Nucleotide sequence and transcriptional analysis of the aerCaerA region of Aeromonas sobria encoding aerolysin and its regulatory region

Molecular Detection, Quantification, and Toxigenicity Profiling of Aeromonas spp. in Source- and Drinking-Water

Aeromonas is ubiquitous in aquatic environments and has been associated with a number of extra-gastrointestinal and gastrointestinal illnesses. This warrants monitoring of raw and processed water sources for pathogenic and toxigenic species of this human pathogen. In this study, a total of 17 different water samples [9 raw and 8 treated samples including 4 basin water (partial sand filtration) and 4 finished water samples] were screened for Aeromonas using selective culturing and a genus-specific real-time quantitative PCR assay. The selective culturing yielded Aeromonas counts ranging 0 - 2 x 10(3)CFU/ml and 15 Aeromonas isolates from both raw and treated water samples. The qPCR analysis indicated presence of a considerable nonculturable population (3.4 x 10(1) - 2.4 x 10(4) cells/ml) of Aeromonas in drinking water samples. Virulence potential of the Aeromonas isolates was assessed by multiplex/singleplex PCR-based profiling of the hemolysin and enterotoxin genes viz cytotoxic heat-labile enterotoxin (act), heat-labile cytotonic enterotoxin (alt), heat-stable cytotonic enterotoxin (ast), and aerolysin (aerA) genes. The water isolates yielded five distinct toxigenicity profiles, viz. act, alt, act+alt, aerA+alt, and aerA+alt+act. The alt gene showed the highest frequency of occurrence (40%), followed by the aerA (20%), act (13%), and ast (0%) genes. Taken together, the study demonstrated the occurrence of a considerable population of nonculturable Aeromonads in water and prevalence of toxigenic Aeromonas spp. potentially pathogenic to humans. This emphasizes the importance of routine monitoring of both source and drinking water for this human pathogen and role of the developed molecular approaches in improving the Aeromonas monitoring scheme for water.

Biomphalysin, a new β pore-forming toxin involved in Biomphalaria glabrata immune defense against Schistosoma mansoni

Aerolysins are virulence factors belonging to the β pore-forming toxin (β-PFT) superfamily that are abundantly distributed in bacteria. More rarely, β-PFTs have been described in eukaryotic organisms. Recently, we identified a putative cytolytic protein in the snail, Biomphalaria glabrata, whose primary structural features suggest that it could belong to this β-PFT superfamily. In the present paper, we report the molecular cloning and functional characterization of this protein, which we call Biomphalysin, and demonstrate that it is indeed a new eukaryotic β-PFT. We show that, despite weak sequence similarities with aerolysins, Biomphalysin shares a common architecture with proteins belonging to this superfamily. A phylogenetic approach revealed that the gene encoding Biomphalysin could have resulted from horizontal transfer. Its expression is restricted to immune-competent cells and is not induced by parasite challenge. Recombinant Biomphalysin showed hemolytic activity that was greatly enhanced by the plasma compartment of B. glabrata. We further demonstrated that Biomphalysin with plasma is highly toxic toward Schistosoma mansoni sporocysts. Using in vitro binding assays in conjunction with Western blot and immunocytochemistry analyses, we also showed that Biomphalysin binds to parasite membranes. Finally, we showed that, in contrast to what has been reported for most other members of the family, lytic activity of Biomphalysin is not dependent on proteolytic processing. These results provide the first functional description of a mollusk immune effector protein involved in killing S. mansoni.

Schistosomiasis is the second most widespread tropical parasitic disease after malaria. It is caused by flatworms of the genus Schistosoma. Its life cycle is complex and requires certain freshwater snail species as intermediate host. Given the limited options for treating S. mansoni infections, much research has focused on a better understanding of the immunobiological interactions between the invertebrate host Biomphalaria glabrata and its parasite S. mansoni. A number of studies published over the last two decades have contributed greatly to our understanding of B. glabrata innate immune mechanisms involved in the defense against parasite. However, most studies have focused on the identification of recognition molecules or immune receptors involved in the host/parasite interplay. In the present study, we report the first functional description of a mollusk immune effector protein involved in killing S. mansoni, a protein related to the β pore forming toxin that we named Biomphalysin.

Simultaneous detection of six diarrhea-causing bacterial pathogens with an in-house PCR-luminex assay

Diarrhea can be caused by a range of pathogens, including several bacteria. Conventional diagnostic methods, such as culture, biochemical tests, and enzyme-linked immunosorbent assay (ELISA), are laborious. We developed a 7-plex PCR-Luminex assay to simultaneously screen for several of the major diarrhea-causing bacteria directly in fecal specimens, including pathogenic Aeromonas, Campylobacter jejuni, Campylobacter coli, Salmonella, Shigella, enteroinvasive Escherichia coli (EIEC), Vibrio, and Yersinia. We included an extrinsic control to verify extraction and amplification. The assay was first validated with reference strains or isolates and exhibited a limit of detection of 10(3) to 10(5) CFU/g of stool for each pathogen as well as quantitative detection up to 10(9) CFU/g. A total of 205 clinical fecal specimens from individuals with diarrhea, previously cultured for enteric pathogens and tested for Campylobacter by ELISA, were evaluated. Using these predicate methods as standards, sensitivities and specificities of the PCR-Luminex assay were 89% and 94% for Aeromonas, 89% and 93% for Campylobacter, 96% and 95% for Salmonella, 94% and 94% for Shigella, 92% and 97% for Vibrio, and 100% and 100% for Yersinia, respectively. All discrepant results were further examined by singleplex real-time PCR assays targeting different gene regions, which revealed 89% (55/62 results) concordance with the PCR-Luminex assay. The fluorescent signals obtained with this approach exhibited a statistically significant correlation with the cycle threshold (C(T)) values from the cognate real-time PCR assays (P < 0.05). This multiplex PCR-Luminex assay enables sensitive, specific, and quantitative detection of the major bacterial causes of gastroenteritis.

Development and application of real-time PCR assays for quantifying total and aerolysin gene-containing aeromonas in source, intermediate, and finished drinking water

Aeromonas spp., opportunistic pathogens, are listed as a microbiological contaminant on the Environmental Protection Agency's (EPA) Drinking Water Contaminant Candidate List. Culture-based methods for identification and quantification of Aeromonas in drinking water are time-consuming and often fail to differentiate pathogenic species from nonpathogenic ones. This study reports successful development and applications of two real-time PCR assays, based on 16S rRNA gene sequences and a virulence gene (aerolysin gene), for rapid and effective quantification of total and aerolysin gene-containing Aeromonas spp. The assays successfully quantified total and aerolysin gene-containing Aeromonas in source, intermediate, and finished water samples collected from seven water works and one pilot plant. The effectiveness of Aeromonas removal by different drinking water treatment processes was examined by comparing the results obtained from the EPA culture-based method and developed real-time PCR assays. Regardless of the methods, our results indicated that conventional water treatment combination (prechlorination/ coagulation/sedimentation/rapid sand filtration) and membrane filtration alone could effectively remove Aeromonas. Slow sand filtration alone might not be effective. The removal efficiencies by different disinfection treatments were not determined, due to the lack of detectable Aeromonas. No Aeromonas was detected in samples with turbidity below 0.06 NTU.

Pore-forming protein toxins: from structure to function

Pore-forming protein toxins (PFTs) are one of Nature's most potent biological weapons. An essential feature of their toxicity is the remarkable property that PFTs can exist either in a stable water-soluble state or as an integral membrane pore. In order to convert from the water-soluble to the membrane state, the toxin must undergo large conformational changes. There are now more than a dozen PFTs for which crystal structures have been determined and the nature of the conformational changes they must undergo is beginning to be understood. Although they differ markedly in their primary, secondary, tertiary and quaternary structures, nearly all can be classified into one of two families based on the types of pores they are thought to form: alpha-PFTs or beta-PFTs. Recent work suggests a number of common features in the mechanism of membrane insertion may exist for each class.

Aeromonas hydrophila beta-hemolysin induces active chloride secretion in colon epithelial cells (HT-29/B6)

The diarrheal mechanisms in Aeromonas enteritis are not completely understood. In this study we investigated the effect of aeromonads and of their secretory products on ion secretion and barrier function of monolayers of human intestinal cells (HT-29/B6). Ion secretion was determined as a short-circuit current (I(SC)) of HT-29/B6 monolayers mounted in Ussing-type chambers. Transepithelial resistance (R(t)) served as a measure of permeability. A diarrheal strain of Aeromonas hydrophila (strain Sb) added to the mucosal side of HT-29/B6 monolayers induced a significant I(SC) (39 +/- 3 microA/cm(2)) and decreased the R(t) to approximately 10% of the initial value. A qualitatively identical response was obtained with sterile supernatant of strain Sb, and Aeromonas supernatant also induced a significant I(SC) in totally stripped human colon. Tracer flux and ion replacement studies revealed the I(SC) to be mainly accounted for by electrogenic Cl(-) secretion. Supernatant applied serosally completely abolished basal I(SC). The supernatant-induced I(SC) was inhibited by the protein kinase C inhibitor chelerythrine, whereas a protein kinase A inhibitor (H8) and a Ca(2+) chelator (BAPTA-AM) had no effect. Physicochemical properties indicated that the supernatant's active compound was an aerolysin-related Aeromonas beta-hemolysin. Accordingly, identical I(SC) and R(t) responses were obtained with Escherichia coli lysates harboring the cloned beta-hemolysin gene from strain SB or the aerA gene encoding for aerolysin. Sequence comparison revealed a 64% homology between aerolysin and the beta-hemolysin cloned from Aeromonas sp. strain Sb. In conclusion, beta-hemolysin secreted by pathogenic aeromonads induces active Cl(-) secretion in the intestinal epithelium, possibly by channel insertion into the apical membrane and by activation of protein kinase C.

Studies of aerolysin promoters from different Aeromonas spp

Aeromonas spp. have been related to food and waterborne diseases. The pore forming toxin aerolysin is regarded as the most important virulence factor in Aeromonas food poisoning. In this work the aerolysin promoters from several Aeromonas spp. strains have been sequenced, and divided into two sequence groups. Further analyses of the promoters were carried out in a reporter-plasmid, pSTINA-II. This plasmid was constructed as a hybrid of pUC4K, pACYC184 and pKK232-8. We could conclude that our constructed reporter-gene plasmid was functional and was used to compare different promoters in an aerolysin negative Aeromonas spp. This construct made it possible to study the expression of the reporter gene using different aerolysin promoters under several conditions. We were able to show that the two obtained sequence groups of aerolysin promoters gave different expression of the reporter gene, and that this expression was dependent of temperature and osmolarity. Reducing the size of one promoter sequence from 254 to 148 bp and 102 bp gave a gradual reduction of the reporter gene expression under all conditions. According to our assay there seems to be more than one functional promoter upstream of the aerolysin gene, although the RT-PCR indicated one transcription starting point, under all test conditions.

The cytotoxic enterotoxin of Aeromonas hydrophila induces proinflammatory cytokine production and activates arachidonic acid metabolism in macrophages

An aerolysin-related cytotoxic enterotoxin (Act) of Aeromonas hydrophila possesses multiple biological activities, which include its ability to lyse red blood cells, destroy tissue culture cell lines, evoke a fluid secretory response in ligated intestinal loop models, and induce lethality in mice. The role of Act in the virulence of the organism has been demonstrated. In this study, we evaluated the potential of Act to induce production of proinflammatory cytokines associated with Act-induced tissue injury and Act's capacity to activate in macrophages arachidonic acid (AA) metabolism that leads to production of eicosanoids (e.g., prostaglandin E(2) [PGE(2)]). Our data indicated that Act stimulated the production of tumor necrosis factor alpha and upregulated the expression of genes encoding interleukin-1beta (IL-1beta) and IL-6 in the murine macrophage cell line RAW264.7. Act also activated transcription of the gene encoding inducible nitric oxide synthase. Act evoked the production of PGE(2) coupled to the cyclooxygenase-2 (COX-2) pathway. AA is a substrate for PGE(2), and Act produced AA from phospholipids by inducing group V secretory phospholipase A(2). We also demonstrated that Act increased cyclic AMP (cAMP) production in macrophages. cAMP, along with PGE(2), could potentiate fluid secretion in animal models because of infiltration and activation of macrophages resulting from Act-induced tissue injury. After Act treatment of RAW cells, we detected an increased translocation of NF-kappaB and cAMP-responsive element binding protein (CREB) to the nucleus using gel shift assays. Act also upregulated production of antiapoptotic protein Bcl-2 in macrophages, suggesting a protective role for Bcl-2 against cell death induced by proinflammatory cytokines. The increased expression of genes encoding the proinflammatory cytokines, COX-2, and Bcl-2 appeared correlated with the activation of NF-kappaB and CREB. This is the first report of the detailed mechanisms of action of Act from A. hydrophila.

Hemolytic activity and siderophore production in different Aeromonas species isolated from fish

The hemolytic activity and siderophore production of several strains of motile aeromonads were determined. The hemolytic activity of Aeromonas caviae and Aeromonas eucrenophila was enhanced after trypsinization of the samples. The enhancement of hemolysis was observed in strains that carried an aerolysin-like gene, detected by a PCR procedure. Siderophore production was demonstrated in all but one strain of Aeromonas jandaei. No apparent relationship was observed between the presence of plasmid DNA and hemolysis or siderophore production.

PCR detection, characterization, and distribution of virulence genes in Aeromonas spp

We found 73.1 to 96.9% similarity by aligning the cytolytic enterotoxin gene of Aeromonas hydrophila SSU (AHCYTOEN; GenBank accession no. M84709) against aerolysin genes of Aeromonas spp., suggesting the possibility of selecting common primers. Identities of 90 to 100% were found among the eight selected primers from those genes. Amplicons obtained from Aeromonas sp. reference strains by using specific primers for each gene or a cocktail of primers were 232 bp long. Of hybridization group 4/5A/5B (HG4/5A/5B), HG9, and HG12 or non-Aeromonas reference strains, none were positive. PCR-restriction fragment length polymorphism (PCR-RFLP) with HpaII yielded three types of patterns. PCR-RFLP 1 contained two fragments (66 and 166 bp) found in HG6, HG7, HG8, HG10, and HG11. PCR-RFLP 2 contained three fragments (18, 66, and 148 bp) found in HG1, HG2, HG3, and HG11. PCR-RFLP 3, with four fragments (7, 20, 66, and 139 bp), was observed only in HG13. PCR-amplicon sequence analysis (PCR-ASA) revealed three main types. PCR-ASA 1 had 76 to 78% homology with AHCYTOEN and included strains in HG6, HG7, HG8, HG10, and HG11. PCR-ASA 2, with 82% homology, was found only in HG13. PCR-ASA 3, with 91 to 99% homology, contained the strains in HG1, HG2, HG3, and HG11. This method indicated that 37 (61%) of the 61 reference strains were positive with the primer cocktail master mixture, and 34 (58%) of 59 environmental isolates, 93 (66%) of 141 food isolates, and 100 (67%) of 150 clinical isolates from around the world carried a virulence factor when primers AHCF1 and AHCR1 were used. In conclusion, this PCR-based method is rapid, sensitive, and specific for the detection of virulence factors of Aeromonas spp. It overcomes the handicap of time-consuming biochemical and other DNA-based methods.

Purification and characterization of enterotoxin produced by Aeromonas sobria

We purified the toxin of Aeromonas sobria capable of inducing a positive response in the mouse intestinal loop assay. The purified toxin showed a positive response not only in the loop assay but also in a hemolytic assay. Subsequently, we cloned the toxin gene and demonstrated that the product of this gene possessed both hemolytic and enterotoxic activities. These results showed that the enterotoxin of A. sobria possesses hemolytic activity. Nucleotide sequence determination of the toxin gene and amino acid sequence analysis of the purified toxin revealed that it is synthesized as a precursor composed of 488 amino acid residues, and that the 24 amino-terminal amino acid residues of the precursor is removed in the mature toxin. As antiserum against the purified toxin neutralized the fluid accumulation induced by living cells not only of A. sobria but also of A. hydrophila, this and antigenically related toxin(s) are thought to play an essential role in the induction of diarrhea by these organisms. The toxin-injured Chinese hamster ovary (CHO) cells induced the release of intracellular lactose dehydrogenase (LDH). The release of LDH from CHO cells and the lysis of erythrocytes by the toxin were repressed by the addition of dextran to the reaction solution, indicating that the toxin forms pores in the membranes and that the cells were injured by the osmotic gradient developed due to pore formation. However, the histopathological examination of intestinal cells exposed to the toxin showed that it caused fluid accumulation in the mouse intestinal loop without causing cellular damage.

Role of a cytotoxic enterotoxin in Aeromonas-mediated infections: development of transposon and isogenic mutants

Transposon and marker exchange mutagenesis were used to evaluate the role of Aeromonas cytotoxic enterotoxin (Act) in the pathogenesis of diarrheal diseases and deep wound infections. The transposon mutants were generated by random insertion of Tn5-751 in the chromosomal DNA of a diarrheal isolate SSU of Aeromonas hydrophila. Some of the transposon mutants had dramatically reduced hemolytic and cytotoxic activities, and such mutants exhibited reduced virulence in mice compared to wild-type Aeromonas when injected intraperitoneally (i.p.). Southern blot data indicated that transposition in these mutants did not occur within the cytotoxic enterotoxin gene (act). The transcription of the act gene was affected drastically in the transposon mutants, as revealed by Northern blot analysis. The altered virulence of these transposon mutants was confirmed by developing isogenic mutants of the wild-type Aeromonas by using a suicide vector. In these mutants, the truncated act gene was integrated in place of a functionally active act gene. The culture filtrates from isogenic mutants were devoid of hemolytic, cytotoxic, and enterotoxic activities associated with Act. These filtrates caused no damage to mouse small intestinal epithelium, as determined by electron microscopy, whereas culture filtrates from wild-type Aeromonas caused complete destruction of the microvilli. The 50% lethal dose of these mutants in mice was 1.0 x 10(8) when injected i. p., compared to 3.0 x 10(5) for the wild-type Aeromonas. Reintegration of the native act gene in place of the truncated toxin gene in isogenic mutants resulted in complete restoration of Act's biological activity and virulence in mice. The animals injected with a sublethal dose of wild-type Aeromonas or the revertant, but not the isogenic mutant, had circulating toxin-specific neutralizing antibodies. Taken together, these studies clearly established a role for Act in the pathogenesis of Aeromonas-mediated infections.

Molecular cloning, nucleotide sequence, and expression in Escherichia coli of a hemolytic toxin (aerolysin) gene from Aeromonas trota

Aeromonas trota AK2, which was derived from ATCC 49659 and produces the extracellular pore-forming hemolytic toxin aerolysin, was mutagenized with the transposon mini-Tn5Km1 to generate a hemolysin-deficient mutant, designated strain AK253. Southern blotting data indicated that an 8.7-kb NotI fragment of the genomic DNA of strain AK253 contained the kanamycin resistance gene of mini-Tn5Km1. The 8.7-kb NotI DNA fragment was cloned into the vector pGEM5Zf(-) by selecting for kanamycin resistance, and the resultant clone, pAK71, showed aerolysin activity in Escherichia coli JM109. The nucleotide sequence of the aerA gene, located on the 1.8-kb ApaI-EcoRI fragment, was determined to consist of 1,479 bp and to have an ATG initiation codon and a TAA termination codon. An in vitro coupled transcription-translation analysis of the 1.8-kb region suggested that the aerA gene codes for a 54-kDa protein, in agreement with nucleotide sequence data. The deduced amino acid sequence of the aerA gene product of A. trota exhibited 99% homology with the amino acid sequence of the aerA product of Aeromonas sobria AB3 and 57% homology with the amino acid sequences of the products of the aerA genes of Aeromonas salmonicida 17-2 and A. sobria 33.

Possible virulence factors of Aeromonas spp. from food and water

Thirty-one isolates of Aeromonas spp. from food and water in Norway were classified and tested for possible virulence factors including cytotoxins (tissue cultures, PCR), enterotoxins (PCR) and invasion ability (Caco-2 cells). Five different species were recorded, A. caviae (9/31), A. hydrophila (15/31), A. schubertii (3/31), A. trota (3/31) and A. veronii biovar veronii (1/31). One of the A. hydrophila strains was probably responsible for a small outbreak of food poisoning caused by ingestion of raw fermented fish. All the A. hydrophila strains produced and secreted cytotoxins at 37 degrees C, as well as two A. trota strains and the single A. veronii biovar veronii strain. In some cases increased cytotoxin secretion was observed under osmotic stress. The majority of the A. caviae strains which produced cytotoxins at 30 degrees C were unable to produce and/or secrete cytotoxins at 37 degrees C. One A. schubertii strain and one A. caviae strain were invasive.

Inactivation of two haemolytic toxin genes in Aeromonas hydrophila attenuates virulence in a suckling mouse model

The contribution of two unrelated Aeromonas hydrophila beta-haemolytic toxins to virulence was assessed in a suckling mouse model. The first haemolysin gene, isolated from an A. hydrophila A6 cosmid bank, encoded a potential gene product of 621 amino acids and a predicted molecular size of 69.0 kDa. The inferred amino acid sequence showed 89% identity to the AHH1 haemolysin of A. hydrophila ATCC 7966, and 51% identity to the HlyA haemolysin of Vibrio cholerae EI Tor strain O17. The second haemolysin gene (designated aerA), which encodes aerolysin, a pore-forming toxin, was partially cloned by PCR for the purpose of mutant construction. This PCR product was a 1040 bp fragment from the C-terminal region of aerA. It is proposed that the 69.0 kDa V. cholerae-HlyA-like haemolysin gene be termed hlyA to contrast with the aerA terminology for the aerolysin. A suicide vector was used to inactivate both the hlyA and aerA genes in A. hydrophila A6. When assessed in the suckling mouse model, only the hlyA aerA double mutant showed a statistically significant reduction in virulence--a 20-fold change in LD50 (Scheffe test, P < 0.05). Cytotoxicity to buffalo green monkey kidney cell monolayers and haemolysis on horse blood agar were eliminated only in the hlyA aerA double mutants. This is the first report of cloning and mutagenesis of two unrelated haemolytic toxin genes in the same strain of a mesophilic aeromonad. For A. hydrophila, a two-toxin model provides a more complete explanation of virulence.

Hyperproduction, purification, and mechanism of action of the cytotoxic enterotoxin produced by Aeromonas hydrophila

A gene encoding the cytotoxic enterotoxin (Act) from Aeromonas hydrophila was hyperexpressed with the pET, pTRX, and pGEX vector systems. Maximum toxin yield was obtained with the pTRX vector. Approximately 40 to 60% of Act was in a soluble form with the pTRX and pET vector systems. The toxin protein was purified to homogeneity by a combination of ammonium sulfate precipitation and fast protein liquid chromatography-based column chromatographies, including hydrophobic, anion-exchange, sizing, and hydroxylapatite chromatographies. Purified mature toxin migrated as a 52-kDa polypeptide on a sodium dodecyl sulfate (SDS)polyacrylamide gel that reacted with Act-specific antibodies in immunoblots. The minimal amount of toxin needed to cause fluid secretion in rat ileal loops was 200 ng, and the 50% lethal dose for mice was 27.5 ng when injected intravenously. Binding of the toxin to erythrocytes was temperature dependent, with no binding occurring at 4 degrees C. However, at 37 degrees C the toxin bound to erythrocytes within 1 to 2 min. It was determined that the mechanism of action of the toxin involved the formation of pores in erythrocyte membranes, and the diameter of the pores was estimated to be 1.14 to 2.8 nm, as determined by the use of saccharides of different sizes and by electron microscopy. Calcium chloride prevented lysis of erythrocytes by the toxin; however, it did not affect the binding and pore-forming capabilities of the toxin. A dose-dependent reduction in hemoglobin release from erythrocytes was observed when Act was preincubated with cholesterol, but not with myristylated cholesterol. With 14C-labeled cholesterol and gel filtration, the binding of cholesterol to Act was demonstrated. None of the other phospholipids and glycolipids tested reduced the hemolytic activity of Act. The toxin also appeared to undergo aggregation when preincubated with cholesterol, as determined by SDS-polyacrylamide gel electorphoresis. As a result of this aggregation, Act's capacity to form pores in the erythrocyte membrane was inhibited.

Plasmids and Aeromonas virulence

Most putative virulence determinants of Aeromonas species are chromosomally encoded. However, several recent reports have indicated that some may be carried on or regulated by plasmids. Therefore, we examined the plasmid carriage rate of a total of 140 clinical and environmental Aeromonas isolates. Plasmid carriage was compared with the ability of an isolate to produce toxins and adhere to HEp-2 cells. Overall, plasmid incidence in Aeromonas species was low (23/140, 16%) and independent of the source of the isolate. Plasmids were, however, more common in environmental isolates of A. veronii biovar sobria than in clinical isolates of this species (P < 0.05). We could find no evidence to support the recent literature findings that plasmids may have a role in Aeromonas virulence.

Characterization of cytotoxic, hemolytic Aeromonas caviae clinical isolates and their identification by determining presence of a unique hemolysin gene

Aeromonas caviae has recently been recognized as an important enteropathogen and its hemolysin is purported to be one of the virulence factors, In this study, a total of 80 clinical isolates of Aeromonas spp. were investigated by PCR with synthetic oligonucleotides targeting a cloned hemolysin-encoding sequence from an A. caviae isolate of clinical origin. Of the 35 clinical A. caviae isolates tested, only 6 contained the target sequence.

Amino-acid residues involved in biological functions of the cytolytic enterotoxin from Aeromonas hydrophila

Some amino acid (aa) residues within the cytolytic enterotoxin (Act) of Aeromonas hydrophila essential for biological activity were identified. Act is a 52-kDa polypeptide, possessing hemolytic, cytotoxic and enterotoxic activities. By deletion analysis, generation of anti-peptide Ab, and site-directed mutagenesis we showed that two regions in Act (aa 245-274 and 361-405) were very important for biological functions. As shown by competitive inhibition assays, peptide 2 (aa 245-274) blocked cytotoxic activity of Act, and aa Tyr256, Trp270 and Gly274 were essential for cytotoxicity. Within peptide 3 (aa 361-405), Trp394 and Trp396 were important for biological activities. Mutations in other regions of the toxin (e.g., Gly169, Asp170, Gly171, Trp172, Asn177,178, Asp179 and His144,209,355) also decreased biological activity. The reactivity of these mutant toxins with Ab in immunoblots was not altered. Data reported in this study suggested the role of some aa residues in biological function(s) of Act.

Homology between the seed cytolysin enterolobin and bacterial aerolysins

Enterolobin, a 55-kDa cytolytic, inflammatory, and insecticidal protein isolated from seeds of the Brazilian tree Enterolobium contortisiliquum (Leguminosae-Mimosoideae) has been further purified and partially sequenced by using both manual and automated methods. A computational search of enterolobin partial amino acid sequence against the PIR database revealed possible sequence similarities with aerolysins, cytolytic proteins from Aeromonas species. An alignment of enterolobin partial sequence to the amino acid sequences of A. hydrophila and A. sobria aerolysins showed several similar regions with many residue identities. The seed protein enterolobin and the bacterial aerolysins may be homologous proteins despite the distant phylogenetic relationship.

Interaction of Serratia marcescens hemolysin (ShlA) with artificial and erythrocyte membranes. Demonstration of the formation of aqueous multistate channels

Pore formation by hemolysin (ShlA) of Serratia marcescens was studied in erythrocytes and in artificial lipid bilayer membranes. The results with erythrocytes demonstrated that hemolysin pores varied in size. In erythrocyte membranes with reduced fluidity (0 degrees C), the toxin formed small pores with diameter 1-1.5 nm. In fluid membranes (above 20 degrees C), hemolysin pores with larger diameters (approximately 2.5-3.0 nm) were observed, which may be caused by association of ShlA monomers into oligomers. Comparison of the channels formed by Staphylococcus aureus alpha-toxin with channels formed by ShlA indicated a slightly smaller pore diameter of ShlA pores. Analysis of ShlA in artificial lipid bilayers showed the formation of pores with a broad distribution of single channel conductances, suggesting variable sizes of the ShlA pore. The lower limit for the pore diameter was approximately 1.0 nm. The ShlA pores did not exhibit pronounced ion selectivity nor voltage dependence, supporting the presence of a large water-filled pore.

The cytolytic toxin aerolysin: from the soluble form to the transmembrane channel

Aerolysin is a cytolytic toxin which forms channels in the plasma membranes of eucaryotic cells. The protein is secreted by Aeromonas hydrophila as an inactive protoxin. Its stability and water solubility are conferred by its ability to dimerize. Maturation of the protein occurs through proteolytic removal of a C-terminal peptide outside the secreting cell. Although the aerolysin which is so produced is still a dimer, it then has the ability to oligomerize. The oligomer is the active form of the toxin, capable of forming the transmembrane channels that disrupt cells. We review here the present knowledge about the structure of aerolysin in relation to the various steps in channel formation.

Nucleotide sequences and characterization of haemolysin genes from Aeromonas hydrophila and Aeromonas sobria

Extracellular haemolysin is thought to be one of the important virulence factors in Aeromonas infection. Two extracellular haemolysin genes (AHH3 and AHH4) from Aeromonas hydrophila strain 28SA, one (AHH5) from A. hydrophila strain AH-1 and one (ASA1) from Aeromonas sobria strain 33 were cloned into cosmid and plasmid vector DNA in Escherichia coli. The nucleotide sequences of the open reading frames of AHH3 and AHH4 are both 1476 basepairs (bp), whereas AHH5 and ASA1 are 1455 and 1467 bp in length, respectively. The deduced amino acid sequences of AHH3, AHH4, AHH5 and the previously reported aerolysin from A. hydrophila showed a significant degree of sequence homology of over 90% each. The amino acid identity of the ASA1 haemolysin and those from A. hydrophila and Aeromonas trota aerolysins ranged from 58-68%. From DNA hybridization analysis using our cloned haemolysin genes as probes, we found that the AHH5 and ASA1 DNA probes hybridized with about 31 and 75% strains of motile Aeromonas species, respectively. The activity of haemolysins of cloned genes were different in medium agar containing various erythrocytes.

Cloning and characterization of fxp, the flexible pilin gene of Aeromonas hydrophila

The flexible pilus of Aeromonas hydrophila is a morphologically and biochemically unique organelle which binds eukaryotic cell surfaces and whose expression is induced by specific physiochemical conditions. fxp, the structural gene coding for the flexible pilus subunit, was localized on a 7.6kb plasmid of A. hydrophila strain AH26. A putative Shine-Dalgarno sequence and -10 and -35 regions were identified, a signal peptide sequence delineated, and the coding sequence compared with other bacterial sequences and found to be unique. Plasmid and chromosomal DNA was prepared from 66 other Aeromonas strains and 12 strains from other bacterial genera and examined by Southern blot hybridization using a labelled fxp oligonucleotide and the 7.6kb plasmid as probes. No hybridizing sequences were identified except in the original strain, AH26. It is proposed that fxp codes for a highly evolved organelle, possibly widely distributed in nature, but that it is carried on a genetic element that is rapidly lost from most strains upon in vitro cultivation and storage.

Structural features of the pore formed by Staphylococcus aureus alpha-toxin inferred from chemical modification and primary structure analysis

Staphylococcus aureus alpha-toxin makes cells and model membranes permeable to ions and uncharged molecules by opening oligomeric pores of uniform size. Its primary sequence reveals peculiar features which give some hints on the structure of the pore. A flexible region separating the toxin into two halves, several amphiphilic beta-strands and two amphiphilic alpha-helices long enough to span the hydrophobic core of the lipid bilayer are predicted. In analogy to bacterial porins, we propose that the inner walls of the pore are, at least in part, built by an amphiphilic beta-barrel. The model is consistent with circular dichroism data and with the electrophysiological properties of the pore. Functional information on this toxin were obtained by chemical modification of its four histidine residues. Specific carbethoxylation suggested they have different roles: one is required for specific receptor binding, one for oligomerisation and two for unspecific lipid binding. A tentative assignment of each histidine to its specific role is done on the basis of the structural predictions. A functionally related hemolysin, Aeromonas hydrophyla aerolysin, reveals remarkably similar features including the presence and location of histidines involved in receptor binding and oligomerisation.

The channel-forming toxin aerolysin

Aeromonas sp. secrete a precursor of the cytolytic protein aerolysin into the culture medium, where it is activated by proteolytic removal of a C-terminal fragment. Activation can be achieved by a variety of mammalian proteases as well as by proteases released by the bacteria itself. Activated toxin binds with high affinity to the transmembrane protein glycophorin on the surface of eucaryotic cells. Binding is followed by oligomerization and the formation of transmembrane channels, leading to cell death. Using chemical modification and site-directed mutagenesis, we have identified regions of the molecule which are important in transfer across the outer membrane of the bacteria, and in proteolytic activation, binding, and oligomerization. A preliminary electron density map of proaerolysin crystals indicates that the protein is organized into three domains. Analysis of two-dimensional crystals of aerolysin suggests that the oligomeric form of the protein is heptameric.

Crossing three membranes. Channel formation by aerolysin

Aerolysin is a channel-forming toxin responsible for the pathogenicity of Aeromonas hydrophila. It crosses the inner and outer membranes of the bacteria in separate steps and is released as a 52-kDa inactive protoxin which is activated by proteolytic removal of approximately 40 amino acids from the C terminus. The toxin binds to the erythrocyte transmembrane protein glycophorin and oligomerizes before inserting into the membrane, producing a voltage gated, anion selective channel about 1 nm in diameter. Remarkably, proaerolysin appears to be dimeric, whereas the oligomer is a heptamer. Using chemical modification and site-directed mutagenesis, we have identified some of the regions of the molecule which appear to be involved in secretion and in channel formation.

Coordinate regulation of virulence genes in Listeria monocytogenes requires the product of the prfA gene

The prfA gene of Listeria monocytogenes encodes a protein that activates transcription of the listeriolysin gene (lisA). In order to explore the role of the prfA gene product in the pathogenesis of listerial infection, we constructed a site-directed insertion mutation in prfA by the chromosomal integration of a novel suicide vector containing a portion of the prfA coding region. This mutation not only transcriptionally silenced the listeriolysin (lisA) gene but also abrogated production of specific RNA transcripts corresponding to the phosphatidylinositol-specific phospholipase C (pic) and metalloprotease (mpl) genes, two further virulence gene products expressed only by pathogenic Listeria strains. The strain was also found to be avirulent when tested in a mouse model of listerial infection. The concomitant loss of multiple characteristics such as production of LisA, Pic, Mpl, and loss of virulence in a mouse infection model is the result of a mutation in a single gene and demonstrates that the prfA gene product is a positive regulator of multiple virulence determinants in L. monocytogenes.

Nucleotide sequence and expression of an extracellular hemolysin gene of Aeromonas hydrophila

The extracellular hemolysin (AHH1) gene of Aeromonas hydrophila ATCC7966 was cloned into Charomid9-28 in Escherichia coli DH1, and its complete nucleotide sequence determined. Escherichia coli carrying this gene expressed an extracellular heat-labile hemolysin for rabbit red blood cells. The minimum size of the coding region of the 2.6 kilobase-pair BamHI-SphI fragment was subcloned into pUC118 and pUC119, selecting for hemolytic activity. The nucleotide sequence of this region contained a single open reading frame of 1734 base pairs, corresponding to a protein of 577 amino acid residues (63,658 daltons). A consensus promoter sequence was present upstream of the AHH1 open reading frame. Maxicell analysis of [35S]methionine-labelled proteins in E. coli CSR603 carrying the AHH1 plasmid suggested that AHH1 gene codes for an approximately 60,000 dalton polypeptide. By colony DNA-DNA hybridization analysis, the AHH1 gene was detected in 43 of 62 hemolysin-producing strains of A. hydrophila (isolated from various sources and areas) and in all 43 hemolysin-producing strains of A. salmonicida (isolated from fish). Three hemolysin-negative strains of A. hydrophila did not react with the AHH1 probe, whereas three non-hemolytic A. salmonicida strains hybridized with the probe.

Pore-forming cytolysins of gram-negative bacteria

A great deal is known about the structure, function and metabolic effects of enzymatic bacterial toxins such as the diphtheria, pertussis and cholera toxins. By comparison, our understanding of the pore-forming, cytolytic toxins, particularly those produced by Gram-negative bacterial pathogens, is far less complete. The genetics and biochemistry of a large, newly discovered family of calcium-dependent, pore-forming cytotoxins (RTX toxins) produced by different genera of the Enterobacteriaceae and Pasteurellaceae are discussed in this review. This toxin family is especially noteworthy because the individual toxins often exhibit different cell- and host-specificity. A brief review is also included of two ancestrally unrelated groups of calcium-independent, pore-forming toxins, the haemolysins produced by Proteus mirabilis and Serratia marcescens and the aerolysins secreted by species of Aeromonas. Their structure and function are contrasted with those of the RTX family members. Emerging questions about the role of cytolysins in pathogenesis are presented. Perhaps the most important issue raised is whether or not less attention should be paid to the lytic capacity of these cytotoxins, with more energy being devoted to the understanding of their non-lytic inhibitory activities against host cells.

Pore-forming bacterial protein hemolysins (cytolysins)

Protein toxins forming pores in biological membranes occur frequently in Gram-positive and Gram-negative bacteria. They kill either bacteria or eukaryotic cells (at most, a few seem to act on both groups of organisms). Most of the toxins affecting eukaryotes have clearly been shown to be related to the pathogenicity of the producing organisms. Toxin formation frequently involves a number of genes which encode the toxin polypeptide as well as proteins for toxin activation and secretion. Regulation of toxin production is usually coupled with that of the synthesis of a number of other virulence factors. Iron is the only known environmental factor that regulates transcription of a number of toxin genes by a Fur repressor-type mechanism, as has been originally described in Escherichia coli. Interestingly, the thiol-activated hemolysins (cytolysins) of Gram-positive bacteria contain a single cysteine which can be replaced by alanine without affecting the cytolytic activity. The Gram-negative hemolysins (cytolysins) are usually synthesized as precursor proteins, then covalently modified to yield an active hemolysin and secreted via specific export systems, which differ for various types of hemolysins.

Detection of the aerolysin gene in Aeromonas hydrophila by the polymerase chain reaction

Synthetic oligonucleotide primers were used in a polymerase chain reaction (PCR) technique to detect the gene for aerolysin in strains of Aeromonas hydrophila and to screen for identical genes in A. caviae, A. sobria, and A. veronii isolated from patients with diarrheal disease. Primers targeted a 209-bp fragment of the aer gene coding for the beta-hemolysin and detected template DNA only in the PCR using nucleic acid (NA) from hemolytic strains of A. hydrophila which were also cytotoxic to Vero and CHO cells and enterotoxic in suckling-mouse assays. PCR amplification of NA from hemolytic A. sobria or nonhemolytic A. hydrophila and A. caviae strains was consistently negative. Primer specificity was determined in the PCR by using NA extracted from 56 strains of bacteria, including hemolytic Escherichia coli and Listeria monocytogenes as well as several recognized enteric pathogens defined in terms of their toxigenicity. The detection limit for the aerolysin gene by PCR amplification was 1 ng of total NA. The PCR clearly identified aerolysin-producing strains of A. hydrophila and may have application as a species-specific virulence test because other hemolytic Aeromonas species tested were negative.

Hemolysin as a marker for Serratia

All Serratia marcescens strains (total of 33) of different sources were hemolytic including clinical strains previously classified as being nonhemolytic. DNA fragments of the two hemolysin genes hybridized with the chromosomal DNA of S. marcescens, S. liquefaciens, S. kiliensis, S. grimesii, S. proteamaculans, S. plymutica, S. rubridaea which were also hemolytic. The restriction pattern of the hemolysin locus differed in each strain. S. ficaria and S. marinorubra expressed a different hemolysin which was much smaller than the S. marcescens hemolysin since it diffused through dialysis membranes. The DNA of the latter strains did not hybridize with the S. marcescens hemolysin DNA probes. Some S. marcescens strains, S. kiliensis and S. liquefaciens also expressed in addition the small hemolysin. No hybridization was found with DNA of Escherichia coli, Salmonella typhimurium, Proteus mirabilis, Proteus vulgaris, Citrobacter freundii, Enterobacter cloacae, Klebsiella arerogenes, Klebsiella pneumoniae, Shigella dysenteriae, Yersinia enterocolitica, Yersinia pseudotuberculosus, Listeria sp., Aeromonas sp., Legionella sp. and a Meninococcus sp., indicating that the hemolysin DNA probes are specific for Serratia, or that the hemolysin genes occur rarely in genera other than Serratia.

Aerolysin of Aeromonas sobria: evidence for formation of ion-permeable channels and comparison with alpha-toxin of Staphylococcus aureus

Aerolysin from Aeromonas sobria AB3 was isolated and purified. The pure toxin formed sodium dodecyl sulfate-insoluble oligomers in a lipidic environment. The addition of aerolysin to the aqueous phase bathing lipid bilayer membranes resulted in the formation of ion-permeable channels which had a single-channel conductance of about 70 pS in 0.1 M KCl. This defines the toxin as a channel-forming component similar to other toxins but without any indication for an association-dissociation reaction, since the channels had a long lifetime at low voltages. At voltages higher than 50 mV, the aerolysin channel switched into a closed state with a low residual conductance. The single-channel conductance was a linear function of the total aqueous conductance, which suggested that the toxin oligomers formed aqueous channels with an estimated minimal diameter of about 0.7 nm. The aerolysin pores were found to be slightly anion selective. The pore-forming properties of aerolysin were compared with those of alpha-toxin of Staphylococcus aureus. Both aerolysin and alpha-toxin share secondary structure features, must oligomerize to form pores in lipid bilayer membranes, and form channels with similar properties.

Pertussis toxin promoter sequences involved in modulation

Previous analysis of the pertussis toxin (PT) promoter has shown that expression of PT requires a trans-activating factor encoded by the vir locus and a 170-base-pair DNA sequence upstream from the transcription start site containing a 21-base-pair direct repeat sequence crucial trans-activation (R. Gross and R. Rappuoli, Proc. Natl. Acad. Sci. USA 85:3913-3917, 1988). In this paper we extend the analysis to the modulative response to environmental stimuli. We show that modulation acts at the transcriptional level and occurs only in phase I bacteria. Modulation also requires a functional vir locus and the same promoter region of 170 base pairs. We show that, in addition to the previously identified direct repeat, even the sequences downstream from position -117 are required for trans-activation and modulation and that the deletion of four cytosine residues at position -31 causes the inactivation of the promoter. The kinetics of the change in transcription show that the PT promoter can be shut off very rapidly by adding 50 mM MgSO4 to the medium, whereas resumption of transcription after removal of the modulative agents from the medium is slow.

Physiological control of amonabactin biosynthesis in Aeromonas hydrophila

Amonabactin is a siderophore from Aeromonas hydrophila which is produced in two biologically active forms composed of the phenolate 2,3-dihydroxybenzoic acid (DHB), lysine, glycine, and either trytophan (amonabactin T) or phenylalanine (amonabactin P). Amonabactin biosynthetic mutants (generated by chemical mutagenesis) that either produced no amonabactin or overproduced the siderophore were isolated and identified on chrome azurol S siderophore detection agar. Amonabactin-negative mutants were of two categories. One type produced no phenolates and used exogenous DHB to synthesize amonabactin (both forms) while the other type excreted DHB but not amonabactin. This suggests an amonabactin biosynthetic pathway composed of two segments, one producing DHB and the other assembling amonabactin from DHB and the amino acids. Overproduction mutants used amonabactin poorly or not at all, indicating that they contained lesions in amonabactin utilization. Adding the analog D-tryptophan to wild-type A. hydrophila cultures reduced synthesis of both amonabactin T and amonabactin P and lengthened the lag phase in iron restricted medium. The tryptophan and phenylalanine forms of amonabactin may be synthesized by a single assembly pathway that contains a novel enzyme (sensitive to D-tryptophan) which inserts either tryptophan or phenylalanine into amonabactin.