Hemolysins Produced byVibrios

Outbreak of vibriosis associated with Vibrio parahaemolyticus in the mud crab Scylla paramamosain cultured in China

In this study, a Gram-negative bacterium was isolated from diseased Scylla paramamosain and tentatively named strain QX17. The bacterial isolate was identified as Vibrio parahaemolyticus based on morphological and biochemical characteristics and molecular identification with the 16S rRNA and HSP60 genes. In the challenge experiment, S. paramamosain injected intramuscularly with the V. parahaemolyticus isolate developed pathological signs similar to the naturally diseased mud crabs. The infection experiment also showed that the median lethal dosage (LD50) for QX17 was 4.79 × 102 CFU g-1 (crab weight). Histopathological analysis of the diseased mud crabs infected with V. parahaemolyticus showed deformation and basement membrane rupture of hepatopancreatic tubules in the hepatopancreas, and disordered and broken muscle fiber in the muscle. Antimicrobial susceptibility tests revealed that QX17 was highly sensitive to most of the tested aminoglycosides, tetracyclines, and quinolones. To the best of our knowledge, this is the first study reporting isolation and antibiotic sensitivities of V. parahaemolyticus from cultured mud crabs. The discovery of V. parahaemolyticus in cultured mud crabs not only adds to the growing list of emerging pathogens in crab aquaculture in China, but also highlights the necessity of developing early detection strategies and appropriate interventions to reduce the damage caused by V. parahaemolyticus in mud crab aquaculture.

Spatiotemporal Regulation of Vibrio Exotoxins by HlyU and Other Transcriptional Regulators

After invading a host, bacterial pathogens secrete diverse protein toxins to disrupt host defense systems. To ensure successful infection, however, pathogens must precisely regulate the expression of those exotoxins because uncontrolled toxin production squanders energy. Furthermore, inappropriate toxin secretion can trigger host immune responses that are detrimental to the invading pathogens. Therefore, bacterial pathogens use diverse transcriptional regulators to accurately regulate multiple exotoxin genes based on spatiotemporal conditions. This review covers three major exotoxins in pathogenic Vibrio species and their transcriptional regulation systems. When Vibrio encounters a host, genes encoding cytolysin/hemolysin, multifunctional-autoprocessing repeats-in-toxin (MARTX) toxin, and secreted phospholipases are coordinately regulated by the transcriptional regulator HlyU. At the same time, however, they are distinctly controlled by a variety of other transcriptional regulators. How this coordinated but distinct regulation of exotoxins makes Vibrio species successful pathogens? In addition, anti-virulence strategies that target the coordinating master regulator HlyU and related future research directions are discussed.

Dietary Cordyceps militaris protects against Vibrio splendidus infection in sea cucumber Apostichopus japonicus

Vibrio splendidus is the common pathogen that causes infectious diseases widely spread in cultured sea cucumber in China. Therefore, we investigated the ability of Cordyceps militaris to protect against infection caused by V. splendidus. In this study, sea cucumbers were fed with a diet containing 0 (control), 1%, 2% or 3% C. militaris for 28 days, and subsequently challenged with V. splendidus by injection with 1.0 × 10(9) cfu per animal. Parameters of immune response such as phagocytosis (PC), lysozyme (LSZ) activity, superoxide dismutase (SOD) activity, alkaline phosphatase (AKP) activity and acid phosphatase (ACP) activity were determined on days 0, 1, 3, 5 and 7 after injection. The results showed that dietary C. militaris at a dose of 2% or 3% significantly up-regulated (P < 0.05) all the immune parameters on day 0. One day after injection with V. splendidus, all the immune indices except ACP exhibited a tendency to decrease and then increase again, returning to the initial level on days 5 and/or 7 after injection. All the immune parameters of those fed with C. militaris were found significantly higher (P < 0.05) than those of the control group on day 1 after injection. Only LSZ activity of those fed with 1%- or 3%-C. militaris diet on day 5 showed significantly increases (P < 0.05) than the controls. As for ACP activity, the values remained steady with time, but with significant increase (P < 0.05) seen in sea cucumbers fed with 2%-C. militaris diet, and lasted for up to 7 days after V. splendidus injection. The cumulative mortality of sea cucumbers fed with the basal diet followed V. splendidus infection was significantly higher (P < 0.05) than those fed with 2% and 3% C. militaris diet. Under the experimental conditions, dietary C. militaris could enhance the immune responses of Apostichopus japonicus and improve its resistance to infection by V. splendidus.

Relationship between heat-induced fibrillogenicity and hemolytic activity of thermostable direct hemolysin and a related hemolysin of Vibrio parahaemolyticus

The formation of nonspecific ion channels by small oligomeric amyloid intermediates is toxic to the host's cellular membranes. Thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH) are major virulence factors of Vibrio parahaemolyticus. We have previously reported the crystal structure of TDH tetramer with the central channel. We have also identified the molecular mechanism underlying the paradoxical responses to heat treatment of TDH, known as the Arrhenius effect, which is the reversible amyloidogenic property. In the present report, we describe the biophysical properties of TRH, which displays 67% amino acid similarity with TDH. Molecular modeling provided a good fit of the overall structure of TDH and TRH. Size-exclusion chromatography, ultracentrifugation, and transmission electron microscopy revealed that TRH formed tetramer in solution. These toxins showed similar hemolytic activity on red blood cells. However, TRH had less amyloid-like structure than TDH analyzed by thioflavin T-binding assay and far-UV circular dichroism spectra. These data indicated that amyloidogenicity upon heating is not essential for the membrane disruption of erythrocytes, but the maintenance of tetrameric structure is indispensable for the hemolytic activity of the TDH and TRH.

The type II secretion system is essential for erythrocyte lysis and gut colonization by the leech digestive tract symbiont Aeromonas veronii

Hemolysin and the type II secretion system (T2SS) have been shown to be important for virulence in many pathogens, but very few studies have shown their importance in beneficial microbes. Here, we investigated the importance of the type II secretion pathway in the beneficial digestive-tract association of Aeromonas veronii and the medicinal leech Hirudo verbana and revealed a critical role for the hemolysis of erythrocytes. A mutant with a miniTn5 insertion in exeM, which is involved in forming the inner membrane platform in the T2SS, was isolated by screening mutants for loss of hemolysis on blood agar plates. A hemolysis assay was used to quantify the mutant's deficiency in lysing sheep erythrocytes and revealed a 99.9% decrease compared to the parent strain. The importance of the T2SS in the colonization of the symbiotic host was assessed. Colonization assays revealed that the T2SS is critical for initial colonization of the leech gut. The defect was tied to the loss of hemolysin production by performing a colonization assay with blood containing lysed erythrocytes. This restored the colonization defect in the mutant. Complementation of the mutant using the promoter region and exeMN revealed that the T2SS is responsible for secreting hemolysin into the extracellular space and that both the T2SS and hemolysin export by the T2SS are critical for initial establishment of A. veronii in the leech gut.

Identification of Vibrio harveyi as a causative bacterium for a tail rot disease of sea bream Sparus aurata from research hatchery in Malta

A bacterial disease was reported from gilthead sea bream (Sparus aurata) within a hatchery environment in Malta. Symptoms included complete erosion of tail, infection in the eye, mucous secretion and frequent mortality. A total of 540 strains were initially isolated in marine agar from different infected body parts and culture water sources. Subsequently 100 isolates were randomly selected, identified biochemically and all were found to be Vibrio harveyi-related organisms; finally from 100 isolates a total of 13 numbers were randomly selected and accurately identified as V. harveyi by 16S rRNA gene sequencing and species-specific PCR. Ribotyping of these strains with HindIII revealed total of six clusters. In vivo challenge study with representative isolates from each cluster proved two clusters each were highly pathogenic, moderately pathogenic and non-pathogenic. All 13 isolates were positive for hemolysin gene, a potential virulence factor. Further analysis revealed probably a single copy of this gene was encoded in all isolates, although not in the same locus in the genome. Although V. harveyi was reported to be an important pathogen for many aquatic organisms, to our knowledge this might be the first report of disease caused by V. harveyi and their systematic study in the sea bream hatchery from Malta.

A single residue change in Vibrio harveyi hemolysin results in the loss of phospholipase and hemolytic activities and pathogenicity for turbot (Scophthalmus maximus)

Vibrio harveyi hemolysin, an important virulence determinant in fish pathogenesis, was further characterized, and the enzyme was identified as a phospholipase B by gas chromatography. Site-directed mutagenesis revealed that a specific residue, Ser153, was critical for its enzymatic activity and for its virulence in fish.

Thermostable direct hemolysin of Vibrio parahaemolyticus is a bacterial reversible amyloid toxin

Thermostable direct hemolysin (TDH), a major virulence factor of Vibrio parahaemolyticus, is detoxified by heating at approximately 60-70 degrees C but is reactivated by additional heating above 80 degrees C. This paradoxical phenomenon, known as the Arrhenius effect, has remained unexplained for approximately 100 years. We now demonstrate that the effect is related to structural changes in the protein that produce fibrils. The native TDH (TDHn) is transformed into nontoxic fibrils rich in beta-strands by incubation at 60 degrees C (TDHi). The TDHi fibrils are dissociated into unfolded states by further heating above 80 degrees C (TDHu). Rapid cooling of TDHu results in refolding of the protein into toxic TDHn, whereas the protein is trapped in the TDHi structure by slow cooling of TDHu. Transmission electron microscopy indicates the fibrillar structures of TDHi. The fibrils show both the property of the nucleation-dependent elongation and the increase in its thioflavin T fluorescence. Formation of beta-rich structures of TDH was also observed in the presence of lipid vesicles containing ganglioside G(T1b), a putative TDH receptor. Congo red was found to inhibit the hemolytic activity of TDH in a dose-dependent manner. These data reveal that the mechanism of the Arrhenius effect which is tightly related to the fibrillogenicity of TDH.