrane fatty acid analyses were performed. At the point of nonculturability of V. vulnificus, the major fatty acid species (C16 and C16:1) had decreased 57% from the T0 level, concomitant with the appearance of several short-chain acids. Although the bacteria were still recoverable, a similar trend was observed with E. coli. Electron microscopy of nonculturable V. vulnificus showed that the cells were rounded and reduced in size and contained fewer ribosomes. Mouse infectivity studies conducted with these cells suggested loss of virulence.

Adsorption kinetics of laterally and polarly flagellated Vibrio

The adsorption of laterally and polarly flagellated bacteria to chitin was measured, and from the data obtained, a modified Langmuir adsorption isotherm was derived. Results indicated that the adsorption of laterally flagellated Vibrio parahaemolyticus follows the Langmuir adsorption isotherm, a type of adsorption referred to as surface saturation kinetics, when conditions are favorable for the production of lateral flagella. When conditions were not favorable for the production of lateral flagella, bacterial adsorption did not follow the Langmuir adsorption isotherm; instead, proportional adsorption kinetics were observed. The adsorption of some polarly flagellated bacteria exhibited surface saturation kinetics. However, the binding index (the product of the number of binding sites and bacterial affinity to the surface) of polarly flagellated bacteria differed significantly from that of laterally flagellated bacteria, suggesting that polarly flagellated bacteria adsorb to chitin by a different mechanism from that used by the laterally flagellated bacteria. From the results of dual-label adsorption competition experiments, in which polarly flagellated V. cholerae competed with increasing concentrations of laterally flagellated V. parahaemolyticus, it was observed that laterally flagellated bacteria inhibited the adsorption of polarly flagellated bacteria. In contrast, polarly flagellated bacteria enhanced the adsorption of V. cholerae. In competition experiments, where V. parahaemolyticus competed against increasing concentrations of other bacteria, polarly flagellated bacteria enhanced V. parahaemolyticus adsorption significantly, whereas laterally flagellated bacteria only slightly enhanced the process. The direct correlation observed between surface saturation kinetics, the production of lateral flagella, and the ability of laterally flagellated bacteria to inhibit the adsorption of polarly flagellated bacteria suggests that lateral flagella represent a component of bacterial structure that is important in the adsorption of laterally flagellated bacteria to surfaces. A model for adsorption events of laterally flagellated bacteria is proposed, based on the evidence presented.

The type IV leader peptidase/N-methyltransferase of Vibrio vulnificus controls factors required for adherence to HEp-2 cells and virulence in iron-overloaded mice

Vibrio vulnificus expresses a number of potential virulence determinants that may contribute to its ability to cause a severe and rapidly disseminating septicemia in susceptible hosts. We have cloned and characterized two genes encoding products related to components of the type IV pilus biogenesis and general secretory (type II) pathways by complementation of a type IV peptidase/N-methyltransferase (PilD) mutant of Pseudomonas aeruginosa with a V. vulnificus genomic library. One of the genes (vvpD) encodes a protein homologous to PilD and other members of the type IV peptidase family that completely restores this activity in a P. aeruginosa mutant deficient in the expression of PilD. The other gene (vvpC) encodes a homolog of PilC from P. aeruginosa, where it is essential for assembly of type IV pili. Phenotypic characterization of a V. vulnificus vvpD mutant, constructed by allelic exchange, showed that VvpD is required for the expression of surface pili, suggesting that the pili observed on V. vulnificus are of the type IV class. This mutant was also unable to secrete at least three extracellular degradative enzymes, and the localization of one of these (the cytolysin/hemolysin) to the periplasmic space indicates that these proteins are normally exported via the type II secretion pathway. Loss of VvpD resulted in significant decreases in CHO cell cytotoxicity, adherence to HEp-2 cells, and virulence in a mouse model. Capsule formation and serum resistance were not affected in the vvpD mutant, indicating that in addition to capsule, virulence of V. vulnificus requires type IV pili and/or extracellular secretion of several exoenzymes.

The orientation of the substrate sites of formate dehydrogenase and fumarate reductase in the membrane of Vibrio succinogenes

Formate dehydrogenase and fumarate reductase are involved in the electron transport phosphorylation system of Vibrio succinogenes. The orientation of the active sites of these enzymes in the cytoplasmic membrane of the bacterium was investigated with the aim of elucidating the mechanism of energy transduction. This was done by measuring the accessiblities of the enzymes to substrates, dyes and inhibitors both in cells and in cell-derived particles obtained with the French press. 1. After treatment of the cells with lysozyme and EDTA, followed by fractionation, both enzymes were found exclusively in the membranous fractions, while the periplasmic as well as the cytoplasmic fractions were devoid of both of the enzymic activities. 2. The sites of dye interaction of fumarate reductase were inaccessible to non-permeant dyes in cells, but were fully accessible in French-press particles. The Km for succinate as measured with the permeant methylene blue as acceptor was increased ten fold on lysis of the cells. The Km measured in the particles was similar to that of lysed cells and was not altered by lysis. 3. The rates of formate oxidation in the presence of non-permeant dyes and the Km for formate were unaffected by cell lysis. On lysis of French-press particles, formate oxidation with both permeant and non-permeant acceptors was increased about three fold. The extent of stimulation was not altered by inhibition of the enzymic activities. 4. Succinate oxidation by particles was fully inhibited by 4-chloromercuriphenyl sulfonate whereas that of cells was fully resistant. Formate dehydrogenase in cells was inhibited by 4-diazophenyl sulfonate when added together with formate. This compound also inhibited the enzyme in the particles when given in the absence of a lytic agent or after its addition. However, most of the enzyme remained active if the inhibitor was added before the lytic agent. 5. Fumarate and succinate were actively taken up by the cells from the medium, while formate did not even penetrate through the membrane of the bacteria. 6. It is concluded that the substrate and the dye-reactive sites of formate dehydrogenase face the outside, while those of fumarate reductase face the inside of the cytoplasmic membrane of cells of V. succinogenes. About 70% of the French-press particles were inverted with respect to the substrate and dye sites of the enzymes. The membrane is impermeable to formate and does not contain a specific transporter. The electrogenic liberation of protons on the outisde and the uptake of protons from the cytoplasm of the bacteria, which accompany electron transport, can be explained on the basis of the orientation of the substrate sites of the enzymes without net transport of protons across the membrane.

Vibrio cyclotrophicus sp. nov., a polycyclic aromatic hydrocarbon (PAH)-degrading marine bacterium

Strain P-2P44T was isolated from creosote-contaminated marine sediments by using a most-probable number procedure in which phenanthrene was the sole carbon and energy source. Growth experiments showed that P-2P44T utilized several two- and three-ring polycyclic aromatic hydrocarbons (PAHs) as substrates, including naphthalene, 2-methylnaphthalene and phenanthrene. Additionally, gas-chromatography experiments showed that P-2P44T degraded several other PAHs, though it was unable to use them as sole sources of carbon and energy. Phylogenetic analyses confirmed that strain P-2P44T is a member of the genus Vibrio, most closely related to Vibrio splendidus. However, strain P-2P44T shared only 98.3% 16S rDNA identity and 35% DNA-DNA reassociation with the type strain of V. splendidus. Strain P-2P44T differed phenotypically from V. splendidus. Together, these differences indicated that strain P-2P44T represents a novel species in the genus Vibrio, for which the name Vibrio cyclotrophicus sp. nov. is proposed; the type strain is P-2P44T (= ATCC 700982T = PICC 106644T).

Production of poly-β-hydroxybutyrate (PHB) by Vibrio spp. isolated from marine environment

Bacteria isolated from marine sediments were screened for their ability to accumulate polyhydroxyalkanoates. Among the isolates, four Vibrio spp. (strain M11, M14, M20 and M31) were studied in detail. All synthesized intracellular lipid inclusions during growth on diverse carbon sources including acetate, glycerol, succinate, glucose and sucrose. The inclusions were identified to be poly-beta-hydroxybutyrate (PHB) using gas chromatography and nuclear magnetic resonance analysis. No other type of polyhydroxyalkanoates (PHAs) was found to be accumulated by these marine isolates, suggesting that the diversity of PHAs produced in marine environments may be not as versatile as found in other environments. Strain M11 accumulated PHB in concentrations as high as 41% of cell dry weight when grown in medium containing 4% of sodium chloride. One of the Vibrio spp. was identified to be closely related to Vibrio natriegens (98% identity) by partial 16S rDNA sequence homology. V. natriegens has the shortest generation time (9.8 min) of any bacterium and this characteristic may be an exploitable trait for the industrial production of PHB.

Phenotypic and genotypic characterization of Vibrio viscosus sp. nov. and Vibrio wodanis sp. nov. isolated from Atlantic salmon (Salmo salar) with 'winter ulcer'

Two groups of Vibrio strains isolated from Atlantic salmon with 'winter ulcer' were characterized phenotypically and genotypically. The data obtained indicated that each of the two groups represented a new species in the genus Vibrio. The names Vibrio viscosus sp. nov. [type strain NVI 88/478T (= NCIMB 13584T)] and Vibrio wodanis sp. nov. [type strain NVI 88/441T (= NCIMB 13582T)] are proposed for the new species. V. viscosus strains exhibited a similar total DNA RFLP pattern and a similar plasmid DNA profile. DNA relatedness (hydroxyapatite method) of the V. viscosus type strain to nine other V. viscosus strains was 81-93% at 60 degrees C. Divergence within related sequences was 0.0-1.5% and relatedness at 75 degrees C was 74-100%. V. wodanis strains exhibited marked heterogeneity on the basis of RFLP analysis and plasmid profiles. DNA relatedness of the V. wodanis type strain to 10 other V. wodanis strains was 66-94% at 60 degrees C. Divergence within related sequences was 0.0-1.5% and relatedness at 75 degrees C was 55-97%. Relatedness between V. viscosus and V. wodanis type strains was approximately 20%. Among other Vibrio species, the closest relative of V. viscosus was Vibrio marinus (ATCC 15381T) (43% relatedness at 60 degrees C) and that of V. wodanis was Vibrio logei (ATCC 15382) (57% relatedness at 60 degrees C). These same pairs were the closest phenotypic relatives. DNA sequence analysis of the 16S rRNA gene of V. viscosus indicated an intimate relationship to V. marinus. A total evaluation of the results, however, supports V. viscosus to be a separate species in the genus Vibrio. The analysis of the sequence of the 16S rRNA gene of V. wodanis supports that V. logei (ATCC 15382) was the most related species. Ability to degrade casein, oxidative production of acid from trehalose and production of lysine decarboxylase are important biochemical tests that will differentiate between V. viscosus, V. wodanis, V. marinus (ATCC 15381T) and V. logei (ATCC 15382).

Vibrio halioticoli sp. nov., a non-motile alginolytic marine bacterium isolated from the gut of the abalone Haliotis discus hannai

Six alginolytic, facultatively anaerobic, non-motile marine bacteria were isolated from the gut of abalone Haliotis discus hannai. DNA-DNA hybridization data showed that the six strains constituted a single genospecies. Phylogenetic analyses of 16S rDNA sequences indicated that the isolates should be assigned to the genus Vibrio. The phenotypic features of the isolates were closely related to Vibrio fischeri and Vibrio pelagius biovar I, but 13 traits (motility, luminescence, alginase production, lipase production, lysine decarboxylase, indole production, growth in 1 and 6% NaCl and assimilation of five carbon compounds) distinguished these strains from V. fischeri, and 17 traits (motility, growth at 37 degrees C, lipase production, indole production, growth in 1 and 6% NaCl, acid from sucrose and D-sorbitol, and assimilation of nine carbon compounds) distinguished these strains from V. pelagius. The G + C content of the isolates was 41.6-43.1 mol%. According to DNA-DNA hybridization data and 16S rDNA phylogenetic analyses, it was concluded that the six isolates constitute a new species different from any other Vibrio species. The name Vibrio halioticoli sp. nov. (type strain IAM 14596T) is proposed. A set of phenotypic features which enables differentiation of the new species from other species of the Vibrionaceae family is described.

Epibiotic Vibrio luminous bacteria isolated from some hydrozoa and bryozoa species

Luminous bacteria are isolated from both Hydrozoa and Bryozoa with chitinous structures on their surfaces. All the specimens of the examined hydroid species (Aglaophenia kirchenpaueri, Aglaophenia octodonta, Aglaophenia tubiformis, Halopteris diaphana, Plumularia setacea, Ventromma halecioides), observed under blue light excitation, showed a clear fluorescence on the external side of the perisarc (chitinous exoskeleton) around hydrocladia. In the bryozoan Myriapora truncata, luminous bacteria are present on the chitinous opercula. All the isolated luminous bacteria were identified on the basis of both phenotypic and genotypic analysis. The isolates from A. tubiformis and H. diaphana were unambiguously assigned to the species Vibrio fischeri. In contrast, the isolates from the other hydroids, phenotypically assigned to the species Vibrio harveyi, were then split into two distinct species by phylogenetic analysis of 16S rRNA gene sequences and DNA-DNA hybridization experiments. Scanning electron microscopy analysis and results of culture-based and culture-independent approaches enabled us to establish that luminous vibrios represent major constituents of the bacterial community inhabiting the A. octodonta surface suggesting that the interactions between luminous bacteria and the examined hydrozoan and bryozoan species are highly specific. These interactions might have epidemiological as well as ecological implications because of the opportunistic pathogenicity of luminous Vibrio species for marine organisms and the wide-distribution of the hydrozoan and bryozoan functioning as carriers.

Mouse skin damage caused by cytolysin from Vibrio vulnificus and by V. vulnificus infection

Light and electron microscopy of mouse skin damage caused by intradermal infection with a virulent strain of Vibrio vulnificus and by a single intradermal injection of the cytolytic toxin produced by the bacterium revealed similar structural alterations. The epidermis was intact; however, the infection and toxin produced acute cellulitis characterized by extensive extracellular edema; disorganization of collagen bundles; large accumulations of cell debris and plasma proteins; damaged or necrotic fat cells, capillary endothelial cells, and muscle cells; and mild inflammatory cell infiltration. The virulent strain of V. vulnificus produced a capsule and was resistant to phagocytosis in vivo, whereas a weakly virulent strain of the bacterium did not produce a capsule and was readily phagocytized and digested. Factors that may be important in the pathogenesis of V. vulnificus wound infections include a capsule that inhibits phagocytosis and an extracellular cytolytic toxin that is responsible, at least in part, for the severe tissue damage characteristic of such wound infections.

Sampling the light-organ microenvironment of Euprymna scolopes: description of a population of host cells in association with the bacterial symbiont Vibrio fischeri

The symbiosis between the squid Euprymna scolopes and the luminous bacterium Vibrio fischeri has a pronounced diel rhythm, one component of which is the venting of the contents of the light organ into the surrounding seawater each day at dawn. In this study, we explored the use of this behavior to sample the microenvironment of the light-organ crypts. Intact crypt contents, which emerge from the lateral pores of the organ as a thick paste-like exudate, were collected from anesthetized host animals that had been exposed to a light cue. Microscopy revealed that the expelled material is composed of a conspicuous population of host cells in association with the bacterial symbionts, all of which are embedded in a dense acellular matrix that strongly resembles the bacteria-based biofilms described in other systems. Assays of the viability of expelled crypt cells revealed no dead bacterial symbionts and a mixture of live and dead host cells. Analyses of the ultrastructure, biochemistry, and phagocytic activity of a subset of the host cell population suggested that some of these cells are macrophage-like molluscan hemocytes.

Differential expression of Vibrio vulnificus capsular polysaccharide

Vibrio vulnificus is a human pathogen whose virulence has been associated with the expression of capsular polysaccharide (CPS). Multiple CPS types have been described; however, virulence does not appear to correlate with a particular CPS composition. Reversible-phase variation for opaque and translucent colony morphologies is characterized by changes in CPS expression, as suggested by electron microscopy of cells stained nonspecifically with ruthenium red. Isolates with opaque colony morphologies are virulent and appear to be more thickly encapsulated than naturally occurring translucent-phase variants, which have reduced, patchy, or absent CPS. Previously, we have shown that the virulence of translucent-phase variants was intermediate between opaque-phase variants and acapsular transposon mutants, suggesting a correlation between virulence and the amount of CPS expressed. In the present study, CPS expression of phase variants and genetically defined mutants of V. vulnificus M06-24/O was examined by using a CPS-specific monoclonal antibody with an enzyme-linked immunosorbent assay, flow cytometry, and immunoelectron microscopy. Semiquantitative analyses of CPS expression correlated well among these assays, confirming that the translucent-phase variant was intermediate in CPS expression and retained type I CPS-specific epitopes. Cell surface expression of CPS varied with the growth phase, increasing during logarithmic growth and declining in stationary culture. Significantly greater CPS expression (P = 0.026) was observed for cells grown at 30 degrees C than for those at 37 degrees C. These studies confirm that phase variation and virulence in V. vulnificus correlate with the amount of CPS expressed and demonstrate the fluidity of bacterial polysaccharide expression in response to environmental conditions.

Histological changes in intestine of Atlantic salmon (Salmo salar L.) following in vitro exposure to pathogenic and probiotic bacterial strains

Furunculosis and vibriosis are diseases that cause severe economic losses in the fish-farming industry. The foregut of the Atlantic salmon (Salmo salar L.) was exposed in vitro to two fish pathogens, Aeromonas salmonicida (causative agent of furunculosis) and Vibrio anguillarum (causative agent of vibriosis), and to one probiotic strain, Carnobacterium divergens, at 6 x 10(4) or 6 x 10(6) viable bacteria per milliliter. Histological changes following bacterial exposure were assessed by light and electron microscopy. Control samples (foregut exposed to Ringer's solution only) and samples exposed only to C. divergens had a similar appearance to intact intestinal mucosal epithelium, with no signs of damage. However, exposure of the foregut to the pathogenic bacteria resulted in damaged epithelial cells, cell debris in the lumen, and disorganization of the microvilli. Co-incubation of the foregut with a pathogen and C. divergens did not reverse the damaging effects caused by the pathogen, although these were alleviated when probiotic bacteria were used. Based on these results, we suggest that the probiotic bacterium, C. divergens, is able to prevent, to some extent, pathogen-induced damage in the Atlantic salmon foregut.

Presence of a capsule in Vibrio vulnificus biotype 2 and its relationship to virulence for eels

Strains of Vibrio vulnificus biotype 2, isolated from internal organs of diseased European eels as pure cultures of opaque cells, together with some reference strains from Japanese eels, were used in this study. Spontaneous translucent-phase variants were obtained from the corresponding parent strains and compared for a variety of phenotypic traits related to virulence for eels. The rate of colony dissociation from opaque to translucent cells was higher (around 10(-2)) than that observed for translucent to opaque cells (10(-3) to 10(-4)). Electron microscopy with ruthenium red revealed the presence of a capsule of variable thickness on opaque cells, whereas translucent-type colonies had no observable capsular materials. No differences in plasmid profiles were detected between the two cell types so that plasmids do not seem to be implicated in the mechanism of phase shift of biotype 2 strains. No apparent difference in outer membrane protein and lipopolysaccharide patterns could be observed between the cell types. Both isogenic morphotypes were able to grow in eel serum and minimal medium supplemented with ethylenediamine di(O-hydroxyphenyl-acetic acid) or transferrin. Therefore, the presence of capsule was not required for the acquisition of iron from iron chelators or for resistance to serum bactericidal action. Both morphotypes were highly virulent for elvers, although the 50% lethal dose for translucent cells was higher than that for the corresponding opaque cells. The latter observation, together with the overall data, suggests that the production of capsular materials by biotype 2 of V. vulnificus is not essential for the development of vibriosis in eels, at least when cells are injected intraperitoneally.

A comparative study of antimicrobial properties of crustinPm1 and crustinPm7 from the black tiger shrimp Penaeus monodon

Several isoforms of crustin have been identified in the black tiger shrimp Penaeus monodon. These cationic cysteine-rich antimicrobial peptides contain a single whey acidic protein (WAP) domain at the C-terminus and exhibit antimicrobial activity against both Gram-positive and Gram-negative bacteria. In this paper, we investigate the binding properties and antimicrobial actions of crustinPm1 and crustinPm7, the two most abundant crustin isoforms found in the haemocyte of P. monodon. Previously, crustinPm1 showed strong inhibition against Gram-positive bacteria, whilst crustinPm7 acted against both Gram-positive and Gram-negative bacteria. A binding study showed that both crustins can bind to Gram-positive and Gram-negative bacterial cells. Enzyme-linked immunosorbent (ELISA) assay suggested that crustins bind to the cell wall components, lipoteichoic acid (LTA) and lipopolysaccharide (LPS) with positive cooperativity of Hill slope (H)>2. This indicates that at least two molecules of crustins interact with one LTA or LPS molecule. In addition, both crustins can induce bacterial agglutination and cause inner membrane permeabilization in Escherichia coli. Scanning Electron Microscopy (SEM) revealed the remarkable change on the cell surface of Staphylococcus aureus, Vibrio harveyi and E. coli after the bacteria were treated with the recombinant crustinPm7. Meanwhile, crustinPm1 can cause a visible change on the cell surface of S. aureus and E. coli only. This is in agreement with the fact that crustinPm1 has shown no antimicrobial activity against V. harveyi. It is likely that the antimicrobial activity of crustins mainly relies on their ability to agglutinate bacterial cells and to disrupt the physiochemical properties of bacterial surface.