Pleiotropic effects of a mutation in rfaC on Escherichia coli hemolysin

Uncovering virulence factors in Cronobacter sakazakii: insights from genetic screening and proteomic profiling

The increasing problem of antibiotic resistance has driven the search for virulence factors in pathogenic bacteria, which can serve as targets for the development of new antibiotics. Although whole-genome Tn5 transposon mutagenesis combined with phenotypic assays has been a widely used approach, its efficiency remains low due to labor-intensive processes. In this study, we aimed to identify specific genes and proteins associated with the virulence of Cronobacter sakazakii, a pathogenic bacterium known for causing severe infections, particularly in infants and immunocompromised individuals. By employing a combination of genetic screening, comparative proteomics, and in vivo validation using zebrafish and rat models, we rapidly screened highly virulent strains and identified two genes, rcsA and treR, as potential regulators of C. sakazakii toxicity toward zebrafish and rats. Proteomic profiling revealed upregulated proteins upon knockout of rcsA and treR, including FabH, GshA, GppA, GcvH, IhfB, RfaC, MsyB, and three unknown proteins. Knockout of their genes significantly weakened bacterial virulence, confirming their role as potential virulence factors. Our findings contribute to understanding the pathogenicity of C. sakazakii and provide insights into the development of targeted interventions and therapies against this bacterium.IMPORTANCEThe emergence of antibiotic resistance in pathogenic bacteria has become a critical global health concern, necessitating the identification of virulence factors as potential targets for the development of new antibiotics. This study addresses the limitations of conventional approaches by employing a combination of genetic screening, comparative proteomics, and in vivo validation to rapidly identify specific genes and proteins associated with the virulence of Cronobacter sakazakii, a highly pathogenic bacterium responsible for severe infections in vulnerable populations. The identification of two genes, rcsA and treR, as potential regulators of C. sakazakii toxicity toward zebrafish and rats and the proteomic profiling upon knockout of rcsA and treR provides novel insights into the mechanisms underlying bacterial virulence. The findings contribute to our understanding of C. sakazakii's pathogenicity, shed light on the regulatory pathways involved in bacterial virulence, and offer potential targets for the development of novel interventions against this highly virulent bacterium.

Complex Multilevel Control of Hemolysin Production by Uropathogenic Escherichia coli

Uropathogenic E. coli (UPEC) is the major cause of urinary tract infections and a frequent cause of sepsis. Nearly half of all UPEC strains produce the potent cytotoxin hemolysin, and its expression is associated with enhanced virulence. In this study, we explored hemolysin variation within the globally dominant UPEC ST131 clone, finding that strains from the ST131 sublineage with the greatest multidrug resistance also possess the strongest hemolytic activity. We also employed an innovative forward genetic screen to define the set of genes required for hemolysin production. Using this approach, and subsequent targeted mutagenesis and complementation, we identified new hemolysin-controlling elements involved in LPS inner core biosynthesis and cytoplasmic chaperone activity, and we show that mechanistically they are required for hemolysin secretion. These original discoveries substantially enhance our understanding of hemolysin regulation, secretion and function.

Uropathogenic Escherichia coli (UPEC) is the major cause of urinary tract infections. Nearly half of all UPEC strains secrete hemolysin, a cytotoxic pore-forming toxin. Here, we show that the prevalence of the hemolysin toxin gene (hlyA) is highly variable among the most common 83 E. coli sequence types (STs) represented on the EnteroBase genome database. To explore this diversity in the context of a defined monophyletic lineage, we contextualized sequence variation of the hlyCABD operon within the genealogy of the globally disseminated multidrug-resistant ST131 clone. We show that sequence changes in hlyCABD and its newly defined 1.616-kb-long leader sequence correspond to phylogenetic designation, and that ST131 strains with the strongest hemolytic activity belong to the most extensive multidrug-resistant sublineage (clade C2). To define the set of genes involved in hemolysin production, the clade C2 strain S65EC was completely sequenced and subjected to a genome-wide screen by combining saturated transposon mutagenesis and transposon-directed insertion site sequencing with the capacity to lyse red blood cells. Using this approach, and subsequent targeted mutagenesis and complementation, 13 genes were confirmed to be specifically required for production of active hemolysin. New hemolysin-controlling elements included discrete sets of genes involved in lipopolysaccharide (LPS) inner core biosynthesis (waaC, waaF, waaG, and rfaE) and cytoplasmic chaperone activity (dnaK and dnaJ), and we show these are required for hemolysin secretion. Overall, this work provides a unique description of hemolysin sequence diversity in a single clonal lineage and describes a complex multilevel system of regulatory control for this important toxin.

On the Antibacterial Activity of Azacarboxylate Ligands: Lowered Metal Ion Affinities for Bis-amide Derivatives of EDTA do not mean Reduced Activity

EDTA is widely used as an inhibitor of bacterial growth, affecting the uptake and control of metal ions by microorganisms. We describe the synthesis and characterisation of two symmetrical bis-amide derivatives of EDTA, featuring glycyl or pyridyl substituents: AmGly₂ and AmPy₂ . Metal ion affinities (logK) have been evaluated for a range of metals (Mg²⁺ , Ca²⁺ , Fe³⁺ , Mn²⁺ , Zn²⁺ ), revealing less avid binding compared to EDTA. The solid-state structures of AmGly₂ and of its Mg²⁺ complex have been determined crystallographically. The latter shows an unusual 7-coordinate, capped octahedral Mg²⁺ centre. The antibacterial activities of the two ligands and of EDTA have been evaluated against a range of health-relevant bacterial species, three Gram negative (Escherichia coli, Pseudomonas aeruginosa and Klebsiella pneumoniae) and a Gram positive (Staphylococcus aureus). The AmPy₂ ligand is the only one that displays a significant inhibitory effect against K. pneumoniae, but is less effective against the other organisms. AmGly₂ exhibits a more powerful inhibitory effect against E. coli at lower concentrations than EDTA (<3 mm) or AmPy₂ , but loses its efficacy at higher concentrations. The growth inhibition of EDTA and AmGly₂ on mutant E. coli strains with defects in outer-membrane lipopolysaccharide (LPS) structures has been assessed to provide insight into the unexpected behaviour. Taken together, the results contradict the assumption of a simple link between metal ion affinity and antimicrobial efficacy.

Characterization of Post-Translational Modifications and Cytotoxic Properties of the Adenylate-Cyclase Hemolysin Produced by Various Bordetella pertussis and Bordetella parapertussis Isolates

Bordetella pertussis and Bordetella parapertussis are the causal agents of whooping cough in humans. They produce diverse virulence factors, including adenylate cyclase-hemolysin (AC-Hly), a secreted toxin of the repeat in toxins (RTX) family with cyclase, pore-forming, and hemolytic activities. Post-translational modifications (PTMs) are essential for the biological activities of the toxin produced by B. pertussis. In this study, we compared AC-Hly toxins from various clinical isolates of B. pertussis and B. parapertussis, focusing on (i) the genomic sequences of cyaA genes, (ii) the PTMs of partially purified AC-Hly, and (iii) the cytotoxic activity of the various AC-Hly toxins. The genes encoding the AC-Hly toxins of B. pertussis and B. parapertussis displayed very limited polymorphism in each species. Most of the sequence differences between the two species were found in the C-terminal part of the protein. Both toxins harbored PTMs, mostly corresponding to palmitoylations of the lysine 860 residue and palmoylations and myristoylations of lysine 983 for B. pertussis and AC-Hly and palmitoylations of lysine 894 and myristoylations of lysine 1017 for B. parapertussis AC-Hly. Purified AC-Hly from B. pertussis was cytotoxic to macrophages, whereas that from B. parapertussis was not.

Characterization of Post-Translational Modifications and Cytotoxic Properties of the Adenylate-Cyclase Hemolysin Produced by Various Bordetella pertussis and Bordetella parapertussis Isolates

Bordetella pertussis and Bordetella parapertussis are the causal agents of whooping cough in humans. They produce diverse virulence factors, including adenylate cyclase-hemolysin (AC-Hly), a secreted toxin of the repeat in toxins (RTX) family with cyclase, pore-forming, and hemolytic activities. Post-translational modifications (PTMs) are essential for the biological activities of the toxin produced by B. pertussis. In this study, we compared AC-Hly toxins from various clinical isolates of B. pertussis and B. parapertussis, focusing on (i) the genomic sequences of cyaA genes, (ii) the PTMs of partially purified AC-Hly, and (iii) the cytotoxic activity of the various AC-Hly toxins. The genes encoding the AC-Hly toxins of B. pertussis and B. parapertussis displayed very limited polymorphism in each species. Most of the sequence differences between the two species were found in the C-terminal part of the protein. Both toxins harbored PTMs, mostly corresponding to palmitoylations of the lysine 860 residue and palmoylations and myristoylations of lysine 983 for B. pertussis and AC-Hly and palmitoylations of lysine 894 and myristoylations of lysine 1017 for B. parapertussis AC-Hly. Purified AC-Hly from B. pertussis was cytotoxic to macrophages, whereas that from B. parapertussis was not.

Bordetella Adenylate Cyclase-Hemolysin Toxins

Adenylate cyclase-hemolysin toxin is secreted and produced by three classical species of the genus Bordetella: Bordetella pertussis, B. parapertussis and B. bronchiseptica. This toxin has several properties such as: (i) adenylate cyclase activity, enhanced after interaction with the eukaryotic protein, calmodulin; (ii) a pore-forming activity; (iii) an invasive activity. It plays an important role in the pathogenesis of these Bordetella species responsible for whooping cough in humans or persistent respiratory infections in mammals, by modulating host immune responses. In contrast with other Bordetella toxins or adhesins, lack of (or very low polymorphism) is observed in the structural gene encoding this toxin, supporting its importance as well as a potential role as a vaccine antigen against whooping cough. In this article, an overview of the investigations undertaken on this toxin is presented.

Uropathogenic Escherichia coli-Associated Exotoxins

Escherichia coli are a common cause of infectious disease outside of the gastrointestinal tract. Several independently evolved E. coli clades are common causes of urinary tract and bloodstream infections. There is ample epidemiological and in vitro evidence that several different protein toxins common to many, but not all, of these strains are likely to aid the colonization and immune-evasion ability of these bacteria. This review discusses our current knowledge and areas of ignorance concerning the contribution of the hemolysin; cytotoxic-necrotizing factor-1; and the autotransporters, Sat, Pic, and Vat, to extraintestinal human disease.

Hemolysin of uropathogenic Escherichia coli: A cloak or a dagger?

Hemolysin from uropathogenic Escherichia coli (UPEC) is a hemolytic and cytotoxic protein active against a broad range of species and cell types. Expression of hemolysin correlates with severity of infection, as up to 78% of UPEC isolates from pyelonephritis cases express hemolysin. Despite decades of research on hemolysin activity, the mechanism of intoxication and the function of hemolysin in UPEC infection remain elusive. Early in vitro research established the role of hemolysin as a lytic protein at high doses. It is hypothesized that hemolysin is secreted at sublytic doses in vivo and recent research has focused on understanding the more subtle effects of hemolysin both in vitro and in elegant infection models in vivo, including inoculation by micropuncture of individual kidney nephrons. As the field continues to evolve, comparisons of hemolysin function in isolates from a range of UTI infections will be important for delineating the role of this toxin. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Outer membrane vesicles mediate transport of biologically active Vibrio cholerae cytolysin (VCC) from V. cholerae strains

Background

Outer membrane vesicles (OMVs) released from Gram-negative bacteria can serve as vehicles for the translocation of virulence factors. Vibrio cholerae produce OMVs but their putative role in translocation of effectors involved in pathogenesis has not been well elucidated. The V. cholerae cytolysin (VCC), is a pore-forming toxin that lyses target eukaryotic cells by forming transmembrane oligomeric β-barrel channels. It is considered a potent toxin that contributes to V. cholerae pathogenesis. The mechanisms involved in the secretion and delivery of the VCC have not been extensively studied.

Methodology/Principal Findings

OMVs from V. cholerae strains were isolated and purified using a differential centrifugation procedure and Optiprep centrifugation. The ultrastructure and the contents of OMVs were examined under the electron microscope and by immunoblot analyses respectively. We demonstrated that VCC from V. cholerae strain V:5/04 was secreted in association with OMVs and the release of VCC via OMVs is a common feature among V. cholerae strains. The biological activity of OMV-associated VCC was investigated using contact hemolytic assay and epithelial cell cytotoxicity test. It showed toxic activity on both red blood cells and epithelial cells. Our results indicate that the OMVs architecture might play a role in stability of VCC and thereby can enhance its biological activities in comparison with the free secreted VCC. Furthermore, we tested the role of OMV-associated VCC in host cell autophagy signalling using confocal microscopy and immunoblot analysis. We observed that OMV-associated VCC triggered an autophagy response in the target cell and our findings demonstrated for the first time that autophagy may operate as a cellular defence mechanism against an OMV-associated bacterial virulence factor.

Conclusion/Significance

Biological assays of OMVs from the V. cholerae strain V:5/04 demonstrated that OMV-associated VCC is indeed biologically active and induces toxicity on mammalian cells and furthermore can induce autophagy.

LPS structure influences protein secretion in Salmonella enterica

In this study we have compared protein secretion in the wild type of S. Typhimurium and the rfaC mutant. We found out that the rfaC mutant was defective in protein secretion. In addition, the rfaC mutant was defective in its invasion into an IPEC-J2 porcine epithelial cell line and also in motility in semisolid agar. Consistent with this, reduced flagella numbers were observed in the rfaC mutant. In the rfaC mutant, there were no defects in flagellin expression as detected by western blot and immune electron microscopy which demonstrated equal amounts of flagellin in the cytoplasm of both the rfaC mutant and the wild-type S. Typhimurium. However, in the wild-type strain only, the flagellin was assembled to spatially restricted areas on the inner side of cytoplasmic membrane. The oligosaccharide core of LPS is therefore required for the assembly of flagella and T3SS secretion machinery followed by protein secretion.

Bacteriophage PhiX174's ecological niche and the flexibility of its Escherichia coli lipopolysaccharide receptor

To determine bacteriophage PhiX174's ecological niche, 783 Escherichia coli isolates were screened for susceptibility. Sensitive strains are diverse regarding their phylogenies and core lipopolysaccharides (LPS), but all have rough phenotypes. Further analysis of E. coli K-12 LPS mutants revealed that PhiX174 can use a wide diversity of LPS structures to initiate its infectious process.

Mutations affecting the extreme C terminus of Escherichia coli haemolysin A reduce haemolytic activity by altering the folding of the toxin

Escherichia coli haemolysin A (HlyA), an RTX toxin, is secreted probably as an unfolded intermediate, by the type I (ABC transporter-dependent) pathway, utilizing a C-terminal secretion signal. However, the mechanism of translocation and post-translocation folding is not understood. We identified a mutation (hlyA99) at the extreme C terminus, which is dominant in competition experiments, blocking secretion of the wild-type toxin co-expressed in the same cell. This suggests that unlike recessive mutations which affect recognition of the translocation machinery, the hlyA99 mutation interferes with some later step in secretion. Indeed, the mutation reduced haemolytic activity of the toxin and the activity of β-lactamase when the latter was fused to a C-terminal 23 kDa fragment of HlyA carrying the hlyA99 mutation. A second mutant (hlyAdel6), lacking the six C-terminal residues of HlyA, also showed reduced haemolytic activity and neither mutant protein regained normal haemolytic activity in in vitro unfolding/refolding experiments. Tryptophan fluorescence spectroscopy indicated differences in structure between the secreted forms of wild-type HlyA and the HlyA Del6 mutant. These results suggested that the mutations affected the correct folding of both HlyA and the β-lactamase fusion. Thus, we propose a dual function for the HlyA C terminus involving an important role in post-translocation folding as well as targeting HlyA for secretion.

Mutation in the LPS outer core biosynthesis gene, galU, affects LPS interaction with the RTX toxins ApxI and ApxII and cytolytic activity of Actinobacillus pleuropneumoniae serotype 1

Lipopolysaccharides (LPS) and Apx toxins are major virulence factors of Actinobacillus pleuropneumoniae, a pathogen of the respiratory tract of pigs. Here, we evaluated the effect of LPS core truncation in haemolytic and cytotoxic activities of this microorganism. We previously generated a highly attenuated galU mutant of A. pleuropneumoniae serotype 1 that has an LPS molecule lacking the GalNAc-Gal II-Gal I outer core residues. Our results demonstrate that this mutant exhibits wild-type haemolytic activity but is significantly less cytotoxic to porcine alveolar macrophages. However, no differences were found in gene expression and secretion of the haemolytic and cytotoxic toxins ApxI and ApxII, both secreted by A. pleuropneumoniae serotype 1. This suggests that the outer core truncation mediated by the galU mutation affects the toxins in their cytotoxic activities. Using both ELISA and surface plasmon resonance binding assays, we demonstrate a novel interaction between LPS and the ApxI and ApxII toxins via the core oligosaccharide. Our results indicate that the GalNAc-Gal II-Gal I trisaccharide of the outer core is fundamental to mediating LPS/Apx interactions. The present study suggests that a lack of binding between LPS and ApxI/II affects the cytotoxicity and virulence of A. pleuropneumoniae.

Inactivation of host Akt/protein kinase B signaling by bacterial pore-forming toxins

Uropathogenic Escherichia coli (UPEC) are the major cause of urinary tract infections (UTIs), and they have the capacity to induce the death and exfoliation of target uroepithelial cells. This process can be facilitated by the pore-forming toxin alpha-hemolysin (HlyA), which is expressed and secreted by many UPEC isolates. Here, we demonstrate that HlyA can potently inhibit activation of Akt (protein kinase B), a key regulator of host cell survival, inflammatory responses, proliferation, and metabolism. HlyA ablates Akt activation via an extracellular calcium-dependent, potassium-independent process requiring HlyA insertion into the host plasma membrane and subsequent pore formation. Inhibitor studies indicate that Akt inactivation by HlyA involves aberrant stimulation of host protein phosphatases. We found that two other bacterial pore-forming toxins (aerolysin from Aeromonas species and alpha-toxin from Staphylococcus aureus) can also markedly attenuate Akt activation in a dose-dependent manner. These data suggest a novel mechanism by which sublytic concentrations of HlyA and other pore-forming toxins can modulate host cell survival and inflammatory pathways during the course of a bacterial infection.

Role of the lipopolysaccharide-CD14 complex for the activity of hemolysin from uropathogenic Escherichia coli

Bacterial pathogens produce a variety of exotoxins, which often become associated with the bacterial outer membrane component lipopolysaccharide (LPS) during their secretion. LPS is a potent proinflammatory mediator; however, it is not known whether LPS contributes to cell signaling induced by those microbial components to which it is attached. This is partly due to the common view that LPS present in bacterial component preparations is an experimental artifact. The Escherichia coli exotoxin hemolysin (Hly) is a known inducer of proinflammatory signaling in epithelial cells, and the signal transduction pathway involves fluctuation of the intracellular-Ca(2+) concentration. Since LPS is known to interact with Hly, we investigated whether it is required as a cofactor for the activity of Hly. We found that the LPS/Hly complex exploits the CD14/LPS-binding protein recognition system to bring Hly to the cell membrane, where intracellular-Ca(2+) signaling is initiated via specific activation of the small GTPase RhoA. Hly-induced Ca(2+) signaling was found to occur independently of the LPS receptor TLR4, suggesting that the role of LPS/CD14 is to deliver Hly to the cell membrane. In contrast, the cytolytic effect triggered by exposure of cells to high Hly concentrations occurs independently of LPS/CD14. Collectively, our data reveal a novel molecular mechanism for toxin delivery in bacterial pathogenesis, where LPS-associated microbial compounds are targeted to the host cell membrane as a consequence of their association with LPS.

Release of the type I secreted alpha-haemolysin via outer membrane vesicles from Escherichia coli

The alpha-haemolysin is an important virulence factor commonly expressed by extraintestinal pathogenic Escherichia coli. The secretion of the alpha-haemolysin is mediated by the type I secretion system and the toxin reaches the extracellular space without the formation of periplasmic intermediates presumably in a soluble form. Surprisingly, we found that a fraction of this type I secreted protein is located within outer membrane vesicles (OMVs) that are released by the bacteria. The alpha-haemolysin appeared very tightly associated with the OMVs as judged by dissociation assays and proteinase susceptibility tests. The alpha-haemolysin in OMVs was cytotoxically active and caused lysis of red blood cells. The OMVs containing the alpha-haemolysin were distinct from the OMVs not containing alpha-haemolysin, showing a lower density. Furthermore, they differed in protein composition and one component of the type I secretion system, the TolC protein, was found in the lower density vesicles. Studies of natural isolates of E. coli demonstrated that the localization of alpha-haemolysin in OMVs is a common feature among haemolytic strains. We propose an alternative pathway for the transport of the type I secreted alpha-haemolysin from the bacteria to the host cells during bacterial infections.

Adenylate cyclase toxin (ACT) from Bordetella hinzii: characterization and differences from ACT of Bordetella pertussis

Bordetella hinzii is a commensal respiratory microorganism in poultry but is increasingly being recognized as an opportunistic pathogen in immunocompromised humans. Although associated with a variety of disease states, practically nothing is known about the mechanisms employed by this bacterium. In this study, we show by DNA sequencing and reverse transcription-PCR that both commensal and clinical strains of B. hinzii possess and transcriptionally express cyaA, the gene encoding adenylate cyclase toxin (ACT) in other pathogenic Bordetella species. By Western blotting, we also found that B. hinzii produces full-length ACT protein in quantities that are comparable to those made by B. pertussis. In contrast to B. pertussis ACT, however, ACT from B. hinzii is less extractable from whole bacteria, nonhemolytic, has a 50-fold reduction in adenylate cyclase activity, and is unable to elevate cyclic AMP levels in host macrophages (nontoxic). The decrease in enzymatic activity is attributable, at least in part, to a decreased binding affinity of B. hinzii ACT for calmodulin, the eukaryotic activator of B. pertussis ACT. In addition, we demonstrate that the lack of intoxication by B. hinzii ACT may be due to the absence of expression of cyaC, the gene encoding the accessory protein required for the acylation of B. pertussis ACT. These results demonstrate the expression of ACT by B. hinzii and represent the first characterization of a potential virulence factor of this organism.

Stability, antigenicity, and aggregation of Moraxella bovis cytolysin after purification and storage

OBJECTIVES

To compare stability, antigenicity, and aggregation characteristics of Moraxella bovis cytolysins among isolates from geographically diverse areas.

STUDY POPULATION

8 isolates of M. bovis.

PROCEDURE

Filter-sterilized broth culture supernatants of M. bovis were concentrated, diafiltered, and chromatographed. The endotoxin and cytolysin activities in samples were measured. Chromatographed cytolysins of M. bovis were examined by immunoblotting. Hemolytic and leukotoxic activities were measured from samples collected at each step of purification and before and after storage. Hemolysis was measured directly by use of washed bovine erythrocyte targets. Leukotoxicity was measured by use of a 51Cr release assay.

RESULTS

Cytolysin was retained by a filter with 100-kd nominal molecular weight limit. Hemolytic activity, leukotoxic activity, and endotoxin were eluted together in void volume of a gel-filtration column (molecular mass exclusion limit = 4 X 10(7) d). Gel-column chromatographed diafiltered retentate had the greatest specific cytolytic activity and the highest endotoxin-to-protein ratio. Frozen diafiltered retentate(-80 degrees C, 4 months) was cytolytic after thawing. Immunoblots of gel-column chromatographed cytolysin contained 4 proteins with molecular masses between 90 and 68 kd. Fractions with high lytic activities also had additional protein bands with molecular masses of 98 and 63 kd. Immunoblots of gel-column chromatographed diafiltered retentate revealed proteins with molecular masses between 90 and 68 kd.

CONCLUSIONS AND CLINICAL RELEVANCE

Diafiltered M. bovis cytolysin is aggregated with endotoxin. Antigenicity and cytolytic activities in diafiltered retentate are conserved among M. bovis isolates. Diafiltration could be useful for bulk semipurification of M. bovis cytolysin. Cytolysin-enriched vaccines of M. bovis could be contaminated by endotoxin.

DegS is necessary for virulence and is among extraintestinal Escherichia coli genes induced in murine peritonitis

Extraintestinal Escherichia coli strains cause meningitis, sepsis, urinary tract infection, and other infections outside the bowel. We examined here extraintestinal E. coli strain CFT073 by differential fluorescence induction. Pools of CFT073 clones carrying a CFT073 genomic fragment library in a promoterless gfp vector were inoculated intraperitoneally into mice; bacteria were recovered by lavage 6 h later and then subjected to fluorescence-activated cell sorting. Eleven promoters were found to be active in the mouse but not in Luria-Bertani (LB) broth culture. Three are linked to genes for enterobactin, aerobactin, and yersiniabactin. Three others are linked to the metabolic genes metA, gltB, and sucA, and another was linked to iha, a possible adhesin. Three lie before open reading frames of unknown function. One promoter is associated with degS, an inner membrane protease. Mutants of the in vivo-induced loci were tested in competition with the wild type in mouse peritonitis. Of the mutants tested, only CFT073 degS was found to be attenuated in peritoneal and in urinary tract infection, with virulence restored by complementation. CFT073 degS shows growth similar to that of the wild type at 37 degrees C but is impaired at 43 degrees C or in 3% ethanol LB broth at 37 degrees C. Compared to the wild type, the mutant shows similar serum survival, motility, hemolysis, erythrocyte agglutination, and tolerance to oxidative stress. It also has the same lipopolysaccharide appearance on a silver-stained gel. The basis for the virulence attenuation is unclear, but because DegS is needed for sigma(E) activity, our findings implicate sigma(E) and its regulon in E. coli extraintestinal pathogenesis.

Loss of regulatory protein RfaH attenuates virulence of uropathogenic Escherichia coli

RfaH is a regulatory protein in Escherichia coli and Salmonella enterica serovar Typhimurium. Although it enhances expression of different factors that are proposed to play a role in bacterial virulence, a direct effect of RfaH on virulence has not been investigated so far. We report that inactivation of rfaH dramatically decreases the virulence of uropathogenic E. coli strain 536 in an ascending mouse model of urinary tract infection. The mortality rate caused by the wild-type strain in this assay is 100%, whereas that of its isogenic rfaH mutant does not exceed 18%. In the case of coinfection, the wild-type strain 536 shows higher potential to colonize the urinary tract even when it is outnumbered 100-fold by its rfaH mutant in the inoculum. In contrast to the wild-type strain, serum resistance of strain 536rfaH::cat is fully abolished. Furthermore, we give evidence that, besides a major decrease in the amount of hemin receptor ChuA (G. Nagy, U. Dobrindt, M. Kupfer, L. Emody, H. Karch, and J. Hacker, Infect. Immun. 69:1924-1928, 2001), loss of the RfaH protein results in an altered lipopolysaccharide phenotype as well as decreased expression of K15 capsule and alpha-hemolysin, whereas levels of other pathogenicity factors such as siderophores, flagella, Prf, and S fimbriae appear to be unaltered in strain 536rfaH::cat in comparison to the wild-type strain. trans complementation of the mutant strain with the rfaH gene restores wild-type levels of the affected virulence factors and consequently restitutes virulence in the mouse model of ascending urinary tract infection.

Expression and characterization of streptococcal rgp genes required for rhamnan synthesis in Escherichia coli

Six genes (rgpA through rgpF) that were involved in assembling the rhamnose-glucose polysaccharide (RGP) in Streptococcus mutans were previously identified (Y. Yamashita, Y. Tsukioka, K. Tomihisa, Y. Nakano, and T. Koga, J. Bacteriol. 180:5803-5807, 1998). The group-specific antigens of Lancefield group A, C, and E streptococci and the polysaccharide antigen of Streptococcus sobrinus have the same rhamnan backbone as the RGP of S. mutans. Escherichia coli harboring plasmid pRGP1 containing all six rgp genes did not synthesize complete RGP. However, E. coli carrying a plasmid with all of the rgp genes except for rgpE synthesized the rhamnan backbone of RGP without glucose side chains, suggesting that in addition to rgpE, another gene is required for glucose side-chain formation. Synthesis of the rhamnan backbone in E. coli required the initiation of transfer of N-acetylglucosamine to a lipid carrier and the expression of the rgpC and rgpD genes encoding the putative ABC transporter specific for RGP. The similarities in RGP synthesis between E. coli and S. mutans suggest common pathways for rhamnan synthesis. Therefore, we evaluated the rhamnosyl polymerization process in E. coli by high-resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the lipooligosaccharide (LOS). An E. coli transformant harboring rgpA produced the LOS modified by the addition of a single rhamnose residue. Furthermore, the rgpA, rgpB, and rgpF genes of pRGP1 were independently mutated by an internal deletion, and the LOS chemotypes of their transformants were examined. The transformant with an rgpA deletion showed the same LOS profile as E. coli without a plasmid. The transformant with an rgpB deletion showed the same LOS profile as E. coli harboring rgpA alone. The transformant with an rgpF deletion showed the LOS band with the most retarded migration. On the basis of these results, we speculated that RgpA, RgpB, and RgpF, in that order, function in rhamnan polymerization.

In vitro and in vivo characterization of a Bordetella bronchiseptica mutant strain with a deep rough lipopolysaccharide structure

Bordetella bronchiseptica is closely related to Bordetella pertussis, which produces respiratory disease primarily in mammals other than humans. However, its importance as a human pathogen is being increasingly recognized. Although a large amount of research on Bordetella has been generated regarding protein virulence factors, the participation of the surface lipopolysaccharide (LPS) during B. bronchiseptica infection is less understood. To get a better insight into this matter, we constructed and characterized the behavior of an LPS mutant with the deepest possible rough phenotype. We generated the defective mutant B. bronchiseptica LP39 on the waaC gene, which codes for a heptosyl transferase involved in the biosynthesis of the core region of the LPS molecule. Although in B. bronchiseptica LP39 the production of the principal virulence determinants adenylate cyclase-hemolysin, filamentous hemagglutinin, and pertactin persisted, the quantity of the two latter factors was diminished, with the levels of pertactin being the most greatly affected. Furthermore, the LPS of B. bronchiseptica LP39 did not react with sera obtained from mice that had been infected with the parental strain, indicating that this defective LPS is immunologically different from the wild-type LPS. In vivo experiments demonstrated that the ability to colonize the respiratory tract is reduced in the mutant, being effectively cleared from lungs within 5 days, whereas the parental strain survived at least for 30 days. In vitro experiments have demonstrated that, although B. bronchiseptica LP39 was impaired for adhesion to human epithelial cells, it is still able to survive within the host cells as efficiently as the parental strain. These results seem to indicate that the deep rough form of B. bronchiseptica LPS cannot represent a dominant phenotype at the first stage of colonization. Since isolates with deep rough LPS phenotype have already been obtained from human B. bronchiseptica chronic infections, the possibility that this phenotype arises as a consequence of selection pressure within the host at a late stage of the infection process is discussed.

Isolation and characterization of Escherichia coli tolC mutants defective in secreting enzymatically active alpha-hemolysin

This study describes the isolation and characterization of a unique class of TolC mutants that, under steady-state growth conditions, secreted normal levels of largely inactive alpha-hemolysin. Unlike the reduced activity in the culture supernatants, the cell-associated hemolytic activity in these mutants was identical to that in the parental strain, thus reflecting a normal intracellular toxin activation event. Treatment of the secreted toxin with guanidine hydrochloride significantly restored cytolytic activity, suggesting that the diminished activity may have been due to the aggregation or misfolding of the toxin molecules. Consistent with this notion, sedimentation and filtration analyses showed that alpha-hemolysin secreted from the mutant strain has a mass greater than that secreted from the parental strain. Experiments designed to monitor the time course of alpha-hemolysin release showed delayed appearance of toxin in the culture supernatant of the mutant strain, thus indicating a possible defect in alpha-hemolysin translocation or release. Eight different TolC substitutions displaying this toxin secretion defect were scattered throughout the protein, of which six localized in the periplasmically exposed alpha-helical domain, while the remaining two mapped within the outer membrane-embedded beta-barrel domain of TolC. A plausible model for the secretion of inactive alpha-hemolysin in these TolC mutants is discussed in the context of the recently determined three-dimensional structure of TolC.

Lipopolysaccharide endotoxins

Bacterial lipopolysaccharides (LPS) typically consist of a hydrophobic domain known as lipid A (or endotoxin), a nonrepeating "core" oligosaccharide, and a distal polysaccharide (or O-antigen). Recent genomic data have facilitated study of LPS assembly in diverse Gram-negative bacteria, many of which are human or plant pathogens, and have established the importance of lateral gene transfer in generating structural diversity of O-antigens. Many enzymes of lipid A biosynthesis like LpxC have been validated as targets for development of new antibiotics. Key genes for lipid A biosynthesis have unexpectedly also been found in higher plants, indicating that eukaryotic lipid A-like molecules may exist. Most significant has been the identification of the plasma membrane protein TLR4 as the lipid A signaling receptor of animal cells. TLR4 belongs to a family of innate immunity receptors that possess a large extracellular domain of leucine-rich repeats, a single trans-membrane segment, and a smaller cytoplasmic signaling region that engages the adaptor protein MyD88. The expanding knowledge of TLR4 specificity and its downstream signaling pathways should provide new opportunities for blocking inflammation associated with infection.

Molecular cloning and functional expression of the rfaE gene required for lipopolysaccharide biosynthesis in Salmonella typhimurium

The rfaE (WaaE) gene of Salmonella typhimurium is known to be located at 76min on the genetic map outside of the rfa gene cluster encoding core oligosaccharide biosynthesis of lipopolysaccharide(LPS). The rfaE mutant synthesizes heptose-deficient LPS; its LPS consists of only lipid A and 3-deoxy-D-manno-octulosonic acid (KDO), and the rfaE gene is believed to be involved in the formation of ADP-L-glycero-D-manno-heptose. Mutants, which make incomplete LPS, are known as rough mutants. Salmonella typhimurium deep-rough mutants affected in the heptose region of the inner core often show reduced growth rate, sensitivity to high temperature and hypersensitivity to hydrophobic antibiotics. We have cloned the rfaE gene of S. typhimurium. The chromosomal region carrying this gene was isolated by screening a genomic library of S. typhimurium using the complementation of S. typhimurium rfaE mutant. The 2.6-Kb insert in the plasmid pHEPs appears to carry a functional rfaE gene. SL1102 (rfaE543) makes heptose-deficient LPS and has a deep rough phenotype, but pHEPs complement the rfaE543 mutation to give the smooth phenotype. The sensitivity of SL1102 to bacteriophages (P22.c2, Felix-O, Br60) which use LPS as their receptor for adsorption is changed to that of wild-type strain. The permeability barrier of SL1102 to hydrophobic antibiotics (novobiocin) is restored to that of wild-type. LPS produced by SL1102 (rfaE543) carrying pHEPs makes LPS indistinguishable from that of smooth strains. The rfaE gene encoded a polypeptide of 477 amino acid residues highly homologous to the S. enterica rfaE protein (98% identity), E. coli (93% identity), Yersenia pestis (85% identity), Haemophilus influenzae (70% identity) and Helicobacter pyroli (41% identity) with a molecular weight 53 kDa.

RTX toxin structure and function: a story of numerous anomalies and few analogies in toxin biology

It can be agreed that RTX toxins contribute to the pathogenesis of different diseases by causing dysfunction of the general cellular reactions of the immune response. The suggestion that RTX toxins induce cytokine production in nonimmune cells that would ultimately cause tissue damage is an expansion of their role in disease pathogenesis (Uhlen et al. 2000). Investigators in the RTX toxin field may not agree with me, but precise and satisfactory answers to the following questions are not yet available. How do RTX toxins mechanistically damage a cell? Do RTX toxins have receptors in the classic sense, in which there is a reversible ligand and receptor complex? What is responsible for the common Ca2+ ion influx in affected cells? The recent observation that an RTX toxin stimulates host-cell-mediated Ca2+ ion oscillation in part challenges the long held concept that these toxins damage cells by the direct formation of pores. Are the Ca2+ ion fluxes truly the noxious cellular insult? What is the final molecular structure of RTX toxins at the time they cause cellular death? How does the common requirement for acyl modification among RTX toxins fit into the toxin structure and mechanism of cellular killing, particularly when mixtures of unusual fatty acids are used by some toxins? There are a number of outstanding laboratories throughout the world that are seeking answers to these questions. We can reasonably expect that during the next decade research on the structure and function of RTX toxins will lead to new chemotherapeutic targets and reagents for basic cell biology and biotechnology.

Bordetella pertussis virulence factors affect phagocytosis by human neutrophils

The interaction between human neutrophils and wild-type Bordetella pertussis or mutants expressing altered lipopolysaccharide or lacking virulence factors-pertussis toxin, adenylate cyclase toxin, dermonecrotic toxin, filamentous hemagglutinin (FHA), pertactin, or BrkA-was examined. In the absence of antibodies, the wild-type strain and the mutants, with the exception of mutants lacking FHA, attached efficiently to neutrophils. The addition of opsonizing antibodies caused a significant reduction (approximately 50%) in attachment of the wild-type strain and most of the mutants expressing FHA, suggesting that bacterium-mediated attachment is more efficient than Fc-mediated attachment. Phagocytosis was also examined. In the absence of antibodies, about 12% of the wild-type bacteria were phagocytosed. Opsonization caused a statistically significant reduction in phagocytosis (to 3%), possibly a consequence of reduced attachment. Phagocytosis of most of the mutants was similar to that of the wild type, with the exception of the mutants lacking adenylate cyclase toxin. About 70% of the adenylate cyclase toxin mutants were phagocytosed, but only in the presence of opsonizing antibody, suggesting that Fc receptor-mediated signaling may be needed for phagocytosis. These studies indicate that FHA mediates attachment of B. pertussis to neutrophils, but adenylate cyclase toxin blocks phagocytosis.

Variable carbohydrate modifications to the catalytic chains of the RgpA and RgpB proteases of Porphyromonas gingivalis W50

Proteases of Porphyromonas gingivalis are considered to be important virulence determinants of this periodontal bacterium. Several biochemical isoforms of arginine-specific proteases are derived from rgpA and rgpB. HRgpA is a heterodimer composed of the catalytic alpha chain noncovalently associated with a beta adhesin chain derived from the C terminus of the initial full-length translation product. The catalytic alpha chain is also present as a monomer (RgpA) either free in solution or associated with membranes. rgpB lacks the coding region for the adhesin domain present in rgpA and yields only monomeric forms (RgpB) which again may be soluble or membrane associated. In this study, the catalytic chains of this unusual group of enzymes are shown to be differentially modified by the posttranslational addition of carbohydrate. A monoclonal antibody (MAb 1B5) raised to the monomeric RgpA did not react with the corresponding recombinant RgpA alpha chain expressed in Escherichia coli but was immunoreactive with P. gingivalis lipopolysaccharide. MAb 1B5 also reacted with the membrane-associated forms of RgpA and RgpB but not with the heterodimeric HRgpA and the soluble form of RgpB. RgpA treated with denaturants was capable of binding to MAb 1B5 whereas treatment with periodate abolished this binding, suggesting the presence of carbohydrate residues within the epitope. Chemical deglycosylation abolished immunoreactivity with MAb 1B5 and caused a approximately 30% reduction in the size of the membrane-associated enzymes. Monosaccharide analysis of HRgpA and RgpA demonstrated 2.1 and 14.4%, respectively, carbohydrate by weight of protein. Furthermore, distinct differences were detected in their monosaccharide compositions, indicating that these protease isoforms are modified not only to different extents but also with different sugars. The variable nature of these additions may have a significant effect on the structure, stability, and immune recognition of these protease glycoproteins.

Molecular basis for structural diversity in the core regions of the lipopolysaccharides of Escherichia coli and Salmonella enterica

Bacterial lipopolysaccharides (LPS) are unique and complex glycolipids that provide characteristic components of the outer membranes of Gram-negative bacteria. In LPS of the Enterobacteriaceae, the core oligosaccharide links a highly conserved lipid A to the antigenic O-polysaccharide. Structural diversity in the core oligosaccharide is limited by the constraints imposed by its essential role in outer membrane stability and provides a contrast to the hypervariable O-antigen. The genetics of core oligosaccharide biosynthesis in Salmonella and Escherichia coli K-12 have served as prototypes for studies on the LPS and lipo-oligosaccharides from a growing range of bacteria. However, despite the wealth of knowledge, there remains a number of unanswered questions, and direct experimental data are not yet available to define the precise mechanism of action of many gene products. Here we present a comparative analysis of the recently completed sequences of the major core oligosaccharide biosynthesis gene clusters from the five known core types in E. coli and the Ra core type of Salmonella enterica serovar Typhimurium and discuss advances in the understanding of the related biosynthetic pathways. Differences in these clusters reflect important structural variations in the outer core oligosaccharides and provide a basis for ascribing functions to the genes in these model clusters, whereas highly conserved regions within these clusters suggest a critical and unalterable function for the inner region of the core.

Acylation of Escherichia coli hemolysin: a unique protein lipidation mechanism underlying toxin function

The pore-forming hemolysin (HlyA) of Escherichia coli represents a unique class of bacterial toxins that require a posttranslational modification for activity. The inactive protoxin pro-HlyA is activated intracellularly by amide linkage of fatty acids to two internal lysine residues 126 amino acids apart, directed by the cosynthesized HlyC protein with acyl carrier protein as the fatty acid donor. This action distinguishes HlyC from all bacterial acyltransferases such as the lipid A, lux-specific, and nodulation acyltransferases, and from eukaryotic transferases such as N-myristoyl transferases, prenyltransferases, and thioester palmitoyltransferases. Most lipids directly attached to proteins may be classed as N-terminal amide-linked and internal ester-linked acyl groups and C-terminal ether-linked isoprenoid groups. The acylation of HlyA and related toxins does not equate to these but does appear related to a small number of eukaryotic proteins that include inflammatory cytokines and mitogenic and cholinergic receptors. While the location and structure of lipid moieties on proteins vary, there are common effects on membrane affinity and/or protein-protein interactions. Despite being acylated at two residues, HlyA does not possess a "double-anchor" motif and does not have an electrostatic switch, although its dependence on calcium binding for activity suggests that the calcium-myristoyl switch may have relevance. The acyl chains on HlyA may provide anchorage points onto the surface of the host cell lipid bilayer. These could then enhance protein-protein interactions either between HlyA and components of a host signal transduction pathway to influence cytokine production or between HlyA monomers to bring about oligomerization during pore formation.