Borrelia burgdorferi oxidative stress regulator BosR directly represses lipoproteins primarily expressed in the tick during mammalian infection

Hitchhiker's Guide to Borrelia burgdorferi

Don't Panic. In the nearly 50 years since the discovery of Lyme disease, Borrelia burgdorferi has emerged as an unlikely workhorse of microbiology. Interest in studying host-pathogen interactions fueled significant progress in making the fastidious microbe approachable in laboratory settings, including the development of culture methods, animal models, and genetic tools. By developing these systems, insight has been gained into how the microbe is able to survive its enzootic cycle and cause human disease. Here, we discuss the discovery of B. burgdorferi and its development as a model organism before diving into the critical lessons we have learned about B. burgdorferi biology at pivotal stages of its lifecycle: gene expression changes during the tick blood meal, colonization of a new vertebrate host, and developing a long-lasting infection in that vertebrate until a new tick feeds. Our goal is to highlight the advancements that have facilitated B. burgdorferi research and identify gaps in our current understanding of the microbe.

A Fur family protein BosR is a novel RNA-binding protein that controls rpoS RNA stability in the Lyme disease pathogen

The σ54-σS sigma factor cascade plays a central role in regulating differential gene expression during the enzootic cycle of Borreliella burgdorferi, the Lyme disease pathogen. In this pathway, the primary transcription of rpoS (which encodes σS) is under the control of σ54 which is activated by a bacterial enhancer-binding protein (EBP), Rrp2. The σ54-dependent activation in B. burgdorferi has long been thought to be unique, requiring an additional factor, BosR, a homologue of classical Fur/PerR repressor/activator. However, how BosR is involved in this σ54-dependent activation remains unclear and perplexing. In this study, we demonstrate that BosR does not function as a regulator for rpoS transcriptional activation. Instead, it functions as a novel RNA-binding protein that governs the turnover rate of rpoS mRNA. We further show that BosR directly binds to the 5' untranslated region (UTR) of rpoS mRNA, and the binding region overlaps with a region required for rpoS mRNA degradation. Mutations within this 5'UTR region result in BosR-independent RpoS production. Collectively, these results uncover a novel role of Fur/PerR family regulators as RNA-binding proteins and redefine the paradigm of the σ54-σS pathway in B. burgdorferi.

BosR and PlzA reciprocally regulate RpoS function to sustain Borrelia burgdorferi in ticks and mammals

The RNA polymerase alternative σ factor RpoS in Borrelia burgdorferi (Bb), the Lyme disease pathogen, is responsible for programmatic-positive and -negative gene regulation essential for the spirochete's dual-host enzootic cycle. RpoS is expressed during tick-to-mammal transmission and throughout mammalian infection. Although the mammalian-phase RpoS regulon is well described, its counterpart during the transmission blood meal is unknown. Here, we used Bb-specific transcript enrichment by tick-borne disease capture sequencing (TBDCapSeq) to compare the transcriptomes of WT and ΔrpoS Bb in engorged nymphs and following mammalian host-adaptation within dialysis membrane chambers. TBDCapSeq revealed dramatic changes in the contours of the RpoS regulon within ticks and mammals and further confirmed that RpoS-mediated repression is specific to the mammalian-phase of Bb's enzootic cycle. We also provide evidence that RpoS-dependent gene regulation, including repression of tick-phase genes, is required for persistence in mice. Comparative transcriptomics of engineered Bb strains revealed that the Borrelia oxidative stress response regulator (BosR), a noncanonical Fur family member, and the cyclic diguanosine monophosphate (c-di-GMP) effector PlzA reciprocally regulate the function of RNA polymerase complexed with RpoS. BosR is required for RpoS-mediated transcription activation and repression in addition to its well-defined role promoting transcription of rpoS by the RNA polymerase alternative σ factor RpoN. During transmission, ligand-bound PlzA antagonizes RpoS-mediated repression, presumably acting through BosR.

Fur-like proteins: Beyond the ferric uptake regulator (Fur) paralog

Proteins belonging to the FUR (ferric uptake regulator) family are the cornerstone of metalloregulation in most prokaryotes. Although numerous reviews have been devoted to these proteins, these reports are mainly focused on the Fur paralog that gives name to the family. In the last years, the increasing knowledge on the other, less ubiquitous members of this family has evidenced their importance in bacterial metabolism. As the Fur paralog, the major regulator of iron homeostasis, Zur, Irr, BosR and PerR are tightly related to stress defenses and host-pathogen interaction being in many cases essential for virulence. Furthermore, the Nur and Mur paralogs largely contribute to control nickel and manganese homeostasis, which are cofactors of pivotal proteins for host colonization and bacterial redox homeostasis. The present review highlights the main features of FUR proteins that differ to the canonical Fur paralog either in the coregulatory metal, such as Zur, Nur and Mur, or in the action mechanism to control target genes, such as PerR, Irr and BosR.

Gene Regulation and Transcriptomics

Borrelia (Borreliella) burgdorferi, along with closely related species, is the etiologic agent of Lyme disease. The spirochete subsists in an enzootic cycle that encompasses acquisition from a vertebrate host to a tick vector and transmission from a tick vector to a vertebrate host. To adapt to its environment and persist in each phase of its enzootic cycle, B. burgdorferi wields three systems to regulate the expression of genes: the RpoN-RpoS alternative sigma factor cascade, the Hk1/Rrp1 two-component system and its product c-di-GMP, and the stringent response mediated by Rel_Bbu and DksA. These regulatory systems respond to enzootic phase-specific signals and are controlled or fine- tuned by transcription factors, including BosR and BadR, as well as small RNAs, including DsrABb and Bb6S RNA. In addition, several other DNA-binding and RNA-binding proteins have been identified, although their functions have not all been defined. Global changes in gene expression revealed by high-throughput transcriptomic studies have elucidated various regulons, albeit technical obstacles have mostly limited this experimental approach to cultivated spirochetes. Regardless, we know that the spirochete, which carries a relatively small genome, regulates the expression of a considerable number of genes required for the transitions between the tick vector and the vertebrate host as well as the adaptation to each.

The RpoS Gatekeeper in Borrelia burgdorferi: An Invariant Regulatory Scheme That Promotes Spirochete Persistence in Reservoir Hosts and Niche Diversity

Maintenance of Borrelia burgdorferi within its enzootic cycle requires a complex regulatory pathway involving the alternative σ factors RpoN and RpoS and two ancillary trans-acting factors, BosR and Rrp2. Activation of this pathway occurs within ticks during the nymphal blood meal when RpoS, the effector σ factor, transcribes genes required for tick transmission and mammalian infection. RpoS also exerts a 'gatekeeper' function by repressing σ⁷⁰-dependent tick phase genes (e.g., ospA, lp6.6). Herein, we undertook a broad examination of RpoS functionality throughout the enzootic cycle, beginning with modeling to confirm that this alternative σ factor is a 'genuine' RpoS homolog. Using a novel dual color reporter system, we established at the single spirochete level that ospA is expressed in nymphal midguts throughout transmission and is not downregulated until spirochetes have been transmitted to a naïve host. Although it is well established that rpoS/RpoS is expressed throughout infection, its requirement for persistent infection has not been demonstrated. Plasmid retention studies using a trans-complemented ΔrpoS mutant demonstrated that (i) RpoS is required for maximal fitness throughout the mammalian phase and (ii) RpoS represses tick phase genes until spirochetes are acquired by a naïve vector. By transposon mutant screening, we established that bba34/oppA5, the only OppA oligopeptide-binding protein controlled by RpoS, is a bona fide persistence gene. Lastly, comparison of the strain 297 and B31 RpoS DMC regulons identified two cohorts of RpoS-regulated genes. The first consists of highly conserved syntenic genes that are similarly regulated by RpoS in both strains and likely required for maintenance of B. burgdorferi sensu stricto strains in the wild. The second includes RpoS-regulated plasmid-encoded variable surface lipoproteins ospC, dbpA and members of the ospE/ospF/elp, mlp, revA, and Pfam54 paralogous gene families, all of which have evolved via inter- and intra-strain recombination. Thus, while the RpoN/RpoS pathway regulates a 'core' group of orthologous genes, diversity within RpoS regulons of different strains could be an important determinant of reservoir host range as well as spirochete virulence.

Genome-wide screen identifies novel genes required for Borrelia burgdorferi survival in its Ixodes tick vector

Borrelia burgdorferi, the causative agent of Lyme disease in humans, is maintained in a complex biphasic life cycle, which alternates between tick and vertebrate hosts. To successfully survive and complete its enzootic cycle, B. burgdorferi adapts to diverse hosts by regulating genes required for survival in specific environments. Here we describe the first ever use of transposon insertion sequencing (Tn-seq) to identify genes required for B. burgdorferi survival in its tick host. We found that insertions into 46 genes resulted in a complete loss of recovery of mutants from larval Ixodes ticks. Insertions in an additional 56 genes resulted in a >90% decrease in fitness. The screen identified both previously known and new genes important for larval tick survival. Almost half of the genes required for survival in the tick encode proteins of unknown function, while a significant portion (over 20%) encode membrane-associated proteins or lipoproteins. We validated the results of the screen for five Tn mutants by performing individual competition assays using mutant and complemented strains. To better understand the role of one of these genes in tick survival, we conducted mechanistic studies of bb0017, a gene previously shown to be required for resistance against oxidative stress. In this study we show that BB0017 affects the regulation of key borrelial virulence determinants. The application of Tn-seq to in vivo screening of B. burgdorferi in its natural vector is a powerful tool that can be used to address many different aspects of the host pathogen interaction.

Borrelia burgdorferi, the causative agent of Lyme disease, must adjust to environmental changes as it moves between its tick and vertebrate hosts. We performed a screen of a B. burgdorferi transposon library using massively parallel sequencing (Tn-seq) to identify fitness defects involved in survival in its tick host. This screen accurately identified genes known to cause decreased fitness for tick survival and identified new genes involved in B. burgdorferi survival in ticks. All of the genes tested individually confirmed the Tn-seq results. One of the genes identified encodes a protein whose function was previously unknown that appears to be involved in regulating expression of proteins known to be involved in environmental adaptation. Tn-seq is a powerful tool for understanding vector-pathogen interactions and may reveal new opportunities for interrupting the infectious cycle of vector-borne diseases.

The Functional and Molecular Effects of Doxycycline Treatment on Borrelia burgdorferi Phenotype

Recent studies have shown that Borrelia burgdorferi can form antibiotic-tolerant persisters in the presence of microbiostatic drugs such as doxycycline. Precisely how this occurs is yet unknown. Our goal was to examine gene transcription by B. burgdorferi following doxycycline treatment in an effort to identify both persister-associated genes and possible targets for antimicrobial intervention. To do so, we performed next-generation RNA sequencing on doxycycline-treated spirochetes and treated spirochetes following regrowth, comparing them to untreated B. burgdorferi. A number of genes were perturbed and most of those which were statistically significant were down-regulated in the treated versus the untreated or treated/re-grown. Genes upregulated in the treated B. burgdorferi included a number of Erp genes and rplU, a 50S ribosomal protein. Among those genes associated with post-treatment regrowth were bba74 (Oms28), bba03, several peptide ABC transporters, ospA, ospB, ospC, dbpA and bba62. Studies are underway to determine if these same genes are perturbed in B. burgdorferi treated with doxycycline in a host environment.

The CXXC Motifs Are Essential for the Function of BosR in Borrelia burgdorferi

BosR, a Fur family member, is essential for the pathogenesis of the Lyme disease pathogen, Borrelia burgdorferi. Unlike typical Fur proteins in which DNA binding represses gene expression, binding of BosR to the rpoS promoter directly activates rpoS transcription in B. burgdorferi. However, virtually nothing is known concerning potential structural features and amino acid residues of BosR that are important for protein function and virulence regulation in B. burgdorferi. Particularly, it remains unknown what structural motifs or residues of BosR coordinate Zn, although previous analyses have indicated that the function of BosR may depend on Zn. To address these information gaps, we herein introduced mutations into four conserved cysteine residues in two putative CXXC motifs of BosR. Our data showed that the ability of BosR to bind Zn was dramatically reduced when the CXXC motifs were mutated. Moreover, we found that the two CXXC motifs contributed to the ability of BosR to form dimers. By using a trans-complementation genetic approach, we additionally demonstrated that both CXXC motifs of BosR were essential for in vivo gene expression regulation. Mutation of any of the four cysteines abolished the transcriptional activation of rpoS. In contrast to wild type BosR, each mutant protein was incapable of binding the rpoS promoter in electrophoretic mobility shift assays. The combined data strongly support that the two CXXC motifs and four cysteines constitute the structural site essential for Zn-coordination, protein dimerization, and the unique regulatory activity of BosR.

The Stringent Response-Regulated sRNA Transcriptome of Borrelia burgdorferi

The Lyme disease spirochete Borrelia (Borreliella) burgdorferi must tolerate nutrient stress to persist in the tick phase of its enzootic life cycle. We previously found that the stringent response mediated by Rel_Bbu globally regulates gene expression to facilitate persistence in the tick vector. Here, we show that Rel_Bbu regulates the expression of a swath of small RNAs (sRNA), affecting 36% of previously identified sRNAs in B. burgdorferi. This is the first sRNA regulatory mechanism identified in any spirochete. Threefold more sRNAs were Rel_Bbu-upregulated than downregulated during nutrient stress and included antisense, intergenic and 5′ untranslated region sRNAs. Rel_Bbu-regulated sRNAs associated with genes known to be important for host infection (bosR and dhhp) as well as persistence in the tick (glpF and hk1) were identified, suggesting potential mechanisms for post-transcriptional regulation of gene expression.

Borrelia Host Adaptation Protein (BadP) Is Required for the Colonization of a Mammalian Host by the Agent of Lyme Disease

Borrelia burgdorferi, the agent of Lyme disease (LD), uses host-derived signals to modulate gene expression during the vector and mammalian phases of infection. Microarray analysis of mutants lacking the B orrelia host adaptation regulator (BadR) revealed the downregulation of genes encoding enzymes whose role in the pathophysiology of B. burgdorferi is unknown. Immunoblot analysis of the badR mutants confirmed reduced levels of these enzymes, and one of these enzymes, encoded by bb0086, shares homology to prokaryotic magnesium chelatase and Lon-type proteases. The BB0086 levels in B. burgdorferi were higher under conditions mimicking those in fed ticks. Mutants lacking bb0086 had no apparent in vitro growth defect but were incapable of colonizing immunocompetent C3H/HeN or immunodeficient SCID mice. Immunoblot analysis revealed reduced levels of proteins critical for the adaptation of B. burgdorferi to the mammalian host, such as OspC, DbpA, and BBK32. Both RpoS and BosR, key regulators of gene expression in B. burgdorferi, were downregulated in the bb0086 mutants. Therefore, we designated BB0086 the B orrelia host adaptation protein (BadP). Unlike badP mutants, the control strains established infection in C3H/HeN mice at 4 days postinfection, indicating an early colonization defect in mutants due to reduced levels of the lipoproteins/regulators critical for initial stages of infection. However, badP mutants survived within dialysis membrane chambers (DMCs) implanted within the rat peritoneal cavity but, unlike the control strains, did not display complete switching of OspA to OspC, suggesting incomplete adaptation to the mammalian phase of infection. These findings have opened a novel regulatory mechanism which impacts the virulence potential of B burgdorferi.

Two Distinct Mechanisms Govern RpoS-Mediated Repression of Tick-Phase Genes during Mammalian Host Adaptation by Borrelia burgdorferi, the Lyme Disease Spirochete

The alternative sigma factor RpoS plays a key role modulating gene expression in Borrelia burgdorferi, the Lyme disease spirochete, by transcribing mammalian host-phase genes and repressing σ⁷⁰-dependent genes required within the arthropod vector. To identify cis regulatory elements involved in RpoS-dependent repression, we analyzed green fluorescent protein (GFP) transcriptional reporters containing portions of the upstream regions of the prototypical tick-phase genes ospAB, the glp operon, and bba74. As RpoS-mediated repression occurs only following mammalian host adaptation, strains containing the reporters were grown in dialysis membrane chambers (DMCs) implanted into the peritoneal cavities of rats. Wild-type spirochetes harboring ospAB- and glp-gfp constructs containing only the minimal (−35/−10) σ⁷⁰ promoter elements had significantly lower expression in DMCs relative to growth in vitro at 37°C; no reduction in expression occurred in a DMC-cultivated RpoS mutant harboring these constructs. In contrast, RpoS-mediated repression of bba74 required a stretch of DNA located between −165 and −82 relative to its transcriptional start site. Electrophoretic mobility shift assays employing extracts of DMC-cultivated B. burgdorferi produced a gel shift, whereas extracts from RpoS mutant spirochetes did not. Collectively, these data demonstrate that RpoS-mediated repression of tick-phase borrelial genes occurs by at least two distinct mechanisms. One (e.g., ospAB and the glp operon) involves primarily sequence elements near the core promoter, while the other (e.g., bba74) involves an RpoS-induced transacting repressor. Our results provide a genetic framework for further dissection of the essential “gatekeeper” role of RpoS throughout the B. burgdorferi enzootic cycle.

Borrelia burgdorferi, the Lyme disease spirochete, modulates gene expression to adapt to the distinctive environments of its mammalian host and arthropod vector during its enzootic cycle. The alternative sigma factor RpoS has been referred to as a “gatekeeper” due to its central role in regulating the reciprocal expression of mammalian host- and tick-phase genes. While RpoS-dependent transcription has been studied extensively, little is known regarding the mechanism(s) of RpoS-mediated repression. We employed a combination of green fluorescent protein transcriptional reporters along with an in vivo model to define cis regulatory sequences responsible for RpoS-mediated repression of prototypical tick-phase genes. Repression of ospAB and the glp operon requires only sequences near their core promoters, whereas modulation of bba74 expression involves a putative RpoS-dependent repressor that binds upstream of the core promoter. Thus, Lyme disease spirochetes employ at least two different RpoS-dependent mechanisms to repress tick-phase genes within the mammal.

BosR Is A Novel Fur Family Member Responsive to Copper and Regulating Copper Homeostasis in Borrelia burgdorferi

The ferric uptake regulator (Fur) family of DNA-binding proteins represses and/or activates gene transcription via divalent metal ion-dependent signal sensing. The Borrelia burgdorferi Fur homologue, also known as Borrelia oxidative stress regulator (BosR), promotes spirochetal adaptation to the mammalian host by directly repressing the lipoproteins required for tick colonization and indirectly activating those required for establishing infection in the mammal. Here, we examined whether the DNA-binding activity of BosR was regulated by any of the four most prevalent transition metal ions in B. burgdorferi, Mn, Fe, Cu, and Zn. Our data indicated that in addition to a structural site occupied by Zn(II), BosR had two regulatory sites that could be occupied by Zn(II), Fe(II), or Cu(II) but not by Mn(II). While Fe(II) had no effect, Cu(II) and Zn(II) had a dose-dependent inhibitory effect on the BosR DNA-binding activity. Competition experiments indicated that Cu(II) had a higher affinity for BosR than Zn(II) or Fe(II). A BosR deficiency in B. burgdorferi resulted in a significant increase in the Cu level but no significant change in the levels of Mn, Fe, or Zn. These data suggest that Cu regulates BosR activity, and BosR in turn regulates Cu homeostasis in B. burgdorferi While this regulatory paradigm is characteristic of the Fur family, BosR is the first one shown to be responsive to Cu(II), which may be an adaptation to the potentially high level of Cu present in the Lyme disease spirochete.IMPORTANCE Transition metal ions serve an essential role in the metabolism of all living organisms. Members of the ferric uptake regulator (Fur) family play critical roles in regulating the cellular homeostasis of transition metals in diverse bacteria, and their DNA-binding activity is often regulated by coordination of the cognate divalent metal ions. To date, regulators with metal ion specificity to Fe(II), Mn(II), Zn(II), and Ni(II) have all been described. In this study, we demonstrate that BosR, the sole Fur homologue in Borrelia burgdorferi, is responsive to Cu(II) and regulates Cu homeostasis in this bacterium, which may be an adaption to potentially Cu-rich milieu in the Lyme disease spirochete. This study has expanded the repertoire of the Fur family's metal ion specificity.

Borrelia burgdorferi Keeps Moving and Carries on: A Review of Borrelial Dissemination and Invasion

Borrelia burgdorferi is the etiological agent of Lyme disease, a multisystemic, multistage, inflammatory infection resulting in patients experiencing cardiac, neurological, and arthritic complications when not treated with antibiotics shortly after exposure. The spirochetal bacterium transmits through the Ixodes vector colonizing the dermis of a mammalian host prior to hematogenous dissemination and invasion of distal tissues all the while combating the immune response as it traverses through its pathogenic lifecycle. The innate immune response controls the borrelial burden in the dermis, but is unable to clear the infection and thereby prevent progression of disease. Dissemination in the mammalian host requires temporal regulation of virulence determinants to allow for vascular interactions, invasion, and colonization of distal tissues. Virulence determinants and/or adhesins are highly heterogenetic among environmental B. burgdorferi strains with particular genotypes being associated with the ability to disseminate to specific tissues and the severity of disease, but fail to generate cross-protective immunity between borrelial strains. The unique motility of B. burgdorferi rendered by the endoflagella serves a vital function for dissemination and protection from immune recognition. Progress has been made toward understanding the chemotactic regulation coordinating the activity of the two polar localized flagellar motors and their role in borrelial virulence, but this regulation is not yet fully understood. Distinct states of motility allow for dynamic interactions between several B. burgdorferi adhesins and host targets that play roles in transendothelial migration. Transmigration across endothelial and blood-brain barriers allows for the invasion of tissues and elicits localized immune responses. The invasive nature of B. burgdorferi is lacking in proactive mechanisms to modulate disease, such as secretion systems and toxins, but recent work has shown degradation of host extracellular matrices by B. burgdorferi contributes to the invasive capabilities of the pathogen. Additionally, B. burgdorferi may use invasion of eukaryotic cells for immune evasion and protection against environmental stresses. This review provides an overview of B. burgdorferi mechanisms for dissemination and invasion in the mammalian host, which are essential for pathogenesis and the development of persistent infection.

A high-throughput genetic screen identifies previously uncharacterized Borrelia burgdorferi genes important for resistance against reactive oxygen and nitrogen species

Borrelia burgdorferi, the causative agent of Lyme disease in humans, is exposed to reactive oxygen and nitrogen species (ROS and RNS) in both the tick vector and vertebrate reservoir hosts. B. burgdorferi contains a limited repertoire of canonical oxidative stress response genes, suggesting that novel gene functions may be important for protection of B. burgdorferi against ROS or RNS exposure. Here, we use transposon insertion sequencing (Tn-seq) to conduct an unbiased search for genes involved in resistance to nitric oxide, hydrogen peroxide, and tertiary-butyl hydroperoxide in vitro. The screens identified 66 genes whose disruption resulted in increased susceptibility to at least one of the stressors. These genes include previously characterized mediators of ROS and RNS resistance (including components of the nucleotide excision repair pathway and a subunit of a riboflavin transporter), as well as novel putative resistance candidates. DNA repair mutants were among the most sensitive to RNS in the Tn-seq screen, and survival assays with individual Tn mutants confirmed that the putative ribonuclease BB0839 is involved in resistance to nitric oxide. In contrast, mutants lacking predicted inner membrane proteins or transporters were among the most sensitive to ROS, and the contribution of three such membrane proteins (BB0017, BB0164, and BB0202) to ROS sensitivity was confirmed using individual Tn mutants and complemented strains. Further analysis showed that levels of intracellular manganese are significantly reduced in the Tn::bb0164 mutant, identifying a novel role for BB0164 in B. burgdorferi manganese homeostasis. Infection of C57BL/6 and gp91phox-/- mice with a mini-library of 39 Tn mutants showed that many of the genes identified in the in vitro screens are required for infectivity in mice. Collectively, our data provide insight into how B. burgdorferi responds to ROS and RNS and suggests that this response is relevant to the in vivo success of the organism.

Regulation of Gene and Protein Expression in the Lyme Disease Spirochete

The infectious cycle of Borrelia burgdorferi necessitates persistent infection of both vertebrates and ticks, and efficient means of transmission between those two very different types of hosts. The Lyme disease spirochete has evolved mechanisms to sense its location in the infectious cycle, and use that information to control production of the proteins and other factors required for each step. Numerous components of borrelial regulatory pathways have been characterized to date. Their effects are being pieced together, thereby providing glimpses into a complex web of cooperative and antagonistic interactions. In this chapter, we present a broad overview of B. burgdorferi gene and protein regulation during the natural infectious cycle, discussions of culture-based methods for elucidating regulatory mechanisms, and summaries of many of the known regulatory proteins and small molecules. We also highlight areas that are in need of substantially more research.

Enzootic origins for clinical manifestations of Lyme borreliosis

Both early localized and late disseminated forms of Lyme borreliosis are caused by Borrelia burgdorferi senso lato. Differentiating between the spirochetes that only cause localized skin infection from those that cause disseminated infection, and tracing the group of medically-important spirochetes to a specific vertebrate host species, are two critical issues in disease risk assessment and management. Borrelia burgdorferi senso lato isolates from Lyme borreliosis cases with distinct clinical manifestations (erythema migrans, neuroborreliosis, acrodermatitis chronica atrophicans, and Lyme arthritis) and isolates from Ixodes ricinus ticks feeding on rodents, birds and hedgehogs were typed to the genospecies level by sequencing part of the intergenic spacer region. In-depth molecular typing was performed by sequencing eight additional loci with different characteristics (plasmid-bound, regulatory, and housekeeping genes). The most abundant genospecies and genotypes in the clinical isolates were identified by using odds ratio as a measure of dominance. Borrelia afzelii was the most common genospecies in acrodermatitis patients and engorged ticks from rodents. Borrelia burgdorferi senso stricto was widespread in erythema migrans patients. Borrelia bavariensis was widespread in neuroborreliosis patients and in ticks from hedgehogs, but rare in erythema migrans patients. Borrelia garinii was the dominant genospecies in ticks feeding on birds. Spirochetes in ticks feeding on hedgehogs were overrepresented in genotypes of the plasmid gene ospC from spirochetes in erythema migrans patients. Spirochetes in ticks feeding on hedgehogs were overrepresented in genotypes of ospA from spirochetes in acrodermatitis patients. Spirochetes from ticks feeding on birds were overrepresented in genotypes of the plasmid and regulatory genes dbpA, rpoN and rpoS from spirochetes in neuroborreliosis patients. Overall, the analyses of our datasets support the existence of at least three transmission pathways from an enzootic cycle to a clinical manifestation of Lyme borreliosis. Based on the observations with these nine loci, it seems to be justified to consider the population structure of B. burgdorferi senso lato as being predominantly clonal.

Lyme borreliosis

Lyme borreliosis is a tick-borne disease that predominantly occurs in temperate regions of the northern hemisphere and is primarily caused by the bacterium Borrelia burgdorferi in North America and Borrelia afzelii or Borrelia garinii in Europe and Asia. Infection usually begins with an expanding skin lesion, known as erythema migrans (referred to as stage 1), which, if untreated, can be followed by early disseminated infection, particularly neurological abnormalities (stage 2), and by late infection, especially arthritis in North America or acrodermatitis chronica atrophicans in Europe (stage 3). However, the disease can present with any of these manifestations. During infection, the bacteria migrate through the host tissues, adhere to certain cells and can evade immune clearance. Yet, these organisms are eventually killed by both innate and adaptive immune responses and most inflammatory manifestations of the infection resolve. Except for patients with erythema migrans, Lyme borreliosis is diagnosed based on a characteristic clinical constellation of signs and symptoms with serological confirmation of infection. All manifestations of the infection can usually be treated with appropriate antibiotic regimens, but the disease can be followed by post-infectious sequelae in some patients. Prevention of Lyme borreliosis primarily involves the avoidance of tick bites by personal protective measures.

Gene bb0318 Is Critical for the Oxidative Stress Response and Infectivity of Borrelia burgdorferi

A greater understanding of the molecular mechanisms that Borrelia burgdorferi uses to survive during mammalian infection is critical for the development of novel diagnostic and therapeutic tools to improve the clinical management of Lyme disease. By use of an in vivo expression technology (IVET)-based approach to identify B. burgdorferi genes expressed in vivo, we discovered the bb0318 gene, which is thought to encode the ATPase component of a putative riboflavin ABC transport system. Riboflavin is a critical metabolite enabling all organisms to maintain redox homeostasis. B. burgdorferi appears to lack the metabolic capacity for de novo synthesis of riboflavin and so likely relies on scavenging riboflavin from the host environment. In this study, we sought to investigate the role of bb0318 in B. burgdorferi pathogenesis. No in vitro growth defect was observed for the Δbb0318 clone. However, the mutant spirochetes displayed reduced levels of survival when exposed to exogenous hydrogen peroxide or murine macrophages. Spirochetes lacking bb0318 were found to have a 100-fold-higher 50% infectious dose than spirochetes containing bb0318 In addition, at a high inoculum dose, bb0318 was found to be important for effective spirochete dissemination to deep tissues for as long as 3 weeks postinoculation and to be critical for B. burgdorferi infection of mouse hearts. Together, these data implicate bb0318 in the oxidative stress response of B. burgdorferi and indicate the contribution of bb0318 to B. burgdorferi mammalian infectivity.

Lipid peroxidation in the pathogenesis of neuroborreliosis

This study analyzed the onset of lipid peroxidation (LPO) in neuroborreliosis and the effects of ceftriaxone therapy on LPO. Twenty-two patients with early neuroborreliosis and 22 healthy subjects were studied. LPO in the cerebrospinal fluid (CSF), as well as the plasma and urine was estimated by the levels of reactive aldehydes: 4-hydroxynonenal (4-HNE), 4-hydroxyhexenal, malondialdehyde, and 4-oxononenal, F2-isoprostanes and A4/J4-neuroprostanes (NPs). The plasma level of 4-HNE-protein adducts arachidonic acid (AA), docosahexaenoic acid (DHA) and vitamin E was determined. Additionally, enzymatic activities of phospholipase A2 (PLA2), platelet-activating factor acetylhydrolase (PAF-AH) and glutathione peroxidase (GSH-Px) were determined. A decrease of AA, DHA levels and GSH-Px activity in plasma was associated with a significant increase of aldehydes in the CSF, plasma and urine. Similarly, the increase of F2-isoprostanes and NPs in the CSF and plasma was associated with the decreased activity of PLA2 and PAF-AH. Ceftriaxone therapy cured patients and reduced the levels of F2-isoprostanes, NPs and reactive aldehydes. However, the activities of PLA2 and PAF-AH increased. Pathophysiological association of neuroborreliosis with systemic LPO was revealed. Effective antibiotic therapy attenuated LPO. Biomarkers of LPO could be useful to monitor the onset of neuroborreliosis and show the effectiveness of pharmacotherapy.

Borrelia burgdorferi: Carbon Metabolism and the Tick-Mammal Enzootic Cycle

Borrelia burgdorferi, the spirochetal agent of Lyme disease, is a zoonotic pathogen that is maintained in a natural cycle that typically involves mammalian reservoir hosts and a tick vector of the Ixodes species. During each stage of the enzootic cycle, B. burgdorferi is exposed to environments that differ in temperature, pH, small molecules, and most important, nutrient sources. B. burgdorferi has a highly restricted metabolic capacity because it does not contain a tricarboxylic acid cycle, oxidative phosphorylation, or any pathways for de novo biosynthesis of carbohydrates, amino acids, or lipids. Thus, B. burgdorferi relies solely on glycolysis for ATP production and is completely dependent on the transport of nutrients and cofactors from extracellular sources. Herein, pathways for carbohydrate uptake and utilization in B. burgdorferi are described. Regulation of these pathways during the different phases of the enzootic cycle is discussed. In addition, a model for differential control of nutrient flux through the glycolytic pathway as the spirochete transits through the enzootic cycle is presented.

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.

Gene Regulation During the Enzootic Cycle of the Lyme Disease Spirochete

Borrelia burgdorferi, the spirochete that causes Lyme disease, exists in an enzootic cycle, alternating between a tick vector and a vertebrate host. To adapt to and survive the environmental changes associated with its enzootic cycle, including nutrient availability, B. burgdorferi uses three different systems to regulate the expression of genes: RpoN-RpoS, histidine kinase (Hk)1/response regulator 1 (Rrp1), and Rel_Bbu. The RpoN-RpoS alternative sigma factor cascade activates genes required for transmission from the tick to the vertebrate, maintenance of the vertebrate infection, and persistence in the tick. Rel_Bbu controls the levels of the alarmones guanosine pentaphosphate and guanosine tetraphosphate, which are necessary for surviving the nutrient-deficient conditions in the midgut of the tick following absorption of the blood meal and the subsequent molt. The Hk1/Rrp1 two-component system produces cyclic dimeric guanosine monophosphate that regulates the genes required for the transitions between the tick and vertebrate as well as protective responses to the blood meal.

The Fur homologue BosR requires Arg39 to activate rpoS transcription in Borrelia burgdorferi and thereby direct spirochaete infection in mice

Borrelia burgdorferi is the causative agent of Lyme disease. In B. burgdorferi, RpoS controls the expression of virulence genes needed for mammalian infection. The Fur homologue BosR regulates the transcription of rpoS and therefore BosR determines, albeit indirectly, the infection status of the spirochaete. Transcription of rpoS in B. burgdorferi is complex: rpoS can be transcribed either from an RpoD-dependent promoter to yield a long transcript or from an RpoN-dependent promoter to yield a short transcript. This study shows that BosR repressed synthesis of the long transcript while at the same time activating synthesis of the short transcript. How BosR does this is unclear. To address this, spirochaetes were engineered to express either BosR or the naturally occurring variant BosRR39K. Mice became infected by the spirochaetes expressing BosR but not by the spirochaetes expressing BosRR39K. Furthermore, the spirochaetes expressing BosR activated rpoS transcription during growth in culture whereas the spirochaetes expressing BosRR39K did not. Thus, BosR's activation of rpoS transcription somehow involves Arg39. This arginine is highly conserved in other FUR proteins and therefore other FUR proteins may also require this arginine to function.

BadR (BB0693) controls growth phase-dependent induction of rpoS and bosR in Borrelia burgdorferi via recognizing TAAAATAT motifs

In Borrelia burgdorferi (Bb), the alternative sigma factor RpoS plays a central role during Bb's adaptation to ticks and mammals. Previous studies have demonstrated that RpoS is not expressed during the early stages of spirochetal growth or when Bb resides in ticks during the intermolt phase, but the molecular details of these events remain unknown. In the current study, biomagnetic bead separation of rpoS promoter-binding proteins, coupled with genetic inactivation, was employed to identify BadR (BB0693) as a negative regulator that controls growth phase-dependent induction of rpoS and bosR in Bb. When badR was inactivated, the expression of rpoS and bosR was induced only during the early stages of bacterial growth, but not during the stationary growth phase. Recombinant BadR bound to the promoter DNA of rpoS and the regulatory region upstream of bosR via AT-rich TAAAATAT motifs. Mutations in this motif markedly inhibited or abolished rBadR binding. These results suggest that BadR directly influences the expression of both rpoS and bosR in Bb. This newly recognized role for BadR to fine-tune the activation of the RpoN-RpoS pathway at strategic times in Bb's life cycle potentially represents another layer of gene control over σ(54)-dependent gene regulation.

Nanoscopic Localization of Surface-Exposed Antigens of Borrelia burgdorferi

Borrelia burgdorferi sensu lato, the causative agent of Lyme disease, is transmitted to humans through the bite of infected Ixodes spp. ticks. Successful infection of vertebrate hosts necessitates sophisticated means of the pathogen to escape the vertebrates' immune system. One strategy employed by Lyme disease spirochetes to evade adaptive immunity involves a highly coordinated regulation of the expression of outer surface proteins that is vital for infection, dissemination, and persistence. Here we characterized the expression pattern of bacterial surface antigens using different microscopy techniques, from fluorescent wide field to super-resolution and immunogold-scanning electron microscopy. A fluorescent strain of B. burgdorferi spirochetes was labeled with monoclonal antibodies directed against various bacterial surface antigens. Our results indicate that OspA is more evenly distributed over the surface than OspB and OspC that were present as punctate areas.

Stage-specific global alterations in the transcriptomes of Lyme disease spirochetes during tick feeding and following mammalian host adaptation

Borrelia burgdorferi, the agent of Lyme disease, is maintained in nature within an enzootic cycle involving a mammalian reservoir and an Ixodes sp. tick vector. The transmission, survival and pathogenic potential of B. burgdorferi depend on the bacterium's ability to modulate its transcriptome as it transits between vector and reservoir host. Herein, we employed an amplification-microarray approach to define the B. burgdorferi transcriptomes in fed larvae, fed nymphs and in mammalian host-adapted organisms cultivated in dialysis membrane chambers. The results show clearly that spirochetes exhibit unique expression profiles during each tick stage and during cultivation within the mammal; importantly, none of these profiles resembles that exhibited by in vitro grown organisms. Profound shifts in transcript levels were observed for genes encoding known or predicted lipoproteins as well as proteins involved in nutrient uptake, carbon utilization and lipid synthesis. Stage-specific expression patterns of chemotaxis-associated genes also were noted, suggesting that the composition and interactivities of the chemotaxis machinery components vary considerably in the feeding tick and mammal. The results as a whole make clear that environmental sensing by B. burgdorferi directly or indirectly drives an extensive and tightly integrated modulation of cell envelope constituents, chemotaxis/motility machinery, intermediary metabolism and cellular physiology. These findings provide the necessary transcriptional framework for delineating B. burgdorferi regulatory pathways throughout the enzootic cycle as well as defining the contribution(s) of individual genes to spirochete survival in nature and virulence in humans.

BosR functions as a repressor of the ospAB operon in Borrelia burgdorferi

The Lyme disease spirochete, Borrelia burgdorferi, must abundantly produce outer surface lipoprotein A (OspA) in the tick vector but downregulate OspA in mammals in order to evade the immune system and maintain its natural enzootic cycle. Here, we show that BosR binds two regulatory elements of the ospAB operon and that increasing BosR expression leads to downregulation of OspA. Both regulatory sequences, cisI and cisII, showed strong BosR-binding and cisII bound much tighter than cisI. A promoterless bosR gene fused with an inducible promoter was introduced into an rpoS mutant and a wild-type strain to assess RpoS-independent and -dependent downregulation of OspA by BosR. With the induction of BosR expression, OspA expression was reduced more significantly in the RpoS-deficient than wild-type background, but not completely repressed. In the presence of constitutive expression of OspC, DbpA and DbpB, increasing BosR production resulted in complete repression of OspA in the RpoS mutant. Taken together, the study clearly demonstrated BosR serves as a repressor that binds both regulatory elements of the ospAB operon and shuts off expression.

Metal-dependent gene regulation in the causative agent of Lyme disease

Borrelia burgdorferi (Bb) is the causative agent of Lyme disease transmitted to humans by ticks of the Ixodes spp. Bb is a unique bacterial pathogen because it does not require iron (Fe²⁺) for its metabolism. Bb encodes a ferritin-like Dps homolog called NapA (also called BicA), which can bind Fe or copper (Cu²⁺), and a manganese (Mn²⁺) transport protein, Borrelia metal transporter A (BmtA); both proteins are required for colonization of the tick vector, but BmtA is also required for the murine host. This demonstrates that Bb's metal homeostasis is a critical facet of the complex enzootic life cycle between the arthropod and murine hosts. Although metals are known to influence the expression of virulence determinants during infection, it is unknown how or if metals regulate virulence in Bb. Recent evidence demonstrates that Bb modulates the intracellular Mn²⁺ and zinc (Zn²⁺) content and, in turn, these metals regulate gene expression through influencing the Ferric Uptake Regulator (Fur) homolog Borrelia Oxidative Stress Regulator (BosR). This mini-review focuses on the burgeoning study of metal-dependent gene regulation within Bb.

Cyclic Di-GMP receptor PlzA controls virulence gene expression through RpoS in Borrelia burgdorferi

As an obligate pathogen, the Lyme disease spirochete Borrelia burgdorferi has a streamlined genome that encodes only two two-component signal transduction systems, Hk1-Rrp1 and Hk2-Rrp2 (in addition to CheA-CheY systems). The output of Hk1-Rrp1 is the production of the second messenger cyclic di-GMP (c-di-GMP), which is indispensable for B. burgdorferi to survive in the tick vector. The output of Hk2-Rrp2 is the transcriptional activation of the global regulator RpoS, which is essential for the pathogen to accomplish its tick-mouse transmission and to establish mammalian infection. Although evidence indicates that these two systems communicate with each other, how they are connected is not fully understood. In this study, we showed that the c-di-GMP-binding protein PlzA, a downstream effector of Rrp1, positively modulates the production of RpoS, a global regulator and downstream target of Rrp2. Thus, PlzA functions as a connector that links Hk1-Rrp1 with Hk2-Rrp2. We further showed that PlzA regulates rpoS expression through modulation of another regulator, BosR, at both the transcriptional and the posttranscriptional levels. In addition, PlzA was also capable of regulating rpoS expression independently of Rrp1, suggesting that besides being a c-di-GMP-binding protein, PlzA has other functions. Along with the previous finding of PlzA controlling motility, these studies demonstrate that PlzA is a multifunctional protein. These findings further reinforce the notion that B. burgdorferi utilizes its limited signaling systems and regulators to govern multiple cellular processes during its complex enzootic cycle between ticks and mammals.