Cloning and identification of a phospholipase gene from vibrio mimicus

Roles of alkyl hydroperoxide reductase subunit C (AhpC) in viable but nonculturable Vibrio parahaemolyticus

Alkyl hydroperoxide reductase subunit C (AhpC) is the catalytic subunit responsible for the detoxification of reactive oxygen species that form in bacterial cells or are derived from the host; thus, AhpC facilitates the survival of pathogenic bacteria under environmental stresses or during infection. This study investigates the role of AhpC in the induction and maintenance of a viable but nonculturable (VBNC) state in Vibrio parahaemolyticus. In this investigation, ahpC1 (VPA1683) and ahpC2 (VP0580) were identified in chromosomes II and I of this pathogen, respectively. Mutants with deletions of these two ahpC genes and their complementary strains were constructed from the parent strain KX-V231. The growth of these strains was monitored on tryptic soy agar-3% NaCl in the presence of the extrinsic peroxides H(2)O(2) and tert-butyl hydroperoxide (t-BOOH) at different incubation temperatures. The results revealed that both ahpC genes were protective against t-BOOH, while ahpC1 was protective against H(2)O(2). The protective function of ahpC2 at 4°C was higher than that of ahpC1. The times required to induce the VBNC state (4.7 weeks) at 4°C in a modified Morita mineral salt solution with 0.5% NaCl and then to maintain the VBNC state (4.7 weeks) in an ahpC2 mutant and an ahpC1 ahpC2 double mutant were significantly shorter than those for the parent strain (for induction, 6.2 weeks; for maintenance, 7.8 weeks) and the ahpC1 mutant (for induction, 6.0 weeks; for maintenance, 8.0 weeks) (P < 0.03). Complementation with an ahpC2 gene reversed the effects of the ahpC2 mutation in shortening the times for induction and maintenance of the VBNC state. This investigation identified the different functions of the two ahpC genes and confirmed the particular role of ahpC2 in the VBNC state of V. parahaemolyticus.

Sigma E regulators control hemolytic activity and virulence in a shrimp pathogenic Vibrio harveyi

Members of the genus Vibrio are important marine and aquaculture pathogens. Hemolytic activity has been identified as a virulence factor in many pathogenic vibrios including V. cholerae, V. parahaemolyticus, V. alginolyticus, V. harveyi and V. vulnificus. We have used transposon mutagenesis to identify genes involved in the hemolytic activity of shrimp-pathogenic V. harveyi strain PSU3316. Out of 1,764 mutants screened, five mutants showed reduced hemolytic activity on sheep blood agar and exhibited virulence attenuation in shrimp (Litopenaeus vannamei). Mutants were identified by comparing transposon junction sequences to a draft of assembly of the PSU3316 genome. Surprisingly none of the disrupted open reading frames or gene neighborhoods contained genes annotated as hemolysins. The gene encoding RseB, a negative regulator of the sigma factor (σ(E)), was interrupted in 2 out of 5 transposon mutants, in addition, the transcription factor CytR, a threonine synthetase, and an efflux-associated cytoplasmic protein were also identified. Knockout mutations introduced into the rpoE operon at the rseB gene exhibited low hemolytic activity in sheep blood agar, and were 3-to 7-fold attenuated for colonization in shrimp. Comparison of whole cell extracted proteins in the rseB mutant (PSU4030) to the wild-type by 2-D gel electrophoresis revealed 6 differentially expressed proteins, including two down-regulated porins (OmpC-like and OmpN) and an upregulated protease (DegQ) which have been associated with σ(E) in other organisms. Our study is the first report linking hemolytic activity to the σ(E) regulators in pathogenic Vibrio species and suggests expression of this virulence-linked phenotype is governed by multiple regulatory pathways within the V. harveyi.

Comparative genomics of clinical and environmental Vibrio mimicus

Whether Vibrio mimicus is a variant of Vibrio cholerae or a separate species has been the subject of taxonomic controversy. A genomic analysis was undertaken to resolve the issue. The genomes of V. mimicus MB451, a clinical isolate, and VM223, an environmental isolate, comprise ca. 4,347,971 and 4,313,453 bp and encode 3,802 and 3,290 ORFs, respectively. As in other vibrios, chromosome I (C-I) predominantly contains genes necessary for growth and viability, whereas chromosome II (C-II) bears genes for adaptation to environmental change. C-I harbors many virulence genes, including some not previously reported in V. mimicus, such as mannose-sensitive hemagglutinin (MSHA), and enterotoxigenic hemolysin (HlyA); C-II encodes a variant of Vibrio pathogenicity island 2 (VPI-2), and Vibrio seventh pandemic island II (VSP-II) cluster of genes. Extensive genomic rearrangement in C-II indicates it is a hot spot for evolution and genesis of speciation for the genus Vibrio. The number of virulence regions discovered in this study (VSP-II, MSHA, HlyA, type IV pilin, PilE, and integron integrase, IntI4) with no notable difference in potential virulence genes between clinical and environmental strains suggests these genes also may play a role in the environment and that pathogenic strains may arise in the environment. Significant genome synteny with prototypic pre-seventh pandemic strains of V. cholerae was observed, and the results of phylogenetic analysis support the hypothesis that, in the course of evolution, V. mimicus and V. cholerae diverged from a common ancestor with a prototypic sixth pandemic genomic backbone.

Cloning and characterization of Photobacterium damselae ssp. piscicida phospholipase: an enzyme that shows haemolytic activity

A phospholipase gene of Photobacterium damselae ssp. piscicida (ppp) was cloned from a genomic library and its nucleotide sequence was determined. The open reading frame consisted of 1218 bp encoding a protein of 405 amino acids with a predicted molecular mass of 46 kDa. The PPP had identities (53-55%) with phospholipase and haemolysin of Vibrio spp., while it showed low identities (23-26%) with glycerophospholipid cholesterol acyltransferase of Aeromonas spp. A recombinant PPP (rPPP) with a His tag at the C-terminus expressed in Escherichia coli and purified showed phospholipase activity. The rPPP also showed lecithin-dependent haemolytic activity against mammalian erythrocytes and direct haemolytic activity against fish erythrocytes. The culture supernatant of wild-type P. damselae ssp. piscicida showed phospholipase activity, while that of a PPP gene knockout mutant did not.

The FAXWXXT motif in the carboxyl terminus ofVibrio mimicusmetalloprotease is involved in binding to collagen

We have shown previously that the C-terminal region of the extracellular metalloprotease of Vibrio mimicus (VMC) is essential for collagenase activity. Here, we demonstrate that deletion of 100 amino acids, but not 67 amino acids, from the C-terminus of the intact VMC protein (VMC61) abolished the collagenase activity. The intervening 33-amino acid region contains a repeated FAXWXXT motif that is essential for insoluble type I collagen binding; the isolated 33-amino acid peptide bound to insoluble type I collagen, while a peptide containing only the first FAXWXXT motif did not. Compared to the VMC61, the 33-amino acid peptide corresponding to the C-terminus exhibited a similar binding affinity and a lower binding capacity.

In vitro antifungal activities of inhibitors of phospholipases from the fungal pathogen Cryptococcus neoformans

Secreted phospholipase B is a proven virulence factor for the pathogenic fungus Cryptococcus neoformans and exhibits three phospholipase activities in the one protein. These are phospholipase B (PLB), lysophospholipase (LPL), and lysophospholipase transacylase (LPTA). Our aim was to investigate the feasibility of using this enzyme as a target for antifungal therapy. We determined in C. neoformans var. grubii strain H99 that 82% of PLB activity was secreted but that 64% of LPL activity and 70% of LPTA activity were cell associated. Cell-associated activities (cytosolic and membrane) were further characterized, since it is likely that any fungicidal effect would depend on inhibition of these enzymes. Four commercially available compounds with structural similarities to phospholipid substrates were tested as inhibitors. These were alexidine dihydrochloride (compound A), dioctadecyldimethylammonium bromide (compound O), 1,12 bis-(tributylphosphonium)dodecane dibromide (compound P), and decamethonium dibromide (compound D). The best phospholipase inhibitors (compounds A and P) were also the most potent antifungal agents by the standard broth microdilution test. Compound A was highly selective for secreted and cell-associated PLB activities and showed no inhibition of mammalian phospholipase A(2) at 0.25 micro M. Compound O, which was specific for secretory and cytosolic LPL and LPTA and membrane-associated PLB, was not antifungal. We conclude that inhibitors of cryptococcal phospholipases can be selective for fungal enzymes and intrinsically antifungal. They also provide tools for assessing the relative importance of the various enzyme activities in virulence. Our results enable further rational structure-function studies to validate the use of phospholipases as antifungal targets.

Characterization of the enzyme activity of an extracellular metalloprotease (VMC) from Vibrio mimicus and its C-terminal deletions

To investigate the enzymatic properties of Vibrio mimicus metalloprotease, the mature metalloprotease gene (vmc) was overexpressed in Escherichia coli and the recombinant protein (rVMC61) was purified by metal affinity chromatography. rVMC61 showed maximum activity at about 37 degrees C, pH 8. The purified rVMC61 was very specific toward collagen substrates, such as gelatin, type I, II, and III collagens and synthetic peptides (Cbz-GPLGP and Cbz-GPGGPA). But it did not show degrading activity toward other biological proteins including lysozyme, lactoferrin and bovine serum albumin. rVMC61 also showed cytotoxicity against CHSE-214 fish cells. To examine the role of the C-terminal region of rVMC61, the 3' end of the metalloprotease gene (vmc) was digested serially with exonuclease III. The truncated vmc derivatives encoding 57-42 kDa of the protease were isolated and overexpressed in E. coli. The collagenase activities of truncated proteins were investigated using gelatin as substrate. Deletion of 100 amino acids from the C-terminus resulted in loss of gelatin degrading activity. However, deletion of 67 amino acids from the C-terminus did not affect its gelatin degrading activity.

Characterization of Vibrio mimicus phospholipase A (PhlA) and cytotoxicity on fish cell

Vibrio mimicus is a typical strain of Vibrio cholerae and produces a phospholipase (PhlA) which shares a highly conserved amino acid sequence with the lecithinase (Lec) of V. cholerae. The recombinant protein (rPhlA) produced from the phlA gene of V. mimicus was expressed in Escherichia coli as His-tag fused protein. The rPhlA was purified by gel filtration and Ni-metal affinity chromatographies. When the action mode was investigated by TLC and GC-MS, the purified rPhlA protein showed a phospholipase A activity, which cleaved the fatty acids at the sn-1 and sn-2 positions of phosphatidylcholine. However, it did not show lysophospholipase, sphingomyelinase, and phospholipase C activities. The rPhlA showed maximum activity at temperature of about 40 degrees C and pH around 8-9. Some divalent cations could affect the activity of PhlA. The addition of Co(2+) increased the activity, whereas Mg(2+) and Zn(2+) did not enhance the enzyme activity. The rPhlA could lyse the erythrocytes obtained from the fish such as rainbow trout and tilapia. A significant cytotoxic activity on a fish cell line, CHSE-214, was observed after 24h exposure to 40 microg rPhlA protein.

Duplication of hemolysin genes in a virulent isolate of Vibrio harveyi

Vibrio harveyi VIB 645, which is very pathogenic towards salmonids and produces extracellular product with a high titer of hemolytic activity towards fish erythrocytes, was found to contain two closely related hemolysin genes (designated vhhA and vhhB), whereas the majority of strains examined (11 of 13) carried only a single hemolysin gene. Both genes from VIB 645 were cloned and sequenced. The open reading frames (ORFs) of vhhA and vhhB shared a high level of identity (98.8%) and were predicted to encode identical polypeptides comprising 418 amino acid residues. The VHH protein shows homology to the lecithinase of V. mimicus and V. cholerae. Transformants of Escherichia coli containing the ORF of either vhhA or vhhB displayed weak hemolytic activity in rainbow trout blood agar. The hemolytic activity was very high when the ORF of vhhB was cloned in E. coli together with the native promoter. Surprisingly, the level of vhh-specific RNA transcript produced by VIB 645 was found to be very low. We conclude that the hemolytic phenotype of VIB 645 is not due to increased expression of one or both copies of the vhh gene.

The Vibrio cholerae O1 chromosomal integron

Until the discovery of the Vibrio cholerae repeat (VCR), the gene capture and expression systems termed integrons had been typically associated with antibiotic-resistance gene cassettes with usually less than five genes in an array. A method is described for the cloning of the ends of large cassette arrays. Conserved restriction sites within VCRs facilitated the mapping by Southern hybridization and cloning of the 5' end of the VCR array, and using appropriate fragments it was possible to develop a physical map of the region of the V. cholerae chromosome. Sequence determination of the predicted beginning of this region revealed intI4, a member of the integron family of integrases. Comparison of these sequences from El Tor, Classical and serotype O134 V. cholerae strains identified the 3' end of the attI site, thereby defining the class 4 integron in one of the V. cholerae chromosomes, and providing the first evidence for integron-like site-specific recombination within V. cholerae. Conduction assays demonstrated IntI1-mediated recombination between VCRs. Restriction mapping places the sequences of intI4 and 26 VCR gene cassettes in arrays within a 120 kb region of the V. cholerae O1 strain 569B genome. This region contains an estimated 150 VCR gene cassettes, dwarfing previously described arrays. Southern analysis of genomic DNA from strains of Vibrio anguillarum, Vibrio mimicus and a number of V. cholerae serotypes revealed fragments that hybridized with VCR-specific probes but showed a high degree of restriction fragment length polymorphism. These data facilitate the identification of part of a new class 5 integron from V. mimicus.