The Spirochetes: Molecular and Cellular Biology Chapter Abstracts
Introduction to Spirochete Evolution, Genome Analyses and Physiology
M.H. Saier, Jr.
This book is devoted to structural, molecular, physiological and evolutionary aspects of spirochetes. The spirochetes are a small cohesive group of chemoheterotrophic bacteria, readily distinguishable from other bacteria on the basis of their unique cell structures. Spirochete cells are generally long and narrow, helical in form, and surrounded by delicate flexible walls. All known spirochetes can swim actively in liquid media by virtue of their axial fibrils: periplasmic flagelli that underlie the cell wall. The cells often bend or flex during movement, and they can attain remarkable directed velocities and even swim through highly viscous solutions (see chapter by Li, Motaleb, Sal, Goldstein and Charon). The motile behavior of these bacteria allows them to respond to chemical and physical stimuli in their environments (see chapter by Lux, Moter and Shi). These organisms comprise a relatively close-knit group of organisms that, based on 16S RNA analyses, diverged from other bacteria during early evolutionary history. They exhibit unusual prokaryotic characteristics such as linear chromosomes and a cytoskeleton. They also are the targets of spirochete-specific bacteriophage which have co-evolved with their hosts over evolutionary time. For this reason, the study of phage can provide clues to the ecology and molecular biology of their host organisms (see chapter by Eggers, Casjens, Hayes, Garon, Damman, Oliver and Samuels).
Spirochetes are currently divided into nine genera (see chapter by Paster and Dewhirst). While some of these genera include free living organisms of bright colors and unusual physiological properties, others are pathogens of humans and animals, the causative agents of syphilis, Lyme disease, relapsing fever, leptospirosis and peridontitis. The pathogens responsible for these diseases exhibit tremendous structural and physiological variability. For example, although they are considered Gram-negative bacteria with a double envelope membrane, many spirochetes including Treponema and Borrelia lack outer membrane lipopolysaccharides (LPS). Others including Leptospira and Brachyspira do synthesize LPS, and LPS is the principle surface antigen displayed by these organisms, of considerable importance to diagnostics and immunity. The chapter by Bulach, Kalambaheti, de la Peña-Moctezuma and Adler discusses these important antigens from structural and biosynthetic standpoints. The importance of vector-host interactions, particularly with respect to tick-borne borreliae, is discussed by Nuttall, Paesen, Lawrie and Wang. The pathogenic properties of many spirochetes have provided much of the scientific impetus for their study.
Genome analyses of two spirochetes, Treponema pallidum and Borrelia burgdorferi, the bacterial determinants of syphilis and Lyme disease, respectively, have revealed a wide range of protein structures. Most protein folds identified in these two organisms have been shown to be common to many other organisms, but a few spirochete-specific folds have been identified (see chapter by Das, Hegyi and Gerstein). Additionally, the metabolic capabilities of these two spirochetes are limited. For example, they rely primarily on glycolysis for energy generation. Transport systems in these two pathogens are highly restricted, showing patterns of transporter types that differ from those found in all other bacteria for which complete genomes are currently available (see chapter by Saier and Paulsen). Information obtained by analyzing the genome of T. pallidum is analyzed in terms of the known biological properties of this important pathogen. Genome analyses of Borrelia species have revealed features unusual to bacteria, often more frequently associated with eukaryotes (see chapter by Casjens). Thus, housekeeping genes are maintained primarily on the large chromosome of B. burgdorferi while the numerous circular and linear plasmids (or small chromosomes) of this organism bear spirochete-specific genes that may prove to be important for pathogenesis. They may be rapidly evolving as compared to the chromosomal genes. These plasmids are greatly enriched for repetitive sequences, the most obvious of which are found in the family of cp32 plasmids (see chapter by Stevenson, Zückert and Akins). Hypervariability within these plasmids as well as the presence of multiple paralogues undoubtedly has survival value for the bacteria in their host organisms. These molecular features may provide mechanisms for avoiding the animal's immune system.
This book thus summarizes important overall aspects of spirochetes as a phylogenetic group of organisms, but it also surveys a few key representative pathogenic spirochetes in greater detail with respect to specific traits. To what extent the specific findings reported will prove to be relevant to other less well studied spirochetes has yet to be determined. However, genome sequences have for the first time provided detailed information about the life processes of two of these important and interesting microorganisms, and genome sequencing projects for other spirochetes that will allow comparative studies are currently in progress. It is an exciting time in the discipline of spirochetology!
Introduction to Genetic Development in Spirochetes
J. García-Lara
Diverse and ubiquitous, the morphologically captivating spirochetes include the causative agents of various important diseases, i.e. syphilis, periodontitis, Lyme disease, Leptospirosis and swine dysentery. The advancement of our knowledge in regard to the biology of this bacterial family has been hampered by the absence of suitable genetic tools. This limitation has been of special significance in the understanding of the mechanism of disease, because the few pathogenic spirochetes that can be cultured in vitro are also particularly intractable genetically. The advent of the new millennium is witnessing a revolution of the genetic methodologies available to study spirochetes. This book presents the relevant milestones of this process.
Besides genetic engineering, the manipulation of DNA needed in investigations of gene regulation and protein function requires the ability to transfer DNA to the desired bacterial recipient. Tilly and coworkers exhaustively discuss the methods available for DNA exchange in spirochetes, and their succesful application to gene inactivation. Obviously, it is necessary to be able to discriminate those spirochetes carrying the recombinant DNA. Addressing this issue, Hardham and Rosey comprehensively cover in their chapter the selective and screening markers proven to be useful in spirochetes, as well as suggest potential alternative antibiotic resistances to be tested.
Although, the transient presence of exogenous DNA fragments has been sufficient to attain homologous recombination and to carry out studies of gene expression, the efficiency, convenience and versatility of these systems is limited when compared with that provided by stable vectors. A remarkable effort has been carried out by a number of laboratories towards obtaining stable episomes capable of shuttling DNA between genetically amenable bacteria and spirochetes. Saint Girons et al. describe the recent advances in this area for Treponema denticola, Leptospira biflexa and Borrelia burgdorferi.
The strategies used to obtain stable maintenance of exogenous DNA have been based on broad host range plasmids or on the random cloning of endogenous sequences containing a native replicon from the spirochetal genome. In addition, an innovative approach guided by genome analysis to locate spirochetal origins of replication is extensively elaborated in the chapter by García-Lara et al. Besides its application to the development of shuttle vectors, this methodology has provided the first origins of replication from the unusual linear chromosome and linear plasmids of Borrelia burgdorferi. It has also unveiled a collection of candidate genes to be involved in replication and/or partitioning.
The contribution of genomics to spirochetal research has been certainly catalyzed by the sequencing of the complete genome of Treponema pallidum and Borrelia burgdorferi. Similarly, the work on outer membrane proteins of Borrelia or the treponemes has evolved at a more rapid pace than that of other genres, and it has patterned, for instance, much of the investigations in Leptospira. The resulting cross-talk generated by the findings obtained from various spirochetal species, based both, on predictive genomics and experimental results, has rendered exciting observations. For example, the definition of a lipobox for spirochetal lipoproteins, presented by Zuerner and coauthors in their chapter. They also describe various aspects of leptospiral research that lead the pack of the spirochetes, such as genetic complementation analysis and on studies pertaining to repetitive DNA sequences in the genome.
A complete genetic system for a bacterial species needs to consider the technology required to study the differential gene expression that will take place as the microorganism encounters different environments. In the case of bacterial pathogens, this would include their corresponding hosts. Seshu and Skare provide a thorough presentation of the methodologies and findings on in vivo gene expression as it relates to Borrelia burgdorferi.
The current knowledge on transcriptional regulators in spirochetes and on the DNA elements that mediate their action is reviewed by Indest et al. Amongst the various mechanisms of gene regulation addressed by these authors, they discuss one of the spotlights of current microbiological research, regulation by quorum sensing. The outcome of studies on differential gene expression impacts the basic understanding of the disease process, the search for therapeutic targets, and the design of biotechnological tools.
As the methodologies presented in this book further develop and the validity of other proposed alternatives is assessed, additional contributions of unquestionable value such as DNA microarray technology will assist in unraveling the mysteries of the biology of spirochetes.
Chapter 1
Phylogenetic Foundation of Spirochetes
B.J. Paster and F.E. Dewhirst
The spirochetes are free-living or host-associated, helical bacteria, some of which are pathogenic to man and animal. Comparisons of 16S rRNA sequences demonstrate that the spirochetes represent a monophyletic phylum within the bacteria. The spirochetes are presently classified in the Class Spirochaetes in the order Spirochetales and are divided into three major phylogenetic groupings, or families. The first family Spirochaetaceae contains species of the genera Borrelia, Brevinema, Cristispira, Spirochaeta, Spironema, and Treponema. The proposed second family "Serpulinaceae" contains the genus Brachyspira (Serpulina). The third family Leptospiraceae contains species of the genera Leptonema and Leptospira. Novel spirochetal species, or phylotypes, that can not be presently cultivated in vitro, have been identified from the human oral cavity, the termite gut, and other host-associated or free-living sources. There are now over 200 spirochetal species or phylotypes, of which more than half is presently not cultivable. It is likely that there is still a significant unrecognized spirochetal diversity that should be evaluated.
Chapter 2
Gyrations, Rotations, Periplasmic Flagella: The Biology of Spirochete Motility
C. Li, Md. A. Motaleb, M. Sal, S.F. Goldstein, and N.W. Charon
Spirochetes have a unique structure, and as a result their motility is different from that of other bacteria. They also have a special attribute: spirochetes can swim in a highly viscous, gel-like medium, such as that found in connective tissue, that inhibits the motility of most other bacteria. In spirochetes, the organelles for motility, the periplasmic flagella, reside inside the cell within the periplasmic space. A given periplasmic flagellum is attached only at one end of the cell, and depending on the species, may or may not overlap in the center of the cell with those attached at the other end. The number of periplasmic flagella varies from species to species. These structures have been shown to be directly involved in spirochete motility, and they function by rotating within the periplasmic space. The mechanics of motility also vary among the spirochetes. In Leptospira, a motility model developed several years ago has been extensively tested, and the evidence supporting this model is convincing. Borrelia burgdorferi swims differently, and a model of its motility has been recently put forward. This model is based on analyzing the motion of swimming cells, high voltage electron microscopy of fixed cells, and mutant analysis. To better understand spirochete motility on a molecular level, the proteins and genes involved in motility are being analyzed. Spirochete periplasmic flagellar filaments are among the most complex of bacterial flagella. They are composed of the FlaA sheath proteins, and in many species, multiple FlaB core proteins. Allelic exchange mutagenesis of the genes which encode these proteins is beginning to yield important information with respect to periplasmic flagellar structure and function. Although we are at an early stage with respect to analyzing the function, organization, and regulation of many of the genes involved in spirochete motility, unique aspects have already become evident. Future studies on spirochete motility should be exciting, as only recently have complete genome sequences and tools for allelic exchange mutagenesis become available.
Chapter 3
The Role of Chemotaxis in Pathogenesis of Spirochetes
R. Lux, A. Moter, and W. Shi
Chemotaxis is an important feature of motile organisms that allows navigation through various environments. It enables them to detect nutrients and to avoid unfavorable or dangerous conditions. Motility and chemotaxis are widely acknowledged as important virulence factors for pathogenic bacteria. In this chapter, we try to explore the role of chemotaxis in the pathogenesis of spirochetes. Chemotaxis might be involved in tissue identification and penetration, and represents a possible mechanism for evasion of the host's immune defense. The recent development of genetic tools for pathogenic spirochetes and "tracking" techniques, employing fluorescent in situ hybridization (FISH), could revolutionize our understanding of the importance of chemotaxis for infection and persistence of these bacteria in their host.
Chapter 4
Bacteriophages of Borrelia burgdorferi and Other Spirochetes
C.H. Eggers, S. Casjens, and D.S. Samuels
A number of bacteriophage-like particles have been observed in association with members of the bacterial order Spirochetales, the spirochetes. Several spirochete bacteriophages have been isolated and characterized at the molecular level. We have characterized a bacteriophage, designated fBB1, of the Lyme disease agent, Borrelia burgdorferi. Here we review the history of the association between the spirochetes and their bacteriophages, with a particular emphasis on fBB1 and its prophage, the 32-kb circular plasmid family of B. burgdorferi.
Chapter 5
The Genetics of Lipopolysaccharide Biosynthesis in Leptospira
D.M. Bulach, T. Kalambaheti, A. de la Peña-Moctezuma and B. Adler
Lipopolysaccharide (LPS) is the major surface antigen of Leptospira. Variation in LPS structure is the basis for the more than 200 serovars that have been identified. Despite the importance of this antigen in immunity and diagnostics, there is relatively little known about the genetics and chemistry of leptospiral LPS, as compared to some members of the Enterobacteriaceae. The nucleotide sequence of the locus encoding enzymes for the biosynthesis of the O-antigen component of leptospiral LPS (rfb locus) has been determined for three serovars namely, L.interrogans serovar Pomona, L.interrogans serovar Hardjo subtype Hardjoprajitno and L.borgpetersenii serovar Hardjo subtype Hardjobovis. In the absence of data relating to the chemical structure or genetic tools to construct isogenic mutants in Leptospira, similarity analysis has been used to provide insight into the mechanisms by which the leptospiral O-antigen is assembled by comparison with characterized systems from other bacteria. In addition, comparison of the gene layout in each of the serovars provides an indication of the genetic basis for serovar diversity.
Chapter 6
Tick-Host Interactions in Spirochete Transmission
P.A. Nuttall, G.C. Paesen, C.H. Lawrie, V. Hajnická, N. Fuchsberger, and H. Wang
Tick-borne spirochetes include borreliae that cause Lyme disease and relapsing fever in humans. They survive in a triangle of parasitic interactions between the spirochete and its vertebrate host, the spirochete and its tick vector, and the host and the tick. Until recently, the significance of vector-host interactions in the transmission of arthropod-borne disease agents has been overlooked. However, there is now compelling evidence that the pharmacological activity of tick saliva can have a profound effect on pathogen transmission both from infected tick to uninfected host, and from infected host to uninfected tick. The salivary glands of ticks provide a pharmacopoeia of anti-inflammatory, anti-haemostatic and anti-immune molecules. These include bioactive proteins that control histamine, bind immunoglobulins, modulate cytokines, and inhibit the alternative complement cascade. The effect of these molecules is to provide a privileged site at the tick-host interface in which borreliae and other tick-borne pathogens are sheltered from the normal innate and acquired host immune mechanisms that combat infections. Understanding the key events at the tick vector-host interface, that promote spirochete infection and transmission, will provide a better understanding of the epidemiology and ecology of these important human pathogens.
Chapter 7
Genome Analyses of Spirochetes: A Study of the ProteinStructures, Functions
and Metabolic Pathways in Treponema pallidum and Borrelia burgdorferi
R. Das, H. Hegyi, and M. Gerstein
We perform a comprehensive genome analysis on two spirochetes, T. pallidum and B. burgdorferi. First, we focus on the occurrence of protein structures in these organisms. We find that there are only a few spirochete-specific folds, relative to those in other types of bacteria. The most common fold, by far, in the spirochetes is the P-loop NTP hydrolase, followed by the TIM barrel. These folds also happen to be amongst the most multifunctional of the known folds. We also survey the membrane-protein structures in T. pallidum and find a notable large family with twelve transmembrane (TM) helices, reflecting the prevalence of 12-TM transporters in bacteria. Then we move to analysis of the metabolic pathways and overall metabolism in the spirochetes, using the metabolic-flux-balancing method. We find that the lipid biosynthesis pathway is absent from the spirochetes. This strongly limits the degree to which these organisms can metabolize NADPH. In turn, we find that the spirochetes distribute flux disproportionately through the glycolytic pathway instead of the NADPH-providing pentose phosphate pathway. Further information is available at <a target="w1" href="http://bioinfo.mbb.yale.edu>bioinfo.mbb.yale.edu</a>
Chapter 8
Whole Genome Analyses of Transporters in Spirochetes: Borrelia burgdorferi and Treponema pallidum
M.H. Saier, Jr. and I.T. Paulsen
The completely sequenced genomes of two spirochetes, Borrelia burgdorferi (Bbu) and Treponema pallidum (Tpa) were analyzed for the distribution of transporter types. Both organisms exhibited fewer proteins with >7 a-helical transmembrane spanners (TMSs), and fewer identified transport systems per megabase pair of DNA than most other prokaryotes analyzed. Each organism exhibits one recognizable ion channel protein of the MscS family. Tpa has twice as many primary carriers as Bbu but lacks PTS permeases that are plentiful in Bbu. Tpa is the only prokaryote so far sequenced which has two F-type ATPases. Large families of secondary nutrient uptake carriers (MFS and APC) that are prevalent in other organisms are essentially lacking in Spirochetes. The largest Spirochete secondary carrier families consist of efflux systems. While both Bbu and Tpa exhibit an unusual degree of transporter diversity, major differences in specificity exist between these two organisms.
Chapter 9
Borrelia Genomes
S. Casjens
All analyzed members of the spirochete genus Borrelia contain a linear chromosome about 910 kbp long. The complete sequence of the B. burgdorferi B31 genome predicts that its chromosome carries essentially all of this organism's housekeeping genes. In accordance with these bacterial species' obligatory parasitic lifestyle, its genes encode enzymes that are capable of only a minimal metabolism, in which all nucleotides, amino acids, fatty acids and enzyme cofactors must be scavenged from the host. In addition to the chromosome, all Borrelia isolates examined carry multiple linear and circular plasmids with lengths between 5 and 200 kbp. The plasmids, which account for over 600 kbp in isolate B31, carry very few genes with homology to genes outside of the Borrelia genus. But they do carry numerous predicted lipoprotein genes, many of which are have been shown to be or are expected to be outer surface proteins. Ten of the linear plasmids have strikingly low protein coding potential for bacterial DNA. These plasmids have enjoyed numerous past duplicative rearrangements, which have resulted in the presence of a substantial fraction of the DNA that appears to be currently undergoing mutational decay, presumably because it is no longer under selection for function.
Chapter 10
Repetition, Conservation, and Variation: the Multiple cp32 Plasmids of Borrelia Species
B. Stevenson, W.R. Zückert and D.R. Akins
Members of the spirochete genus Borrelia contain large numbers of extrachromosomal DNAs. Sequence analysis of the B. burgdorferi strain B31 genome indicated that its many plasmids contain large quantities of repeated sequences, the most obvious of which are the cp32 plasmid family. Individual spirochetes may carry nine or more different, but homologous, cp32 plasmids. Every other species of Borrelia examined thus far also contains multiple plasmids related to the B. burgdorferi cp32s. These plasmids are arguably the best characterized of all the borrelial plasmids, and epitomize the apparent redundancy evident in the many plasmids carried by these bacteria. Despite their extensive similarities, cp32 plasmids contain some open reading frames whose sequences often vary between plasmids, and which encode proteins synthesized by the bacteria during vertebrate infection. In this chapter, we analyze the hypervariable and conserved regions of the cp32 plasmid family, and discuss possible reasons why borreliae harbor multiple gene paralogs.
Chapter 11
Antibiotic Selective Markers and Spirochete Genetics
J.M. Hardham and E.L. Rosey
Until very recently, the pathogenic spirochetes have been refractory to genetic manipulation. This has been due, in part, to difficulties with in vitro growth and the genetic distance that spirochetes are from typical Gram-negative and Gram-positive organisms. Insertional mutagenesis and other genetic techniques are now possible in some of the pathogenic spirochetes such as Borrelia burgdorferi, Brachyspira (Serpulina) hyodysenteriae, Leptospira sp., and Treponema denticola. However, organisms such as Treponema pallidum, which cannot be grown in vitro, are still not amenable to genetic manipulation. These recent advances have paved the way for more detailed genetic studies of transcriptional regulation, protein function, protein localization, metabolic capabilities, motility, and pathogenic nature of this group of spirochetes. This chapter will discuss the current repertoire of antibiotic markers that are useful for spirochetal genetic manipulation. Further advances in selectable markers and shuttle vectors will allow researchers to complete Koch's molecular hypothesis for various virulence genes of the pathogenic spirochetes and increase the overall understanding of these challenging bacteria.
Chapter 12
DNA Exchange and Insertional Inactivation in Spirochetes
K. Tilly, A.F. Elias, J.L. Bono, P. Stewart, and P. Rosa
Spirochetes have complex life cycles and are associated with a number of diseases in humans and animals. Despite their significance as pathogens, spirochete genetics are in their early stages. However, gene inactivation has been achieved in Borrelia burgdorferi, Brachyspira hyodysenteriae, Leptospira biflexa, and Treponema denticola. Here, we review methods that have been used in spirochetes for gene inactivation and DNA exchange, with a primary focus on B. burgdorferi. We also describe factors influencing electrotransformation in B. burgdorferi. In summary, optimal transformation frequencies are obtained with log phase bacteria, large amounts of DNA (up to 50 mg per transformation), and high field strength (12.5-37.5 kV/cm). Infectious B. burgdorferi isolates transform with frequencies 100-fold lower than those found for high passage, non-infectious strains. Surface characteristics of the bacteria, which often correlate with infectivity, are among the obstacles to effective transformation by electroporation.
Chapter 13
Shuttle Vectors for Spirochetes
I. Saint Girons, B. Chi, and H. Kuramitsu
Constructions of Escherichia coli-spirochete shuttle vectors are based on naturally occurring plasmids, broad host range plasmids or bacteriophages. This chapter primarily focuses on genetic tools for Treponema denticola which is associated with periodontal diseases. The T. pallidum FlaA protein, E. coli b-galactosidase, and the green fluorescent protein were successfully expressed in T. denticola from a shuttle vector system. Shuttle vectors for Borrelia burgdorferi and Leptospira biflexa were reported, opening the way to further genetic manipulation.
Chapter 14
The Role of Genomics in Approaching the Study of Borrelia DNA Replication
J. García-Lara, M. Picardeau, B.J. Hinnebusch, W.M. Huang, and S. Casjens
The identification of chromosomal and episomal origins of replication in the genome of the causative agent of Lyme disease, the spirochete Borrelia burgdorferi, has been greatly facilitated by genomics. Analysis of genome features, including strand compositional asymmetries, organizational similarities to other bacterial origins of replication, and the presence of homologues of genes involved in replication and partitioning, have contributed to the identification of a collection of putative origins of replication within the Borrelia genome. This analysis has provided the basis for the experimental verification of origins in the linear chromosome and in the linear plasmid lp28-2. Information generated during the study of these origins will significantly contribute to the understanding of the mechanisms of replication and partitioning in Borrelia.
Chapter 15
Technological Advances in the Molecular Biology of Leptospira
R. Zuerner, D. Haake, B. Adler, and R. Segers
Pathogenic members of the genus Leptospira have been refractory to genetic study due to lack of known mechanisms of genetic exchange. To bypass this limitation, several techniques have been useful for Leptospira gene discovery, including heterologous complementation of Escherichia coli mutants, screening of DNA libraries with probes, and random sequence analysis. Construction of combined physical and genetic maps revealed the presence of two circular chromosomal replicons. The organization of the L. interrogans genome is quite variable, with genetically similar strains differentiated by many rearrangements. These rearrangements likely occur through recombination between repetitive DNA elements found scattered throughout the genome. Analysis of intervening sequences and genes encoding LPS biosynthetic enzymes provide evidence of lateral transfer of DNA between Leptospira spp. We have also gained insight into the biology of these bacteria by analyzing genes encoding LPS and outer membrane proteins (OMPs). Some of these OMPs are differentially expressed. Characterization of mechanisms governing the expression of the OMP genes should provide insight into host-parasite interactions. Furthermore, recent advances in heterologous expression of leptospiral OMP genes are opening new avenues of vaccine development.
Chapter 16
The Many Faces of Borrelia burgdorferi
J. Seshu and J.T. Skare
In this chapter we describe several genetic regulatory mechanisms adopted by the agent of Lyme disease, Borrelia burgdorferi, to sense and adapt to different host and environmental conditions either in vitro or in vivo. This regulation results in the increased or decreased synthesis of several proteins whose levels are believed to play key roles in the ability of B. burgdorferi to cycle between both arthropod and mammalian hosts. Moreover, the differential synthesis of these proteins serves to modulate the response of B. burgdorferi to signals in the requisite host and may also, in some cases, function as virulence determinants of this spirochete. Elucidation of these mechanisms will help in the understanding of the pathogenicity of B. burgdorferi as well as aid in identifying proteins that are important during different stages of infection.
Chapter 17
Transcriptional Regulation in Spirochetes of Medical Importance
K.J. Indest, R. Ramamoorthy, and M.T. Philipp
Spirochetes belong to a widely diverse family of bacteria. Several species in this family can cause a variety of illnesses including syphilis and Lyme disease. Despite the fact that the complete genome sequence of two species, Borrelia burgdorferi and Treponema pallidum, have been deciphered, much remains to be understood about spirochetal gene regulation. In this chapter we focus on the environmental transitions that spirochetes undergo during their life cycles and the mechanisms of transcriptional regulation that might possibly mediate spirochetal adaptations to such changes.
Chapter 18
Biology of Treponema pallidum: Correlation of Functional Activities With Genome Sequence Data
Steven J. Norris, David L. Cox, and George M. Weinstock
Aspects of the biology of T. pallidum subsp. pallidum, the agent of syphilis, are examined in the context of a century of experimental studies and the recently determined genome sequence. T. pallidum and a group of closely related pathogenic spirochetes have evolved to become highly invasive, persistent pathogens with little toxigenic activity and an inability to survive outside the mammalian host. Analysis of the genome sequence confirms morphologic studies indicating the lack of lipopolysaccharide and lipid biosynthesis mechanisms, as well as a paucity of outer membrane protein candidates. The metabolic capabilities and adaptability of T. pallidum are minimal, and this relative deficiency is reflected by the absence of many pathways, including the tricarboxylic acid cycle, components of oxidative phosphorylation, and most biosynthetic pathways. Although multiplication of T. pallidum has been obtained in a tissue culture system, continuous in vitro culture has not been achieved. The balance of oxygen utilization and toxicity is key to the survival and growth of T. pallidum, and the genome sequence reveals a similarity to lactic acid bacteria that may be useful in understanding this relationship. The identification of relatively few genes potentially involved in pathogenesis reflects our lack of understanding of invasive pathogens relative to toxigenic organisms. The genome sequence will provide useful raw data for additional functional studies on the structure, metabolism, and pathogenesis of this enigmatic organism.
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