Non-encapsulated strains reveal novel insights in invasion and survival of Streptococcus suis in epithelial cells

Streptococcus suis is a porcine and human pathogen causing invasive diseases, such as meningitis or septicaemia. Host cell interactions of S. suis have been studied mainly with serotype 2 strains, but multiple capsular serotypes as well as non-typeable strains exist with diverse virulence features. At present, S. suis is considered an extracellular pathogen. However, whether or not it can also invade host cells is a matter of controversial discussions. We have assessed adherence and invasion of S. suis for HEp-2 epithelial cells by comparing 10 serotype 2 strains and four non-typeable (NT) strains. Only the NT strains and a non-encapsulated serotype 2 mutant strain, but none of the serotype 2 strains, adhered strongly and were invasive. Invasion seemed to be affected by environmental signals, as suggested from comparison of strains grown in different media. Further phenotypic and genotypic characterization revealed a high diversity among the different strains. Electron microscopic analysis of invasion of selected invasive NT strains indicated different uptake mechanisms. One strain induced large invaginations comparable to those seen in 'caveolae' mediated uptake, whereas invasion of the other strains was accompanied by formation of filipodia-like membrane protrusions. Invasion of all strains, however, was similarly susceptible to hypertonic sucrose, which inhibits receptor-mediated endocytosis. Irrespective of the uptake pathway, streptococci resided in acidified phago-lysosome like vacuoles. All strains, except one, survived intracellularly as well as extracellular acidic conditions. Survival seemed to be associated with the AdiS protein, an environmentally regulated arginine deiminase of S. suis. Concluding, invasion and survival of NT strains of S. suis in epithelial cells revealed novel evidence that S. suis exhibits a broad variety of virulence-associated features depending on genetic variation and regulation.

Oceanisphaera litoralis gen. nov., sp. nov., a novel halophilic bacterium from marine bottom sediments

A polyphasic taxonomic study was performed to characterize a new bacterial isolate, designated KMM 3654(T), from a marine bottom sand sample. The strain was Gram-negative, encapsulated, aerobic, moderately halophilic and grew between 0.5 and 10 % NaCl and at 4-42 degrees C. Its DNA G+C content was 56.4 mol%. Isolate KMM 3654(T) was phylogenetically closely related to members of the genus Oceanimonas, showing 96.7 and 95.6 % 16S rRNA gene sequence similarity to Oceanimonas doudoroffii DSM 7028(T) and Oceanimonas baumannii ATCC 700832(T), respectively. Strain KMM 3654(T) shared some physiological and chemotaxonomic properties with these two Oceanimonas species, but differed from them in morphology, growth at 4 degrees C, urease activity, weak phenol degradation and utilization of phenylacetate. On the basis of phenotypic and phylogenetic evidence, Oceanisphaera litoralis gen. nov., sp. nov. is proposed, with the type strain KMM 3654(T) (=DSM 15406(T)).

Improved bounce-back methods for no-slip walls in lattice-Boltzmann schemes: theory and simulations

A detailed analysis is presented for the accuracy of several bounce-back methods for imposing no-slip walls in lattice-Boltzmann schemes. By solving the lattice-BGK (Bhatnagar-Gross-Krook) equations analytically in the case of plane Poiseuille flow, it is found that the volumetric approach by Chen et al. is first-order accurate in space, and the method of Bouzidi et al. second-order accurate in space. The latter method, however, is not mass conservative because of errors associated with interpolation of densities residing on grid nodes. Therefore, similar interpolations are applied to Chen's volumetric scheme, which indeed improves the accuracy in the case of plane Poiseuille flow with boundaries parallel to the underlying grid. For skew boundaries, however, it is found that the accuracy remains first order. An alternative volumetric approach is proposed with a more accurate description of the geometrical surface. This scheme is demonstrated to be second-order accurate, even in the case of skew channels. The scheme is mass conservative in the propagation step because of its volumetric description, but still not in the collision step. However, the deviation in the mass is, in general, found to be small and proportional to the second-order terms in the standard BGK equilibrium distribution. Consequently, the scheme is a priori mass conservative for Stokes flow.

Structural and functional cellular changes induced by Burkholderia pseudomallei rhamnolipid

In this study we report that extracellular Burkholderia pseudomallei rhamnolipid induced cytopathic changes characterized by retraction, rounding up, and, finally, detachment in phagocytic and nonphagocytic cell lines. These changes were due to a progressive reorganization of the F-actin network resulting in impaired cell cycle progression and a reduced phagocytic function of macrophages.

Fatal outcome of lung transplantation in cystic fibrosis patients due to small-colony variants of the Burkholderia cepacia complex

The aim of this study was to investigate the possible role of small-colony variant morphotypes of Burkholderia cepacia-like organisms in infectious complications in cystic fibrosis patients following lung transplantation. Respiratory tract specimens from 470 cystic fibrosis patients were screened over a 22-month period for Burkholderia cepacia-like organisms. Nineteen patients were positive for these organisms, eight of whom harboured small-colony-variant morphotypes. Three patients underwent bilateral lung transplantation during the study, two of whom harboured small-colony variants in addition to clonally identical wildtypes of Burkholderia multivorans and Burkholderia cepacia genomovar III prior to lung transplantation. Both patients developed fatal systemic infections post transplantation due to small-colony variants. In vitro testing revealed that small-colony variants exhibited increased serum resistance in comparison to wildtypes. The results of this study indicate that diagnostic efforts should be undertaken to carefully identify small-colony variants of Burkholderia cepacia complex, since they might be an indicator of poor post-transplantation outcome in patients with cystic fibrosis.

Halomonas halocynthiae sp. nov., isolated from the marine ascidian Halocynthia aurantium

The marine bacterium strain KMM 1376T was isolated from gill tissue of the ascidian Halocynthia aurantium, an inhabitant of the coastal waters of the Sea of Japan. Strain KMM 1376T is an aerobic, gram-negative, non-motile, encapsulated, non-pigmented, slightly halophilic bacterium that is characterized by fimbria-like structures, growth in 0.5-15% NaCl at 7-35 degrees C and absence of acid production from many carbohydrates. The DNA G+C content is 54 mol%. The main fatty acids are C16:0, C16.1omega7c and C18:1omega9c. Comparative 16S rDNA sequence analysis of strain KMM 1376T revealed 92.7-95.3% sequence similarity to members of Halomonas. Based on phenotypic and phylogenetic characteristics, it is proposed that the novel isolate be classified as Halomonas halocynthiae sp. nov., with the type strain KMM 1376T (= DSM 14573T).

Volumetric method for calculating the flow around moving objects in lattice-Boltzmann schemes

A method for calculating the fluid flow around moving objects is presented, based on a volumetric representation of the lattice-Boltzmann scheme and surfaces defined by facets. It enables us to move objects of arbitrary shape and orientation independent of the position of the grid nodes. To represent the motion of the object, additional momentum is added to the reflected particles from each facet in the propagation step. These particles are redistributed on nodes in the vicinity of the surface, depending on the position and orientation of the facet. Because the surface is considered to be closed, additional techniques need to be used to guarantee the conservation of mass. The flow field of a moving periodic cubic array of cubes at two Re numbers (Re=0.5 and Re=50) is compared with that of a fixed array. For Re=0.5, no significant deviations are found for the velocity field, pressure field, and the drag force. For Re=50, the drag and pressure field exhibit small fluctuations that relate to the position of the surface relative to the position of the grid. However, the influence of the pressure fluctuations on the velocity field is very small. Results on the velocity for a moving array of cubes show second-order accuracy in the lattice spacing. For physical consistency, the drag force on a periodic cubic array of moving spheres at Re=0.5 is compared with Hasimoto's analytical solution. The dependence on the grid spacing, the resolution of the surface of the object, and the viscosity have been studied. The discrepancies between simulations and the analytical results are smaller than 1.5%. For Re=50, the drag force, the streamline pattern, and the pressure field around a moving sphere in a large periodic domain showed good agreement with data from literature on a single sphere in an infinitely large flow field.

Microbial diversity of cultivatable bacteria associated with the North Sea bryozoan Flustra foliacea

The microbial diversity of cultivatable bacteria associated with the bryozoan species Flustra foliacea from the North Sea was investigated by a molecular approach. Amplified ribosomal RNA restriction analyses (ARDRA) and 16S rDNA partial sequence analysis revealed differences in the composition of cultivatable bacteria populations from single bryozoan colonies collected from two different sampling sites in the North Sea as well from one site taken at different points in time. Whereas gamma-Proteobacteria identified as Shewanella frigidimarina, Pseudoalteromonas ssp. and Psycbrobacter ssp. were predominant on samples of Flustra I (taken near the island of Helgoland), most bacteria isolated from Flustra II, originating from the Steingrund, could be affiliated to Gram-positive taxa. Survey of the bryozoan samples from the latter site in February 2000 led to the detection of a phylogenetically mixed bacterial population, consisting of gamma-, and alpha-Proteobacteria and Gram-positive bacteria with low and high GC-content (Flustra III). As these bacteria are among the most widely isolated organisms from the marine environment, it may be concluded that the bryozoan Flustra foliacea accepts colonization of surfaces by bacteria which are common inhabitants of the marine environment and which may have been transferred into this environment from terrestrial sites.

Muricauda ruestringensis gen. nov., sp. nov., a facultatively anaerobic, appendaged bacterium from German North Sea intertidal sediment

A gram-negative, facultatively anaerobic bacterium with appendages was isolated from continuous cultures with a seawater-sediment suspension containing hexadecane as the sole carbon source. Although this organism was isolated from a hexadecane-degrading bacterial community, it was not able to degrade hexadecane. However, this bacterium was able to use different sugars and amino acids for growth, indicating that it probably profits from the lysis or from products like surfactants of other cells in the community. 16S rDNA analysis demonstrated that the isolated strain is phylogenetically related to the family Flavobacteriaceae of the phylum 'Cytophaga-Flavobacterium-Bacteroides'. Evidence based on phenotypic characteristics and 16S rDNA analysis supports the conclusion that this bacterium is distinct from its nearest relative, Zobellia uliginosa (90.72% similarity in 16S rRNA gene sequence), and from the other genera of the Flavobacteriaceae. It is therefore proposed that the isolated marine bacterium represents a novel taxon, designated Muricauda ruestringensis gen. nov., sp. nov. The type strain is strain B1T (= DSM 13258T = LMG 19739T).

Genetic control of susceptibility to group A streptococcal infection in mice

The influence of genetic background on the ability to control infection with group A streptococci was investigated in different inbred strains of mice. Whereas BALB/c, C57BL/10, and DBA/2 mice were the most resistant strains, with lower bacteria loads and higher survival times, C3H/HeN and CBA/J mice exhibited substantially higher bacterial growth and 100% mortality. Differences in susceptibility were not dependent on the inoculum size. Resistance was influenced by sex, with males being much more susceptible than females. B cell- and T cell-deficient mice from the resistant background were as resistant to infection as were immunocompetent mice, which suggests that the effector mechanisms are independent of adaptive immunity. These results demonstrate for the first time the influence of genetic background and sex on susceptibility to infection with Streptococcus pyogenes in mice. The use of this mouse model of group A streptococcal infection will allow for a better definition of parameters involved in the outcome of the disease.

The pavA gene of Streptococcus pneumoniae encodes a fibronectin-binding protein that is essential for virulence

Streptococcus pneumoniae colonizes the nasopharynx in up to 40% of healthy subjects, and is a leading cause of middle ear infections (otitis media), meningitis and pneumonia. Pneumococci adhere to glycosidic receptors on epithelial cells and to immobilized fibronectin, but the bacterial adhesins mediating these reactions are largely uncharacterized. In this report we describe a novel pneumococcal protein PavA, which binds fibronectin and is associated with pneumococcal adhesion and virulence. The pavA gene, present in 64 independent isolates of S. pneumoniae tested, encodes a 551 amino acid residue polypeptide with 67% identical amino acid sequence to Fbp54 protein in Streptococcus pyogenes. PavA localized to the pneumococcal cell outer surface, as demonstrated by immunoelectron microscopy, despite lack of conventional secretory or cell-surface anchorage signals within the primary sequence. Full-length recombinant PavA polypeptide bound to immobilized human fibronectin in preference to fluid-phase fibronectin, in a heparin-sensitive interaction, and blocked binding of wild-type pneumococcal cells to fibronectin. However, a C-terminally truncated PavA' polypeptide (362 aa residues) failed to bind fibronectin or block pneumococcal cell adhesion. Expression of pavA in Enterococcus faecalis JH2-2 conferred > sixfold increased cell adhesion levels to fibronectin over control JH2-2 cells. Isogenic mutants of S. pneumoniae, either abrogated in PavA expression or producing a 42 kDa C-terminally truncated protein, showed up to 50% reduced binding to immobilized fibronectin. Inactivation of pavA had no effects on growth rate, cell morphology, cell-surface physico-chemical properties, production of pneumolysin, autolysin, or surface proteins PspA and PsaA. Isogenic pavA mutants of encapsulated S. pneumoniae D39 were approximately 104-fold attenuated in virulence in the mouse sepsis model. These results provide evidence that PavA fibronectin-binding protein plays a direct role in the pathogenesis of pneumococcal infections.

Salana multivorans gen. nov., sp. nov., a novel actinobacterium isolated from an anaerobic bioreactor and capable of selenate reduction

Three facultatively anaerobic, Gram-positive bacteria, strains Se-3111T, Se-13111 and Se-1311A, were isolated from an anaerobic, dechlorinating bioreactor culture enriched from sediment of the River Saale in Germany. All strains were isolated from the dechlorinating mixed culture through their ability to reduce selenate anaerobically to elemental selenium. All three strains shared identical 16S rDNA sequences and phylogenetic analysis revealed that strain Se-3111T forms a novel taxon within the suborder Micrococcineae of the class Actinobacteria, related most closely to Beutenbergia cavernae. On the basis of genotypic, chemotaxonomic and physiological characteristics, it is proposed that the novel strains Se-3111T, Se-13111 and Se-1311A be classified in a new genus as Salana multivorans gen. nov., sp. nov. The type strain of the novel species is Se-3111T (= DSM 13521T = NRRL B-24118T).

Eukaryotic expression plasmid transfer from the intracellular bacterium Listeria monocytogenes to host cells

The facultative intracellular, Gram-positive bacterium Listeria monocytogenes invades phagocytic and non-phagocytic cells from the tissues and organs of a wide variety of animals and humans. Here, we report the use of these bacteria as vehicles for gene transfer. Eukaryotic expression plasmids were introduced into the nucleus of host cells following lysis of the intracytosolic, plasmid-carrying bacteria with antibiotics. Cell lines of different tissues and species could be transfected in this way. We examined bacterial properties required for delivery of the expression plasmids and found that this was strictly dependent on the ability of these bacteria to both invade eukaryotic cells and egress from the vacuole into the cytosol of the infected host cells. Macrophage-like cell lines or primary, peritoneal macrophages proved to be almost refractory to Listeria-mediated gene transfer. Thus, attenuated L. monocytogenes represents a serious candidate for consideration as a DNA-transfer vehicle for in vivo somatic gene therapy. The potential for oral administration of L. monocytogenes and the ease in producing and cultivating recombinant strains are further attributes that make its use as a gene transfer vehicle attractive.

Characterization of the intracellular survival of Mycobacterium avium ssp. paratuberculosis: phagosomal pH and fusogenicity in J774 macrophages compared with other mycobacteria

The phagosomes containing viable pathogenic mycobacteria, such as Mycobacterium (M.) tuberculosis and Mycobacterium avium ssp. avium (M. avium), are known to be limited in their ability to both acidify and fuse with late (but not early) endocytic organelles. Here, we analysed the pH and fusogenicity of phagosomes containing M. avium ssp. paratuberculosis (M. ptb), the causative agent of paratuberculosis in ruminants. Using the murine J774 macrophage cell line, we compared viable and heat-killed M. ptb and, in addition, viable or dead M. avium, as well as two non-pathogenic mycobacteria, Mycobacterium smegmatis and Mycobacterium gordonae. Electron microscopic analysis revealed that M. ptb persisted intracellularly in phagosomes for up to 15 days. The phagosomes containing live M. ptb and M. avium were significantly reduced in their ability to acquire some markers for the endocytic pathway, such as internalized calcein, BSA-gold or the membrane protein Lamp 2. However, they were almost completely accessible to 70 kDa fluorescein isothiocyanate (FITC)-dextran and Lamp 1. Overall, the phagosomes containing dead pathogenic mycobacteria behaved similarly to the ones containing live non-pathogenic mycobacteria in all experiments. Using FITC-dextran in a novel fluorescence-activated cell sorting (FACS)-based method, we could also show that the bulk of endocytic compartments, including phagosomes, were only very mildly acidified to approximately pH 6.3 over at least 72 h in J774 cells infected with live M. ptb and M. avium. In contrast, J774 cells treated with heat-killed M. ptb or BSA-coated latex beads showed substantial acidification of the phagosome/endocytic compartments to a pH value of approximately 5.2. After infection with M. smegmatis and M. gordonae, acidification was initially (1-5 h after infection) inhibited, but increased after longer infection to levels similar to those with dead mycobacteria.

alpha-Enolase of Streptococcus pneumoniae is a plasmin(ogen)-binding protein displayed on the bacterial cell surface

Binding of human plasminogen to Streptococcus pneumoniae and its subsequent activation promotes penetration of bacteria through reconstituted basement membranes. In this study, we have characterized a novel pneumococcal surface protein with a molecular mass of 47 kDa, designated Eno, which specifically binds human plasmin(ogen), exhibits alpha-enolase activity and is necessary for viability. Using enzyme assays, we have confirmed the alpha-enolase activity of both pneumococcal surface-displayed Eno and purified recombinant Eno protein. Immunoelectron microscopy indicated the presence of Eno in the cytoplasm as well as on the surface of encapsulated and unencapsulated pneumococci. Plasminogen-binding activity was demonstrated with whole pneumococcal cells and purified Eno protein. Binding of activated plasminogen was also shown for Eno; however, the affinity for plasmin is significantly reduced compared with plasminogen. Results from competitive inhibition assays indicate that binding is mediated through the lysine binding sites in plasmin(ogen). Carboxypeptidase B treatment and amino acid substitutions of the C-terminal lysyl residues of Eno indicated that the C-terminal lysine is pivotal for plasmin(ogen)-binding activity. Eno is ubiquitously distributed among pneumococcal serotypes, and binding experiments suggested the reassociation of secreted Eno to the bacterial cell surface. The reassociation was also confirmed by immunoelectron microscopy. The results suggest a mechanism of plasminogen activation for human pathogens that might contribute to their virulence potential in invasive infectious processes.

Anti-VEGFR-2 scFvs for cell isolation. Single-chain antibodies recognizing the human vascular endothelial growth factor receptor-2 (VEGFR-2/flk-1) on the surface of primary endothelial cells and preselected CD34+ cells from cord blood

Five specific single-chain antibodies recognizing the human vascular endothelial growth factor receptor-2 (VEGFR-2/KDR) were selected from a V-gene phage display library constructed from mice immunized with the extracellular domain of VEGFR-2 (Ig-like domain 1-7). All five scFv antibodies (A2, A7, B11, G3, and H1) bound to the purified native antigen in enzyme-linked immunosorbent assay and Dot Blot, and showed no crossreactivity to the human VEGF-receptor 1 (VEGFR-1). The selected antibodies recognize a conformation-dependent epitope of the native receptor and do not recognize denatured antigen in Western blots, as well as linear overlapping peptides comprising the sequence of the human VEGFR-2. The five scFv antibodies bind to the surface of endothelial cells overexpressing human VEGFR-2 c-DNA (PAE/VEGFR-2 cells) as detected by surface immunofluorescence using confocal microscopy. In addition scFv A7 specifically detected VEGFR-2 expressing endothelial cells in the glomerulus of frozen human kidney tissue sections. Therefore, A7 has potential clinical application as a marker for angiogenesis in cryosections of different human tissues. Additionally, two recombinant scFvs (A2 and A7) very efficiently recognize VEGFR-2 on PAE/VEGFR-2 cells and freshly prepared human umbilical vein endothelial cells by fluorescence-activated cell sorter (FACS) analysis. The scFv fragment A7, which was the most sensitive antibody in FACS analysis, recognizes human CD34+VEGFR-2+ hematopoietic immature cells within the population of enriched CD34+ cells isolated from human cord blood. The dissociation constant of A7 was determined to be K(d) = 3.8 x 10(-9) M by BIAcore analysis. In conclusion, scFv fragment A7 seems to be an important tool for FACS analysis and cell sorting of vascular endothelial cells, progenitor cells and hematopoitic stem cells, which are positive for VEGFR-2 gene expression.

TGF-beta1 promotes microglial amyloid-beta clearance and reduces plaque burden in transgenic mice

Abnormal accumulation of the amyloid-beta peptide (Abeta) in the brain appears crucial to pathogenesis in all forms of Alzheimer disease (AD), but the underlying mechanisms in the sporadic forms of AD remain unknown. Transforming growth factor beta1 (TGF-beta1), a key regulator of the brain's responses to injury and inflammation, has been implicated in Abeta deposition in vivo. Here we demonstrate that a modest increase in astroglial TGF-beta1 production in aged transgenic mice expressing the human beta-amyloid precursor protein (hAPP) results in a three-fold reduction in the number of parenchymal amyloid plaques, a 50% reduction in the overall Abeta load in the hippocampus and neocortex, and a decrease in the number of dystrophic neurites. In mice expressing hAPP and TGF-beta1, Abeta accumulated substantially in cerebral blood vessels, but not in parenchymal plaques. In human cases of AD, Abeta immunoreactivity associated with parenchymal plaques was inversely correlated with Abeta in blood vessels and cortical TGF-beta1 mRNA levels. The reduction of parenchymal plaques in hAPP/TGF-beta1 mice was associated with a strong activation of microglia and an increase in inflammatory mediators. Recombinant TGF-beta1 stimulated Abeta clearance in microglial cell cultures. These results demonstrate that TGF-beta1 is an important modifier of amyloid deposition in vivo and indicate that TGF-beta1 might promote microglial processes that inhibit the accumulation of Abeta in the brain parenchyma.

The role played by the group A streptococcal negative regulator Nra on bacterial interactions with epithelial cells

Group A streptococci (GAS) specifically attach to and internalize into human epithelial host cells. In some GAS isolates, fibronectin-binding proteins were identified as being responsible for these virulence traits. In the present study, the previously identified global negative regulator Nra was shown to control the binding of soluble fibronectin probably via regulation of protein F2 and/or SfbII expression in the serotype M49 strain 591. According to results from a conventional invasion assay based on the recovery of viable intracellular bacteria, the increased fibronectin binding did not affect bacterial adherence to HEp-2 epithelial cells, but was associated with a reduction in the internalization rates. However, when examined by confocal and electron microscopy techniques, the nra-mutant bacteria were shown to exhibit higher adherence and internalization rates than the corresponding wild type. The mutant bacteria escaped from the phagocytic vacuoles much faster, promoting consistent morphological changes which resulted in severe host cell damage. The apoptotic and lytic processes observed in nra-mutant infected host cells were correlated with an increased expression of the genes encoding superantigen SpeA, the cysteine protease SpeB, and streptolysin S in the nra-mutant bacteria. Adherence and internalization rates of a nra/speB-double mutant at wild-type levels indicated that the altered speB expression in the nra mutant contributed to the observed changes in both processes. The Nra-dependent effects on bacterial virulence were confined to infections carried out with stationary growth phase bacteria. In conclusion, the obtained results demonstrated that the global GAS regulator Nra modulates virulence genes, which are involved in host cell damage. Thus, by helping to achieve a critical balance of virulence factor expression that avoids the injury of target cells, Nra may facilitate GAS persistence in a safe intracellular niche.

The multicellular morphotypes of Salmonella typhimurium and Escherichia coli produce cellulose as the second component of the extracellular matrix

Production of cellulose has been thought to be restricted to a few bacterial species such as the model organism Acetobacter xylinus. We show by enzymatic analysis and mass spectrometry that, besides thin aggregative fimbriae, the second component of the extracellular matrix of the multicellular morphotype (rdar) of Salmonella typhimurium and Escherichia coli is cellulose. The bcsA, bcsB, bcsZ and bcsC genes responsible for cellulose biosynthesis are not regulated by AgfD, the positive transcriptional regulator of the rdar morphotype. Transcription of the bcs genes was not co-expressed with the rdar morphotype under any of the environmental conditions examined. However, cellulose biosynthesis was turned on by the sole expression of adrA, a gene encoding a putative transmembrane protein regulated by agfD, indicating a novel pathway for the activation of cellulose synthesis. The co-expression of cellulose and thin aggregative fimbriae leads to the formation of a highly hydrophobic network with tightly packed cells aligned in parallel in a rigid matrix. As the production of cellulose would now appear to be a property widely distributed among bacteria, the function of the cellulose polymer in bacteria will have to be considered in a new light.

Chromosomal insertion and excision of a 30 kb unstable genetic element is responsible for phase variation of lipopolysaccharide and other virulence determinants in Legionella pneumophila

We recently described the phase-variable expression of a virulence-associated lipopolysaccharide (LPS) epitope in Legionella pneumophila. In this study, the molecular mechanism for phase variation was investigated. We identified a 30 kb unstable genetic element as the molecular origin for LPS phase variation. Thirty putative genes were encoded on the 30 kb sequence, organized in two putative opposite transcription units. Some of the open reading frames (ORFs) shared homologies with bacteriophage genes, suggesting that the 30 kb element was of phage origin. In the virulent wild-type strain, the 30 kb element was located on the chromosome, whereas excision from the chromosome and replication as a high-copy plasmid resulted in the mutant phenotype, which is characterized by alteration of an LPS epitope and loss of virulence. Mapping and sequencing of the insertion site in the genome revealed that the chromosomal attachment site was located in an intergenic region flanked by genes of unknown function. As phage release could not be induced by mitomycin C, it is conceivable that the 30 kb element is a non-functional phage remnant. The protein encoded by ORF T on the 30 kb plasmid could be isolated by an outer membrane preparation, indicating that the genes encoded on the 30 kb element are expressed in the mutant phenotype. Therefore, it is conceivable that the phenotypic alterations seen in the mutant depend on high-copy replication of the 30 kb element and expression of the encoded genes. Excision of the 30 kb element from the chromosome was found to occur in a RecA-independent pathway, presumably by the involvement of RecE, RecT and RusA homologues that are encoded on the 30 kb element.

Co-operative binding of human fibronectin to Sfbl protein triggers streptococcal invasion into respiratory epithelial cells

Streptococcal fibronectin binding protein I (SfbI) mediates adherence to and invasion of Streptococcus pyogenes into human epithelial cells. In this study, we analysed the binding activity of distinct domains of SfbI protein towards its ligand, the extracellular matrix component fibronectin, as well as the biological implication of the binding events during the infection process. By using purified recombinant SfbI derivatives as well as in vivo expressed SfbI domains on the surface of heterologous organism Streptococcus gordonii, we were able to dissociate the two major streptococcal target domains on the human fibronectin molecule. The SfbI repeat region exclusively bound to the 30 kDa N-terminal fragment of fibronectin, whereas the SfbI spacer region exclusively bound to the 45 kDa collagen-binding fragment of fibronectin. In the case of native surface-expressed SfbI protein, an induced fit mode of bacteria-fibronectin interaction was identified. We demonstrate that binding of the 30 kDa fibronectin fragment to the repeat region of SfbI protein co-operatively activates the adjacent SfbI spacer domain to bind the 45 kDa fibronectin fragment. The biological consequence arising from this novel mode of fibronectin targeting was analysed in eukaryotic cell invasion assays. The repeat region of SfbI protein is mediating adherence and constitutes a prerequisite for subsequent invasion, whereas the SfbI spacer domain efficiently triggers the invasion process of streptococci into the eukaryotic cell. Thus, we were able to dissect bacterial adhesion from invasion by manipulating one protein. SfbI protein therefore represents a highly evolved prokaryotic molecule that exploits the host factor fibronectin not only for extracellular targeting but also for its subsequent activation that leads to efficient cellular invasion.