PubMed Health⌕ Search

Biomedical subjects

I Ofek

Publications and source records attributed to I Ofek.

At least 37 records · Page 2Linked to original sources
PubMed1998

Pathogenic adaptation of Escherichia coli by natural variation of the FimH adhesin.

Conventional wisdom regarding mechanisms of bacterial pathogenesis holds that pathogens arise by external acquisition of distinct virulence factors, whereas determinants shared by pathogens and commensals are considered to be functionally equivalent and have been ignored as genes that could become adapted specifically for virulence. It is shown here, however, that genetic variation in an originally commensal trait, the FimH lectin of type 1 fimbriae, can change the tropism of Escherichia coli, shifting it toward a urovirulent phenotype. Random point mutations in fimH genes that increase binding of the adhesin to mono-mannose residues, structures abundant in the oligosaccharide moieties of urothelial glycoproteins, confer increased virulence in the mouse urinary tract. These mutant FimH variants, however, are characterized by increased sensitivity to soluble inhibitors bathing the oropharyngeal mucosa, the physiological portal of E. coli. This functional trade-off seems to be detrimental for the intestinal ecology of the urovirulent E. coli. Thus, bacterial virulence can be increased by random functional mutations in a commensal trait that are adaptive for a pathologic environment, even at the cost of reduced physiological fitness in the nonpathologic habitat.

Adhesins, Bacterial↗
PubMed1998

Inhibiting interspecies coaggregation of plaque bacteria with a cranberry juice constituent [published erratam appear in J Am Dent Assoc 1999 Jan;130(1):36 and 1999 Mar;130(3):332].

Dental plaque stability depends on bacterial adhesion to acquired pellicle, and on interspecies adhesion (or coaggregation). A high-molecular-weight cranberry constituent at 0.6 to 2.5 milligrams per milliliter reversed the coaggregation of 49 (58 percent) of 84 coaggregating bacterial pairs tested. It acted preferentially on pairs in which one or both members are gram-negative anaerobes frequently involved in periodontal diseases. Thus, the anticoaggregating cranberry constituent has the potential for altering the subgingival microbiota, resulting in conservative control of gingival and periodontal diseases. However, the high dextrose and fructose content of the commercially available cranberry juice makes it unsuitable for oral hygiene use, and the beneficial effect of the high-molecular-weight constituent requires animal and clinical studies.

Bacterial Adhesion↗
PubMed1997

M protein mediated adhesion of M type 24 Streptococcus pyogenes stimulates release of interleukin-6 by HEp-2 tissue culture cells.

We investigated the contributions of lipoteichoic acid and M protein to reversible and irreversible adhesion of group A streptococci and the effects of such adhesion on release of interleukin-6. Streptococci in which lipoteichoic acid was masked by the hyaluronate capsule were readily washed from HEp-2 cells, indicating no attachment. Unencapsulated, M-negative streptococci in which lipoteichoic acid was exposed were removed more slowly, indicating loose attachment. Only unencapsulated streptococci that expressed both lipoteichoic acid and M protein remained stably adherent to HEp-2 cells throughout multiple washes. Streptococci expressing both M protein and lipoteichoic acid induced release of interleukin-6 from HEp-2 cells, whereas an isogenic, M-negative mutant failed to induce release of interleukin-6. These data suggest that lipoteichoic acid mediates reversible adhesion and that M protein is required for irreversible adhesion and for inducing release of interleukin-6 from HEp-2 cells.

Antigens, Bacterial↗
PubMed1997

Insertional inactivation of streptolysin S expression is associated with altered riboflavin metabolism in Streptococcus pyogenes.

Transposon Tn916 mutagenesis was used to create a mutant of Streptococcus pyogenes M type 3, designated ISS417, in which the ability to produce streptolysin S (SLS) and several other exoproteins was impaired. Concomitantly, the mutant became dependent upon riboflavin for growth and was able to grow in Todd Hewitt broth (THB) when supplemented with riboflavin or riboflavinrich yeast extract. The parent strain was apparently able to utilize THB-derived components as a substitute for riboflavin, while the mutant was not. Although the parent strain grew well in synthetic medium, it was unable to produce SLS, except when it was supplemented with a small amount of THB. Thus, a component of THB was able to "trigger" SLS formation in the parent strain. The mutant grew well in this medium, but was unable to produce SLS even when it was supplemented with THB. Southern hybridization analysis revealed that the ISS417 mutant harbours a single transposon insertion in its chromosome. Phage transduction experiments showed that the riboflavin dependency and the inability to make SLS phenotypes are co-transducible. The pleotrophic properties of the ISS417 mutant differ from those reported for insertional inactivation of the mga locus which regulates production of a number of surface proteins in S. pyogenes and the sar locus which regulates production of a number of exoproteins in Staphylococcus aureus. In view of the possibility that there exist a genetic linkage between the riboflavin biosynthetic pathway and expression of the oxygen-stable SLS, we hypothesize that SLS has a role in the growth economy of S. pyogenes.

Bacterial Proteins↗
PubMed1997

SP-A enhances phagocytosis of Klebsiella by interaction with capsular polysaccharides and alveolar macrophages.

We found that surfactant protein A (SP-A) enhances phagocytosis of Klebsiella pneumoniae K21a but not of K2 serotypes by alveolar macrophages. SP-A interacted with the capsule of K21a (containing Man alpha1 Man sequences) as shown by SP-A-induced agglutination of the bacteria, by binding of SP-A-coated particles onto the bacterial surface, and by binding of SP-A to immobilized parent K21a strain and recombinant strains that switched their capsule from K2 to K21a. In contrast, only marginal binding of SP-A to K2 parent strain (lacking this sequence) could be detected. Furthermore, binding of capsular polysaccharide of K21a to immobilized SP-A was inhibited by mannan but not by lipopolysaccharide and K2 capsular polysaccharide. SP-A-treated macrophages bound increased numbers of parent K21a strain and recombinant strains of K21a capsule type but considerably less parent K2 strain. SP-A also enhanced killing of K21a strains by macrophages. The enhanced binding of K21a by macrophages pretreated with SP-A was inhibited by mannan, suggesting that binding is mediated by the mannose receptor on macrophages. We conclude that SP-A increases phagocytosis of the Klebsiella by two mechanisms, one of which is by serving as an opsonin, which binds to the capsular polysaccharides of the bacteria and potentially to SP-A receptors on the macrophages, and the other by activating the macrophages, resulting in increased activity of the mannose receptor.

Animals↗
PubMed1997

Phagocyte-bacteria interactions.

Recognition and phagocytosis of micro-organisms in a serum-poor environment represent innate immunity against many extracellular pathogens. As a paradigm for such processes, we discuss the recognition of Klebsiella pneumoniae by alveolar macrophages and monocyte-derived macrophages in the absence of serum. Macrophages recognize and subsequently kill Klebsiella expressing Man-alpha 2/3-Man or Rha-alpha 2/3-Rha sequences in their capsular polysaccharides by two mechanisms: (a) recognition of the capsular structures by macrophage mannose receptors, and (b) opsonization by the lung surfactant protein A (SP-A), which binds to the capsular polysaccharides of Klebsiella and to SP-A receptors on the macrophages. Sp-A may also enhance phagocytosis by increasing the activity of macrophage mannose receptors. We conclude that a specific microbial surface structure may be a target for recognition by macrophages via several mechanisms, as exemplified in the case of Klebsiella capsular polysaccharides. Multiple recognition mechanisms of pathogens by macrophages may be essential to provide innate immunity to reduce the frequency of infections caused by a relatively less virulent bacterium in the immuno-compromised host.

Animals↗
PubMed1995

Nonopsonic phagocytosis of microorganisms.

Nonopsonic phagocytosis mediated by phagocyte receptors that recognize corresponding adhesins on microbial surfaces has attracted increasing interest as a potential host defense mechanism against extracellular pathogens and as a means of survival in the host for intracellular pathogens. Three types of nonopsonic phagocytosis involving carbohydrate-protein interactions (also termed lectinophagocytosis), protein-protein interactions, and hydrophobic interactions are discussed. A prominent receptor on phagocytic cells involved in recognizing pathogens belongs to the CD11/CD18 integrins. It mediates both opsonophagocytosis and nonopsonic phagocytosis and exhibits multiple specificity for different microbial adhesins. In other cases, similar specificity toward a microbial ligand (e.g. the Klebsiella pneumoniae capsule) is shared by dual molecules, one of which (e.g. the mannose-binding protein in serum) mediates opsonophagocytosis and the other (e.g. the macrophage mannose receptor) mediates nonopsonic phagocytosis of the microorganisms. In addition, we discuss how nonopsonic phagocytosis can trigger the phagocytes to release inflammatory agents and cause tissue injury. Further studies of the molecular mechanisms of nonopsonic phagocytosis, in particular those underlying the up-regulation of the phagocytic receptors by various agents, should lead to the development of new approaches for the prevention of infectious diseases.

Amino Acid Sequence↗
PubMed1994

Antibacterial synergism of polymyxin B nonapeptide and hydrophobic antibiotics in experimental gram-negative infections in mice.

Polymyxin B nonapeptide, derived by cleavage of the fatty acyl diaminobutyric acid from polymyxin B, is considerably less toxic, lacks bactericidal activity, and retains its ability to render gram-negative bacteria susceptible to several antibiotics by permeabilizing their outer membranes. The peptide rendered all 53 polymyxin-susceptible strains tested more susceptible to novobiocin, lowering the MIC of novobiocin eightfold or more. The combination of polymyxin B nonapeptide with novobiocin or with erythromycin administered intraperitoneally in multiple doses synergistically protected mice infected with gram-negative bacteria. This combination may be clinically useful because of the apparent rarity of the acquisition of resistance.

Animals↗
PubMed1994

Type 1 fimbrial shafts of Escherichia coli and Klebsiella pneumoniae influence sugar-binding specificities of their FimH adhesins.

The type 1 fimbriae of enterobacteria comprise FimA, which constitutes most of the fimbrial shaft, and a cassette of three minor ancillary subunits including FimH, the mannose-binding moiety. The sugar-binding specificities of Escherichia coli and Klebsiella pneumoniae type 1 fimbriae were examined by determining the relative activities of two aromatic mannosides in inhibiting the yeast aggregation caused by the fimbriated bacteria. 4-Methylumbelliferyl alpha-mannoside (MeUmb alpha Man) was approximately 10-fold more effective than p-nitrophenyl alpha-mannoside (p-NP alpha Man) in inhibiting the yeast aggregation caused by the recombinant expressing native E. coli type 1 fimbriae. In contrast, MeUmb alpha Man was only fourfold more effective than p-NP alpha Man in assays employing the recombinant expressing native K. pneumoniae type 1 fimbriae. In order to elucidate the molecular mechanisms underlying the sugar-binding specificities of type 1 fimbriae in the two species, transcomplementation studies were performed and resulted in the creation of recombinants expressing two types of hybrid fimbriae: one consisting of a cassette of minor subunits of E. coli fimbriae borne on a filamentous shaft of K. pneumoniae FimA subunits and the other consisting of a cassette of K. pneumoniae minor fimbrial subunits borne on a shaft of E. coli FimA subunits. Although the heterologous FimH was incorporated into the fimbrial filaments in amounts comparable to those observed in native fimbriae, the hemagglutination activities of recombinants expressing hybrid fimbriae were significantly lower than those of their counterparts bearing native fimbriae. The sugar-binding specificity of the recombinant expressing hybrid fimbriae consisting of an E. coli shaft bearing K. pneumoniae FimH was different from those of recombinants expressing native K. pneumoniae fimbriae in its affinity for the two aromatic sugars but was remarkably similar to the specificities exhibited by recombinants expressing native E. coli fimbriae. Conversely, the sugar-binding specificity of the recombinant expressing hybrid fimbriae consisting of a K. pneumoniae shaft bearing E. coli FimH was different from that of the recombinant expressing native E. coli fimbriae but was very similar to those of recombinants expressing native K. pneumoniae fimbriae. We conclude that the differences in the sugar-binding specificity between E. coli and K. pneumoniae FimH fimbrial subunits is influenced by the fimbrial shafts which carry the adhesin molecules in a functionally competent form at the distal tips.

Adhesins, Escherichia coli↗
PubMed1994

Anti-lipoteichoic acid antibodies enhance release of cytokines by monocytes sensitized with lipoteichoic acid.

Lipoteichoic acid (LTA) from gram-positive bacteria can stimulate monocytes to produce cytokines. To ascertain whether aggregation of LTA receptors can contribute to this effect, human monocytes were sensitized with LTA from Streptococcus pyogenes, washed, and treated with anti-LTA antibodies. The addition of anti-LTA antibodies or F(ab')2 fragments markedly enhanced the aggregation of LTA receptors, as evidenced by indirect immunofluorescence and the release of tumor necrosis factor alpha and interleukin-1 beta. These findings suggest that aggregation of LTA receptors of monocytes is required for triggering marked cytokine responses.

Animals↗
PubMed1993

Genetic exchange of determinants for capsular polysaccharide biosynthesis between Klebsiella pneumoniae strains expressing serotypes K2 and K21a.

The production of a capsular polysaccharide (CPS; K antigen) is characteristic of Klebsiella pneumoniae, but CPS structure varies among strains, and many different serotypes are now known. In this study, cps gene clusters encoding the elements of capsular polysaccharide biosynthesis were exchanged by homologous recombination between strains expressing different serotypes. The wild-type K. pneumoniae strains used for genetic exchange were KPA1 (cpsK2), expressing K2 CPS, and KPB1 (cpsK21a), expressing K21a CPS. Plasmid R68.45 was used to mobilize fragments of chromosomal DNA from auxotrophic derivatives of donor strains. Auxotrophic his alleles introduced into recipient strains provided selectable markers to coinherit the adjacent cps gene clusters from donors expressing a heterologous CPS. Each of the capsule-switched recombinants, KPA5 (cpsK21a) and KPB20 (cpsK2), was shown to have a CPS that was immunologically identical to the serotype of the respective donor. The recombinants retained their respective recipient strain background, as evidenced by a genetic marker and demonstration of a distinctive restriction fragment length polymorphism in genomic DNA. KPB1 CPS contained a sequence (mannose-alpha-2-mannose) that binds to a macrophage lectin and may be responsible for their higher susceptibility to macrophage binding and phagocytosis compared with KPA1, whose CPS lacked such sequences. The recombinant strains expressing heterologous cps genes inherited the macrophage-binding phenotype of the donor, thus confirming that relative susceptibility to phagocytosis was determined by the capsule type expressed. KPA1 was highly virulent in a mouse lethality assay, which is a feature typical of K2 strains, whereas KPB1 was not virulent in mice. Recombinant KPA5 retained relatively high virulence in mice, even though it produced the heterologous K21a CPS, which suggests that a virulence factor other than capsule biosynthesis is encoded by the KPA genomic strain background. In contrast, KPB20 gained marginal virulence in the mouse lethality assay through the inheritance and expression of the K2 CPS from the virulent strain. Thus, pathogenesis in K. pneumoniae may be multifactorial. Specific antibody was used to stabilize the CPS on the surface of K. pneumoniae, and the structural organization of the homologous and heterologous capsules was examined by electron microscopy. Recombinant KPB20, expressing heterologous K2 CPS, had a uniform layer of capsule surrounding the organism that was similar to that seen on the surfaces of the parental strains. However, KPA5, expressing the heterologous K21a CPS, was unusual in that the uniform capsular layer was physically separated from the cell wall by approximately 50 nm.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Adhesion of Trichomonas vaginalis to plastic surfaces: requirement for energy and serum constituents.

The ability of Trichomonas vaginalis to adhere to plastic surfaces in the presence of various agents and under different growth conditions was examined in wells of microtitre plates containing unsupplemented TYI medium or the same, with various supplements. Following incubation, the wells were thoroughly washed and adhesion was determined by microscopic counting of the adherent organisms. There was no detectable adhesion in the absence of both serum and carbohydrate. Optimal adhesion (about 10-20% of the total number of parasites) was obtained throughout the growth curve in culture media supplemented with either serum or serum Cohn fractions IV-I (rich in alpha-globulin) or IV-4 (rich in alpha + beta-globulin) and 25 mM glucose, maltose or fructose, but not in plates pre-coated with the Cohn fractions. Cohn fraction II + III (rich in beta + gamma-globulin) moderately enhanced adhesion while Cohn fractions II (rich in gamma-globulin) or V (albumin), fibronectin, Tamm-Horsfall glycoproteins and polylysine were without effect. Non-metabolizable sugars (methyl derivatives of glucose, mannose or fucose) did not support growth, but, surprisingly, enhanced adhesion. At 4 degrees C, the trichomonads were not able to adhere and pre-adherent organisms detached from the plastic surface. Optimal adhesion was obtained at a pH range of 6.5-7.5 but was already detectable at pH 5.5. Cytochalasin E markedly suppressed adhesion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗