Secreted enzymes of Aeromonas

Toxicity of Crude Extracellular Products of Aeromonas hydrophila on Rohu, Labeo rohita (Ham.)

In the present study the haemolytic and proteolytic activity of extracellular products (ECP) secreted from Aeromonas hydrophila (CAHH14 strain) were studied with respect to temperature and different time of incubation as well as its lethal toxicity on rohu, Labeo rohita. The strain was isolated from Catla catla (showing abdominal dropsy symptom) collected from the pond of Central Institute of Freshwater Aquaculture (CIFA), Bhubaneswar, India and was characterized on the basis of biochemical tests. The highest production of haemolysin was achieved when the bacteria was grown at 35°C for 30 h. The proteolytic activity was found to be highest when the bacterium was grown at 30°C for 36 h. The haemolytic and proteolytic toxin produced by Aeromonas hydrophila was found to be lethal to rohu (LD(50) 1.7 × 10(4) cfu/ml). The lethality of ECP was decreased by heating and completely inactivated by boiling at 100°C for 10 min. This indicates that protease activity and haemolytic activity of A. hydrophila ECP was temperature dependant.

Molecular quantification of virulence gene-containing Aeromonas in water samples collected from different drinking water treatment processes

Pathogenic species of Aeromonas produce a range of virulence factors, including aerolysin, cytotonic enterotoxins, and serine protease, to cause acute gastroenteritis and wound infections in humans and animals. Recognizing that not all Aeromonas strains are pathogenic, in this study, we proposed to evaluate Aeromonas removal effectiveness based on the presence of virulence gene-containing Aeromonas as a proper means to assess microbial risk of Aeromonas. We developed and applied real-time PCR assays to quantify serine protease (ser) gene- and heat-labile cytotonic enterotoxin (alt) gene-containing Aeromonas in water samples. Among 18 Aeromonas isolates from the source water, only three isolates possessed all three genes (aer, ser, and alt). A higher percent of isolates has either ser gene (89%) or alt gene (72%) compared to the percent of isolates containing aer gene (44%). Results of this study suggested that several different conventional and unconventional drinking water treatment processes could effectively remove Aeromonas from source water. As the comprehensive knowledge of the distribution of virulence factors in different Aeromonas species is currently not available, using real-time PCR to quantify various virulence factor genes in water samples and/or isolates can be a practical means for better assessment of microbial risks in water.

Isolation and characterization of virulent Aeromonas veronii from ascitic fluid of oscar Astronotus ocellatus showing signs of infectious dropsy

The cichlid oscar Astronotus ocellatus has worldwide commercial value in the pet fish industry because of its early maturation, relatively high fecundity, ability to identify its caretaker and also to alter colouration amongst conspecifics. Pathogenic strains of Aeromonas veronii resistant to multiple antibiotics were isolated from A. ocellatus individuals showing signs of infectious abdominal dropsy. The moribund fish showed haemorrhage in all internal organs, and pure cultures could be obtained from the abdominal fluid. The isolates recovered were biochemically identified as A. veronii biovar sobria and genetically confirmed as A. veronii based on 16S rRNA gene sequence analysis (GenBank accession no. FJ573179). The RAPD profile using 3 primers (OPA-3, OPA-4 and OPD-20) generated similar banding patterns for all isolates. They displayed cytotoxic and haemolytic activity and produced several exoenzymes which were responsible for the pathogenic potential of the isolates. In the representative isolate MCCB 137, virulence genes such as enterotoxin act, haemolytic toxin aerA, type 3 secretion genes such as aexT, ascVand ascF-ascG, and gcat (glycerophospholipid-cholesterol acyltransferase) could be amplified. MCCB 137 exhibited a 50% lethal dose (LD50) of 10(5.071) colony-forming units ml(-1) in goldfish and could be subsequently recovered from lesions as well as from the internal organs. This is the first description of a virulent A. veronii from oscar.

A differential proteomics study of Caenorhabditis elegans infected with Aeromonas hydrophila

The striking similarities between the innate defences of vertebrates and invertebrates as well as the amenability of Caenorhabditis elegans for genetic analysis, have made this free-living ground nematode a popular model system in the study of bacterial pathogenesis. Although genetic studies have brought new insights, showing the inducibility and pathogen-specificity of the immune response, there is still much to be discovered about the exact mechanisms underlying resistance to infection. In this paper a different angle was adopted to study host-pathogen interactions in C. elegans. We report the application of differential gel electrophoresis (DIGE), combined with mass spectrometry to search for proteins that are differentially synthesised in the worm after infection with the gram-negative bacterium Aeromonas hydrophila. Given the dynamic nature of an immune response, the proteome of C. elegans was investigated at three different time-points after infection. A total of 65 differential proteins were identified. This study confirms the involvement of galectins, C-type lectins and lipid binding proteins in the immunity of C. elegans. In addition a number of unknown proteins, which might represent important players of the worm's defence system, were isolated and identified. This work gives a first indication of the complex changes that occur at the protein level during infection.

Diversity of extracellular proteases among Aeromonas determined by zymogram analysis

AIMS

The current research was aimed at comparing extracellular proteolytic activities and zymogram profiles among Aeromonas spp.

METHODS AND RESULTS

Extracellular proteases of 47 strains of Aeromonas were analyzed by substrate (casein and gelatin) co-polymerized SDS-PAGE, and caseinolytic activity was determined using azocasein. Large variation on caseinolytic activity was evidenced. In general, the caseinolytic activity of Aeromonas hydrophila strains was significantly higher than that of other Aeromonas species. Several caseinolytic and gelatinolytic profiles were detected in Aeromonas. Cluster analysis allowed separating Aeromonas strains in four and three groups, based on their caseinolytic and gelatinolytic profiles, respectively. Although not specific patterns were evident, most Aer. hydrophila strains were clustered together and differed from Aeromonas caviae strains. The main caseinases of Aer. hydrophila were a serine protease with an apparent molecular weight (AMW) of 56 kDa and a metalloprotease with AMW of 22 kDa. Gelatinase profiles were characterized by the presence of high molecular weight metalloproteases (84 and 93 kDa), although the most active enzyme was a serine protease with AMW of 56 kDa. Other new caseinases and gelatinases were detected in specific Aeromonas strains.

CONCLUSIONS

Aeromonas strains exhibited several extracellular proteolytic profiles, with a larger inter than intraspecific variation. Moreover, zymogram analyses allowed identifying new caseinases and gelatinases in Aeromonas.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report on the intra- and interspecific variation of proteolytic profiles in Aeromonas determined by zymogram analysis, including the detection of new caseinases and gelatinases in this genus.

Polyhydroxyalkanoates production from carbohydrates by a genetic recombinant Aeromonas sp

AIMS

To develop an Aeromonas strain able to utilize inexpensive carbon sources such as starch for the synthesis of polyhydroxyalkanoates (PHA).

METHODS AND RESULTS

A recombinant Aeromonas sp. (strain KC007-1) was constructed by introducing the PHB synthesis genes (phaCAB) into the bacterium. Strain KC001-R1 can not only use carbohydrate (including starch) for growth but also accumulate significant amounts of polyhydroxybutyrate (PHB) in the cells.

CONCLUSIONS

One of the present focuses on PHA production has been on lowering the production costs. Starch is an example of an inexpensive carbohydrate for use in industrial production of PHA. We have demonstrated that by introducing the phaCAB operon into Aeromonas sp. allowed the bacterium able to accumulated PHB using this substrate.

SIGNIFICANCE AND IMPACT OF THE STUDY

Aeromonas spp. are able to synthesize PHA using fatty acids as carbon source. Although good robust growth results with use of starch as sole carbon source for Aeromonas, PHA synthesis does not occur. Strain KC007-R1 showed the ability to accumulate PHA in relative high amount with both carbohydrates and fatty acids as carbon source, and can be cultivated to a significant amount of cell mass and hence is a potential strain for further development for industrial applications.

Detection of aerolysin gene in Aeromonas hydrophila isolated from fish and pond water

Aerolysin is a hemolytic toxin encoded by aerolysin gene (1482 bp) that plays a key role in the pathogenesis of Aeromonas hydrophila infection in fish. New speciesspecific primers were designed to amplify 326 bp conserved region of aerolysin gene for A. hydrophila. Twenty-five isolates of A. hydrophila recovered from fish and pond water were studied for detection of aerolysin gene. Aerolysin gene was detected in 85% of the isolates during the study. The designed primers were highly specific and showed no cross reactivity with Escherichia coli, Aeromonas veronii, Vibrio cholerae, Flavobacterium spp., Chyseobacterium spp. and Staphylococcus aureus. The sensitivity limit of primers for detection of aerolysin gene in the genomic DNA of A. hydrophila was 5 pg.

Gene cloning, expression and homology modeling of hemolysin gene from Aeromonas hydrophila

Hemolysin is a significant toxin secreted by Aeromonas hydrophila, which contributes pathogenicity of fish to humans. The complete ORF of hemolysin gene (1886 bp) was amplified using PCR. It was cloned in TA and sub-cloned in pET28a vector then transformed into Escherichia coli BL21(DE3) codon plus RP cells expressed by the induction with 1.0 mM of IPTG. The expected size of expressed protein was 68.0 kDa estimated by migration in 12% SDS-PAGE. Anti-His monoclonal antibodies were used to substantiate the recombinant protein by Western blotting. The percent similarity between hemolysin of A. hydrophila with other hemolytic toxins revealed that the hemolysin/aerolysin/cytotoxin sequence varied from 99.35 to 50.40%. Homology modeling was used to construct 3-D structure of hemolysin of A. hydrophila with the known crystal 3-D structure (PDB: 1XEZ). This protein can be used for immunoassays and it is suitable for vaccine candidate against A. hydrophila infection.

Cold active microbial lipases: some hot issues and recent developments

Lipases are glycerol ester hydrolases that catalyze the hydrolysis of triglycerides to free fatty acids and glycerol. Lipases catalyze esterification, interesterification, acidolysis, alcoholysis and aminolysis in addition to the hydrolytic activity on triglycerides. The temperature stability of lipases has regarded as the most important characteristic for use in industry. Psychrophilic lipases have lately attracted attention because of their increasing use in the organic synthesis of chiral intermediates due to their low optimum temperature and high activity at very low temperatures, which are favorable properties for the production of relatively frail compounds. In addition, these enzymes have an advantage under low water conditions due to their inherent greater flexibility, wherein the activity of mesophilic and thermophilic enzymes are severely impaired by an excess of rigidity. Cold-adapted microorganisms are potential source of cold-active lipases and they have been isolated from cold regions and studied. Compared to other lipases, relatively smaller numbers of cold active bacterial lipases were well studied. Lipases isolated from different sources have a wide range of properties depending on their sources with respect to positional specificity, fatty acid specificity, thermostability, pH optimum, etc. Use of industrial enzymes allows the technologist to develop processes that closely approach the gentle, efficient processes in nature. Some of these processes using cold active lipase from C. antarctica have been patented by pharmaceutical, chemical and food industries. Cold active lipases cover a broad spectrum of biotechnological applications like additives in detergents, additives in food industries, environmental bioremediations, biotransformation, molecular biology applications and heterologous gene expression in psychrophilic hosts to prevent formation of inclusion bodies. Cold active enzymes from psychrotrophic microorganisms showing high catalytic activity at low temperatures can be highly expressed in such recombinant strains. Thus, cold active lipases are today the enzymes of choice for organic chemists, pharmacists, biophysicists, biochemical and process engineers, biotechnologists, microbiologists and biochemists.

Phylogenetic analysis of intestinal bacteria and their adhesive capability in relation to the intestinal mucus of carp

AIMS

The aims of the present study are to characterize the intestinal microbial community displaying a high-adhesive capability in fish, and to evaluate the relationship between mucosal adhesion of intestinal bacteria and fish health and disease.

METHODS AND RESULTS

A total of 707 aerobic bacteria isolated from carp intestine that were maintained under either feeding (feeding group) or no-feeding (no-feeding group) conditions and were performed adhesive assay. Isolates were divided into three categories on the basis of adhesive capability: high-, medium-, and low- adhesive capabilities. The average proportions of isolates with high-adhesive capability in the feeding and no-feeding groups were 30% and 32%, respectively. A phylogenetic analysis using a partial 16S rRNA gene demonstrated that most isolates with high-adhesive capability in both groups were classified as belonging to an Aeromonas group, and populations of isolates within high- and low-adhesive categories were markedly different.

CONCLUSIONS

Intestinal bacteria with a high-adhesive capability in relation to intestinal mucous always colonize on the surface of intestinal mucosa and grow in the intestinal tract of feeding carp. The adhesive capability of intestinal bacteria is essential for colonization and growth in the intestinal tract of fish.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results indicate that members of the Aeromonas group with adhesive capability always colonize on the surface of intestinal mucosa.

Molecular cloning and sequence analysis of lamB encoding outer membrane maltose-inducible porin of Aeromonas hydrophila

Aeromonas hydrophila is a significantly important pathogen causing major diseases in humans and fresh water fish. The outer membrane proteins (OMP) which are strong immunogens have been reported to act as adhesins aiding in the attachment of enteropathogenic bacteria. It is of interest to investigate the role of OMP in pathogenesis and their potential as vaccine candidates. In our laboratory, we cloned the gene encoding channel protein LamB porin of A. hydrophila. DNA sequence analysis revealed a full length gene of 1345 bp having a high level of homology with the lamB gene of different bacteria. Open reading frame of A. hydrophila lamB consists of a signal peptide of 25 amino acids, two protein translation start sites ATG present at the 31st and 37th base pairs, a translation termination codon, TAA at 1333rd base pair.

Characterization of extracellular proteins produced by Aeromonas hydrophila AH-1

Aeromonas hydrophila is a ubiquitous Gram-negative bacterium which can cause motile aeromonad septicemia in both fish and humans. A. hydrophila secretes many extracellular proteins associated with pathogenicity and environmental adaptability. In this study, an extracellular proteome map of A. hydrophila AH-1 was constructed. The major extracellular virulence factors were characterized by comparing the proteomes of various deletion mutants with that of the wild type. The results suggested that serine protease was involved in the processing of a toxin and secreted enzymes such as hemolysin, glycerophospholipid-cholesterol acyltransferase and metalloprotease. We also showed that expressions of polar and lateral flagellins were under the control of temperature, FlhA, LafK, and RpoN. In addition, three novel proteins (potential effector proteins including one ExoT-like protein) were revealed to be secreted via the type III secretion system (TTSS) of A. hydrophila AH-1. Another novel finding was the demonstration of a crosstalk between the lateral flagellar system and the TTSS in A. hydrophila. These results showed that proteomics is a powerful tool for characterizing virulence factors. The construction of proteome maps will provide a valuable means of finding potential candidates for developing suitable diagnostics and therapeutics for this emerging pathogen.

Identification and characterization of putative virulence genes and gene clusters in Aeromonas hydrophila PPD134/91

Aeromonas hydrophila is a gram-negative opportunistic pathogen of animals and humans. The pathogenesis of A. hydrophila is multifactorial. Genomic subtraction and markers of genomic islands (GIs) were used to identify putative virulence genes in A. hydrophila PPD134/91. Two rounds of genomic subtraction led to the identification of 22 unique DNA fragments encoding 19 putative virulence factors and seven new open reading frames, which are commonly present in the eight virulence strains examined. In addition, four GIs were found, including O-antigen, capsule, phage-associated, and type III secretion system (TTSS) gene clusters. These putative virulence genes and gene clusters were positioned on a physical map of A. hydrophila PPD134/91 to determine their genetic organization in this bacterium. Further in vivo study of insertion and deletion mutants showed that the TTSS may be one of the important virulence factors in A. hydrophila pathogenesis. Furthermore, deletions of multiple virulence factors such as S-layer, serine protease, and metalloprotease also increased the 50% lethal dose to the same level as the TTSS mutation (about 1 log) in a blue gourami infection model. This observation sheds light on the multifactorial and concerted nature of pathogenicity in A. hydrophila. The large number of putative virulence genes identified in this study will form the basis for further investigation of this emerging pathogen and help to develop effective vaccines, diagnostics, and novel therapeutics.

Role of Microfilaments and Microtubules in the Invasion of EPC Cells by Aeromonas hydrophila

The influence of the cytoskeleton on the invasion of Aeromonas hydrophila strain AhJ-1, isolated from diseased fish, in the monolayer cell of epithelioma papillosum cells of carp (EPC) was evaluated by the recovery of gentamicin-resistant bacteria from Triton X-100 cell lysates. The depolymerization of microfilaments (MF) by cytochalasin B and D inhibited the uptake of A. hydrophila in a dose-dependent manner and that of microtubules (MT) by colchicines and nocodazole did not affect the invasion of A. hydrophila in EPC cells significantly. The invasion frequency decreased approximately 62% with the addition of 0.1 microg/ml cytochalasin D and nearly 86% by the addition of 5.0 microg/ml. Invasion decreased approximately 49% and 83% by addition of cytochalasin B in a concentration of 2.5 microg/ml and 10.0 microg/ml. Colchicine and nocodazole, inhibitors of MT formation appears to have little effect on the invasion of EPC cells by strain Ah J-1. Thus MF formation, but not MT formation seems to play an important role in the internalization of A. hydrophila J-1.

Complete type III secretion system of a mesophilic Aeromonas hydrophila strain

We have investigated the existence and genetic organization of a functional type III secretion system (TTSS) in a mesophilic Aeromonas strain by initially using the Aeromonas hydrophila strain AH-3. We report for the first time the complete TTSS DNA sequence of an Aeromonas strain that comprises 35 genes organized in a similar disposition as that in Pseudomonas aeruginosa. Using several gene probes, we also determined the presence of a TTSS in clinical or environmental strains of different Aeromonas species: A. hydrophila, A. veronii, and A. caviae. By using one of the TTSS genes (ascV), we were able to obtain a defined insertion mutant in strain AH-3 (AH-3AscV), which showed reduced toxicity and virulence in comparison with the wild-type strain. Complementation of the mutant strain with a plasmid vector carrying ascV was fully able to restore the wild-type toxicity and virulence.

Population patterns and antimicrobial resistance ofAeromonasin urban playa lakes

Bacteria belonging to the genus Aeromonas are indigenous to aquatic environments. Once regarded as unimportant human pathogens, reports of opportunistic infections caused by these organisms have appeared increasingly in the medical literature. To estimate the potential for human infection by Aeromonas where limited water resources are being used intensively, we studied the spatial and temporal variation and incidence of antimicrobial resistance among environmental isolates of Aeromonas from two urban playa lakes in Lubbock, Texas. Aeromonas population densities varied seasonally, with the highest densities occurring from mid-April to late October. The greatest range of densities was 100-fold, from 2.50 to 255.17 colony-forming units per 0.1 mL of water sample. Densities also varied with water depth, although the variation did not display a consistent pattern. One hundred fifty-one Aeromonas isolates were divided into 10 species or subspecies groups by using the BIOLOG identification system. Nine isolates displayed resistance to co-trimoxazole, tetracycline, and cefuroxime, and none was resistant to more than one of these antimicrobial agents. In summary, the results of this study showed that the densities of Aeromonas peak in the late spring and again in late summer, times when human activity around the playa lakes is also high. Thus, we infer that human exposure to these potential pathogens varies seasonally. Compared to other published studies, the incidence of antimicrobial-resistant Aeromonas is relatively low in urban playa lakes in Lubbock, Texas. Nevertheless, resistant organisms were detected.Key words: Aeromonas, water, playa, antibiotic resistance, population dynamics.

A type III secretion system is required for Aeromonas hydrophila AH-1 pathogenesis

Aeromonas hydrophila is a gram-negative opportunistic pathogen in fish and humans. Many bacterial pathogens of animals and plants have been shown to inject anti-host virulence determinants into the hosts via a type III secretion system (TTSS). Degenerate primers based on lcrD family genes that are present in every known TTSS allowed us to locate the TTSS gene cluster in A. hydrophila AH-1. A series of genome walking steps helped in the identification of 25 open reading frames that encode proteins homologous to those in TTSSs in other bacteria. PCR-based analysis showed the presence of lcrD homologs (ascV) in all of the 33 strains of A. hydrophila isolated from various sources. Insertional inactivation of two of the TTSS genes (aopB and aopD) led to decreased cytotoxicity in carp epithelial cells, increased phagocytosis, and reduced virulence in blue gourami. These results show that a TTSS is required for A. hydrophila pathogenesis. This is the first report of sequencing and characterization of TTSS gene clusters from A. hydrophila. The TTSS identified here may help in developing suitable vaccines as well as in further understanding of the pathogenesis of A. hydrophila.

Molecular characterization of a glucose-inhibited division gene, gidA, that regulates cytotoxic enterotoxin of Aeromonas hydrophila

By using a mini-transposon, we obtained two mutated strains of a diarrheal isolate, SSU, of Aeromonas hydrophila that exhibited a 50 to 53% reduction in the hemolytic activity and 83 to 87% less cytotoxic activity associated with the cytotoxic enterotoxin (Act). Act is a potent virulence factor of A. hydrophila and has been shown to contribute significantly to the development of both diarrhea and septicemia in animal models. Subsequent cloning and DNA sequence analysis revealed that transposon insertion occurred at different locations in these two mutants within the same 1,890-bp open reading frame for the glucose-inhibited division gene (gidA). A similar reduction in hemolytic (46%) and cytotoxic (81%) activity of Act was noted in the gidA isogenic mutant of A. hydrophila that was generated by marker exchange mutagenesis. Northern blot analysis revealed that the transcription of the cytotoxic enterotoxin gene (act) was not altered in the gidA transposon and isogenic mutants. However, by generating a chromosomal act::alkaline phosphatase gene (phoA) reporter construct, we demonstrated significantly reduced phosphatase activity in these mutants, indicating the effect of glucose-inhibited division (GidA) protein in modulating act gene expression at the translational level. The biological effects of Act in the gidA mutants were restored by complementation. The virulence of the gidA mutants in mice was dramatically reduced compared to the those of the wild-type (WT) and complemented strains of A. hydrophila. The histopathological examination of lungs, in particular, indicated severe congestion, alveolar hemorrhage, and acute inflammatory infiltrate in the interstitial compartment and the alveolar spaces when mice were infected with the WT and complemented strains. Minimal-to-mild changes were noted in the lungs with the gidA mutants. Taken together, our data indicate for the first time that GidA regulates the most-potent virulence factor of A. hydrophila, Act.

Characterisation of Aeromonas spp. isolated from frozen fish intended for human consumption in Mexico

A total of 82 strains of presumptive Aeromonas spp. were identified biochemically and genetically (16S rDNA-RFLP). The strains were isolated from 250 samples of frozen fish (Tilapia, Oreochromis niloticus niloticus) purchased in local markets in Mexico City. In the present study, we detected the presence of several genes encoding for putative virulence factors and phenotypic activities that may play an important role in bacterial infection. In addition, we studied the antimicrobial patterns of those strains. Molecular identification demonstrated that the prevalent species in frozen fish were Aeromonas salmonicida (67.5%) and Aeromonas bestiarum (20.9%), accounting for 88.3% of the isolates, while the other strains belonged to the species Aeromonas veronii (5.2%), Aeromonas encheleia (3.9%) and Aeromonas hydrophila (2.6%). Detection by polymerase chain reaction (PCR) of genes encoding putative virulence factors common in Aeromonas, such as aerolysin/hemolysin, lipases including the glycerophospholipid-cholesterol acyltransferase (GCAT), serine protease and DNases, revealed that they were all common in these strains. Our results showed that first generation quinolones and second and third generation cephalosporins were the drugs with the best antimicrobial effect against Aeromonas spp. In Mexico, there have been few studies on Aeromonas and its putative virulence factors. The present work therefore highlights an important incidence of Aeromonas spp., with virulence potential and antimicrobial resistance, isolated from frozen fish intended for human consumption in Mexico City.

Sequence analysis and biochemical characterisation of chromosomal CAV-1 (Aeromonas caviae), the parental cephalosporinase of plasmid-mediated AmpC 'FOX' cluster

Aeromonas caviae CIP 74.32 was resistant to amoxicillin, ticarcillin and cephalothin, and susceptible to cefoxitin, cefotaxime, ceftazidime, aztreonam and imipenem. This strain produced a cephalosporinase (pI 7.2) and an oxacillinase (pI 8.5). The cephalosporinase gene cav-1 was cloned and sequenced. Unlike A. caviae donor, Escherichia coli pNCE50 transformant producing CAV-1 beta-lactamase was resistant to cefoxitin. The deduced protein sequence CAV-1 contained 382 amino acids, and shared >96% homology with FOX-1 to FOX-5 cephalosporinase. CAV-1 presented only two amino acid substitutions (Thr270Ser and Arg271Ala) with FOX-1. CAV-1 is the chromosomal putative ancestor of the FOX family, a cluster of class C/group 1 plasmidic cephalosporinases spreading in Klebsiella and E. coli clinical isolates via conjugative plasmids.

Stress related changes of cell surface hydrophilicity in Bacillus subtilis

The changes of cell surface hydrophilicity in Bacillus subtilis were analyzed in response to oxygen-limitation, heat shock, salt stress, pH-shock, phosphate- and carbon-limitation. Although cell surface hydrophilicity varied during growth phases, an increase of surface hydrophilicity was observed under several of these stress conditions. An observed drop in intracellular GTP and/or ATP may be an element of the signal transduction pathway leading to an increase in surface hydrophilicity in response to environmental stresses. Attachment of cells to soil particles under salt stress conditions is strongly influenced by the degS/degU two-component system, which thereby provides a mechanism for the bacteria to escape from the hostile environment.

Potential virulence and antimicrobial susceptibility of Aeromonas popoffii recovered from freshwater and seawater

Aeromonas popoffii is the most recent species within the genus Aeromonas described from freshwater. In our study this species was also recovered from this habitat and for the first time from seawater. Most of the virulence factors known in Aeromonas spp. (aerolysin/hemolysin, serine protease, lipases and DNases) were highly prevalent in this species. Third-generation cephalosporins and quinolones were the most active antimicrobial agents against A. popoffii.

Comparsion of selected growth media for culturing Serratia marcescens, Aeromonas sp., and Pseudomonas aeruginosa as pathogens of adult Stomoxys calcitrans (Diptera: Muscidae)

Stable flies, Stomoxys calcitrans (L.), were orally infected with Aeromonas sp., Pseudomonas aeruginosa (Schroeter), and Serratia marcescens Bizio that were cultured on egg-yolk media, nutrient broth, and fly egg media. Aeromonas and Serratia caused mortality when the bacteria were originally grown on egg-yolk medium. Pseudomonas was equally lethal regardless of the media on which it was cultured. A wild isolate of Aeromonas caused greater death than an isolate that had been passed through host flies and had been reisolated from killed flies. Mortality increased with bacterial dose for all species. Aeromonas and Serratia caused mortality within several days after ingestion, whereas Pseudomonas caused a gradual increase in mortality 3-7 d after ingestion. The pathologic activity of Aeromonas and Serratia required extracellular products produced when cells were grown in egg yolk medium. Aeromonas required both supernatant and cells from egg yolk medium, wereas Serratia required supernatant from egg yolk medium and cells from either nutrient broth or egg yolk medium. Mortality due to ingestion of Aeromonas was correlated with the presence of enzymes that cause alpha- and beta-hemolysis, while mortality following ingestion of Serratia was associated with alpha-hemolysins, elastases, and chitinases.

Production of serine protease of Aeromonas sobria is controlled by the protein encoded by the gene lying adjacent to the 3' end of the protease gene

We cloned a protease gene of Aeromonas sobria and determined its nucleotide sequence. The protease is composed of 624 amino acid residues and its calculated molecular weight is 66,737.7. The amino acid sequence showed the characteristic features of a bacterial serine protease. We expressed the protease gene in Vibrio parahaemolyticus from which the synthesized protease is secreted into the culture medium as the mature form, and purified the mature protease by successive column chromatographies. The size of the mature protease is 65,000 daltons and the amino acid sequence analysis revealed that a 24-amino acid peptide at the amino terminal of the precursor is removed from the mature protease. This peptide might function as a signal peptide in translocation across the inner membrane. Subsequently, we found that the protein, designated ORF2 protein, encoded by the gene lying adjacent to the 3' end of the protease gene plays an important role in production of the protease. Mutation of the ORF2 gene did not affect transcription of the protease gene, but resulted in degradation of the protease in the cell. This shows that ORF2 protein is required for the successful production of the serine protease by cell.

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.

Molecular analysis of genetic differences between virulent and avirulent strains of Aeromonas hydrophila isolated from diseased fish

Aeromonas hydrophila, a normal inhabitant of aquatic environments, is an opportunistic pathogen of a variety of aquatic and terrestrial animals, including humans. A. hydrophila PPD134/91 is defined as virulent whereas PPD35/85 is defined as avirulent on the basis of their different LD50 values in fish. Suppression subtractive hybridization (SSH) was used to identify genetic differences between these two strains. Sixty-nine genomic regions of differences were absent in PPD35/85, and the DNA sequences of these regions were determined. Sixteen ORFs encoded by 23 fragments showed high homology to known proteins of other bacteria. ORFs encoded by the remaining 46 fragments were identified as new proteins of A. hydrophila, showing no significant homology to any known proteins. Among these PPD134/91-specific genes, 22 DNA fragments (21 ORFs) were present in most of the eight virulent strains studied but mostly absent in the seven avirulent strains, suggesting that they are universal virulence genes in A. hydrophila. The PPD134/91-specific genes included five known virulence factors of A. hydrophila: haemolysin (hlyA), protease (oligopeptidase A), outer-membrane protein (Omp), multidrug-resistance protein and histone-like protein (HU-2). Another 47 DNA fragments (44 ORFs) were mainly present in PPD134/91, indicating the heterogeneity among motile aeromonads. Some of these fragments encoded virulence determinants. These included genes for the synthesis of O-antigen and type II restriction/modification system. The results indicated that SSH is successful in identifying genetic differences and virulence genes among different strains of A. hydrophila.

Enterotoxins in Aeromonas-associated gastroenteritis

Aeromonas species produce an array of virulence factors, and the pathogenesis of Aeromonas infections is therefore complex and multifactorial. Aeromonas-associated gastroenteritis is especially a problem in young children. The potential involvement of enterotoxins in the pathogenesis of Aeromonas infections is discussed.

Quorum sensing-dependent regulation and blockade of exoprotease production in Aeromonas hydrophila

In Aeromonas hydrophila, the ahyI gene encodes a protein responsible for the synthesis of the quorum sensing signal N-butanoyl-L-homoserine lactone (C4-HSL). Inactivation of the ahyI gene on the A. hydrophila chromosome abolishes C4-HSL production. The exoprotease activity of A. hydrophila consists of both serine protease and metalloprotease activities; in the ahyI-negative strain, both are substantially reduced but can be restored by the addition of exogenous C4-HSL. In contrast, mutation of the LuxR homolog AhyR results in the loss of both exoprotease activities, which cannot be restored by exogenous C4-HSL. Furthermore, a substantial reduction in the production of exoprotease by the ahyI+ parent strain is obtained by the addition of N-acylhomoserine lactone analogs that have acyl side chains of 10, 12, or 14 carbons. The inclusion of N-(3-oxododecanoyl)-L-homoserine lactone or N-(3-oxotetradecanoyl)-L-homoserine lactone at 10 microM in overnight cultures of A. hydrophila abolishes exoprotease production in azocasein assays and reduces the activity of all the exoprotease species seen in zymograms.

Cloning, sequencing, and role in virulence of two phospholipases (A1 and C) from mesophilic Aeromonas sp. serogroup O:34

Two different representative recombinant clones encoding Aeromonas hydrophila lipases were found upon screening on tributyrin (phospholipase A1) and egg yolk agar (lecithinase-phospholipase C) plates of a cosmid-based genomic library of Aeromonas hydrophila AH-3 (serogroup O34) introduced into Escherichia coli DH5alpha. Subcloning, nucleotide sequencing, and in vitro-coupled transcription-translation experiments showed that the phospholipase A1 (pla) and C (plc) genes code for an 83-kDa putative lipoprotein and a 65-kDa protein, respectively. Defined insertion mutants of A. hydrophila AH-3 defective in either pla or plc genes were defective in phospholipase A1 and C activities, respectively. Lecithinase (phospholipase C) was shown to be cytotoxic but nonhemolytic or poorly hemolytic. A. hydrophila AH-3 plc mutants showed a more than 10-fold increase in their 50% lethal dose on fish and mice, and complementation of the plc single gene on these mutants abolished this effect, suggesting that Plc protein is a virulence factor in the mesophilic Aeromonas sp. serogroup O:34 infection process.

Construction of a physical and preliminary genetic map of Aeromonas hydrophila JMP636

A physical and preliminary genetic map of the Aeromonas hydrophila JMP636 chromosome has been constructed. The topology of the genome was predicted to be circular as chromosomal DNA did not migrate from the origin during PFGE unless linearized by S1 nuclease. Cleavage of the chromosome with PacI and PmeI produced 23 and 14 fragments, respectively, and enabled calculation of the genome size at 4.5 Mb. Digestion of the chromosome with I-CeuI produced 10 fragments, indicating that 10 rrl (23S) genes were likely to be present. Hybridizations between DNA fragments generated with PacI, PmeI and I-CeuI were used to initially determine the relationship between these segments. To accurately map genes previously characterized from JMP636, the suicide vector pJP5603 was modified to introduce restriction sites for PacI and PmeI, producing pJP9540. Following cloning of genes into this vector and recombinational insertion into the JMP636 chromosome, PacI and PmeI cleavage determined the location of genes within macrorestriction fragments with the additional bands produced forming hybridization probes. From the data generated, it was possible to form a physical map comprising all the fragments produced by PacI and PmeI, and assign the contig of I-CeuI fragments on this map. The preliminary genetic map defines the location of six loci for degradative enzymes previously characterized from JMP636, while the locations of the 10 sets of ribosomal genes were assigned with less accuracy from hybridization data.