The pili of Aeromonas hydrophila: identification of an environmentally regulated "mini pilin"

Autoinducer-2 promotes adherence of Aeromonas veronii through facilitating the expression of MSHA type IV pili genes mediated by c-di-GMP

IMPORTANCE

Aeromonas veronii can adhere to host cells through different adherence factors including outer-membrane proteins (OMPs), lipopolysaccharide (LPS), and pili, but its adherence mechanisms are still unclear. Here, we evaluated the effect of autoinducer-2 (AI-2) on adherence of A. veronii and its regulation mechanism. After determination of the promotion effect of AI-2 on adherence, we investigated which adherence factor was regulated by AI-2, and the results show that AI-2 only limits the formation of pili. Among the four distinct pili systems, only the mannose-sensitive hemagglutinin (MSHA) type IV pili genes were significantly downregulated after deficiency of AI-2. MshE, an ATPase belonged to MSHA type IV pilin, was confirmed as c-di-GMP receptor, that can bind with c-di-GMP which is positively regulated by AI-2, and the increase of c-di-GMP can promote the expression of MSHA type IV pili genes and adherence of A. veronii. Therefore, this study confirms that c-di-GMP positively regulated by AI-2 binds with MshE, then increases the expression of MSHA pili genes, finally promoting adherence of A. veronii, suggesting a multilevel positive regulatory adhesion mechanism that is responsible for A. veronii adherence.

Secretion Systems in Gram-Negative Bacterial Fish Pathogens

Bacterial fish pathogens are one of the key challenges in the aquaculture industry, one of the fast-growing industries worldwide. These pathogens rely on arsenal of virulence factors such as toxins, adhesins, effectors and enzymes to promote colonization and infection. Translocation of virulence factors across the membrane to either the extracellular environment or directly into the host cells is performed by single or multiple dedicated secretion systems. These secretion systems are often key to the infection process. They can range from simple single-protein systems to complex injection needles made from dozens of subunits. Here, we review the different types of secretion systems in Gram-negative bacterial fish pathogens and describe their putative roles in pathogenicity. We find that the available information is fragmented and often descriptive, and hope that our overview will help researchers to more systematically learn from the similarities and differences between the virulence factors and secretion systems of the fish-pathogenic species described here.

Aeromonas flagella and colonisation mechanisms

Aeromonas species are inhabitants of aquatic environments and are able to cause disease in humans and fish among other animals. In aquaculture, they are responsible for the economically important diseases of furunculosis and motile Aeromonas septicaemia (MAS). Whereas gastroenteritis and wound infections are the major human diseases associated with the genus. As they inhabit and survive in diverse environments, aeromonads possess a wide range of colonisation factors. The motile species are able to swim in liquid environments through the action of a single polar flagellum, the flagellin subunits of which are glycosylated; although essential for function the biological role of glycan addition is yet to be determined. Approximately 60% of aeromonads possess a second lateral flagella system that is expressed in viscous environments for swarming over surfaces; both flagellar systems have been shown to be important in the initial colonisation of surfaces. Subsequently, other non-flagellar colonisation factors are employed; these can be both filamentous and non-filamentous. The aeromonads possess a number of fimbrial systems with the bundle-forming MSHA type IV pilus system, having a major role in human cell adherence. Furthermore, a series of outer-membrane proteins have also been implicated in the aeromonad adhesion process. A number of strains are also capable of cell invasion and that maybe linked with the more invasive diseases of bacteraemia or wound infections. These strains employ cell surface factors that allow the colonisation of these niches that protect them from the host's immune system such as S-layers, capsules or particular lipopolysaccharides.

Aeromonas hydrophila sepsis with septic embolism and rhabdomyolysis in a chronic iron overload haemodialysis patient treated with deferoxamine

Aeromonas infection in humans is associated with certain underlying diseases, especially chronic liver disease or malignancy. However, Aeromonas infection associated with iron overload is rarely reported. We report a case of a 47-year-old female with end-stage renal disease on haemodialysis and on deferoxamine treatment for iron overload who developed Aeromonas sepsis with septic embolism and rhabdomyolysis. Although the patients with Aeromonas infection and rhabdomyolysis have been correlated with high mortality, this reported case survived. We suggest that a chronic haemodialysis patient on deferoxamine treatment for iron overload is vulnerable to Aeromonas infection. In such cases, the clinician should be alerted to the possibility of rhabdomyolsis, and frequent haemodialysis is necessary.

Investigation of the role of type IV Aeromonas pilus (Tap) in the pathogenesis of Aeromonas gastrointestinal infection

Although there is substantial evidence that type IV pili purified from diarrhea-associated Aeromonas species (designated Bfp for bundle-forming pilus) are intestinal colonization factors (S. M. Kirov, L. A. O'Donovan, and K. Sanderson, Infect. Immun. 67:5447-5454, 1999), nothing is known regarding the function of a second family of Aeromonas type IV pili (designated Tap for type IV Aeromonas pilus), identified following the cloning of a pilus biogenesis gene cluster tapABCD. Related pilus gene clusters are widely conserved among gram-negative bacteria, but their significance for virulence has been controversial. To investigate the role of Tap pili in Aeromonas pathogenesis, mutants of Aeromonas strains (a fish isolate of A. hydrophila and a human dysenteric isolate of A. veronii bv. sobria) were prepared by insertional inactivation of the tapA gene which encodes the type IV pilus subunit protein, TapA. Exotoxic activities were unaffected by the mutation in tapA. Inactivation of tapA had no effect on the bacterial adherence of these two isolates to HEp-2 cells. For the A. veronii bv. sobria isolate, adhesion to Henle 407 intestinal cells and to human intestinal tissue was also unaffected. There was no significant effect on the duration of colonization or incidence of diarrhea when the A. veronii bv. sobria strain was tested in the removable intestinal tie adult rabbit diarrhea model or on its ability to colonize infant mice. Evidence was obtained that demonstrated that TapA was expressed by both Aeromonas species and was present on the cell surface, although if assembled into pili this pilus type appears to be an uncommon one under standard bacterial growth conditions. Further studies into factors which may influence Tap expression are required, but the present study suggests that Tap pili may not be as significant as Bfp pili for Aeromonas intestinal colonization.

Expression of Vibrio cholerae zonula occludens toxin and analysis of its subcellular localization

Vibrio cholerae elaborates zonula occludens toxin (Zot), a protein that increases the permeability of small intestinal mucosa by opening intercellular tight junctions. The zot gene is located, together with the genes encoding CT and Ace enterotoxins, within the genome of V. cholerae filamentous phage CTXsmall ef, Cyrillic. Interestingly, Zot appears to be structurally and functionally related to the gene I product of other filamentous phages and it has been shown to be required for CTXsmall ef, Cyrillic morphogenesis. In this study we described the cloning of zot in several expression plasmid systems and we examined the subcellular localization of Zot by using affinity purified anti-Zot antibodies. We found that Zot localizes in the V. cholerae cell envelope with M(r);45 kDa which is consistent with the predicted primary translation product from the first methionine of zot (44.8 kDa). A second molecule, corresponding to the 33 kDa N-terminal region of Zot, was also detected. Both molecules are exposed at the bacterial cell surface. The production of the 33 kDa Zot, that might represent a processing product, was abolished in mutant ZotG59. N-terminal tagged 6xHis-Zot fusion protein retained the capability to reach the outer membrane and the 6xHis tag was not cleaved off during the translocation to the periplasm, whereas the presence of the tag partially blocked the formation of the 33 kDa molecule. Zot secretion and anchorage to the bacterial outer membrane was also observed in E. coli strains expressing Zot, suggesting that the toxin may be directed to the outer membrane via the same pathway in E. coli and V. cholerae. Zot cleavage might be due to a V. cholerae specific protease activity, since the 33 kDa protein was not efficiently produced in E. coli. On the basis of these data and Zot amino acid sequence analysis, we suggest that while the N-terminal part of the molecule is involved in the morphogenesis of CTXsmall ef, Cyrillic, the C-terminal region might carry the domain(s) responsible for Zot enterotoxic activity.

Functional characterization of type IV pili expressed on diarrhea-associated isolates of Aeromonas species

Our past work has shown that long, flexible type IV pili (single or in bundles) are the predominant pili expressed on fecal isolates of diarrhea-associated species of Aeromonas (Aeromonas veronii biovar sobria and A. caviae). They represent a family of type IV pili which we have designated Bfp (for bundle-forming pili). Reports from Japan suggest that Bfp are intestinal colonization factors. This study presents compelling evidence to support this conclusion. Aeromonas bacteria and/or Bfp purified from a strain of A. veronii biovar sobria were shown to adhere to epithelial and intestinal cell lines, freshly isolated human enterocytes, and fresh and fixed human and rabbit intestinal tissues, as determined by light and electron microscopy and immunohistochemical detection. Removal of Bfp by mechanical means decreased adhesion to cell lines by up to 80%. Purified Bfp blocked adhesion of the test strain to intestinal cells in a dose-dependent manner. Adhesion was also blocked by the Fab fraction of anti-Bfp immunoglobulin G. Moreover, ultrastructural studies (ruthenium red staining and transmission and scanning electron microscopy) demonstrated for the first time that Aeromonas adhesion to human enterocytes is pilus mediated and suggested that Bfp may also promote colonization by forming bacterium-to-bacterium linkages. Bfp-positive isolates examined for type IV pilus-mediated twitching motility in agar and slide culture assays developed for Pseudomonas aeruginosa did not, however, exhibit this function.

Structural predictions of AgfA, the insoluble fimbrial subunit of Salmonella thin aggregative fimbriae

The unusually stable and multifunctional, thin aggregative fimbriae common to all Salmonella spp. are principally polymers of the fimbrin subunit, AgfA. AgfA of Salmonella enteritidis consists of two domains: a protease-sensitive, 22 amino acid residue N-terminal region and a protease-resistant, 109 residue C-terminal core. The unusual amino acid sequence of the AgfA core region comprises two-, five- and tenfold internal sequence homology patterns reflected in five conserved, 18-residue tandem repeats. These repeats have the consensus sequence, Sx5QxGx2NxAx3Q and are linked together by four or five residues, (x)xAx2. The predicted secondary structure for this unusual arrangement of tandem repeats in AgfA indicates mainly extended conformation with the beta strands linked by four to six residues. Candidate proteins of known structure with motifs of alternating beta strands and short loops were selected from folds described in SCOP as a source of coordinates for AgfA model construction. Three all-beta class motifs selected from the Serratia marcescens metalloprotease, myelin P2 protein or vitelline membrane outer protein I were used for initial AgfA homology build-up procedures ultimately resulting in three structural models; beta barrel, beta prism and parallel beta helix. The beta barrel model is a compact, albeit irregular structure, with the beta strands arranged in two antiparallel beta sheet faces. The beta prism model does not reflect the 5 or 10-fold symmetry of the AgfA primary sequence. However, the favored, parallel beta helix model is a compact coil of ten helically arranged beta strands forming two parallel beta sheet faces. This arrangement predicts a regular, potentially stable, C-terminal core region consistent with the observed tandem repeat sequences, protease-resistance and strong tendency of this fimbrin to oligomerize and aggregate. Positional conservation of amino acid residues in AgfA and the Escherichia coli AgfA homologue, CsgA, provides strong support for this model. The parallel beta helix model of AgfA offers an interesting solution to a multifunctional fimbrin molecular surface having solvent exposed areas, regions for major and minor subunit interactions as well as fiber-fiber interactions common to many bacterial fimbriae.

In vitro inhibition of epithelial cell invasion by Aeromonashydrophila Vibrio species by fish Aeromonas hydrophila major adhesin

A virulent strain of Aeromonas hydrophila (PPD 134/91) was obtained from the Primary Production Department, Singapore. Its major adhesin was isolated and purified by potassium thiocyanate extraction and Bio-Gel P-100 gel filtration. The ability of the protein in peak 1, termed major adhesin, to inhibit bacteria from adhering to and invading host cells was studied in vitro using epithelioma papillosum cells of carp (EPC). Results showed that a concentration of 10 μg ml^-1 of this major adhesin could competitively inhibit 28% of A. hydrophila PPD 134/91 from invading EPC cells in vitro. When the concentration was increased to 40 μg ml^-1 , the major adhesin significantly cross-inhibited nine other virulent or weakly virulent strains of A. hydrophila. In addition, the major adhesin significantly inhibited not only another bacterial strain from the same family, Aeromonas sobria, but also strains of Vibrio spp. tested. Therefore, we suggest that the major adhesin of this virulent A. hydrophila strain has the potential to be used as a vaccine against the heterogeneous Aeromonas and Vibrio species.

Characterization of filamentous phages of Vibrio cholerae O139 and O1

We have analyzed our collection of Vibrio cholerae O139 strains to determine whether filamentous phages are produced in their culture supernatants, and whether any replicative form of DNA is detectable in cell lysates. Two types of filamentous phage, designated fs1 (6.4 kb) and fs2 (8.5 kb), were found in strains of Vibrio cholerae O139, fs1 was commonly produced from clinical isolates of Vibrio cholerae O1. Infectious particles (filamentous phages) were inducible by subculture, mitomycin C, and cultivation in a ligated ileal loop of a rabbit. Type 4 fimbriae of Vibrio cholerae O1 sensitive to D-glucose and D-mannose were suggested to be receptors for fs1 and fs2. The genome of fs1 was revealed to encode a potential new enterotoxin homologous to zonula occludens toxin. Clarification of the relation of type 4 fimbriae and these filamentous phages will provide a new understanding of the colonization of Vibrio-cholerae O1 and O139. Thus the presence of a new enterotoxin encoded by the genome of filamentous phage like fs1 may clarify the pathogenesis of cholera toxin negative clinical isolates of Vibrio cholerae O1 and non-O1. Our findings combined with the earlier report by Ehara et al. [Microbio. Immunol. 37 (1993) 679-688] suggest that type 4 fimbriae of Vibrio cholerae O1 are important for the development of an effective vaccine against cholera.

Molecular characterization of the genes involved in O-antigen modification, attachment, integration and excision in Shigella flexneri bacteriophage SfV

Bacteriophage SfV is a temperate phage of Shigella flexneri responsible for converting serotype Y (3,4) to serotype 5a (V; 3,4) through its glucosyl transferase gene. The glucosyl transferase (gtr) gene of SfV has been cloned and shown to partially convert S. flexneri serotype Y to serotype 5a. In this study, we found that the serotype-converting region of SfV was approximately 2.5 kb in length containing three continuous ORFs. The recombinant strain carrying the three complete ORFs expressed the type V and group antigen 3,4, both indistinguishable from that of S. flexneri 5a wild-type strain. The interruption of orf5 or orf6 gave partial conversion in the S. flexneri recombinant strain indicated by the incomplete replacement of group antigen 3,4. The region adjacent to the serotype-conversion genes was found to be identical to the attP-int-xis region of phage P22. Altogether, an approximately 2.2-kb sequence covering a portion of the serotype-conversion (approximately 500 nt)-attP-int-xis regions of SfV was remarkably similar to that of P22.

Molecular analysis and expression of the extracellular lipase of Aeromonas hydrophila MCC-2

The structural gene encoding the extracellular lipase of Aeromonas hydrophila MCC-2 was cloned and found to be expressed in Escherichia coli using its own promoter. When the cloned gene (lip) was expressed in E. coli minicells, an 80 kDa protein was identified. Subcellular fractionation of E. coli carrying the lip gene indicated that the Lip protein was mainly associated with the membrane fraction. Nucleotide sequence analysis revealed that the gene is 2253 bp long, coding for a 79-9 kDa protein with an estimated pl of 10.36. The deduced protein contains two putative signal peptide cleavage sites: one is a typical signal peptidase cleavage site and the other bears a strong resemblance to known lipoprotein leader sequences. Radioactivity from [3H]palmitate was incorporated into the Lip protein when expressed in E. coli. The deduced protein contains a sequence of VHFLGHSLGA which is very well conserved among lipases. It shows 67% and 65% overall identity to the amino acid sequences of lipase from A. hydrophila strains H3 and JMP636, respectively, but shows little homology to those of other lipases. The Lip protein was purified to homogeneity from both A. hydrophila and recombinant E. coli. In hydrolysis of p-nitrophenyl esters and triacylglycerols, using purified enzyme, the optimum chain lengths for the acyl moiety on the substrate were C10 to C12 for ester hydrolysis and C8 to C10 for triacylglycerol hydrolysis.

Virulence factors-pathogenicity relationships for Aeromonas species from clinical and food isolates

The presence of virulence factors in 96 Aeromonas strains isolated from food and clinical samples was studied. Neither cytotoxic activity and hydrophobicity, not the presence of pili or an extra surface layer made it possible to establish differences between food and clinical strains. Statistical studies showed that cytotoxin production was associated with a positive Voges-Proskauer reaction, inability to ferment arabinose and a positive lysine decarboxylation. Therefore, when comparing cytotoxic clinical and food strains with lysine decarboxylation phenotype, there was a significant difference (p < 0.05) between the two groups. The association of a cytotoxin production and lysine decarboxylation character should thus be considered as a possible virulence marker.

Lysogenic conversion by a filamentous phage encoding cholera toxin

Vibrio cholerae, the causative agent of cholera, requires two coordinately regulated factors for full virulence: cholera toxin (CT), a potent enterotoxin, and toxin-coregulated pili (TCP), surface organelles required for intestinal colonization. The structural genes for CT are shown here to be encoded by a filamentous bacteriophage (designated CTXphi), which is related to coliphage M13. The CTXphi genome chromosomally integrated or replicated as a plasmid. CTXphi used TCP as its receptor and infected V. cholerae cells within the gastrointestinal tracts of mice more efficiently than under laboratory conditions. Thus, the emergence of toxigenic V. cholerae involves horizontal gene transfer that may depend on in vivo gene expression.

An environmentally regulated pilus-like appendage involved in Campylobacter pathogenesis

Examination of strains of Campylobacter jejuni, Campylobacter coli, and Campylobacter fetus by electron microscopy revealed that they produced peritrichous pilus-like appendages when the bacteria were grown in the presence of bile salts. Various bile-salt supplements were used and it was found that deoxycholate and chenodeoxycholic acid caused a significant enhancement of pilus production and resulted in a highly aggregative phenotype. Morphologically, the pili were between 4 and 7 nm in width and were greater than 1 micron in length. A gene, termed pspA, which encodes a predicted protein resembling protease IV of Escherichia coli, was identified in C. jejuni strain 81-176. A site-specific insertional mutation within this gene resulted in the loss of pilus synthesis as determined by electron microscopy. Insertions upstream and downstream of the gene had no effect on pilus production. The non-piliated mutant of strain 81-176 showed no reduction in adherence to or invasion of INT 407 cells in vitro. However, this mutant, while still possessing the ability to colonize ferrets, caused significantly reduced disease symptoms in this animal model.

Aeromonas trota strains, which agglutinate with Vibrio cholerae O139 Bengal antiserum, possess a serologically distinct fimbrial colonization factor

Pili of Aeromonas trota strain 1220, which agglutinates with Vibrio cholerae O139 Bengal antiserum, were purified and characterized. The molecular mass of the subunit protein was estimated to be 20 kDa and the pl was 5 center dot 4. The pili were immunologically unrelated to the other Aeromonas pili reported so far. However, the N-terminal amino acid sequence of the subunit pilin was similar to those of the pilins from other Aeromonas pili reported previously. Neither A. trota cells nor pili purified from strain 1220 agglutinated human and rabbit erythrocytes, but both adhered to the rabbit intestine. Bacterial cells pretreated with antipilus antibody (Fab portion) failed to adhere to the rabbit intestine. Moreover, bacteria did not adhere to the rabbit intestine pretreated with the purified pili. This pilus antigen was not detected in V. cholerae O139 Bengal and other Aeromonas spp. These findings suggest that the pilus of the A. trota strain is a novel colonization factor of Aeromonas spp.

Adhesion of Aeromonas sp. to cell lines used as models for intestinal adhesion

Adhesion to HEp-2 cells has been shown to correlate with enteropathogenicity for Aeromonas species. Such adhesion is thought to reflect the ability of strains to adhere to human intestinal enterocytes, although HEp-2 cells are not of intestinal origin. In this study strains of Aeromonas veronii biotype sobria isolated from various sources were investigated in parallel assays for their ability to adhere to HEp-2 cells and to an intestinal cell line (Caco-2). Quantitative assays showed identical adhesion values were obtained with both cell lines. Adhesion was best when bacteria were grown at 22 degrees C compared with 37 degrees C and 7 degrees C. Some environmental isolates showed greater adhesion when grown at 7 degrees C than when grown at 37 degrees C. Filamentous structures on these strains are also optimally expressed under the above conditions (reported elsewhere). Mechanical shearing or trypsin treatment to remove surface structures from several adhesive strains grown at 22 degrees C decreased adhesion to cell lines by 50-80% providing further indirect evidence that filamentous adhesins may play a role in cell adhesion for this Aeromonas species.

Electron microscopic examination of factors influencing the expression of filamentous surface structures on clinical and environmental isolates of Aeromonas veronii Biotype sobria

Strains of Aeromonas veronii biotype sobria isolated from clinical and environmental sources were examined for their expression of surface structures under a variety of culture conditions. When grown on solid media at 37 degrees C, more than 95% of bacteria from the majority of strains isolated from human diarrheal feces and chicken carcasses were non-piliated or expressed only a few pili of long, flexible morphology per cell. Strains isolated from water or other foods were much more likely to express pili. Heavily piliated strains (all sources) possessed pili of several morphological types, including long, flexible pili of varying widths and rigid pili of varying lengths. Expression of Pili was favored by growth at temperatures ca. 20 degrees C and below and growth in liquid medium. Most fecal strains expressed some pili under these conditions. In addition, other surface structures (fibrillar aggregates, fibrillar networks bundle-forming pili) were seen on some strains from most sources. These were also seen most frequently when bacteria were grown in liquid media at temperatures ca. 20 degrees C and below. Pili expression was not dramatically influenced by growth under anaerobic conditions, or in iron-depleted media, or by combinations of the above conditions. The role of the above surface structures in Aeromonas pathogenicity remains to be elucidated.

The Salmonella ompC gene: structure and use as a carrier for heterologous sequences

The Salmonella typhi (St) ompC gene codes for a major outer membrane protein (OMP) that is highly expressed in both low and high osmolarity. By hybridization studies with the entire gene or with segments thereof, ompC was found to be highly conserved within 11 different Salmonella serotypes, with the exception of S. arizonae. The study included several St isolates from Mexico and Indonesia. Variation was only detected in two (e and f) of the seven regions previously found to vary between St and E. coli ompC. Chimeric OmpC proteins, carrying a rotavirus VP4 capsid protein epitope, are well recognized by a specific monoclonal antibody (mAb) against this epitope, either in OMP preparations (by enzyme-linked immunosorbent assay; ELISA) or intact cells (by ELISA and immunogold-labelling), indicating that regions c and f are oriented towards the cell surface and are probably exposed. As has been shown before for other regulated OMP, this experimental approach could be useful for the presentation of heterologous epitopes in order to gain knowledge about porin topology, for testing the effect of altered porin surface epitopes on bacterial physiology, or else, in the development of multivalent vaccines.

Toxin production, adherence and protein expression by clinical Aeromonas spp. isolates in broth and human pooled ileostomy fluid

The physiological behaviour of clinical Aeromonas spp. isolates was compared following culture in a conventional broth and human pooled ileostomy fluid (PIF). Protein expression was markedly affected by the growth medium, with an overall reduction in whole cell proteins in bacteria grown in ileostomy fluid. In addition, novel outer membrane proteins were produced in PIF but not in broth. The majority of A. hydrophila and A. sobria isolates produced toxin in both broth and PIF, whereas no cytotoxin positive A. caviae were found. Toxin titres were at least two doubling dilutions higher in 40% and 21% of A. hydrophila and A. sobria isolates, respectively, following culture in brain heart infusion broth compared with PIF. Bacterial adherence to Vero and A-549 cells was significantly more common in A. hydrophila (53%) and A. sobria (64%) than in A. caviae (15%) (P < 0.01). We observed increased adherence by 6 aeromonas strains previously classified as adherence-positive, but not by 6 non-adherers, in PIF compared with brain heart infusion broth. The influence of growth medium on the expression of potential virulence determinants by Aeromonas spp. provides a rationale for the use of human ileostomy fluid in future in vitro studies, in order to simulate the nutrient conditions found in vivo.

Variable expression of O-antigen and the role of lipopolysaccharide as an adhesin in Aeromonas sobria

On initial isolation of Aeromonas sobria 3767 from a diarrhoeal stool specimen, two colony types were obtained: opaque (3767O) and translucent (3767T). Strain 3767O consistently produced lipopolysaccharide (LPS) core and O-antigen side chain, detectable by SDS-PAGE and by Western blotting with an O-antigen-specific monoclonal antibody. Strain 3767T produced LPS core but the amount of O-antigen was dependent on factors including growth medium and bacterial growth phase. Strain 3767T exhibited significantly lower levels of adhesion to HEp-2 cells than 3767O and this correlated with the level of LPS expression, with the greatest reduction (61%) at stationary phase when no LPS was detectable. The results implicate LPS as an adhesin for A. sobria 3767.

Purification and characterization of vibrio cholerae O139 fimbriae

A Vibrio cholerae O139 (strain Al-1841) isolated from a patient with a cholera-like disease in Bangladesh predominantly produced new curved, wavy fimbriae (Al-1841 fimbriae) and small numbers of previously reported V. cholerae non-O1 S7-like pili. The former was purified and characterized. The molecular mass of the Al-1841 fimbrial subunit was less than 2.5 kDa, and it was immunologically different from that of V. cholerae non-O1 S7 pili. This novel fimbrial antigen was detected in all 182 Gram-negative strains from five genera tested but was absent from the Gram-positive bacteria tested. The purified Al-1841 fimbriae did not agglutinate human or rabbit erythrocytes.

The public health significance of Aeromonas spp. in foods

There is now evidence that some strains of Aeromonas species are enteropathogens. Such strains possess virulence properties, such as the ability to produce enterotoxins, cytotoxins, haemolysins and/or the ability to invade epithelial cells. Strains with these properties are common contaminants of drinking water and a wide range of foods. Contact or consumption of contaminated water, especially in summer, is a major risk factor in Aeromonas-associated gastroenteritis. Aeromonas-contaminated foods may also be vehicles of infection. Given the properties of strains that have been described in foods it has been suggested that food-borne illness could result not only from colonization and in vivo expression of virulence factors, but possibly also by intoxication following ingestion of foods that have been stored for a period of time, even under refrigeration. This paper reviews what is known about Aeromonas spp. in foods, their expression of virulence determinants, particularly at refrigeration temperatures, and the questions remaining to be answered to evaluate the risk they pose, so that an appropriate public health response can be determined.

Iron uptake mechanisms of pathogenic bacteria

Most of the iron in a mammalian body is complexed with various proteins. Moreover, in response to infection, iron availability is reduced in both extracellular and intracellular compartments. Bacteria need iron for growth and successful bacterial pathogens have therefore evolved to compete successfully for iron in the highly iron-stressed environment of the host's tissues and body fluids. Several strategies have been identified among pathogenic bacteria, including reduction of ferric to ferrous iron, occupation of intracellular niches, utilisation of host iron compounds, and production of siderophores. While direct evidence that high affinity mechanisms for iron acquisition function as bacterial virulence determinants has been provided in only a small number of cases, it is likely that many if not all such systems play a central role in the pathogenesis of infection.

Pili and the interaction of Aeromonas species with human peripheral blood polymorphonuclear cells

The interaction of differentially piliated Aeromonas strains expressing pili of two broadly different morphologic types (short, rigid (S/R) and/or long, wavy (L/W)) with human peripheral blood mononuclear leukocytes (PMN) was investigated to determine whether host defense cells might exert a selective pressure on pili expression in vivo accounting for the different pili phenotypes of clinical and environmental strains. A majority of Aeromonas veronii biotype sobria strains from water (6/6) and faeces (8/11) readily associated with PMN (> 60% PMN with adherent and/or internalised bacteria), irrespective of their degree, or predominant type, of piliation. Rigid pili of Aeromonas species did not promote interaction with PMN. However, the majority (55%) of strains which interacted well with PMN were adherent to HEp-2 cells. Interaction with PMN is unlikely to be the reason few S/R pili are seen on faecal strains, but it may be a selective pressure on L/W adhesive pili, or other OMP adhesins, resulting in the shedding of strains which have lost critical adhesins.

Purification and characterization of Aeromonas sobria Ae24 pili: a possible new colonization factor

Pili of Aeromonas sobria Ae24 were purified and characterized. The molecular mass of the pilin was estimated to be about 19 kDa by SDS-PAGE. The Ae24 pili were electrophoretically distinguishable from previously reported Aeromonas hydrophila Ae6 W pili and A. sobria Ae1 pili, although all three had indistinguishable morphology and shared a high degree of homology in the N-terminal amino acid sequences. Strain Ae24 and its purified pili adhered to rabbit intestine and agglutinated human and rabbit erythrocytes. Hemagglutination was inhibited by D-galactose and D-mannose, but not by L-fucose. Organisms pretreated with Fab fraction of the antipilus antibody failed to adhere to the intestine. Organisms did not adhere to intestine pretreated with the purified pili. These findings suggest that the pili are a colonization factor of A. sobria Ae24 for the rabbit intestine, and that the receptor is galactose- and mannose-containing structure.

CTX genetic element encodes a site-specific recombination system and an intestinal colonization factor

In Vibrio cholerae, the genes encoding cholera toxin (ctxAB) are located on a segment of DNA (termed the "core" region) that is flanked by two or more copies of a repeated sequence called RS1. Together these DNA units comprise the CTX genetic element. Evidence presented here suggests that RS1 sequences encode a site-specific recombination system, which allows integration of a suicide plasmid carrying RS1 into an 18-base-pair sequence (attRS1) located on the chromosome of nontoxigenic V. cholerae strains. Strains of V. cholerae with large deletions removing attRS1 and the entire CTX genetic element no longer undergo site-specific recombination with the RS1 sequence. Additionally, these deletion strains show a defect in intestinal colonization. Recombination experiments localize the gene responsible for enhancing colonization to a portion of the core region of the CTX element. The identified gene encodes a peptide that is highly similar in amino acid sequence to the flexible pilin of Aeromonas hydrophila. These results have important implications in the construction of stable, live attenuated cholera vaccines.

Adherence to HEp-2 cells and enteropathogenic potential of Aeromonas spp

Aeromonas strains (total = 60) of clinical, water and food origin were tested for adherence to HEp-2 cells. Environmental strains were selected (except for A. caviae) to include primarily those expressing other virulence-associated properties. Adhesion was markedly species-dependent (A. veronii biotype sobria, 15 of 26 [58%]. A caviae, 4 of 12 [33%] and A. hydrophila, 2 of 8 [11%]). A. veronii biotype sobria were adhesive, irrespective of source (62 and 54% for clinical and environmental strains, respectively). Adherent strains of this species were enterotoxin-positive and most (13 of 15) grew at 43 degrees C. A. caviae isolated from clinical specimens contained a higher proportion (75%) of adherent strains than environmental strains (13%). Virulent subsets of A. veronii biotype sobria and A. caviae are adherent to HEp-2 cells. The HEp-2 assay is a useful model for investigating mechanisms of adherence and enteropathogenicity of virulent Aeromonas species.

Isolation of carbohydrate-reactive outer membrane proteins of Aeromonas hydrophila

Outer membrane proteins of Aeromonas hydrophila A6 were isolated by affinity chromatography on the basis of their reactivity with trisaccharide structures analogous to the terminal trisaccharide of the H antigen of the human ABO(H) blood group system and were characterized by using antisera raised against the isolate. The outer membrane extract for affinity chromatography was prepared from pressure-disrupted outer membranes by differential centrifugation, followed by solubilization of outer membrane components in a nondenaturing, nonionic detergent. Carbohydrate-reactive outer membrane proteins (CROMPs) were then purified by affinity chromatography on two different affinity matrices composed of trisaccharides resembling the terminal trisaccharide of the H antigen, attached to inert silica beads. The relative efficiencies of H type 1 and 2 terminal trisaccharides as affinity adsorbents were established. Reactive proteins were eluted under alkaline conditions (pH 11.0) and in the presence of soluble H substance prepared from group O secretor saliva, but not by 60 mM alpha-L-fucose or under acid conditions (pH 3.0). The eluate contained at least three components (M(r)s, 43,000, 40,000, and < 14,000), as detected by immunoblot analysis with a polyvalent, polyspecific rabbit antiserum to A. hydrophila A6 (serum 3/83). A specific antiserum (serum 3/91) prepared in a rabbit by repeated immunizations with nitrocellulose containing the 43,000-Da band reacted with three bands (M(r)s, 43,000, 40,000, and < 14,000) in immunoblot analysis of solubilized outer membranes of A. hydrophila A6, suggesting that the 40,000- and < 14,000-Da elements are immunologically related to components of the 43,000-Da protein. Furthermore, pretreatment of A. hydrophila A6 with serum 3/91 reduced the strength of bacterial hemagglutination. The purified CROMPs did not agglutinate human group O erythrocytes. The reactivity of isolated CROMPs with a second CROMP-specific antibody (lipopolysaccharide-absorbed serum 3/83) was investigated. CROMPs, proteinase K-treated CROMPs, and bovine serum albumin were bound to latex beads and reacted with lipopolysaccharide-absorbed serum 3/83. Antibodies eluted from CROMP-latex inhibited hemagglutination of human erythrocytes by A. hydrophila A6 to a titer of 4. Antibody eluted from proteinase K-treated CROMP-latex beads showed hemagglutination inhibition activity only when undiluted. There was no hemagglutination inhibition antibody activity detectable in the eluate from bovine serum albumin-latex beads. These results show that antibodies which react with the isolated CROMPs also react with an H-antigen-reactive hemagglutinin of A. hydrophila A6. The possibility that CROMPs act as an adhesin, or adhesins, and contribute to the virulence of this organism is discussed.

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.

Aerokey II: a flexible key for identifying clinical Aeromonas species

A small subset (n = 18) of highly discriminatory tests was derived from the feature frequency of 50 tests used in the study of 167 predominantly clinical Aeromonas strains. Seven of these eighteen tests were used to construct a flexible, dichotomous key, Aerokey II, for identifying clinical aerontonads: esculin hydrolysis, gas from glucose, acid from arabinose, indole production, acid from sucrose, Voges-Proskauer reaction, and resistance to cephalothin (30 micrograms). This schema was initially evaluated in a single-blind trial of 60 well-characterized clinical Aeromonas hydrophila (n = 21), A. caviae (n = 19), and A. veronii bv. sobria (n = 20) strains from an independent laboratory. Of the 60 strains tested, 58 (97%) were accurately identified to the species level. Aerokey II was further evaluated with 18 additional American Type Culture Collection and reference strains representing the more recently proposed taxa A. veronii bv. veronii, A. schubertii, A. jandaei, and A. trota and accurately identified all of these strains.

Localized adherence by enteropathogenic Escherichia coli is an inducible phenotype associated with the expression of new outer membrane proteins

Enteropathogenic Escherichia coli grow as discrete colonies on the mucous membranes of the small intestine. A similar pattern can be demonstrated in vitro; termed localized adherence (LA), it is characterized by the presence of circumscribed clusters of bacteria attached to the surfaces of cultured epithelial cells. The LA phenotype was studied using B171, an O111:NM enteropathogenic E. coli (EPEC) strain, and HEp-2 cell monolayers. LA could be detected 30-60 min after exposure of HEp-2 cells to B171. However, bacteria transferred from infected HEp-2 cells to fresh monolayers exhibited LA within 15 min, indicating that LA is an inducible phenotype. Induction of the LA phenotype was found to be associated with de novo protein synthesis and changes in the outer membrane proteins, including the production of a new 18.5-kD polypeptide. A partial NH2-terminal amino acid sequence of this polypeptide was obtained and showed it to be identical through residue 12 to the recently described bundle-forming pilus subunit of EPEC. Expression of the 18.5-kD polypeptide required the 57-megadalton enteropathogenic E. coli adherence plasmid previously shown to be required for the LA phenotype in vitro and full virulence in vivo. This observation, the correspondence of the 18.5-kD polypeptide to an EPEC-specific pilus protein, and the temporal correlation of its expression with the development of the LA phenotype suggest that it may contribute to the EPEC colonial mode of growth.

An inducible bundle-forming pilus of enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC), a cause of childhood diarrhea, grow on the surface of the small intestine and on cultured epithelial cells as colonies of adherent bacteria. When propagated on solid medium containing blood or attached to HEp-2 cells, EPEC express ropelike bundles of filaments, termed bundle-forming pili (BFP), that create a network of fibers that bind together the individual organisms. BFP were found to be expressed by five EPEC serogroups, each harboring a approximately 92-kilobase plasmid previously known to be important for virulence in humans. When two of these strains were cured of this plasmid, they neither expressed BFP nor grew as adherent colonies. An antiserum to BFP reduced the capacity of EPEC to infect cultured epithelial cells. BFP are composed of a repeating subunit of 19,500 daltons, the amino-terminal amino acid sequence of this subunit is homologous to that of the toxin-coregulated pilin of Vibrio cholerae.

Recent advances in the study of the taxonomy, pathogenicity, and infectious syndromes associated with the genus Aeromonas

Over the past decade, the emergence of Aeromonas species as bona fide human pathogens and their probable role as etiologic agents of bacterial gastroenteritis have resulted in an explosion of scientific interest in the genus. Major accomplishments occurring in this field during that interval include a more refined taxonomy, identification of new cell-associated factors (surface layers, pili), and the molecular analysis of selected extracellular gene products that may play a critical role in pathogenesis (hemolysins, enterotoxins). This review provides an updated overview of recent systematic, clinical, and pathophysiologic advances and defines key areas of medical and scientific interest in which major questions remain unanswered.

Purification and characterization of Aeromonas sobria pili, a possible colonization factor

Pili of Aeromonas sobria Ae1 were purified and characterized. The molecular mass of the pilin was estimated to be about 23 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The Ae1 pili were electrophoretically and immunologically distinguishable from the W pili of A. hydrophila Ae6, although the two pili were morphologically indistinguishable. The N-terminal amino acid sequences of the two pilins were identical in the first 10 residues. Strain Ae1 and its purified pili adhered to human and rabbit intestines and agglutinated human and rabbit erythrocytes. Hemagglutination was inhibited by D-galactose and D-mannose, but not by L-fucose. Organisms pretreated with the Fab fraction of the antipilus antibody failed to adhere to the intestines. Organisms did not adhere to intestines pretreated with the purified pili. These findings suggest that the pili are a colonization factor of A. sobria Ae1.

Purification and characterization of thin, aggregative fimbriae from Salmonella enteritidis

Novel fimbriae were isolated and purified from the human enteropathogen Salmonella enteritidis 27655. These fimbriae were thin (measuring 3 to 4 nm in diameter), were extremely aggregative, and remained cell associated despite attempts to separate them from blended cells by centrifugation. The thin fimbriae were not solubilized in 5 M NaOH or in boiling 0.5% deoxycholate, 8 M urea, or 1 to 2% sodium dodecyl sulfate (SDS) with or without 5% beta-mercaptoethanol. Therefore, an unconventional purification procedure based on the removal of contaminating cell macromolecules in sonicated cell extracts by enzymatic digestion and preparative SDS-polyacrylamide gel electrophoresis (PAGE) was used. The insoluble fimbriae recovered from the well of the gel required depolymerization in formic acid prior to analysis by SDS-PAGE. Acid depolymerization revealed that the fimbriae were composed of fimbrin subunits, each with an apparent molecular mass of 17 kDa. Although their biochemical characteristics and amino acid composition were typical of fimbriae in general, these thin fimbriae were clearly distinct from other previously characterized fimbriae. Moreover, their fimbrin subunits had a unique N-terminal amino acid sequence. Native fimbriae on whole cells were specifically labeled with immune serum raised to the purified fimbriae. This immune serum also reacted with the denatured 17-kDa fimbrin protein in Western blots. The polyclonal immune serum did not cross-react with the other two native fimbrial types produced by this strain or with their respective fimbrins on Western blots (immunoblots). Therefore, these fimbriae represent the third fimbrial type produced by the enteropathogen S. enteritidis.

Iron and bacterial virulence--a brief overview

Iron is now recognized as playing a vital role in infection. Not only does it restricted availability in tissue fluids present microbial pathogens with the problem of acquiring sufficient for multiplication in vivo, but it also constitutes a major environmental signal which co-ordinately regulates the expression of a number of virulence and metabolic genes. Progress in understanding the strategies used by pathogens for acquiring iron in vivo, and their responses to iron restriction, is providing a fresh insight into microbial pathogenicity.