Axial fibrils of anaerobic spirochetes: ultrastructure and chemical characteristics

Polypeptides of Treponema pallidum: progress toward understanding their structural, functional, and immunologic roles. Treponema Pallidum Polypeptide Research Group

Treponema pallidum subsp. pallidum, the spirochete that causes syphilis, is unusual in a number of respects, including its small genome size, inability to grow under standard in vitro culture conditions, microaerophilism, apparent paucity of outer membrane proteins, structurally complex periplasmic flagella, and ability to evade the host immune responses and cause disease over a period of years to decades. Many of these attributes are related ultimately to its protein content. Our knowledge of the activities, structure, and immunogenicity of its proteins has been expanded by the application of recombinant DNA, hybridoma, and structural fractionation techniques. The purpose of this monograph is to summarize and correlate this new information by using two-dimensional gel electrophoresis, monoclonal antibody reactivity, sequence data, and other properties as the bases of polypeptide identification. The protein profiles of the T. pallidum subspecies causing syphilis, yaws, and endemic syphilis are virtually indistinguishable but differ considerably from those of other treponemal species. Among the most abundant polypeptides are a group of lipoproteins of unknown function that appear to be important in the immune response during syphilitic infection. The periplasmic flagella of T. pallidum and other spirochetes are unique with regard to their protein content and ultrastructure, as well as their periplasmic location. They are composed of three core proteins (homologous to the other members of the eubacterial flagellin family) and a single, unrelated sheath protein; the functional significance of this arrangement is not understood at present. Although the bacterium contains the chaperonins GroEL and DnaK, these proteins are not under the control of the heat shock regulon as they are in most organisms. Studies of the immunogenicity of T. pallidum proteins indicate that many may be useful for immunodiagnosis and immunoprotection. Future goals in T. pallidum polypeptide research include continued elucidation of their structural locations and functional activities, identification and characterization of the low-abundance outer membrane proteins, further study of the immunoprotective and immunodiagnostic potential of T. pallidum proteins, and clarification of the roles of treponemal proteins in pathogenesis.

The Borrelia burgdorferi flagellum-associated 41-kilodalton antigen (flagellin): molecular cloning, expression, and amplification of the gene

Monoclonal antibodies directed against the major Borrelia burgdorferi flagellar protein, the 41-kilodalton (kDa) protein flagellin, were used to monitor cloning and expression of the flagellin gene from a Borrelia burgdorferi genomic library. The structure of the gene was analyzed, and recombinant nonfusion flagellin was produced in Escherichia coli. A DNA sequence analysis of the 41-kDa flagellin gene revealed the presence of an open reading frame that encoded a protein having 336 amino acid residues and a calculated molecular mass of 35.8 kDa, indicating that there was posttranslational modification of the natural 41-kDa flagellin protein. Upstream from the AUG start codon sequence we identified motifs corresponding to consensus procaryotic promoter elements which could be utilized by the cloned flagellin gene when it was expressed in E. coli MC1061. The deduced flagellin protein sequence exhibited high levels of homology to sequences of flagellin proteins from Bacillus subtilis and Salmonella typhimurium. The levels of sequence similarity for the amino- and carboxy-terminal portions were about 65 and 56%, respectively. DNA sequence information on the flagellin gene was used to design oligonucleotides for gene amplification by the polymerase chain reaction method, and by using this method 0.01 pg of Borrelia burgdorferi DNA could be detected. Our results provide a basis for further biochemical analysis of the 41-kDa flagellin protein, investigation of the role of this protein in host-pathogen interactions, and development of a standardized reagent for diagnostic systems for Borrelia burgdorferi infections.

Cloning and sequence analysis of flaA, a gene encoding a Spirochaeta aurantia flagellar filament surface antigen

Spirochaeta aurantia DNA that coded for an antigenic determinant of the flagellin associated with the filament surface of the periplasmic flagella was isolated. When expressed in Escherichia coli, the antigenic polypeptide had an apparent molecular weight of 37,000. Sequence analysis of the antigen-encoding DNA revealed the presence of an open reading frame that determined a polypeptide with a predicted molecular weight of 31,241. This polypeptide showed a region of identity with the N-amino-terminal region of the 39,000- and 37,000-dalton flagellins of the distantly related spirochetes Treponema phagedenis and Treponema pallidum, respectively (S. J. Norris, N. W. Charon, R. G. Cook, M. D. Fuentes, and R. J. Limberger, J. Bacteriol. 170:4072-4082, 1988). The region of identity in the deduced S. aurantia polypeptide was preceded by a possible signal sequence and signal peptidase cleavage site.

Antigenic relatedness and N-terminal sequence homology define two classes of periplasmic flagellar proteins of Treponema pallidum subsp. pallidum and Treponema phagedenis

The periplasmic flagella of many spirochetes contain multiple proteins. In this study, two-dimensional electrophoresis, Western blotting (immunoblotting), immunoperoxidase staining, and N-terminal amino acid sequence analysis were used to characterize the individual periplasmic flagellar proteins of Treponema pallidum subsp. pallidum (Nichols strain) and T. phagedenis Kazan 5. Purified T. pallidum periplasmic flagella contained six proteins (Mrs = 37,000, 34,500, 33,000, 30,000, 29,000, and 27,000), whereas T. phagedenis periplasmic flagella contained a major 39,000-Mr protein and a group of two major and two minor 33,000- to 34,000-Mr polypeptide species; 37,000- and 30,000-Mr proteins were also present in some T. phagedenis preparations. Immunoblotting with monospecific antisera and monoclonal antibodies and N-terminal sequence analysis indicated that the major periplasmic flagellar proteins were divided into two distinct classes, designated class A and class B. Class A proteins consisted of the 37-kilodalton (kDa) protein of T. pallidum and the 39-kDa polypeptide of T. phagedenis; class B included the T. pallidum 34.5-, 33-, and 30-kDa proteins and the four 33- and 34-kDa polypeptide species of T. phagedenis. The proteins within each class were immunologically cross-reactive and possessed similar N-terminal sequences (67 to 95% homology); no cross-reactivity or sequence homology was evident between the two classes. Anti-class A or anti-class B antibodies did not react with the 29- or 27-kDa polypeptides of T. pallidum or the 37- and 30-kDa T. phagedenis proteins, indicating that these proteins are antigenically unrelated to the class A and class B proteins. The lack of complete N-terminal sequence homology among the major periplasmic flagellar proteins of each organism indicates that they are most likely encoded by separate structural genes. Furthermore, the N-terminal sequences of T. phagedenis and T. pallidum periplasmic flagellar proteins are highly conserved, despite the genetic dissimilarity of these two species.

Biochemical and cytological analysis of the complex periplasmic flagella from Spirochaeta aurantia

The periplasmic flagella of Spirochaeta aurantia were isolated and were found to be ultrastructurally and biochemically complex. Generally, flagellar filaments were 18 to 20 nm in diameter and appeared to consist of an 11 to 13-nm-wide inner region and an outer layer. The hook-basal body region consisted of two closely apposed disks connected to a hook by a rod. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of purified flagella together with a Western blot analysis of a motility mutant that produces hooks and basal bodies but not flagellar filaments revealed that the filaments were composed of three major polypeptides of 37,500, 34,000, and 31,500 apparent molecular weight (37.5K, 34K, and 31.5K polypeptides) and three minor polypeptides of 36,000, 33,000, and 32,000 apparent molecular weight (36K, 33K, and 32K polypeptides). Purified hook-basal body preparations were greatly enriched in three polypeptides in the range of 62,000 to 66,000 apparent molecular weight. Immunogold labeling experiments with a monoclonal antibody specific for the 37.5K flagellin and one that reacts with an epitope common to the 36K, 34K, 33K, 32K, and 31.5K flagellins revealed that the 37.5K major polypeptide was a component of the outer layer, whereas one or more of the other polypeptides constituted the core.

Motility of the spirochete Leptospira

Spirochetes are a group of bacteria with a unique ultrastructure and a fascinating swimming behavior. This article reviews the hydrodynamics of spirochete motility, and examines the motility of the spirochete Leptospira in detail. Models of Leptospira motility are discussed, and future experiments are proposed. The outermost structure of Leptospira is a membrane sheath, and within this sheath are a helically shaped cell cylinder and two periplasmic flagella. One periplasmic flagellum is attached subterminally at either end of the cell cylinder and extends partway down the length of the cell. In swimming cells, each end of the cell may assume either a spiral or a hook shape. Translational cells have the anterior end spiral shaped, and the posterior end hook shaped. In the model of Berg et al., the periplasmic flagella are believed to rotate between the sheath and the cell cylinder. Rotation of the anterior periplasmic flagellum causes the generation of a gyrating spiral-shaped wave. This wave is believed sufficient to propel the cells forward in a low-viscosity medium. The cell cylinder concomitantly rolls around the periplasmic flagella in the opposite direction--which allows the cell to literally screw through a gel-like viscous medium without slippage. This model is presented, and it is contrasted to previous models of Leptospira motility.

Treponema phagedenis has at least two proteins residing together on its periplasmic flagella

Treponema phagedenis is an anaerobic, motile spirochete with several periplasmic flagella (PFs) at each cell end. This study provides the first genetic evidence that multiple protein species are associated with the PFs. In addition, these proteins were found to reside together on a given PF. Nonmotile mutants which lacked the PFs were isolated, and spontaneous revertants to motility regained the PFs. These results suggest that the PFs are involved in the motility of T. phagedenis. Isolated PFs had two major protein bands with molecular weights of 33,000 and 39,800, as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Western blots with monoclonal and polyclonal antibodies indicated that both proteins were absent in the PF mutants but present in the revertants. Immunoelectron microscopy revealed that the 39,800-molecular-weight protein was distributed along the entire PF. Immunoprecipitation analysis suggested that the 39,800- and 33,000-molecular-weight proteins were closely associated in situ.

Chemotaxis of Spirochaeta aurantia: involvement of membrane potential in chemosensory signal transduction

The effects of valinomycin and nigericin on sugar chemotaxis in Spirochaeta aurantia were investigated by using a quantitative capillary assay, and the fluorescent cation, 3,3'-dipropyl-2,2'-thiodicarbocyanine iodide was used as a probe to study effects of chemoattractants on membrane potential. Addition of a chemoattractant, D-xylose, to cells in either potassium or sodium phosphate buffer resulted in a transient membrane depolarization. In the presence of valinomycin, the membrane potential of cells in potassium phosphate buffer was reduced, and the transient membrane depolarization that resulted from the addition of D-xylose was eliminated. Although there was no detectable effect of valinomycin on motility, D-xylose taxis of cells in potassium phosphate buffer was completely inhibited by valinomycin. In sodium phosphate buffer, valinomycin had little effect on membrane potential or D-xylose taxis. Nigericin is known to dissipate the transmembrane pH gradient of S. aurantia in potassium phosphate buffer. This compound did not dissipate the membrane potential or the transient membrane depolarization observed upon addition of D-xylose to cells in either potassium or sodium phosphate buffer. Nigericin did not inhibit D-xylose taxis in either potassium or sodium phosphate buffer. This study indicates that the membrane potential but not the transmembrane pH gradient of S. aurantia is somehow involved in chemosensory signal transduction.

Relationship between proton motive force and motility in Spirochaeta aurantia

The effects of various metabolic inhibitors on the motility of Spirochaeta aurantia were investigated. After 15 min in sodium arsenate buffer, 90% of cells remained motile even though adenosine triphosphate levels dropped from 5.6 to 0.1 nmol/mg (dry weight) of cells. After 70 min in sodium arsenate, 5% of cells were motile. Addition of phenazine methosulfate plus ascorbate at this time resulted in motility of 95% of cells, but adenosine triphosphate levels remained at 0.1 nmol/mg of cell dry weight. Carbonyl cyanide-m-chlorophenyl hydrazone rapidly (within 1 min) and completely inhibited motility of metabolizing cells in potassium phosphate buffer. However, after 15 min in the presence of carbonyl cyanide m-chlorophenyl hydrazone the cellular adenosine triphosphate level was 3.4 nmol/mg (dry weight) of cells, and the rate of oxygen uptake was 44% of the rate measured in the absence of carbonyl cyanide m-chlorophenyl hydrazone. Cells remained motile under conditions where either the electrical potential or the pH gradient across the membrane of S. aurantia was dissipated. However, if both gradients were simultaneously dissipated, motility was rapidly inhibited. This study indicates that a proton motive force, in the form of either a transmembrane electrical potential or a transmembrane pH gradient, is required for motility in S. aurantia. Adenosine triphosphate does not appear to directly activate the motility system in this spirochete.

Involvement of periplasmic fibrils in motility of spirochetes

Nonmotile (Mot-) strains of Spirochaeta aurantia and Spirochaeta halophila were isolated with a procedure involving mutagenesis of motile wild-type cells. Electron microscopy showed that a Mot- mutant strain of S. halophia possessed incomplete periplasmic fibrils, inasmuch as most or all of the filamentous portion of the periplasmic fibrils was absent. Some of the cells of this Mot-, fibril-defective mutant strain lacked the filamentous portion of the periplasmic fibrils and formed proximal hooks, whereas other cells appeared to have a very small segment of the filamentous portion of the periplasmic fibrils attached to the proximal hooks. Motile revertants were isolated repeatedly from cultures of the Mot-, fibril-defective mutant and from S. halophila Mot- mutants that completely lacked periplasmic fibrils. The motile revertants possessed periplasmic fibrils ultrastructurally indistinguishable from wild-type periplasmic fibrils. This study indicates that periplasmic fibrils play an essential role in the motility of spirochetes.

Characterization of the cytoplasmic fibrils of Treponema refringens (Nichols)

The cytoplasmic fibrils of Treponema refringens were studied in situ by electron microscopy of thin sectioned and negatively stained cells. From 5 to 21 parallel fibrils ran through the cell in a band adjacent to the inner side of the cytoplasmic membrane, on the inner sides of the curves of the spirochete. The nuclear areas of cells were adjacent to the fibrils. Cross sections of fibrils isolated from cells which had been lysed were polygonal and not uniformly electron dense. Polyacrylamide gel electrophoresis of partially purified fibril preparations indicated their main component to be a protein with a molecular weight of 97,000. Fibrils were solubilized by 1% trypsin, 1% pronase, 6 M urea, 1 N HCl, 0.005 N NaOH or 1.3% sodium dodecyl sulfate. By electron microscopy of negatively stained isolated fibrils, each fibril was found to be a complex arrangement of strands rather than a single tubule.

Relationship between cell coiling and motility of spirochetes in viscous environments

The lowest viscosity that stops translational motility of cells (minimum immobilizing viscosity [MIV] was determined for various spirochetes. The viscous agent used was polyvinylpyrrolidone, The MIV for either Spirochaeta halophila P1 or Spirochaeta aurantia J4T was approximately 1,000 centipoise (cp), and for Leptospira interrogans (biflexa) B16 the MIV was greater than 500 cp. In comparison, the MIV for the flagellated bacteria Escherichia coli and Spirillum serpens was 60 cp. MIV values for two S. halophila mutant strains lacking the characteristic cell coiling (Hel-mutants) were 70 and 120 cp, approximately one-tenth the MIV for the wild-type strain. MIV values for cells of S. aurantia strains with fewer coils than comparably long cells of S. aurantia J4T were 300 to 600 cp. The average velocity of strains of S. aurantia and S. halophila decreased at viscosities higher than 2 to 3 cp. At 2 cp the average velocity of S. halophila P1 was 16 micron/s, whereas the average velocities of Hel-mutant strains were 7 to 9 micron/s. This study indicates that the coiling of spirochetes plays a role in their ability to move through environments of realtively high viscosity. Among the spirochetes we investigated, this ability is greater in the more extensively coiled strains.

Chemotaxis in Spirochaeta aurantia

Cell of Spirochaeta aurantia M1 suspended in isotropic buffer solution swam in nearly straight lines and appeared to spin around their longitudinal axis. Occasionally, cells stopped and flexed, and then resumed translational motility, usually in a different direction. The average cell velocity was 26 micron/s. A quantitative assay for chemotaxis was used to test various chemicals for their ability to attract S. aurantia M1. The cells exhibited a tactic response toward 5 X 10(-2) M D-glucose between 10 and 35degree C; the optimum response was at 25degree C. At 5 degree C motility was not impaired, but D-glucose taxis was abolished. Chemotaxis toward D-glucose was stimulated by L-cysteine (2 X 10(-4) M). D-Glucose, 2-deoxy-D-glucose, alpha-methyl-D-glucoside, D-galactose, D-fucose, D-mannose, D-fructose, D-xylose, maltose, cellobiose, and D-glucosamine were effectve attractants for S. aurantia M1. D-Galactose taxis and D-fucose taxis were induced by the presence of D-galactose in the growth medium. The amino acids tested did not serve as attractants, tgrowing cells of S. aurantia M1 exhibited an aerotactic response.

Spirochaeta halophila sp. n., a facultative anaerobe from a high-salinity pond

A facultatively anaerobic spirochete isolated from a high-salinity pond grew optimally when 0.75 M NaCl, 0.2 M MgSO4, and 0.01 M CaCl2 were present in media containing yeast extract, peptone, and a carbohydrate. The organism failed to grow when any one of these three salts was omitted from the medium. Aerobically-grown colonies of the spirochete were red, whereas anaerobically-grown colonies showed no pigmentation. Non-pigmented mutants of the spirochete were isolated. The spirochete used carbohydrates, but not amino acids, as energy sources. Glucose was fermented to CO2, H2, ethanol, acetate, and a small amount of lactate. Determinations of radioactivity in products formed from glucose-1-14C and enzymatic assays indicated that glucose was dissimilated to pyruvate mainly via the Embden-Meyerhof pathway. Pyruvate was metabolized through a clostridial-type clastic reaction. Cells growing aerobically performed an incomplete oxidation of glucose mainly to CO2 and acetate. Comparison of aerobic and anaerobic growth yields indicated that oxidative phosphorylation occurred in cells growing aerobically. The guanine + cytosine content of the DNA of the spirochete was 62 moles %. It is proposed that the spirochete described herein be considered a new species and that it be named Spirochaeta halophila.

The ultrastructure of cultivable treponemes

Cells of Treponema genitalis were studied in the electron microscope by means of negative staining and ultrathin sectioning techniques. All cells were covered by a regularly structured surface layer. This layer appeared to consist of pairs of thin fibrils attached to an amorphous layer. This amorphous layer in turn is probably identical with the exterior part of the outer membrane of the organism. The pairs of thin fibrils located on this surface were interconnected by polygons. The treponemes were regularly coiled and had somewhat tapered ends with 2-4 flagella inserted at each end. The two bundles of flagella were entwined around the cytoplasmic body of the cell and interdigitated in the middle region of the organism. Treatment of cells of T. genitalis with Myxobacter AL-1 protease 1, or with deoxycholate did not reveal intracytoplasmic tubules. This is in contrast to the results obtained with similar treatments of all other strains of species of Treponema hitherto examined.

Isolation and antigenic characteristics of axial filaments from the Reiter Treponeme

Axial filaments were isolated and purified from Reiter treponemes after detergent solubilization of the cells" outer envelope. The axial filaments were separated from the spirochetal cells by shearing, purified by density gradient centrifugation, and fragmented by ultrasonication. Acrylamide gel electrophoresis of dissociated filaments revealed two major protein bands. Gel diffusion precipitin tests and immunoelectrophoresis between a purified axial filament suspension and anti-Reiter treponeme serum gave a single precipitin line. Checkerboard complement fixation tests also gave results consistent with a single antigen-antibody system. Tests with immune sera to other cultivable spirochetes were positive with some and negative with others. In addition, strongly positive reactions were obtained in complement fixation and precipitin tests with sera from rabbits and humans with syphilis and other treponematoses. However, both serological tests gave reactions of partial identity between the antigen(s) of Reiter treponeme axial filaments and those of the pathogenic treponemes. It was concluded from these studies that the axial filaments were probably the cellular locus of the so-called "Reiter protein" antigen of syphilis serology.

Purification and characterization of axial filaments from Treponema phagedenis biotype reiterii (the Reiter treponeme)

Axial filaments have been purified from Treponema phagedenis biotype reiterii (the Reiter treponeme) and partially characterized chemically. The preparations consist largely of protein but also contain small amounts of hexose (3%). Filaments dissociate to subunits in acid, alkali, urea, guanidine, and various detergents. Amino acid analyses show an overall resemblance to other spirochetal axial filaments and to bacterial flagella. Dissociated filaments migrate as a single band upon acrylamide gel electrophoresis at pH 4.3 (in 4 M urea and 10 (3) M ethylenediaminetetraacetate) and at pH 12, but in sodium dodecyl sulfate gels, three bands are obtained under a wide variety of conditions. Two of these bands migrate very close together, with molecular weights of 33,000 +/- 500. The other band has a molecular weight of 36,500 +/- 500. Analysis of axial filaments by the dansyl chloride method yields both methionine and glutamic acid as amino terminal end groups. Sedimentation equilibrium measurements on dissociated axial filaments in 7 M guanidine hydrochloride yield plots of log C against varkappa(2) which vary with the speed and initial protein concentration used. Molecular weight values calculated from these plots are consistent with a model in which axial filament subunits are heterogeneous with respect to molecular weight in the approximate range of 32,000 to 36,000.

Peptidoglycan of free-living anaerobic spirochetes

Electron microscope examination of negatively stained or thin-sectioned cells of Spirochaeta stenostrepta treated with penicillin or lysozyme showed that the peptidoglycan was present as a thin, electron-dense layer adjacent and external to the cytoplasmic membrane. The peptidoglycan was isolated from cells of S. stenostrepta and Spirochaeta litoralis by a procedure including treatments with sodium lauryl sulfate and Pronase. Hydrolysates of the isolated S. stenostrepta and S. litoralis peptidoglycans contained glucosamine, muramic acid, glutamic acid, l-ornithine, and alanine in molar ratios of 0.90:0.85:1.00:1.00:1.40 and of 0.63:0.63:0.99:1.00:1.41, respectively. Determination of N-terminal residues suggested that nearly 50% of the ornithine in S. stenostrepta and S. litoralis peptidoglycans was involved in peptide cross-linkage. The peptidoglycan layer of S. stenostrepta was sensitive to lysozyme and myxobacter AL-1 protease.

Fatty acid composition of Spirochaeta stenostrepta

The fatty acid composition of Spirochaeta stenostrepta consists primarily of saturated, branch-chained fatty acids. Iso-C(15), anteiso-C(15), iso-C(17), and anteiso-C(17) represent 66% of the total fatty acids.