Examination of the Borrelia burgdorferi transcriptome in Ixodes scapularis during feeding

Longitudinal map of transcriptome changes in the Lyme pathogen Borrelia burgdorferi during tick-borne transmission

Borrelia burgdorferi (Bb), the causative agent of Lyme disease, adapts to vastly different environments as it cycles between tick vector and vertebrate host. During a tick bloodmeal, Bb alters its gene expression to prepare for vertebrate infection; however, the full range of transcriptional changes that occur over several days inside of the tick are technically challenging to capture. We developed an experimental approach to enrich Bb cells to longitudinally define their global transcriptomic landscape inside nymphal Ixodes scapularis ticks during a transmitting bloodmeal. We identified 192 Bb genes that substantially change expression over the course of the bloodmeal from 1 to 4 days after host attachment. The majority of upregulated genes encode proteins found at the cell envelope or proteins of unknown function, including 45 outer surface lipoproteins embedded in the unusual protein-rich coat of Bb. As these proteins may facilitate Bb interactions with the host, we utilized mass spectrometry to identify candidate tick proteins that physically associate with Bb. The Bb enrichment methodology along with the ex vivo Bb transcriptomes and candidate tick interacting proteins presented here provide a resource to facilitate investigations into key determinants of Bb priming and transmission during the tick stage of its unique transmission cycle.

Identification and evaluation of midgut protein RL12 of Dermacentor silvarum interacting with Anaplasma ovis VirD4

Anaplasma ovis, a tick-borne intra-erythrocytic Gram-negative bacterium, is a causative agent of ovine anaplasmosis. It is known that Dermacentor ticks act as biological vectors for A. ovis. VirD4 is the machine component of Type IV Secretion System of A. ovis. To better understand the pathogen-vector interaction, VirD4 was used as a bait protein for screening midgut proteins of Dermacentor silvarum via yeast two-hybrid mating assay. As a result, a ribosomal protein RL12 was identified from the midgut cDNA library of D. silvarum. For further validation, using in vitro Glutathione S-transferase (GST) pull-down assay, interaction between the proteins, GST-RL12 and HIS-VirD4, was observed in Western blot analysis. The study is first of its kind reporting a D. silvarum midgut protein interaction with VirD4 from A. ovis. Functional annotations showed some important cellular processes are attributed to the protein, particularly in the stringent response and biogenesis. The results of the study suggest the involvement of the VirD4-RL12 interaction in the regulation of signaling pathways, which is a tool for understanding the pathogen-vector interaction.

Molecular Interactions During Borrelia burgdorferi Migration from the Vector to the Mammalian Nervous System

Lyme disease (LD) is an infectious disease caused by the spirochetes of genus borrelia, which are transmitted by the ticks of the genus ixodes. LD is transmitted by the spirochete B. burgdorferi sensu lato. Once in contact with the host through a tick bite, the pathogen comes into contact with the host defense, and must escape this machinery to establish LD, thus using a large number of mechanisms involving the vector of the pathogen, the pathogen itself and also the host. The initial diagnosis of the disease can be made based on the clinical symptoms of LD and the disease can be treated and cured with antibiotics if the diagnosis is made early in the beginning of the disease. Contrariwise, if LD is left untreated, the pathogen disseminates throughout the tissues and organs of the body, where it establishes different types of disease manifestations. In the nervous system, the inflammation caused by B. burgdorferi is known as Lyme neuroborreliosis (LNB). LNB is one of the principal manifestations of LD. In this review, we systematically describe the different molecular interactions among B. burgdorferi, the vector (tick) and the mammalian host.

Interactions Between Ticks and Lyme Disease Spirochetes

Borrelia burgdorferi sensu lato causes Lyme borreliosis in a variety of animals and humans. These atypical bacterial pathogens are maintained in a complex enzootic life cycle that primarily involves a vertebrate host and Ixodes spp. ticks. In the Northeastern United States, I. scapularis is the main vector, while wild rodents serve as the mammalian reservoir host. As B. burgdorferi is transmitted only by I. scapularis and closely related ticks, the spirochete-tick interactions are thought to be highly specific. Various borrelial and arthropod proteins that directly or indirectly contribute to the natural cycle of B. burgdorferi infection have been identified. Discrete molecular interactions between spirochetes and tick components also have been discovered, which often play critical roles in pathogen persistence and transmission by the arthropod vector. This review will focus on the past discoveries and future challenges that are relevant to our understanding of the molecular interactions between B. burgdorferi and Ixodes ticks. This information will not only impact scientific advancements in the research of tick- transmitted infections but will also contribute to the development of novel preventive measures that interfere with the B. burgdorferi life cycle.

Immune Response to Borrelia: Lessons from Lyme Disease Spirochetes

The mammalian host responds to infection with Borrelia spirochetes through a highly orchestrated immune defense involving innate and adaptive effector functions aimed toward limiting pathogen burdens, minimizing tissue injury, and preventing subsequent reinfection. The evolutionary adaptation of Borrelia spirochetes to their reservoir mammalian hosts may allow for its persistence despite this immune defense. This review summarizes our current understanding of the host immune response to B. burgdorferi sensu lato, the most widely studied Borrelia spp. and etiologic agent of Lyme borreliosis. Pertinent literature will be reviewed with emphasis on in vitro, ex vivo and animal studies that influenced our understanding of both the earliest responses to B. burgdorferi as it enters the mammalian host and those that evolve as spirochetes disseminate and establish infection in multiple tissues. Our focus is on the immune response of inbred mice, the most commonly studied animal model of B. burgdorferi infection and surrogate for one of this pathogen's principle natural reservoir hosts, the white-footed deer mouse. Comparison will be made to the immune responses of humans with Lyme borreliosis. Our goal is to provide an understanding of the dynamics of the mammalian immune response during infection with B. burgdorferi and its relation to the outcomes in reservoir (mouse) and non-reservoir (human) hosts.

The Lyme disease spirochete's BpuR DNA/RNA-binding protein is differentially expressed during the mammal-tick infectious cycle, which affects translation of the SodA superoxide dismutase

When the Lyme disease spirochete, Borrelia burgdorferi, transfers from a feeding tick into a human or other vertebrate host, the bacterium produces vertebrate‐specific proteins and represses factors needed for arthropod colonization. Previous studies determined that the B. burgdorferi BpuR protein binds to its own mRNA and autoregulates its translation, and also serves as co‐repressor of erp transcription. Here, we demonstrate that B. burgdorferi controls transcription of bpuR, expressing high levels of bpuR during tick colonization but significantly less during mammalian infection. The master regulator of chromosomal replication, DnaA, was found to bind specifically to a DNA sequence that overlaps the bpuR promoter. Cultured B. burgdorferi that were genetically manipulated to produce elevated levels of BpuR exhibited altered levels of several proteins, although BpuR did not impact mRNA levels. Among these was the SodA superoxide dismutase, which is essential for mammalian infection. BpuR bound to sodA mRNA in live B. burgdorferi, and a specific BpuR‐binding site was mapped 5′ of the sodA open reading frame. Recognition of posttranscriptional regulation of protein levels by BpuR adds another layer to our understanding of the B. burgdorferi regulome, and provides further evidence that bacterial protein levels do not always correlate directly with mRNA levels.

The Emerging Role of Microbial Biofilm in Lyme Neuroborreliosis

Lyme borreliosis (LB) is the most common tick-borne disease caused by the spirochete Borrelia burgdorferi in North America and Borrelia afzelii or Borrelia garinii in Europe and Asia, respectively. The infection affects multiple organ systems, including the skin, joints, and the nervous system. Lyme neuroborreliosis (LNB) is the most dangerous manifestation of Lyme disease, occurring in 10-15% of infected individuals. During the course of the infection, bacteria migrate through the host tissues altering the coagulation and fibrinolysis pathways and the immune response, reaching the central nervous system (CNS) within 2 weeks after the bite of an infected tick. The early treatment with oral antimicrobials is effective in the majority of patients with LNB. Nevertheless, persistent forms of LNB are relatively common, despite targeted antibiotic therapy. It has been observed that the antibiotic resistance and the reoccurrence of Lyme disease are associated with biofilm-like aggregates in B. burgdorferi, B. afzelii, and B. garinii, both in vitro and in vivo, allowing Borrelia spp. to resist to adverse environmental conditions. Indeed, the increased tolerance to antibiotics described in the persisting forms of Borrelia spp., is strongly reminiscent of biofilm growing bacteria, suggesting a possible role of biofilm aggregates in the development of the different manifestations of Lyme disease including LNB.

A bite so sweet: the glycobiology interface of tick-host-pathogen interactions

Vector-borne diseases constitute 17% of all infectious diseases in the world; among the blood-feeding arthropods, ticks transmit the highest number of pathogens. Understanding the interactions between the tick vector, the mammalian host and the pathogens circulating between them is the basis for the successful development of vaccines against ticks or the tick-transmitted pathogens as well as for the development of specific treatments against tick-borne infections. A lot of effort has been put into transcriptomic and proteomic analyses; however, the protein-carbohydrate interactions and the overall glycobiology of ticks and tick-borne pathogens has not been given the importance or priority deserved. Novel (bio)analytical techniques and their availability have immensely increased the possibilities in glycobiology research and thus novel information in the glycobiology of ticks and tick-borne pathogens is being generated at a faster pace each year. This review brings a comprehensive summary of the knowledge on both the glycosylated proteins and the glycan-binding proteins of the ticks as well as the tick-transmitted pathogens, with emphasis on the interactions allowing the infection of both the ticks and the hosts by various bacteria and tick-borne encephalitis virus.

RNA-Seq of Borrelia burgdorferi in Multiple Phases of Growth Reveals Insights into the Dynamics of Gene Expression, Transcriptome Architecture, and Noncoding RNAs

Borrelia burgdorferi, the agent of Lyme disease, differentially expresses numerous genes and proteins as it cycles between mammalian hosts and tick vectors. Insights on regulatory mechanisms have been provided by earlier studies that examined B. burgdorferi gene expression patterns during cultivation. However, prior studies examined bacteria at only a single time point of cultivation, providing only a snapshot of what is likely a dynamic transcriptional program driving B. burgdorferi adaptations to changes during culture growth phases. To address that concern, we performed RNA sequencing (RNA-Seq) analysis of B. burgdorferi cultures at early-exponential, mid-exponential, and early-stationary phases of growth. We found that expression of nearly 18% of annotated B. burgdorferi genes changed significantly during culture maturation. Moreover, genome-wide mapping of the B. burgdorferi transcriptome in different growth phases enabled insight on transcript boundaries, operon structures, and identified numerous putative non-coding RNAs. These RNA-Seq data are discussed and presented as a resource for the community of researchers seeking to better understand B. burgdorferi biology and pathogenesis.

Microarray-Based Comparative Genomic and Transcriptome Analysis of Borrelia burgdorferi

Borrelia burgdorferi, the spirochetal agent of Lyme disease, is maintained in nature in a cycle involving a tick vector and a mammalian host. Adaptation to the diverse conditions of temperature, pH, oxygen tension and nutrient availability in these two environments requires the precise orchestration of gene expression. Over 25 microarray analyses relating to B. burgdorferi genomics and transcriptomics have been published. The majority of these studies has explored the global transcriptome under a variety of conditions and has contributed substantially to the current understanding of B. burgdorferi transcriptional regulation. In this review, we present a summary of these studies with particular focus on those that helped define the roles of transcriptional regulators in modulating gene expression in the tick and mammalian milieus. By performing comparative analysis of results derived from the published microarray expression profiling studies, we identified composite gene lists comprising differentially expressed genes in these two environments. Further, we explored the overlap between the regulatory circuits that function during the tick and mammalian phases of the enzootic cycle. Taken together, the data indicate that there is interplay among the distinct signaling pathways that function in feeding ticks and during adaptation to growth in the mammal.

Apparent role for Borrelia burgdorferi LuxS during mammalian infection

The Lyme disease spirochete, Borrelia burgdorferi, controls protein expression patterns during its tick-mammal infection cycle. Earlier studies demonstrated that B. burgdorferi synthesizes 4,5-dihydroxy-2,3-pentanedione (autoinducer-2 [AI-2]) and responds to AI-2 by measurably changing production of several infection-associated proteins. luxS mutants, which are unable to produce AI-2, exhibit altered production of several proteins. B. burgdorferi cannot utilize the other product of LuxS, homocysteine, indicating that phenotypes of luxS mutants are not due to the absence of that molecule. Although a previous study found that a luxS mutant was capable of infecting mice, a critical caveat to those results is that bacterial loads were not quantified. To more precisely determine whether LuxS serves a role in mammalian infection, mice were simultaneously inoculated with congenic wild-type and luxS strains, and bacterial numbers were assessed using quantitative PCR. The wild-type bacteria substantially outcompeted the mutants, suggesting that LuxS performs a significant function during mammalian infection. These data also provide further evidence that nonquantitative infection studies do not necessarily provide conclusive results and that regulatory factors may not make all-or-none, black-or-white contributions to infectivity.

Insights into the biology of Borrelia burgdorferi gained through the application of molecular genetics

Borrelia burgdorferi, the vector-borne bacterium that causes Lyme disease, was first identified in 1982. It is known that much of the pathology associated with Lyme borreliosis is due to the spirochete's ability to infect, colonize, disseminate, and survive within the vertebrate host. Early studies aimed at defining the biological contributions of individual genes during infection and transmission were hindered by the lack of adequate tools and techniques for molecular genetic analysis of the spirochete. The development of genetic manipulation techniques, paired with elucidation and annotation of the B. burgdorferi genome sequence, has led to major advancements in our understanding of the virulence factors and the molecular events associated with Lyme disease. Since the dawn of this genetic era of Lyme research, genes required for vector or host adaptation have garnered significant attention and highlighted the central role that these components play in the enzootic cycle of this pathogen. This chapter covers the progress made in the Borrelia field since the application of mutagenesis techniques and how they have allowed researchers to begin ascribing roles to individual genes. Understanding the complex process of adaptation and survival as the spirochete cycles between the tick vector and vertebrate host will lead to the development of more effective diagnostic tools as well as identification of novel therapeutic and vaccine targets. In this chapter, the Borrelia genes are presented in the context of their general biological roles in global gene regulation, motility, cell processes, immune evasion, and colonization/dissemination.

Assessment of the potential contribution of the highly conserved C-terminal motif (C10) of Borrelia burgdorferi outer surface protein C in transmission and infectivity

OspC is produced by all species of the Borrelia burgdorferi sensu lato complex and is required for infectivity in mammals. To test the hypothesis that the conserved C-terminal motif (C10) of OspC is required for function in vivo, a mutant B. burgdorferi strain (B31::ospCΔC10) was created in which ospC was replaced with an ospC gene lacking the C10 motif. The ability of the mutant to infect mice was investigated using tick transmission and needle inoculation. Infectivity was assessed by cultivation, qRT-PCR, and measurement of IgG antibody responses. B31::ospCΔC10 retained the ability to infect mice by both needle and tick challenge and was competent to survive in ticks after exposure to the blood meal. To determine whether recombinant OspC protein lacking the C-terminal 10 amino acid residues (rOspCΔC10) can bind plasminogen, the only known mammalian-derived ligand for OspC, binding analyses were performed. Deletion of the C10 motif resulted in a statistically significant decrease in plasminogen binding. Although deletion of the C10 motif influenced plasminogen binding, it can be concluded that the C10 motif is not required for OspC to carry out its critical in vivo functions in tick to mouse transmission.

Measurements of mRNA degradation in Borrelia burgdorferi

The importance of gene regulation in the enzootic cycle of Borrelia burgdorferi, the spirochete that causes Lyme disease, is well established. B. burgdorferi regulates gene expression in response to changes in environmental stimuli associated with changing hosts. In this study, we monitored mRNA decay in B. burgdorferi following transcriptional arrest with actinomycin D. The time-dependent decay of transcripts encoding RNA polymerase subunits (rpoA and rpoS), ribosomal proteins (rpsD, rpsK, rpsM, rplQ, and rpsO), a nuclease (pnp), outer surface lipoproteins (ospA and ospC), and a flagellar protein (flaB) have different profiles and indicate half-lives ranging from approximately 1 min to more than 45 min in cells cultured at 35°C. Our results provide a first step in characterizing mRNA decay in B. burgdorferi and in investigating its role in gene expression and regulation.

Impact of climate trends on tick-borne pathogen transmission

Recent advances in climate research together with a better understanding of tick-pathogen interactions, the distribution of ticks and the diagnosis of tick-borne pathogens raise questions about the impact of environmental factors on tick abundance and spread and the prevalence and transmission of tick-borne pathogens. While undoubtedly climate plays a role in the changes in distribution and seasonal abundance of ticks, it is always difficult to disentangle factors impacting on the abundance of tick hosts from those exerted by human habits. All together, climate, host abundance, and social factors may explain the upsurge of epidemics transmitted by ticks to humans. Herein we focused on tick-borne pathogens that affect humans with epidemic potential. Borrelia burgdorferi s.l. (Lyme disease), Anaplasma phagocytophilum (human granulocytic anaplasmosis), and tick-borne encephalitis virus (tick-borne encephalitis) are transmitted by Ixodes spp. Crimean-Congo hemorrhagic fever virus (Crimean-Congo hemorrhagic fever) is transmitted by Hyalomma spp. In this review, we discussed how vector tick species occupy the habitat as a function of different climatic factors, and how these factors impact on tick survival and seasonality. How molecular events at the tick-pathogen interface impact on pathogen transmission is also discussed. Results from statistically and biologically derived models are compared to show that while statistical models are able to outline basic information about tick distributions, biologically derived models are necessary to evaluate pathogen transmission rates and understand the effect of climatic variables and host abundance patterns on pathogen transmission. The results of these studies could be used to build early alert systems able to identify the main factors driving the subtle changes in tick distribution and seasonality and the prevalence of tick-borne pathogens.

OspC is potent plasminogen receptor on surface of Borrelia burgdorferi

Host-derived proteases are crucial for the successful infection of vertebrates by several pathogens, including the Lyme disease spirochete bacterium, Borrelia burgdorferi. B. burgdorferi must traverse tissue barriers in the tick vector during transmission to the host and during dissemination within the host, and it must disrupt immune challenges to successfully complete its infectious cycle. It has been proposed that B. burgdorferi can accomplish these tasks without an endogenous extra-cytoplasmic protease by commandeering plasminogen, the highly abundant precursor of the vertebrate protease plasmin. However, the molecular mechanism by which B. burgdorferi immobilizes plasminogen to its surface remains obscure. The data presented here demonstrate that the outer surface protein C (OspC) of B. burgdorferi is a potent plasminogen receptor on the outer membrane of the bacterium. OspC-expressing spirochetes readily bind plasminogen, whereas only background levels of plasminogen are detectable on OspC-deficient strains. Furthermore, plasminogen binding by OspC-expressing spirochetes can be significantly reduced using anti-OspC antibodies. Co-immunofluorescence staining assays demonstrate that wild-type bacteria immobilize plasminogen only if they are actively expressing OspC regardless of the expression of other surface proteins. The co-localization of plasminogen and OspC on OspC-expressing spirochetes further implicates OspC as a biologically relevant plasminogen receptor on the surface of live B. burgdorferi.

Selective Capture of Transcribed Sequences: A Promising Approach for Investigating Bacterium-Insect Interactions

Bacterial interactions with eukaryotic hosts are complex processes which vary from pathogenic to mutualistic. Identification of bacterial genes differentially expressed in the host, promises to unravel molecular mechanisms driving and maintaining such interactions. Several techniques have been developed in the past 20 years to investigate bacterial gene expression within their hosts. The most commonly used techniques include in-vivo expression technology, signature-tagged mutagenesis, differential fluorescence induction, and cDNA microarrays. However, the limitations of these techniques in analyzing bacterial in-vivo gene expression indicate the need to develop alternative tools. With many advantages over the other methods for analyzing bacterial in-vivo gene expression, selective capture of transcribed sequences (SCOTS) technique has the prospect of becoming an elegant tool for discovery of genes involved in the bacterium-host interaction. Here, we summarize the advances in SCOTS technique, including its current and potential applications in bacterial gene expression studies under a variety of conditions from in-vitro to in-vivo and from mammals to insects.

Gene regulation in Borrelia burgdorferi

Borrelia burgdorferi, the spirochete that causes Lyme disease, is maintained in nature via an enzootic cycle that comprises a tick vector and a vertebrate host. Transmission from the tick to the mammal, acquisition from the mammal back to the tick, and adaptation to the two disparate environments require sensing signals and responding by regulating programs of gene expression. The molecular mechanisms utilized to effect these lifestyle changes have begun to be elucidated and feature an alternative sigma factor cascade in which RpoN (σ(54)) and RpoS (σ(S)) globally control the genes required for the different phases of the enzootic cycle. The RpoN-RpoS pathway is surprisingly complex, entailing Rrp2, an unusual enhancer-binding protein and two-component regulatory system response regulator activated by acetyl phosphate; BosR, an unorthodox DNA-binding protein; DsrA(Bb), a small noncoding RNA; and Hfq and CsrA, two RNA-binding proteins. B. burgdorferi also has a c-di-GMP signaling system that regulates the tick side of the enzootic cycle and whose function is only beginning to be appreciated.

HrpA, a DEAH-box RNA helicase, is involved in global gene regulation in the Lyme disease spirochete

Spirochetes causing Lyme borreliosis are obligate parasites that can only be found in a tick vector or a vertebrate host. The ability to survive in these two disparate environments requires up and downregulation of specific genes by regulatory circuits that remain largely obscure. In this work on the Lyme spirochete, B. burgdorferi, we show that a disruption of the hrpA gene, which encodes a putative RNA helicase, results in a complete loss in the ability of the spirochetes to infect mice by needle inoculation. Studies of protein expression in culture by 2D gels revealed a change in the expression of 33 proteins in hrpA clones relative to the wild-type parent. Quantitative characterization of protein expression by iTRAQ analysis revealed a total of 187 differentially regulated proteins in an hrpA background: 90 downregulated and 97 upregulated. Forty-two of the 90 downregulated and 65 of the 97 upregulated proteins are not regulated under any conditions by the previously reported regulators in B. burgdorferi (bosR, rrp2, rpoN, rpoS or rrp1). Downregulated and upregulated proteins also fell into distinct functional categories. We conclude that HrpA is part of a new and distinct global regulatory pathway in B. burgdorferi gene expression. Because an HrpA orthologue is present in many bacteria, its participation in global regulation in B. burgdorferi may have relevance in other bacterial species where its function remains obscure. We believe this to be the first report of a role for an RNA helicase in a global regulatory pathway in bacteria. This finding is particularly timely with the recent growth of the field of RNA regulation of gene expression and the ability of RNA helicases to modulate RNA structure and function.

Molecular interactions that enable movement of the Lyme disease agent from the tick gut into the hemolymph

Borrelia burgdorferi, the causative agent of Lyme disease, is transmitted to humans by bite of Ixodes scapularis ticks. The mechanisms by which the bacterium is transmitted from vector to host are poorly understood. In this study, we show that the F(ab)₂ fragments of BBE31, a B.burgdorferi outer-surface lipoprotein, interfere with the migration of the spirochete from tick gut into the hemolymph during tick feeding. The decreased hemolymph infection results in lower salivary glands infection, and consequently attenuates mouse infection by tick-transmitted B. burgdorferi. Using a yeast surface display approach, a tick gut protein named TRE31 was identified to interact with BBE31. Silencing tre31 also decreased the B. burgdorferi burden in the tick hemolymph. Delineating the specific spirochete and arthropod ligands required for B. burgdorferi movement in the tick may lead to new strategies to interrupt the life cycle of the Lyme disease agent.

Lyme disease, the most common tick-borne illness in North America, is caused by Borrelia burgdorferi. Currently, spirochete and tick molecules that facilitate Borrelia migration within the vector, a key step for mammalian infection by tick-transmitted spirochetes, have not yet been identified. In this study, we show that F(ab)₂ fragments of BBE31, a B.burgdorferi outer-surface lipoprotein, interfere with the spirochete migration from the tick gut into the hemolymph. Our results indicated that decreased hemolymph infection by blocking BBE31 resulted in lower salivary glands infection, which eventually attenuated murine infection by tick-transmitted B.burgdorferi. We also found that a tick gut protein TRE31 enables Borrelia movement by interacting with BBE31. This finding provides novel insights into the transmission of spirochete within the vector and provides potential vaccine targets to block the microbial life cycle within the vector.

Proteome analysis of Borrelia burgdorferi response to environmental change

We examined global changes in protein expression in the B31 strain of Borrelia burgdorferi, in response to two environmental cues (pH and temperature) chosen for their reported similarity to those encountered at different stages of the organism's life cycle. Multidimensional nano-liquid chromatographic separations coupled with tandem mass spectrometry were used to examine the array of proteins (i.e., the proteome) of B. burgdorferi for different pH and temperature culture conditions. Changes in pH and temperature elicited in vitro adaptations of this spirochete known to cause Lyme disease and led to alterations in protein expression that are associated with increased microbial pathogenesis. We identified 1,031 proteins that represent 59% of the annotated genome of B. burgdorferi and elucidated a core proteome of 414 proteins that were present in all environmental conditions investigated. Observed changes in protein abundances indicated varied replicon usage, as well as proteome functional distributions between the in vitro cell culture conditions. Surprisingly, the pH and temperature conditions that mimicked B. burgdorferi residing in the gut of a fed tick showed a marked reduction in protein diversity. Additionally, the results provide us with leading candidates for exploring how B. burgdorferi adapts to and is able to survive in a wide variety of environmental conditions and lay a foundation for planned in situ studies of B. burgdorferi isolated from the tick midgut and infected animals.

Characterization of unique regions of Borrelia burgdorferi surface-located membrane protein 1

The pathogen of Lyme disease, Borrelia burgdorferi, produces a putative surface protein termed "surface-located membrane protein 1" (Lmp1). Lmp1 has been shown previously to assist the microbe in evasion of host-acquired immune defenses and in the establishment of persistent infection of mammals. Here, we show that Lmp1 is an integral membrane protein with surface-exposed N-terminal, middle, and C-terminal regions. During murine infection, antibodies recognizing these three protein regions were produced. Separate immunization of mice with each of the discrete regions exerted differential effects on spirochete survival during infection. Notably, antibodies against the C-terminal region primarily interfered with B. burgdorferi persistence in the joints, while antibodies specific to the N-terminal region predominantly affected pathogen levels in the heart, including the development of carditis. Genetic reconstitution of lmp1 deletion mutants with the lmp1 N-terminal region significantly enhanced its ability to resist the bactericidal effects of immune sera and also was observed to increase pathogen survival in vivo. Taken together, the combined data suggest that the N-terminal region of Lmp1 plays a distinct role in spirochete survival and other parts of the protein are related to specific functions corresponding to pathogen persistence and tropism during infection that is displayed in an organ-specific manner. The findings reported here underscore the fact that surface-exposed regions of Lmp1 could potentially serve as vaccine targets or antigenic regions that could alter the course of natural Lyme disease.

BBA52 facilitates Borrelia burgdorferi transmission from feeding ticks to murine hosts

Borrelia burgdorferi, the pathogen of Lyme borreliosis, persists in nature through a tick-rodent transmission cycle. A selective assessment of the microbial transcriptome, limited to gene-encoding putative membrane proteins, reveals that bba52 transcription in vivo is strictly confined to the vector-specific portion of the microbial life cycle, with the highest levels of expression noted in feeding ticks and with swift down-regulation noted in mice. bba52 deletion did not affect murine disease as assessed by the genesis of arthritis and carditis or long-term persistence of pathogens in mice or ticks. However, bba52 deficiency did impair microbial transitions between hosts and vector, defects that could be fully rescued when bba52 expression was genetically restored to the original genomic locus. These studies establish that BBA52 facilitates vector-host transitions by the pathogen and therefore is a potential antigenic target for interference with transmission of B. burgdorferi from ticks to mammalian hosts.

The bba64 gene of Borrelia burgdorferi, the Lyme disease agent, is critical for mammalian infection via tick bite transmission

The spirochetal agent of Lyme disease, Borrelia burgdorferi, is transmitted by bites of Ixodes ticks to mammalian reservoir hosts and humans. The mechanism(s) by which the organism is trafficked from vector to host is poorly understood. In this study, we demonstrate that a B. burgdorferi mutant strain deficient in the synthesis of the bba64 gene product was incapable of infecting mice via tick bite even though the mutant was (i) infectious in mice when introduced by needle inoculation, (ii) acquired by larval ticks feeding on infected mice, and (iii) able to persist through tick molting stages. This finding of a B. burgdorferi gene required for pathogen transfer and/or survival from the tick to the susceptible host represents an important breakthrough toward understanding transmission mechanisms involved for the Lyme disease agent.

Live imaging reveals a biphasic mode of dissemination of Borrelia burgdorferi within ticks

Lyme disease is caused by transmission of the spirochete Borrelia burgdorferi from ticks to humans. Although much is known about B. burgdorferi replication, the routes and mechanisms by which it disseminates within the tick remain unclear. To better understand this process, we imaged live, infectious B. burgdorferi expressing a stably integrated, constitutively expressed GFP reporter. Using isolated tick midguts and salivary glands, we observed B. burgdorferi progress through the feeding tick via what we believe to be a novel, biphasic mode of dissemination. In the first phase, replicating spirochetes, positioned at varying depths throughout the midgut at the onset of feeding, formed networks of nonmotile organisms that advanced toward the basolateral surface of the epithelium while adhering to differentiating, hypertrophying, and detaching epithelial cells. In the second phase of dissemination, the nonmotile spirochetes transitioned into motile organisms that penetrated the basement membrane and entered the hemocoel, then migrated to and entered the salivary glands. We designated the first phase of dissemination "adherence-mediated migration" and provided evidence that it involves the inhibition of spirochete motility by one or more diffusible factors elaborated by the feeding tick midgut. Our studies, which we believe are the first to relate the transmission dynamics of spirochetes to the complex morphological and developmental changes that the midgut and salivary glands undergo during engorgement, challenge the conventional viewpoint that dissemination of Lyme disease-causing spirochetes within ticks is exclusively motility driven.

Borrelia burgdorferi small lipoprotein Lp6.6 is a member of multiple protein complexes in the outer membrane and facilitates pathogen transmission from ticks to mice

Borrelia burgdorferi lipoprotein Lp6.6 is a differentially produced spirochete antigen. An assessment of lp6.6 expression covering representative stages of the infectious cycle of spirochetes demonstrates that the gene is solely expressed during pathogen persistence in ticks. Deletion of lp6.6 in infectious B. burgdorferi did not influence in vitro growth, or its ability to persist and induce inflammation in mice, migrate to larval or nymphal ticks or survive through the larval-nymphal molt. However, Lp6.6-deficient spirochetes displayed significant impairment in their ability to transmit from infected ticks to naïve mice, which was restored upon genetic complementation of the mutant with a wild-type copy of lp6.6, establishing that Lp6.6 plays a role in pathogen transmission from ticks to mammals. Lp6.6 is a subsurface, yet highly abundant, outer membrane antigen. Two-dimensional blue native/SDS-PAGE coupled with liquid chromatography-mass spectrometry (LC-MS/MS) analysis and protein cross-linking studies independently shows that Lp6.6 exists in multiple protein complexes in the outer membrane. We speculate that the function of Lp6.6 is connected to the physiological processes of these membrane complexes. Further characterization of differentially produced membrane antigens and associated protein complexes will likely aid in our understanding of the molecular details of B. burgdorferi persistence and transmission through a complex enzootic cycle.

Reviewing molecular adaptations of Lyme borreliosis spirochetes in the context of reproductive fitness in natural transmission cycles

Lyme borreliosis (LB) is caused by a group of pathogenic spirochetes – most often Borrelia burgdorferi, B. afzelii, and B. garinii – that are vectored by hard ticks in the Ixodes ricinus-persulcatus complex, which feed on a variety of mammals, birds, and lizards. Although LB is one of the best-studied vector-borne zoonoses, the annual incidence in North America and Europe leads other vector-borne diseases and continues to increase. What factors make the LB system so successful, and how can researchers hope to reduce disease risk – either through vaccinating humans or reducing the risk of contacting infected ticks in nature? Discoveries of molecular interactions involved in the transmission of LB spirochetes have accelerated recently, revealing complex interactions among the spirochete-tick-vertebrate triad. These interactions involve multiple, and often redundant, pathways that reflect the evolution of general and specific mechanisms by which the spirochetes survive and reproduce. Previous reviews have focused on the molecular interactions or population biology of the system. Here molecular interactions among the LB spirochete, its vector, and vertebrate hosts are reviewed in the context of natural maintenance cycles, which represent the ecological and evolutionary contexts that shape these interactions. This holistic system approach may help researchers develop additional testable hypotheses about transmission processes, interpret laboratory results, and guide development of future LB control measures and management.

A chromosomally encoded virulence factor protects the Lyme disease pathogen against host-adaptive immunity

Borrelia burgdorferi, the bacterial pathogen of Lyme borreliosis, differentially expresses select genes in vivo, likely contributing to microbial persistence and disease. Expression analysis of spirochete genes encoding potential membrane proteins showed that surface-located membrane protein 1 (lmp1) transcripts were expressed at high levels in the infected murine heart, especially during early stages of infection. Mice and humans with diagnosed Lyme borreliosis also developed antibodies against Lmp1. Deletion of lmp1 severely impaired the pathogen's ability to persist in diverse murine tissues including the heart, and to induce disease, which was restored upon chromosomal complementation of the mutant with the lmp1 gene. Lmp1 performs an immune-related rather than a metabolic function, as its deletion did not affect microbial persistence in immunodeficient mice, but significantly decreased spirochete resistance to the borreliacidal effects of anti-B. burgdorferi sera in a complement-independent manner. These data demonstrate the existence of a virulence factor that helps the pathogen evade host-acquired immune defense and establish persistent infection in mammals.

The pathogen of Lyme borreliosis, Borrelia burgdorferi, causes disease in many parts of the world, resulting in multi-system complications in infected humans and animals. The microbe produces certain antigens in response to host environments that potentially allow it to persist and cause disease. Here, we analyzed the expression of B. burgdorferi genes encoding potential membrane proteins in infected hosts and show that one of them, termed Lmp1, is dramatically expressed in infected mice, most prominently in cardiac tissue during early infection. Mice and humans diagnosed with Lyme borreliosis also develop antibodies against Lmp1. Deletion of lmp1 in an infectious isolate of B. burgdorferi impairs the pathogen's ability to persist in murine tissues, especially the heart, and to induce disease, which was reversed when the gene was inserted back into the chromosome of the mutant. Lmp1 performs an immune-related, rather than a metabolic, function as its deletion does not affect microbial persistence in immunodeficient mice, but decreases the spirochete's ability to resist the borreliacidal effects of anti-B. burgdorferi sera. These data identify the existence of a surface-located antigen of B. burgdorferi that helps the pathogen evade host-acquired immune defense and establish persistent infection and disease in mammals.

Rrp1, a cyclic-di-GMP-producing response regulator, is an important regulator of Borrelia burgdorferi core cellular functions

Two-component systems (TCS) are universal among bacteria and play critical roles in gene regulation. Our understanding of the contributions of TCS in the biology of the Borrelia is just now beginning to develop. Borrelia burgdorferi, a causative agent of Lyme disease, harbours a TCS comprised of open reading frames (ORFs) BB0419 and BB0420. BB0419 encodes a response regulator designated Rrp1, and BB0420 encodes a hybrid histidine kinase-response regulator designated Hpk1. Rrp1, which contains a conserved GGDEF domain, undergoes phosphorylation and produces the secondary messenger, cyclic diguanylate (c-di-GMP), a critical signaling molecule in numerous organisms. However, the regulatory role of the Rrp1-Hpk1 TCS and c-di-GMP signaling in Borrelia biology are unexplored. In this study, the distribution, conservation, expression and potential global regulatory capability of Rrp1 were assessed. rrp1 was found to be universal and highly conserved among isolates, co-transcribed with hpk1, constitutively expressed during in vitro cultivation, and significantly upregulated upon tick feeding. Allelic exchange replacement and microarray analyses revealed that the Rrp1 regulon consists of a large number of genes encoded by the core Borrelia genome (linear chromosome, linear plasmid 54 and circular plasmid 26) that encode for proteins involved in central metabolic processes and virulence mechanisms including immune evasion.

sodA is essential for virulence of Borrelia burgdorferi in the murine model of Lyme disease

Borrelia burgdorferi, the causative agent of Lyme disease, has a limited set of genes to combat oxidative/nitrosative stress encountered in its tick vector or mammalian hosts. We inactivated the gene encoding for superoxide dismutase A (sodA, bb0153), an enzyme mediating the dismutation of superoxide anions and examined the in vitro and in vivo phenotype of the mutant. There were no significant differences in the in vitro growth characteristics of the sodA mutant compared with the control strains. Microscopic analysis of viability of spirochaetes revealed greater percentage of cell death upon treatment of sodA mutant with superoxide generators compared with its controls. Infectivity analysis in C3H/HeN mice following intradermal needle inoculation of 10(3) or 10(5) spirochaetes per mouse revealed complete attenuation of infectivity for the sodA mutant compared with control strains at 21 days post infection. The sodA mutant was more susceptible to the effects of activated macrophages and neutrophils, suggesting that its in vivo phenotype is partly due to the killing effects of activated immune cells. These studies indicate that SodA plays an important role in combating oxidative stress and is essential for the colonization and dissemination of B. burgdorferi in the murine model of Lyme disease.

Differential expression of a putative CarD-like transcriptional regulator, LtpA, in Borrelia burgdorferi

The availability of microbial genome information has provided a fruitful opportunity for studying regulatory networks in a variety of pathogenic bacteria. In an initial effort to elucidate regulatory networks potentially involved in differential gene expression by the Lyme disease pathogen Borrelia burgdorferi, we have been investigating the functions and regulation of putative transcriptional regulatory factors predicted to be encoded within the B. burgdorferi genome. Herein we report the regulation of one of the predicted transcriptional regulators, LtpA (BB0355), which is homologous to the transcriptional regulator CarD from Myxococcus xanthus. LtpA expression was assessed in response to various environmental stimuli. Immunoblot and quantitative reverse transcription-PCR analyses revealed that unlike many well-characterized differentially regulated Borrelia genes whose expression is induced by elevated temperature, the expression of LtpA was significantly downregulated when spirochetes were grown at an elevated temperature (37 degrees C), as well as when the bacteria were cultivated in a mammalian host-adapted environment. In contrast, LtpA was induced at a lower culture temperature (23 degrees C). Further analyses indicated that the downregulation of LtpA was not dependent on the Rrp2-RpoN-RpoS regulatory pathway, which is involved in the downregulation of OspA when B. burgdorferi is grown in a mammalian host-adapted environment. LtpA protein levels in B. burgdorferi were unaltered in response to changes in the pH in the borrelial cultures. Multiple attempts to generate an LtpA-deficient mutant were unsuccessful, which has hampered the elucidation of its role in pathogenesis. Given that LtpA is exclusively expressed during borrelial cultivation at a lower temperature, a parameter that has been widely used as a surrogate condition to mimic B. burgdorferi in unfed (flat) ticks, and because LtpA is homologous to a known transcriptional regulator, we postulate that LtpA functions as a regulator modulating the expression of genes important to B. burgdorferi's survival within its arthropod vector.

Biology of infection with Borrelia burgdorferi

The spirochete Borrelia burgdorferi is a tick-borne obligate parasite whose normal reservoir is a variety of small mammals. Although infection of these natural hosts does not lead to disease, infection of humans can result in Lyme disease as a consequence of the human immunopathologic response to B burgdorferi. Consistent with the pathogenesis of Lyme disease, bacterial products that allow B burgdorferi to replicate and survive seem to be primarily what is required for the bacterium to cause disease in a susceptible host. This article describes the basic biology of B burgdorferi and reviews some of the bacterial components required for infection of and survival in the mammalian and tick hosts.

Borrelia burgdorferi basic membrane proteins A and B participate in the genesis of Lyme arthritis

Lyme arthritis results from colonization of joints by Borrelia burgdorferi and the ensuing host response. Using gene array–based differential analysis of B. burgdorferi gene expression and quantitative reverse trancription-polymerase chain reaction, we identified two paralogous spirochete genes, bmpA and bmpB, that are preferentially up-regulated in mouse joints compared with other organs. Transfer of affinity-purified antibodies against either BmpA or BmpB into B. burgdorferi–infected mice selectively reduced spirochete numbers and inflammation in the joints. B. burgdorferi lacking bmpA/B were therefore generated to further explore the role of these proteins in the pathogenesis of Lyme disease. B. burgdorferi lacking bmpA/B were infectious in mice, but unable to persist in the joints, and they failed to induce severe arthritis. Complementation of the mutant spirochetes with a wild-type copy of the bmpA and bmpB genes partially restored the original phenotype. These data delineate a role for differentially produced B. burgdorferi antigens in spirochete colonization of mouse joints, and suggest new strategies for the treatment of Lyme arthritis.

Regulated synthesis of the Borrelia burgdorferi inner-membrane lipoprotein IpLA7 (P22, P22-A) during the Lyme disease spirochaete's mammal-tick infectious cycle

Results of previous immunological studies suggested that Borrelia burgdorferi regulates synthesis of the IpLA7 lipoprotein during mammalian infection. Through combined use of quantitative reverse transcription PCR, immunofluorescence analyses, ELISA and immunoblotting, it is now demonstrated that IpLA7 is actually expressed throughout mammalian infection, as well as during transmission both from feeding ticks to naïve mice and from infected mice to naïve, feeding ticks. However, proportions of IpLA7-expressing B. burgdorferi within tick midguts declined significantly with time following completion of blood feeding. Cultured bacteria differentially expressed IpLA7 in response to changes in temperature, pH and concentration of 4,5-dihydroxy-2,3-pentanedione, the precursor of autoinducer 2, indicative of mechanisms governing IpLA7 expression. Previous studies also reported mixed results as to the cellular localization of IpLA7. It is now demonstrated that IpLA7 localizes primarily to the borrelial inner membrane and is not surface-exposed, consistent with the ability of these bacteria to produce IpLA7 throughout mammalian infection despite being the target of a robust immune response.

Genetic and physiological characterization of the Borrelia burgdorferi ORF BB0374-pfs-metK-luxS operon

The Lyme disease spirochaete, Borrelia burgdorferi, produces the LuxS enzyme both in vivo and in vitro; this enzyme catalyses the synthesis of homocysteine and 4,5-dihydroxy-2,3-pentanedione (DPD) from a by-product of methylation reactions. Unlike most bacteria, B. burgdorferi is unable to utilize homocysteine. However, DPD levels alter expression levels of a subset of B. burgdorferi proteins. The present studies demonstrate that a single B. burgdorferi operon encodes both of the enzymes responsible for synthesis of DPD, as well as the enzyme for production of the Lyme spirochaete's only activated-methyl donor and a probable phosphohydrolase. Evidence was found for only a single transcriptional promoter, located 5' of the first gene, which uses the housekeeping sigma(70) subunit for RNA polymerase holoenzyme function. All four genes are co-expressed, and mRNA levels are growth-rate dependent, being produced during the exponential phase. Thus, high metabolic activity is accompanied by increased cellular levels of the only known borrelial methyl donor, enhanced detoxification of methylation by-products, and increased production of DPD. Therefore, production of DPD is directly correlated with cellular metabolism levels, and may thereby function as an extracellular and/or intracellular signal of bacterial health.

Tick-host-pathogen interactions in Lyme borreliosis

Borrelia burgdorferi, the spirochetal agent of Lyme borreliosis, is predominantly transmitted by Ixodes ticks. Spirochetes have developed many strategies to adapt to the different environments that are present in the arthropod vector and the vertebrate host. This review focuses on B. burgdorferi genes that are preferentially expressed in the tick and the vertebrate host, and describes how selected gene products facilitate spirochete survival throughout the enzootic life cycle. Interestingly, B. burgdorferi also enhances expression of specific Ixodes scapularis genes, such as TROSPA and salp15. The importance of these genes and their products for B. burgdorferi survival within the tick, and during the transmission process, will also be reviewed. Moreover, we discuss how such vector molecules could be used to develop vector-antigen-based vaccines to prevent the transmission of B. burgdorferi and, potentially, other arthropod-borne microbes.

Sigma factor selectivity in Borrelia burgdorferi: RpoS recognition of the ospE/ospF/elp promoters is dependent on the sequence of the -10 region

Members of the ospE/ospF/elp lipoprotein gene families of Borrelia burgdorferi, the Lyme disease agent, are transcriptionally upregulated in response to the influx of blood into the midgut of an infected tick. We recently have demonstrated that despite the high degree of similarity between the promoters of the ospF (P(ospF)) and ospE (P(ospE)) genes of B. burgdorferi strain 297, the differential expression of ospF is RpoS-dependent, while ospE is controlled by sigma(70). Herein we used wild-type and RpoS-deficient strains of B. burgdorferi and Escherichia coli to analyse transcriptional reporters consisting of a green fluorescent protein (gfp) gene fused to P(ospF), P(ospE), or two hybrid promoters in which the -10 regions of P(ospF) and P(ospE) were switched [P(ospF ) ((E - 10)) and P(ospE) ((F - 10)) respectively]. We found that the P(ospF)-10 region is both necessary and sufficient for RpoS-dependent recognition in B. burgdorferi, while sigma(70) specificity for P(ospE) is dependent on elements outside of the -10 region. In E. coli, sigma factor selectivity for these promoters was much more permissive, with expression of each being primarily due to sigma(70). Alignment of the sequences upstream of each of the ospE/ospF/elp genes from B. burgdorferi strains 297 and B31 revealed that two B31 ospF paralogues [erpK (BBM38) and erpL (BBO39)] have -10 regions virtually identical to that of P(ospF). Correspondingly, expression of gfp reporters based on the erpK and erpL promoters was RpoS-dependent. Thus, the sequence of the P(ospF)-10 region appears to serve as a motif for RpoS recognition, the first described for any B. burgdorferi promoter. Taken together, our data support the notion that B. burgdorferi utilizes sequence differences at the -10 region as one mechanism for maintaining the transcriptional integrity of RpoS-dependent and -independent genes activated at the onset of tick feeding.

Borrelia burgdorferi–Traveling incognito?

We outline in this review how Borrelia burgdorferi, the causative agent of Lyme disease, moves from the tick to the vertebrate host, and what molecules are potentially involved in this challenging commute. The survival strategies utilized by the spirochete during transmission and the initial stages of infection are discussed.

Functionality of Borrelia burgdorferi LuxS: the Lyme disease spirochete produces and responds to the pheromone autoinducer-2 and lacks a complete activated-methyl cycle

Borrelia burgdorferi produces Pfs and LuxS enzymes for breakdown of the toxic byproducts of methylation reactions, producing 4,5-dihydroxy-2,3-pentanedione (DPD), adenine, and homocysteine. DPD and its spontaneously rearranged derivatives constitute a class of bacterial pheromones named autoinducer-2 (AI-2). We describe that B. burgdorferi produces DPD during laboratory cultivation. Furthermore, addition of in vitro synthesized DPD to cultured B. burgdorferi resulted in altered expression levels of a specific set of bacterial proteins, among which is the outer surface lipoprotein VlsE. While a large number of bacteria utilize homocysteine, the other LuxS product, for synthesis of methionine as part of the activated-methyl cycle, B. burgdorferi was found to lack that ability. We propose that the main function of B. burgdorferi LuxS is to synthesize DPD and that the Lyme disease spirochete utilizes a form of DPD as a pheromone to control gene expression.

Alternate sigma factor RpoS is required for the in vivo-specific repression of Borrelia burgdorferi plasmid lp54-borne ospA and lp6.6 genes

While numerous positively regulated loci have been characterized during the enzootic cycle of Borrelia burgdorferi, very little is known about the mechanism(s) involved in the repression of borrelial loci either during tick feeding or within the mammalian host. Here, we report that the alternative sigma factor RpoS is required for the in vivo-specific repression of at least two RpoD-dependent B. burgdorferi loci, ospA and lp6.6. The downregulation of ospA and Ip6.6 appears to require either a repressor molecule whose expression is RpoS dependent or an accessory factor which enables RpoS to directly interact with the ospA and Ip6.6 promoter elements, thereby blocking transcription by RpoD. The central role for RpoS during the earliest stages of host adaptation suggests that tick feeding imparts signals to spirochetes that trigger the RpoS-dependent repression, as well as expression, of in vivo-specific virulence factors critical for the tick-to-mammalian host transition.

Comparative transcriptional profiling of Borrelia burgdorferi clinical isolates differing in capacities for hematogenous dissemination

Borrelia burgdorferi, the etiologic agent of Lyme disease, is genetically heterogeneous. Previous studies have shown a significant association between the frequency of hematogenous dissemination in Lyme disease patients and the genotype of the infecting B. burgdorferi strain. Comparative transcriptional profiling of two representative clinical isolates with distinct genotypes (BL206 and B356) was undertaken. A total of 78 open reading frames (ORFs) had expression levels that differed significantly between the two isolates. A number of genes with potential involvement in nutrient uptake (BB0603, BBA74, BB0329, BB0330, and BBB29) have significantly higher expression levels in isolate B356. Moreover, nearly 25% of the differentially expressed genes are predicted to be localized on the cell surface, implying that these two isolates have cell surface properties that differ considerably. One of these genes, BBA74, encodes a protein of 257 amino acid residues that has been shown to possess porin activity. BBA74 transcript level was >20-fold higher in B356 than in BL206, and strain B356 contained three- to fivefold more BBA74 protein. BBA74 was disrupted by the insertion of a kanamycin resistance cassette into the coding region. The growth rates of both wild-type and mutant strains were essentially identical, and cultures reached the same final cell densities. However, the mutant strains consistently showed prolonged lags of 2 to 5 days prior to the induction of log-phase growth compared to wild-type strains. It is tempting to speculate that the absence of BBA74 interferes with the enhanced nutrient uptake that may be required for the entry of cells into log-phase growth. These studies demonstrate the value of comparative transcriptional profiling for identifying differences in the transcriptomes of B. burgdorferi clinical isolates that may provide clues to pathogenesis. The 78 ORFs identified here are a good starting point for the investigation of factors involved in the hematogenous dissemination of B. burgdorferi.

Synthesis of autoinducer 2 by the lyme disease spirochete, Borrelia burgdorferi

Defining the metabolic capabilities and regulatory mechanisms controlling gene expression is a valuable step in understanding the pathogenic properties of infectious agents such as Borrelia burgdorferi. The present studies demonstrated that B. burgdorferi encodes functional Pfs and LuxS enzymes for the breakdown of toxic products of methylation reactions. Consistent with those observations, B. burgdorferi was shown to synthesize the end product 4,5-dihydroxy-2,3-pentanedione (DPD) during laboratory cultivation. DPD undergoes spontaneous rearrangements to produce a class of pheromones collectively named autoinducer 2 (AI-2). Addition of in vitro-synthesized DPD to cultured B. burgdorferi resulted in differential expression of a distinct subset of proteins, including the outer surface lipoprotein VlsE. Although many bacteria can utilize the other LuxS product, homocysteine, for regeneration of methionine, B. burgdorferi was found to lack such ability. It is hypothesized that B. burgdorferi produces LuxS for the express purpose of synthesizing DPD and utilizes a form of that molecule as an AI-2 pheromone to control gene expression.

Differential infectivity of the Lyme disease spirochete Borrelia burgdorferi derived from Ixodes scapularis salivary glands and midgut

Blood fed nymphal Ixodes scapularis Say infected with Borrelia burgdorferi were dissected to obtain salivary gland and midgut extracts. Extracts were inoculated into C3H/HeJ mice, and ear, heart, and bladder were cultured to determine comparative infectivity. Aliquots of extracts were then analyzed by quantitative polymerase chain reaction to determine the number of spirochetes inoculated into mice. A comparative median infectious dose (ID50) was determined for both salivary gland and midgut extract inoculations. Our data demonstrated a statistically significant difference (P < 0.002) in the ID50 derived from salivary gland (average = 18) versus midgut (average = 251) extracts needed to infect susceptible mice. A rationale for the differential infectivity of salivary and midgut derived spirochetes is discussed.

Borrelia burgdorferi sigma54 is required for mammalian infection and vector transmission but not for tick colonization

Previous studies have shown that a sigma54-sigma(S) cascade regulates the expression of a few key lipoproteins in Borrelia burgdorferi, the agent of Lyme disease. Here, we demonstrate that these sigma factors, both together and independently, regulate a much more extensive number of genes and cellular processes. Microarray analyses of sigma54 and sigma(S) mutant strains identified 305 genes regulated by sigma54 and 145 regulated by sigma(S), whereas the sigma54-sigma(S) regulatory cascade appears to control 48 genes in B. burgdorferi. In silico analyses revealed that nearly 80% of genes with altered expression in the sigma54 mutant were linked to potential sigma54-dependent promoters. Many sigma54-regulated genes are expressed in vivo, and through genetic complementation of the mutant, we demonstrated that sigma54 was required by B. burgdorferi to infect mammals. Surprisingly, sigma54 mutants were able to infect Ixodes scapularis ticks and be maintained for at least 24 wk after infection, suggesting the sigma54-sigma(S) regulatory network was not involved in long-term survival in ticks. However, sigma54 mutants did not enter the salivary glands during tick feeding, indicating that sigma54-regulated genes were involved in the transmission process.

The burgeoning molecular genetics of the Lyme disease spirochaete

Lyme disease is the most commonly reported vector-borne disease in North America and Europe, yet we know little about which components of the causative agent, Borrelia burgdorferi, are critical for infection or virulence. Molecular genetics has provided a powerful means by which to address these topics in other bacterial pathogens. Certain features of B. burgdorferi have hampered the development of an effective system of genetic analysis, but basic tools are now available and their application has begun to provide information about the identities and roles of key bacterial components in both the tick vector and the mammalian host. Increased genetic analysis of B. burgdorferi should advance our understanding of the infectious cycle and the pathogenesis of Lyme disease.

Extracellular secretion of the Borrelia burgdorferi Oms28 porin and Bgp, a glycosaminoglycan binding protein

Borrelia burgdorferi, the Lyme disease pathogen, cycles between its Ixodes tick vector and vertebrate hosts, adapting to vastly different biochemical environments. Spirochete gene expression as a function of temperature, pH, growth phase, and host milieu is well studied, and recent work suggests that regulatory networks are involved. Here, we examine the release of Borrelia burgdorferi strain B31 proteins into conditioned medium. Spirochetes intrinsically radiolabeled at concentrations ranging from 10(7) to 10(9) cells per ml secreted Oms28, a previously characterized outer membrane porin, into RPMI medium. As determined by immunoblotting, this secretion was not associated with outer membrane blebs or cytoplasmic contamination. A similar profile of secreted proteins was obtained for spirochetes radiolabeled in mixtures of RPMI medium and serum-free Barbour-Stoenner-Kelly (BSK II) medium. Proteomic liquid chromatography-tandem mass spectrometry analysis of tryptic fragments derived from strain B31 culture supernatants confirmed the identity of the 28-kDa species as Oms28 and revealed a 26-kDa protein as 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidase (Pfs-2), previously described as Bgp, a glycosaminoglycan-binding protein. The release of Oms28 into the culture medium is more selective when the spirochetes are in logarithmic phase of growth compared to organisms obtained from stationary phase. As determined by immunoblotting, stationary-phase spirochetes released OspA, OspB, and flagellin. Oms28 secreted by strains B31, HB19, and N40 was also recovered by radioimmunoprecipitation. This is the first report of B. burgdorferi protein secretion into the extracellular environment. The possible roles of Oms28 and Bgp in the host-pathogen interaction are considered.

RpoS is not central to the general stress response in Borrelia burgdorferi but does control expression of one or more essential virulence determinants

Borrelia burgdorferi, the Lyme disease spirochete, undergoes dramatic changes in antigenic composition as it cycles between its arthropod and mammalian hosts. A growing body of evidence suggests that these changes reflect, at least in part, the need for spirochetes to adapt to the physiological stresses imposed by abrupt changes in environmental conditions and nutrient availability. In many microorganisms, global responses are mediated by master regulators such as alternative sigma factors, with Escherichia coli RpoS (sigmaS) serving as a prototype. The importance of this transcriptional activator in other bacteria, coupled with the report by Hubner et al. (A. Hubner, X. Yang, D. M. Nolen, T. G. Popova, F. C. Cabello, and M. V. Norgard, Proc. Natl. Acad. Sci. USA 98:12724-12729, 2001) demonstrating that the borrelial RpoS ortholog controls expression of OspC and decorin-binding protein A (DbpA), prompted us to examine more closely the roles of RpoS-dependent and -independent differential gene expression in physiological adaptation by the Lyme disease spirochete. We observed that B. burgdorferi rpoS (rpoSBb) was induced following temperature shift and transcript levels were further enhanced by reduced pH (pH 6.8). Using quantitative real-time reverse transcription-PCR (RT-PCR), we demonstrated that, in contrast to its ortholog (rpoSEc) in Escherichia coli, rpoSBb was expressed at significant levels in B. burgdorferi throughout all phases of growth following temperature shift. By comparing a B. burgdorferi strain 297 rpoSBb mutant to its wild-type counterpart, we determined that RpoSBb was not required for survival following exposure to a wide range of environmental stresses (i.e., temperature shift, serum starvation, increased osmolality, reactive oxygen intermediates, and increased or reduced oxygen tension), although the mutant was more sensitive to extremes of pH. While B. burgdorferi strains lacking RpoS were able to survive within intraperitoneal dialysis membrane chambers at a level equivalent to that of the wild type, they were avirulent in mice. Lastly, RT-PCR analysis of the ospE-ospF-elp paralogous lipoprotein families complements earlier findings that many temperature-inducible borrelial loci are controlled in an RpoSBb-independent manner. Together, these data point to fundamental differences between the role(s) of RpoS in B. burgdorferi and that in E. coli. Rather than functioning as a master regulator, RpoSBb appears to serve as a stress-responsive activator of a subset of virulence determinants that, together with the RpoS-independent, differentially expressed regulon, encompass the spirochete's genetic programs required for mammalian host adaptation.

Combined effects of blood and temperature shift on Borrelia burgdorferi gene expression as determined by whole genome DNA array

Borrelia burgdorferi undergoes differential gene expression during transmission from its tick vector to a vertebrate host. The addition of blood to a spirochete culture at 35 degrees C for 48 h had a dramatic effect on gene expression of this organism. Utilizing B. burgdorferi whole genome DNA arrays, we compared the transcriptomes of the spirochetes following a 2-day temperature shift with blood and without blood. Using combined data from three independent RNA isolations we demonstrated that the addition of blood led to a differential expression of 154 genes. Of these, 75 genes were upregulated, with 49 (65%) of them encoded on plasmids. Blood supplementation of cultures also resulted in the downregulation of 79 genes, where 56 (70%) were plasmid encoded. We verified our results by reverse transcriptase PCR of several genes in both flat and feeding ticks. In the 2-day experiment we observed the effect that exposure to increased temperature and blood combined had on B. burgdorferi gene expression at this crucial time when the spirochetes begin to move from the vector to a new vertebrate host. These changes, among others, coincide with the upregulation of the chemotaxis and sensing regulons, of the lp38-encoded ABC transporter, of proteases capable of remodeling the outer surface of the spirochetes, and of the recombination genes of cp32 as a transient or initial part of the stress response of the phage. These are all functions that could cause or facilitate the changes that spirochetes undergo following a blood meal in the tick.