DNA analysis of Borrelia burgdorferi NCH-1, the first northcentral U.S. human Lyme disease isolate

An atlas of human vector-borne microbe interactions reveals pathogenicity mechanisms

Vector-borne diseases are a leading cause of death worldwide and pose a substantial unmet medical need. Pathogens binding to host extracellular proteins (the "exoproteome") represents a crucial interface in the etiology of vector-borne disease. Here, we used bacterial selection to elucidate host-microbe interactions in high throughput (BASEHIT)-a technique enabling interrogation of microbial interactions with 3,324 human exoproteins-to profile the interactomes of 82 human-pathogen samples, including 30 strains of arthropod-borne pathogens and 8 strains of related non-vector-borne pathogens. The resulting atlas revealed 1,303 putative interactions, including hundreds of pairings with potential roles in pathogenesis, including cell invasion, tissue colonization, immune evasion, and host sensing. Subsequent functional investigations uncovered that Lyme disease spirochetes recognize epidermal growth factor as an environmental cue of transcriptional regulation and that conserved interactions between intracellular pathogens and thioredoxins facilitate cell invasion. In summary, this interactome atlas provides molecular-level insights into microbial pathogenesis and reveals potential host-directed targets for next-generation therapeutics.

Exploring a possibility of using Raman spectroscopy for detection of Lyme disease

Lyme disease (LD), one of the most prevalent tick-borne diseases in the United States (US), is caused by Borreliella burgdorferi sensu stricto (Bb). To date, in the US, LD diagnostics is primarily based on validated two-tiered serological testing, which overall exhibits low sensitivity among other drawbacks. In the present study, a potential of Raman spectroscopy (RS) to detect Bb infection in mice has been explored. For that, C3H mice were infected with wild-type Bb strains, 297, B31, or B31-derived mutant, ∆vlsE. Blood samples taken prior to and post Bb infection were subjected to RS. The data demonstrated that RS did not directly detect Bb spirochetes in blood, but rather sensed biochemical changes associated with Bb infection. Despite Bb infection-associated blood changes detectable by RS were very limited, the partial least square discriminant analysis showed that the average true positive rates were 86% for 297 and 89% for B31 and ∆vlsE.

The BB0345 Hypothetical Protein of Borrelia burgdorferi Is Essential for Mammalian Infection

During the natural enzootic life cycle of Borrelia burgdorferi (also known as Borreliella burgdorferi), the bacteria must sense conditions within the vertebrate and arthropod and appropriately regulate expression of genes necessary to persist within these distinct environments. bb0345 of B. burgdorferi encodes a hypothetical protein of unknown function that is predicted to contain an N-terminal helix-turn-helix (HTH) domain. Because HTH domains can mediate protein-DNA interactions, we hypothesized that BB0345 might represent a previously unidentified borrelial transcriptional regulator with the ability to regulate events critical for the B. burgdorferi enzootic cycle. To study the role of BB0345 within mammals, we generated a bb0345 mutant and assessed its virulence potential in immunocompetent mice. The bb0345 mutant was able to initiate localized infection and disseminate to distal tissues but was cleared from all sites by 14 days postinfection. In vitro growth curve analyses revealed that the bb0345 mutant grew similar to wild-type bacteria in standard Barbour-Stoenner-Kelley II (BSK-II) medium; however, the mutant was not able to grow in dilute BSK-II medium or dialysis membrane chambers (DMCs) implanted in rats. Proteinase K accessibility assays and whole-cell partitioning indicated that BB0345 was intracellular and partially membrane associated. Comparison of protein production profiles between the wild-type parent and the bb0345 mutant revealed no major differences, suggesting BB0345 may not be a global transcriptional regulator. Taken together, these data show that BB0345 is essential for B. burgdorferi survival in the mammalian host, potentially by aiding the spirochete with a physiological function that is required by the bacterium during infection.

Antimicrobial action of calprotectin that does not involve metal withholding

Calprotectin is a potent antimicrobial that inhibits the growth of pathogens by tightly binding transition metals such as Mn and Zn, thereby preventing their uptake and utilization by invading microbes. At sites of infection, calprotectin is abundantly released from neutrophils, but calprotectin is also present in non-neutrophil cell types that may be relevant to infections. We show here that in patients infected with the Lyme disease pathogen Borreliella (Borrelia) burgdorferi, calprotectin is produced in neutrophil-free regions of the skin, in both epidermal keratinocytes and in immune cells infiltrating the dermis, including CD68 positive macrophages. In culture, B. burgdorferi's growth is inhibited by calprotectin, but surprisingly, the mechanism does not involve the classical withholding of metal nutrients. B. burgdorferi cells exposed to calprotectin cease growth with no reduction in intracellular Mn and no loss in activity of Mn enzymes including the SodA superoxide dismutase. Additionally, there is no obvious loss in intracellular Zn. Rather than metal depletion, we find that calprotectin inhibits B. burgdorferi growth through a mechanism that requires physical association of calprotectin with the bacteria. By comparison, calprotectin inhibited E. coli growth without physically interacting with the microbe, and calprotectin effectively depleted E. coli of intracellular Mn and Zn. Our studies with B. burgdorferi demonstrate that the antimicrobial capacity of calprotectin is complex and extends well beyond simple withholding of metal micronutrients.

Assessing the Contribution of an HtrA Family Serine Protease During Borrelia turicatae Mammalian Infection

Tick-borne relapsing fever (TBRF), characterized by recurring febrile episodes, is globally distributed and among the most common bacterial infections in some African countries. Despite the public health concern that this disease represents, little is known regarding the virulence determinants required by TBRF Borrelia during infection. Because the chromosomes of TBRF Borrelia show extensive colinearity with those of Lyme disease (LD) Borrelia, the exceptions represent unique genes encoding proteins that are potentially essential to the disparate enzootic cycles of these two groups of spirochetes. One such exception is a gene encoding an HtrA family protease, BtpA, that is present in TBRF Borrelia, but not in LD spirochetes. Previous work suggested that btpA orthologs may be important for resistance to stresses faced during mammalian infection. Herein, proteomic analyses of the TBRF spirochete, Borrelia turicatae, demonstrated that BtpA, as well as proteins encoded by adjacent genes in the B. turicatae genome, were produced in response to culture at mammalian body temperature, suggesting a role in mammalian infection. Further, transcriptional analyses revealed that btpA was expressed with the genes immediately upstream and downstream as part of an operon. To directly assess if btpA is involved in resistance to environmental stresses, btpA deletion mutants were generated. btpA mutants demonstrated no growth defect in response to heat shock, but were more sensitive to oxidative stress produced by t-butyl peroxide compared to wild-type B. turicatae. Finally, btpA mutants were fully infectious in a murine relapsing fever (RF) infection model. These results indicate that BtpA is either not required for mammalian infection, or that compensatory mechanisms exist in TBRF spirochetes to combat environmental stresses encountered during mammalian infection in the absence of BtpA.

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.

DksA Controls the Response of the Lyme Disease Spirochete Borrelia burgdorferi to Starvation

The pathogenic spirochete Borrelia burgdorferi senses and responds to changes in the environment, including changes in nutrient availability, throughout its enzootic cycle in Ixodes ticks and vertebrate hosts. This study examined the role of DnaK suppressor protein (DksA) in the transcriptional response of B. burgdorferi to starvation. Wild-type and dksA mutant B. burgdorferi strains were subjected to starvation by shifting cultures grown in rich complete medium, Barbour-Stoenner-Kelly II (BSK II) medium, to a defined mammalian tissue culture medium, RPMI 1640, for 6 h under microaerobic conditions (5% CO₂, 3% O₂). Microarray analyses of wild-type B. burgdorferi revealed that genes encoding flagellar components, ribosomal proteins, and DNA replication machinery were downregulated in response to starvation. DksA mediated transcriptomic responses to starvation in B. burgdorferi, as the dksA-deficient strain differentially expressed only 47 genes in response to starvation compared to the 500 genes differentially expressed in wild-type strains. Consistent with a role for DksA in the starvation response of B. burgdorferi, fewer CFU of dksA mutants were observed after prolonged starvation in RPMI 1640 medium than CFU of wild-type B. burgdorferi spirochetes. Transcriptomic analyses revealed a partial overlap between the DksA regulon and the regulon of Rel_Bbu, the guanosine tetraphosphate and guanosine pentaphosphate [(p)ppGpp] synthetase that controls the stringent response; the DksA regulon also included many plasmid-borne genes. Additionally, the dksA mutant exhibited constitutively elevated (p)ppGpp levels compared to those of the wild-type strain, implying a regulatory relationship between DksA and (p)ppGpp. Together, these data indicate that DksA, along with (p)ppGpp, directs the stringent response to effect B. burgdorferi adaptation to its environment.IMPORTANCE The Lyme disease bacterium Borrelia burgdorferi survives diverse environmental challenges as it cycles between its tick vectors and various vertebrate hosts. B. burgdorferi must withstand prolonged periods of starvation while it resides in unfed Ixodes ticks. In this study, the regulatory protein DksA is shown to play a pivotal role controlling the transcriptional responses of B. burgdorferi to starvation. The results suggest that DksA gene regulatory activity impacts B. burgdorferi metabolism, virulence gene expression, and the ability of this bacterium to complete its natural life cycle.

Whole genome sequence and comparative analysis of Borrelia burgdorferi MM1

Lyme disease is caused by spirochaetes of the Borrelia burgdorferi sensu lato genospecies. Complete genome assemblies are available for fewer than ten strains of Borrelia burgdorferi sensu stricto, the primary cause of Lyme disease in North America. MM1 is a sensu stricto strain originally isolated in the midwestern United States. Aside from a small number of genes, the complete genome sequence of this strain has not been reported. Here we present the complete genome sequence of MM1 in relation to other sensu stricto strains and in terms of its Multi Locus Sequence Typing. Our results indicate that MM1 is a new sequence type which contains a conserved main chromosome and 15 plasmids. Our results include the first contiguous 28.5 kb assembly of lp28-8, a linear plasmid carrying the vls antigenic variation system, from a Borrelia burgdorferi sensu stricto strain.

Immune escape strategies of Borrelia burgdorferi

The borrelial resurge demonstrates that Borrelia burgdorferi is a persistent health problem. This spirochete is responsible for a global public health concern called Lyme disease. B. burgdorferi faces diverse environmental conditions of its vector and host during its life cycle. To circumvent the host immune system is a prominent feature of B. burgdorferi. To date, numerous studies have reported on the various mechanisms used by this pathogen to evade the host defense mechanisms. This current review attempts to consolidate this information to describe the immunological and molecular methods used by B. burgdorferi for its survival.

Investigation of ospC Expression Variation among Borrelia burgdorferi Strains

Outer surface protein C (OspC) is the most studied major virulence factor of Borrelia burgdorferi, the causative agent of Lyme disease. The level of OspC varies dramatically among B. burgdorferi strains when cultured in vitro, but little is known about what causes such variation. It has been proposed that the difference in endogenous plasmid contents among strains contribute to variation in OspC phenotype, as B. burgdorferi contains more than 21 endogenous linear (lp) and circular plasmids (cp), and some of which are prone to be lost. In this study, we analyzed several clones isolated from B. burgdorferi strain 297, one of the most commonly used strains for studying ospC expression. By taking advantage of recently published plasmid sequence of strain 297, we developed a multiplex PCR method specifically for rapid plasmid profiling of B. burgdorferi strain 297. We found that some commonly used 297 clones that were thought having a complete plasmid profile, actually lacked some endogenous plasmids. Importantly, the result showed that the difference in plasmid profiles did not contribute to the ospC expression variation among the clones. Furthermore, we found that B. burgdorferi clones expressed different levels of BosR, which in turn led to different levels of RpoS and subsequently, resulted in OspC level variation among B. burgdorferi strains.

Plasmid diversity and phylogenetic consistency in the Lyme disease agent Borrelia burgdorferi

Background

Bacteria from the genus Borrelia are known to harbor numerous linear and circular plasmids. We report here a comparative analysis of the nucleotide sequences of 236 plasmids present in fourteen independent isolates of the Lyme disease agent B. burgdorferi.

Results

We have sequenced the genomes of 14 B. burgdorferi sensu stricto isolates that carry a total of 236 plasmids. These individual isolates carry between seven and 23 plasmids. Their chromosomes, the cp26 and cp32 circular plasmids, as well as the lp54 linear plasmid, are quite evolutionarily stable; however, the remaining plasmids have undergone numerous non-homologous and often duplicative recombination events. We identify 32 different putative plasmid compatibility types among the 236 plasmids, of which 15 are (usually) circular and 17 are linear. Because of past rearrangements, any given gene, even though it might be universally present in these isolates, is often found on different linear plasmid compatibility types in different isolates. For example, the arp gene and the vls cassette region are present on plasmids of four and five different compatibility types, respectively, in different isolates. A majority of the plasmid types have more than one organizationally different subtype, and the number of such variants ranges from one to eight among the 18 linear plasmid types. In spite of this substantial organizational diversity, the plasmids are not so variable that every isolate has a novel version of every plasmid (i.e., there appears to be a limited number of extant plasmid subtypes).

Conclusions

Although there have been many past recombination events, both homologous and nonhomologous, among the plasmids, particular organizational variants of these plasmids correlate with particular chromosomal genotypes, suggesting that there has not been rapid horizontal transfer of whole linear plasmids among B. burgdorferi lineages. We argue that plasmid rearrangements are essentially non-revertable and are present at a frequency of only about 0.65% that of single nucleotide changes, making rearrangement-derived novel junctions (mosaic boundaries) ideal phylogenetic markers in the study of B. burgdorferi population structure and plasmid evolution and exchange.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3553-5) contains supplementary material, which is available to authorized users.

Evidence that BosR (BB0647) Is a Positive Autoregulator in Borrelia burgdorferi

Borrelia burgdorferi survives in nature through a complex tick-mammalian life cycle. During its transit between ticks and mammalian hosts, B. burgdorferi must dramatically alter its outer surface profile in order to interact with and adapt to these two diverse niches. It has been established that the regulator BosR (BB0647) in B. burgdorferi plays important roles in modulating borrelial host adaptation. However, to date, how bosR expression itself is controlled in B. burgdorferi remains largely unknown. Previously, it has been shown that DNA sequences upstream of BosR harbor multiple sites for the binding of recombinant BosR, suggesting that BosR may influence its own expression in B. burgdorferi However, direct experimental evidence supporting this putative autoregulation of BosR has been lacking. Here, we investigated the expression of bosR throughout the tick-mammal life cycle of B. burgdorferi via quantitative reverse transcription (RT)-PCR analyses. Our data indicated that bosR is expressed not only during mouse infection, but also during the tick acquisition, intermolt, and transmission phases. Further investigation revealed that bosR expression in B. burgdorferi is influenced by environmental stimuli, such as temperature shift and pH change. By employing luciferase reporter assays, we also identified two promoters potentially driving bosR transcription. Our study offers strong support for the long-postulated function of BosR as an autoregulator in B. burgdorferi.

Outer surface protein OspC is an antiphagocytic factor that protects Borrelia burgdorferi from phagocytosis by macrophages

Outer surface protein C (OspC) is one of the major lipoproteins expressed on the surface of Borrelia burgdorferi during tick feeding and the early phase of mammalian infection. OspC is required for B. burgdorferi to establish infection in both immunocompetent and SCID mice and has been proposed to facilitate evasion of innate immune defenses. However, the exact biological function of OspC remains elusive. In this study, we showed that the ospC-deficient spirochete could not establish infection in NOD-scid IL2rγ(null) mice that lack B cells, T cells, NK cells, and lytic complement. The ospC mutant also could not establish infection in anti-Ly6G-treated SCID and C3H/HeN mice (depletion of neutrophils). However, depletion of mononuclear phagocytes at the skin site of inoculation in SCID and C3H/HeN mice allowed the ospC mutant to establish infection in vivo. In phagocyte-depleted mice, the ospC mutant was able to colonize the joints and triggered neutrophilia during dissemination. Furthermore, we found that phagocytosis of green fluorescent protein (GFP)-expressing ospC mutant spirochetes by murine peritoneal macrophages and human THP-1 macrophage-like cells, but not in PMN-HL60, was significantly higher than parental wild-type B. burgdorferi strains, suggesting that OspC has an antiphagocytic property. In addition, overproduction of OspC in spirochetes also decreased the uptake of spirochetes by murine peritoneal macrophages. Together, our findings provide evidence that mononuclear phagocytes play a key role in clearance of the ospC mutant and that OspC promotes spirochetes' evasion of macrophages during early Lyme borreliosis.

Evaluation of the Importance of VlsE Antigenic Variation for the Enzootic Cycle of Borrelia burgdorferi

Efficient acquisition and transmission of Borrelia burgdorferi by the tick vector, and the ability to persistently infect both vector and host, are important elements for the life cycle of the Lyme disease pathogen. Previous work has provided strong evidence implicating the significance of the vls locus for B. burgdorferi persistence. However, studies involving vls mutant clones have thus far only utilized in vitro-grown or host-adapted spirochetes and laboratory strains of mice. Additionally, the effects of vls mutation on tick acquisition and transmission has not yet been tested. Thus, the importance of VlsE antigenic variation for persistent infection of the natural reservoir host, and for the B. burgdorferi enzootic life cycle in general, has not been examined to date. In the current work, Ixodes scapularis and Peromyscus maniculatus were infected with different vls mutant clones to study the importance of the vls locus for the enzootic cycle of the Lyme disease pathogen. The findings highlight the significance of the vls system for long-term infection of the natural reservoir host, and show that VlsE antigenic variability is advantageous for efficient tick acquisition of B. burgdorferi from the mammalian reservoir. The data also indicate that the adaptation state of infecting spirochetes influences B. burgdorferi avoidance from host antibodies, which may be in part due to its respective VlsE expression levels. Overall, the current findings provide the most direct evidence on the importance of VlsE for the enzootic cycle of Lyme disease spirochetes, and underscore the significance of VlsE antigenic variation for maintaining B. burgdorferi in nature.

BB0238, a presumed tetratricopeptide repeat-containing protein, is required during Borrelia burgdorferi mammalian infection

The Lyme disease spirochete, Borrelia burgdorferi, occupies both a tick vector and mammalian host in nature. Considering the unique enzootic life cycle of B. burgdorferi, it is not surprising that a large proportion of its genome is composed of hypothetical proteins not found in other bacterial pathogens. bb0238 encodes a conserved hypothetical protein of unknown function that is predicted to contain a tetratricopeptide repeat (TPR) domain, a structural motif responsible for mediating protein-protein interactions. To evaluate the role of bb0238 during mammalian infection, a bb0238-deficient mutant was constructed. The bb0238 mutant was attenuated in mice infected via needle inoculation, and complementation of bb0238 expression restored infectivity to wild-type levels. bb0238 expression does not change in response to varying culture conditions, and thus, it appears to be constitutively expressed under in vitro conditions. bb0238 is expressed in murine tissues during infection, though there was no significant change in expression levels among different tissue types. Localization studies indicate that BB0238 is associated with the inner membrane of the spirochete and is therefore unlikely to promote interaction with host ligands during infection. B. burgdorferi clones containing point mutations in conserved residues of the putative TPR motif of BB0238 demonstrated attenuation in mice that was comparable to that in the bb0238 deletion mutant, suggesting that BB0238 may contain a functional TPR domain.

Identification of lysine residues in the Borrelia burgdorferi DbpA adhesin required for murine infection

Decorin-binding protein A (DbpA) of Borrelia burgdorferi mediates bacterial adhesion to heparin and dermatan sulfate associated with decorin. Lysines K82, K163, and K170 of DbpA are known to be important for in vitro interaction with decorin, and the DbpA structure, initially solved by nuclear magnetic resonance (NMR) spectroscopy, suggests these lysine residues colocalize in a pocket near the C terminus of the protein. In the current study, we solved the structure of DbpA from B. burgdorferi strain 297 using X-ray crystallography and confirmed the existing NMR structural data. In vitro binding experiments confirmed that recombinant DbpA proteins with mutations in K82, K163, or K170 did not bind decorin, which was due to an inability to interact with dermatan sulfate. Most importantly, we determined that the in vitro binding defect observed upon mutation of K82, K163, or K170 in DbpA also led to a defect during infection. The infectivity of B. burgdorferi expressing individual dbpA lysine point mutants was assessed in mice challenged via needle inoculation. Murine infection studies showed that strains expressing dbpA with mutations in K82, K163, and K170 were significantly attenuated and could not be cultured from any tissue. Proper expression and cellular localization of the mutated DbpA proteins were examined, and NMR spectroscopy determined that the mutant DbpA proteins were structurally similar to wild-type DbpA. Taken together, these data showed that lysines K82, K163, and K170 potentiate the binding of DbpA to dermatan sulfate and that an interaction(s) mediated by these lysines is essential for B. burgdorferi murine infection.

Synthesis of RpoS is dependent on a putative enhancer binding protein Rrp2 in Borrelia burgdorferi

The RpoN-RpoS regulatory pathway plays a central role in governing adaptive changes by B. burgdorferi when the pathogen shuttles between its tick vector and mammalian hosts. In general, transcriptional activation of bacterial RpoN (σ54)-dependent genes requires an enhancer binding protein. B. burgdorferi encodes the putative enhancer binding protein Rrp2. Previous studies have revealed that the expression of σ54-dependent rpoS was abolished in an rrp2 point mutant. However, direct evidence linking the production of Rrp2 in B. burgdorferi and the expression of rpoS has been lacking, primarily due to the inability to inactivate rrp2 via deletion or insertion mutagenesis. Herein we introduced a regulatable (IPTG-inducible) rrp2 expression shuttle plasmid into B. burgdorferi, and found that the controlled up-regulation of Rrp2 resulted in the induction of σ54-dependent rpoS expression. Moreover, we created an rrp2 conditional lethal mutant in virulent B. burgdorferi. By exploiting this conditional mutant, we were able to experimentally manipulate the temporal level of Rrp2 expression in B. burgdorferi, and examine its direct impact on activation of the RpoN-RpoS regulatory pathway. Our data revealed that the synthesis of RpoS was coincident with the IPTG-induced Rrp2 levels in B. burgdorferi. In addition, the synthesis of OspC, a lipoprotein required by B. burgdorferi to establish mammalian infection, was rescued in the rrp2 point mutant when RpoS production was restored, suggesting that Rrp2 influences ospC expression indirectly via its control over RpoS. These data demonstrate that Rrp2 is required for the synthesis of RpoS, presumably via its action as an enhancer binding protein for the activation of RpoN and subsequent transcription of rpoS in B. burgdorferi.

CsrA (BB0184) is not involved in activation of the RpoN-RpoS regulatory pathway in Borrelia burgdorferi

Borrelia burgdorferi encodes a homologue of the bacterial carbon storage regulator A (CsrA). Recently, it was reported that CsrA contributes to B. burgdorferi infectivity and is required for the activation of the central RpoN-RpoS regulatory pathway. However, many questions concerning the function of CsrA in B. burgdorferi gene regulation remain unanswered. In particular, there are conflicting reports concerning the molecular details of how CsrA may modulate rpoS expression and, thus, how CsrA may influence the RpoN-RpoS pathway in B. burgdorferi. To address these key discrepancies, we examined the role of CsrA in differential gene expression in the Lyme disease spirochete. Upon engineering an inducible csrA expression system in B. burgdorferi, controlled hyperexpression of CsrA in a merodiploid strain did not significantly alter the protein and transcript levels of bosR, rpoS, and RpoS-dependent genes (such as ospC and dbpA). In addition, we constructed isogenic csrA mutants in two widely used infectious B. burgdorferi strains. When expression of bosR, rpoS, ospC, and dbpA was compared between the csrA mutants and their wild-type counterparts, no detectable differences were observed. Finally, animal studies indicated that the csrA mutants remained infectious for and virulent in mice. Analyses of B. burgdorferi gene expression in mouse tissues showed comparable levels of rpoS transcripts by the csrA mutants and the parental strains. Taken together, these results constitute compelling evidence that CsrA is not involved in activation of the RpoN-RpoS pathway and is dispensable for mammalian infectious processes carried out by B. burgdorferi.

Pyruvate protects pathogenic spirochetes from H2O2 killing

Pathogenic spirochetes cause clinically relevant diseases in humans and animals, such as Lyme disease and leptospirosis. The causative agent of Lyme disease, Borrelia burgdorferi, and the causative agent of leptospirosis, Leptospria interrogans, encounter reactive oxygen species (ROS) during their enzootic cycles. This report demonstrated that physiologically relevant concentrations of pyruvate, a potent H2O2 scavenger, and provided passive protection to B. burgdorferi and L. interrogans against H2O2. When extracellular pyruvate was absent, both spirochetes were sensitive to a low dose of H2O2 (≈0.6 µM per h) generated by glucose oxidase (GOX). Despite encoding a functional catalase, L. interrogans was more sensitive than B. burgdorferi to H2O2 generated by GOX, which may be due to the inherent resistance of B. burgdorferi because of the virtual absence of intracellular iron. In B. burgdorferi, the nucleotide excision repair (NER) and the DNA mismatch repair (MMR) pathways were important for survival during H2O2 challenge since deletion of the uvrB or the mutS genes enhanced its sensitivity to H2O2 killing; however, the presence of pyruvate fully protected ΔuvrB and ΔmutS from H2O2 killing further demonstrating the importance of pyruvate in protection. These findings demonstrated that pyruvate, in addition to its classical role in central carbon metabolism, serves as an important H2O2 scavenger for pathogenic spirochetes. Furthermore, pyruvate reduced ROS generated by human neutrophils in response to the Toll-like receptor 2 (TLR2) agonist zymosan. In addition, pyruvate reduced neutrophil-derived ROS in response to B. burgdorferi, which also activates host expression through TLR2 signaling. Thus, pathogenic spirochetes may exploit the metabolite pyruvate, present in blood and tissues, to survive H2O2 generated by the host antibacterial response generated during infection.

Rrp2, a prokaryotic enhancer-like binding protein, is essential for viability of Borrelia burgdorferi

The Lyme disease spirochete, Borrelia burgdorferi, exists in two diverse niches (i.e., an arthropod tick vector and mammalian host) during its enzootic life cycle. To effectively adapt to these unique environments, the bacterium alters the expression of numerous genes, including several major outer surface (lipo)proteins that are required for infection and transmission. An enhancer-binding protein (EBP), known as Rrp2, is one identified activator of the RpoN/RpoS alternative sigma factor cascade. Because initial efforts to generate an rrp2 deletion strain were unsuccessful, the role of Rrp2 in the activation of the RpoN/RpoS pathway was first defined using a strain of B. burgdorferi carrying an rrp2 point mutant that was defective in its ability to activate RpoN-dependent transcription. The fact that subsequent attempts to disrupt rrp2 have also been unsuccessful has led investigators to hypothesize that Rrp2 has other undefined functions which are essential for B. burgdorferi survival and independent of its EBP function. We used a lac-based inducible expression system to generate a conditional rrp2 mutant in virulent B. burgdorferi. In this strain, an isopropyl-β-D-thiogalactopyranoside-inducible copy of the rrp2 gene is expressed in trans from a borrelial shuttle vector. We found that the chromosomal copy of rrp2 could be inactivated only when rrp2 was induced, and the maintenance of rrp2 expression was required for the growth of the mutants. In addition, the overexpression of rrp2 is detrimental to B. burgdorferi growth in a manner that is independent of the RpoN/RpoS pathway. These studies provide the first direct evidence that rrp2 is an essential gene in B. burgdorferi.

Genome stability of Lyme disease spirochetes: comparative genomics of Borrelia burgdorferi plasmids

Lyme disease is the most common tick-borne human illness in North America. In order to understand the molecular pathogenesis, natural diversity, population structure and epizootic spread of the North American Lyme agent, Borrelia burgdorferi sensu stricto, a much better understanding of the natural diversity of its genome will be required. Towards this end we present a comparative analysis of the nucleotide sequences of the numerous plasmids of B. burgdorferi isolates B31, N40, JD1 and 297. These strains were chosen because they include the three most commonly studied laboratory strains, and because they represent different major genetic lineages and so are informative regarding the genetic diversity and evolution of this organism. A unique feature of Borrelia genomes is that they carry a large number of linear and circular plasmids, and this work shows that strains N40, JD1, 297 and B31 carry related but non-identical sets of 16, 20, 19 and 21 plasmids, respectively, that comprise 33–40% of their genomes. We deduce that there are at least 28 plasmid compatibility types among the four strains. The B. burgdorferi ∼900 Kbp linear chromosomes are evolutionarily exceptionally stable, except for a short ≤20 Kbp plasmid-like section at the right end. A few of the plasmids, including the linear lp54 and circular cp26, are also very stable. We show here that the other plasmids, especially the linear ones, are considerably more variable. Nearly all of the linear plasmids have undergone one or more substantial inter-plasmid rearrangements since their last common ancestor. In spite of these rearrangements and differences in plasmid contents, the overall gene complement of the different isolates has remained relatively constant.

BosR (BB0647) controls the RpoN-RpoS regulatory pathway and virulence expression in Borrelia burgdorferi by a novel DNA-binding mechanism

In Borrelia burgdorferi (Bb), the Lyme disease spirochete, the alternative σ factor σ⁵⁴ (RpoN) directly activates transcription of another alternative σ factor, σ^S (RpoS) which, in turn, controls the expression of virulence-associated membrane lipoproteins. As is customary in σ⁵⁴-dependent gene control, a putative NtrC-like enhancer-binding protein, Rrp2, is required to activate the RpoN-RpoS pathway. However, recently it was found that rpoS transcription in Bb also requires another regulator, BosR, which was previously designated as a Fur or PerR homolog. Given this unexpected requirement for a second activator to promote σ⁵⁴-dependent gene transcription, and the fact that regulatory mechanisms among similar species of pathogenic bacteria can be strain-specific, we sought to confirm the regulatory role of BosR in a second virulent strain (strain 297) of Bb. Indeed, BosR displayed the same influence over lipoprotein expression and mammalian infectivity for strain Bb 297 that were previously noted for Bb strain B31. We subsequently found that recombinant BosR (rBosR) bound to the rpoS gene at three distinct sites, and that binding occurred despite the absence of consensus Fur or Per boxes. This led to the identification of a novel direct repeat sequence (TAAATTAAAT) critical for rBosR binding in vitro. Mutations in the repeat sequence markedly inhibited or abolished rBosR binding. Taken together, our studies provide new mechanistic insights into how BosR likely acts directly on rpoS as a positive transcriptional activator. Additional novelty is engendered by the facts that, although BosR is a Fur or PerR homolog and it contains zinc (like Fur and PerR), it has other unique features that clearly set it apart from these other regulators. Our findings also have broader implications regarding a previously unappreciated layer of control that can be involved in σ⁵⁴–dependent gene regulation in bacteria.

Lyme disease, caused by the bacterium Borrelia burgdorferi (Bb), remains the most common arthropod-borne illness in the United States. A critical strategy for Bb to maintain its presence in nature is adaptation to its diverse tick and mammalian (mouse) hosts. To accomplish this, Bb encodes a potential gene regulator, BB0647 (BosR). Herein, we confirmed that BosR is essential for Bb to establish mammalian infection. We then found that purified recombinant BosR bound to the promoter DNA (regulatory region) of rpoS, suggesting that BosR directly controls the expression of the rpoS gene. This study has revealed a new mechanism of bacterial gene control. The discovery that BosR governs Bb's virulence may lead to new strategies to interrupt the bacterium's complex life cycle.

High-throughput plasmid content analysis of Borrelia burgdorferi B31 by using Luminex multiplex technology

Borrelia burgdorferi, the causative agent of Lyme disease in North America, is an invasive pathogen that causes persistent multiorgan manifestations in humans and other mammals. Genetic studies of this bacterium are complicated by the presence of multiple plasmid replicons, many of which are readily lost during in vitro culture. The analysis of B. burgdorferi plasmid content by plasmid-specific PCR and agarose gel electrophoresis or other existing techniques is informative, but these techniques are cumbersome and challenging to perform in a high-throughput manner. In this study, a PCR-based Luminex assay was developed for determination of the plasmid content of the strain B. burgdorferi B31. This multiplex, high-throughput method allows simultaneous detection of the plasmid contents of many B. burgdorferi strains in a 96-well format. The procedure was used to evaluate the occurrence of plasmid loss in 44 low-passage B. burgdorferi B31 clones and in a library of over 4,000 signature-tagged mutagenesis (STM) transposon mutant clones. This analysis indicated that only 40% of the clones contained all plasmids, with (in order of decreasing frequency) lp5, lp56, lp28-1, lp25, cp9, lp28-4, lp28-2, and lp21 being the most commonly missing plasmids. These results further emphasize the need for careful plasmid analysis in Lyme disease Borrelia studies. Adaptations of this approach may also be useful in the evaluation of plasmid content and chromosomal gene variations in additional Lyme disease Borrelia strains and other organisms with variable genomes and in the correlation of these genetic differences with pathogenesis and other biological properties.

Invariant NKT cell development requires a full complement of functional CD3 zeta immunoreceptor tyrosine-based activation motifs

Invariant NKT (iNKT) cells regulate early immune responses to infections, in part because of their rapid release of IFN-gamma and IL-4. iNKT cells are proposed to reduce the severity of Lyme disease following Borrelia burgdorferi infection. Unlike conventional T cells, iNKT cells express an invariant alphabeta TCR that recognizes lipids bound to the MHC class I-like molecule, CD1d. Furthermore, these cells are positively selected following TCR interactions with glycolipid/CD1d complexes expressed on CD4+CD8+ thymocytes. Whereas conventional T cell development can proceed with as few as 4/10 CD3 immunoreceptor tyrosine-based activation motifs (ITAMs), little is known about the ITAM requirements for iNKT cell selection and expansion. We analyzed iNKT cell development in CD3 zeta transgenic lines with various tyrosine-to-phenylalanine substitutions (YF) that eliminated the functions of the first (YF1,2), third (YF5,6), or all three (YF1-6) CD3 zeta ITAMs. iNKT cell numbers were significantly reduced in the thymus, spleen, and liver of all YF mice compared with wild type mice. The reduced numbers of iNKT cells resulted from significant reductions in the expression of the early growth response 2 and promyelocytic leukemia zinc finger transcription factors. In the mice with few to no iNKT cells, there was no difference in the severity of Lyme arthritis compared with wild type controls, following infections with the spirochete B. burgdorferi. These findings indicate that a full complement of functional CD3 zeta ITAMs is required for effective iNKT cell development.

A manganese transporter, BB0219 (BmtA), is required for virulence by the Lyme disease spirochete, Borrelia burgdorferi

Borrelia burgdorferi (Bb), the causative agent of Lyme disease, is transmitted to mammalian hosts through an arthropod (tick) vector. To establish infection, Bb must acquire essential nutrients, including transition metals, from its mammalian and tick hosts. Thus far, no metal transporter has been identified in Bb. Here, we report the identification of the first metal transporter, BmtA (BB0219), in Bb. BmtA-deficient mutants of virulent Bb were readily generated, and the mutants grew slightly slower than wild-type Bb in vitro. However, BmtA mutants were sensitive to the chelating actions of EDTA, suggesting a role for BmtA in metal utilization. Intracellular accumulation of manganese (Mn) was substantially diminished in the bmtA mutant, indicating that BmtA was operative in Mn uptake. Given that BmtA lacks homology to any known Mn transporter, we postulate that BmtA is part of a novel mechanism for Mn acquisition by a bacterial pathogen. BmtA also was essential to the infectious life cycle of Bb in ticks and mammals, thereby qualifying BmtA as a new borrelial virulence factor. In addition, the bmtA mutant was sensitive to treatment with t-butyl hydroperoxide, implying that BmtA, and thus Mn, is important to Bb for detoxifying reactive oxygen species, including those potentially liberated by immune effector cells during the innate immune response. Our discovery of the first molecule involved in metal transport in Bb provides a foundation for further elucidating metal homeostasis in this important human pathogen, which may lead to new strategies for thwarting Lyme disease.

Transcriptional interplay among the regulators Rrp2, RpoN and RpoS in Borrelia burgdorferi

The RpoN-RpoS alternative sigma factor pathway is essential for key adaptive responses by Borrelia burgdorferi, particularly those involved in the infection of a mammalian host. A putative response regulator, Rrp2, is ostensibly required for activation of the RpoN-dependent transcription of rpoS. However, questions remain regarding the extent to which the three major constituents of this pathway (Rrp2, RpoN and RpoS) act interdependently. To assess the functional interplay between Rrp2, RpoN and RpoS, we employed microarray analyses to compare gene expression levels in rrp2, rpoN and rpoS mutants of parental strain 297. We identified 98 genes that were similarly regulated by Rrp2, RpoN and RpoS, and an additional 47 genes were determined to be likely regulated by this pathway. The substantial overlap between genes regulated by RpoS and RpoN provides compelling evidence that these two alternative sigma factors form a congruous pathway and that RpoN regulates B. burgdorferi gene expression through RpoS. Although several known B. burgdorferi virulence determinants were regulated by the RpoN-RpoS pathway, a defined function has yet to be ascribed to most of the genes substantially regulated by Rrp2, RpoN and RpoS.

Assessment of decorin-binding protein A to the infectivity of Borrelia burgdorferi in the murine models of needle and tick infection

Background

Decorin-binding proteins (Dbps) A and B of Borrelia burgdorferi, the agent of Lyme disease, are surface-exposed lipoproteins that presumably bind to the extracellular matrix proteoglycan, decorin. B. burgdorferi infects various tissues including the bladder, heart, joints, skin and the central nervous system, and the ability of B. burgdorferi to bind decorin has been hypothesized to be important for this disseminatory pathogenic strategy.

Results

To determine the role of DbpBA in the infectious lifecycle of B. burgdorferi, we created a DbpBA-deficient mutant of B. burgdorferi strain 297 and compared the infectious phenotype of the mutant to the wild-type strain in the experimental murine model of Lyme borreliosis. The mutant strain exhibited a 4-log decrease in infectivity, relative to the wild-type strain, when needle inoculated into mice. Upon complementation of the DbpBA-mutant strain with DbpA, the wild-type level of infectivity was restored. In addition, we demonstrated that the DbpBA-deficient mutant was able to colonize Ixodes scapularis larval ticks after feeding on infected mice and persist within the ticks during the molt to the nymphal state. Moreover, surprisingly, the DbpBA-mutant strain was capable of being transmitted to naïve mice via tick bite, giving rise to infected mice.

Conclusion

These results suggest that DbpBA is not required for the natural tick-transmission process to mammals, despite inferences from needle-inoculation experiments implying a requirement for DbpBA during mammalian infection. The combined findings also send a cautionary note regarding how results from needle-inoculation experiments with mice should be interpreted.

bptA (bbe16) is essential for the persistence of the Lyme disease spirochete, Borrelia burgdorferi, in its natural tick vector

Borrelia burgdorferi (Bb), the agent of Lyme disease, is a zoonotic spirochetal bacterium that depends on arthropod (Ixodes ticks) and mammalian (rodent) hosts for its persistence in nature. The quest to identify borrelial genes responsible for Bb's parasitic dependence on these two diverse hosts has been hampered by limitations in the ability to genetically manipulate virulent strains of Bb. Despite this constraint, we report herein the inactivation and genetic complementation of a linear plasmid-25-encoded gene (bbe16) to assess its role in the virulence, pathogenesis, and survival of Bb during its natural life cycle. bbe16 was found to potentiate the virulence of Bb in the murine model of Lyme borreliosis and was essential for the persistence of Bb in Ixodes scapularis ticks. As such, we have renamed bbe16 a gene encoding borrelial persistence in ticks (bpt)A. Although protease accessibility experiments suggested that BptA as a putative lipoprotein is surface-exposed on the outer membrane of Bb, the molecular mechanism(s) by which BptA promotes Bb persistence within its tick vector remains to be elucidated. BptA also was shown to be highly conserved (>88% similarity and >74% identity at the deduced amino acid levels) in all Bb sensu lato strains tested, suggesting that BptA may be widely used by Lyme borreliosis spirochetes for persistence in nature. Given Bb's absolute dependence on and intimate association with its arthropod and mammalian hosts, BptA should be considered a virulence factor critical for Bb's overall parasitic strategy.

Molecualar survival strategies of the Lyme disease spirochete Borrelia burgdorferi

Lyme disease is a tick-transmitted disease caused by the spirochete Borrelia burgdorferi. The bacterium adopts different strategies for its survival inside the immunocompetent host from the time of infection until dissemination in different parts of body tissues. The success of this spirochete depends on its ability to colonise the host tissues and counteract the host's defence mechanisms. During this process borrelia seems to maintain its vitality to ensure long-term survival in the host. Borrelia's proteins are encoded by plasmid and chromosomal genes. These genes are differentially regulated and expressed by different environmental factors in ticks as well as in the mammalian host during infection. In addition, antigenic diversity enables the spirochete to escape host defence mechanisms and maintain infection. In this review we focus on the differential expression of proteins and genes, and further molecular mechanisms used by borrelia to maintain its survival in the host. In light of these pathogenetic mechanisms, further studies on spirochete host interaction are needed to understand the complex interplay that finally lead to host autoimmunity.

The luxS gene is not required for Borrelia burgdorferi tick colonization, transmission to a mammalian host, or induction of disease

luxS mutants of Borrelia burgdorferi strain 297 naturally colonized their arthropod (Ixodes scapularis) vector, were maintained in ticks throughout the molting process (larvae to nymphs), were tick transmitted to uninfected mice, and elicited histopathology in mice indistinguishable from that induced by wild-type B. burgdorferi.

The response regulator Rrp2 is essential for the expression of major membrane lipoproteins in Borrelia burgdorferi

Borrelia burgdorferi (Bb), the agent of Lyme disease, exists in nature through a complex enzootic life cycle that involves both ticks and mammals. As Bb transitions between its two diverse niches, profound adaptive changes occur that are reflected in differential patterns of gene expression, particularly involving lipoprotein genes. Using a mutagenesis approach, we show that Rrp2 (gene BB0763), one of the proteins predicted by the Bb genome (www.tigr.org) to be a response regulator of a two-component sensory transduction system, is a pivotal regulator governing the expression of major membrane lipoproteins such as OspC, DbpA, and Mlp8, as well as many other mammalian infection-associated immunogens of Bb. Sequence analysis additionally suggested that Rrp2 is a bacterial enhancer-binding protein, essential for sigma54-dependent gene activation. Mutagenesis of a key amino acid residue within a putative activation domain revealed that Rrp2 controlled lipoprotein expression by governing the expression of the alternative sigma-factor sigmas in a sigma54-dependent manner. We therefore propose a signal transduction pathway involving Rrp2, sigma54, and sigmas, which in concert control the expression of key lipoproteins and other infection-associated immunogens in Bb.

Regulation of expression of the paralogous Mlp family in Borrelia burgdorferi

The Mlp (multicopy lipoproteins) family is one of many paralogous protein families in Borrelia burgdorferi. To examine the extent to which the 10 members of the Mlp family in B. burgdorferi strain 297 might be differentially regulated, antibodies specific for each of the Mlps were developed and used to analyze the protein expression profiles of individual Mlps when B. burgdorferi replicated under various cultivation conditions. All of the Mlps were upregulated coordinately when B. burgdorferi was cultivated at either elevated temperature, reduced culture pH, or increased spirochete cell density. Inasmuch as the expression of OspC is influenced by a novel RpoN-RpoS regulatory pathway, similar induction patterns for OspC and the Mlp paralogs prompted an assessment of whether the RpoN-RpoS pathway also was involved in Mlp expression. In contrast to wild-type B. burgdorferi, both RpoN- and RpoS-deficient mutants were unable to upregulate OspC or the Mlp paralogs when grown at lower pH (6.8), indicating that pH-mediated regulation of OspC and Mlp paralogs is dependent on the RpoN-RpoS pathway. However, when RpoN- or RpoS-deficient mutants were shifted from 23 degrees C to 37 degrees C or were cultivated to higher spirochete densities, some induction of the Mlps still occurred, whereas OspC expression was abolished. The combined findings suggest that the Mlp paralogs are coordinately regulated as a family and have an expression profile similar, but not identical, to that of OspC. Although Mlp expression as a family is influenced by the RpoN-RpoS regulatory pathway, there exists at least one additional layer of gene regulation, yet to be elucidated, contributing to Mlp expression in B. burgdorferi.

Expression of a luxS gene is not required for Borrelia burgdorferi infection of mice via needle inoculation

The luxS gene product is an integral component of LuxS/autoinducer-2 (AI-2) quorum-sensing systems in bacteria. A putative luxS gene was expressed at comparable levels by Borrelia burgdorferi strain 297 cultivated either in vitro or in dialysis membrane chambers implanted in rat peritoneal cavities. Although the borrelial luxS gene functionally complemented a LuxS deficiency in Escherichia coli DH5 alpha, AI-2-like activity could not be detected within B. burgdorferi culture supernatants or concentrated cell lysates. Finally, a luxS-deficient mutant of B. burgdorferi was infectious at wild-type levels when it was intradermally needle inoculated into mice, indicating that expression of luxS probably is not required for infectivity but, at the very least, is not essential for mammalian host adaptation. Our findings also challenge the notion that a LuxS/AI-2 quorum-sensing system is operative in B. burgdorferi.

Immunological and genetic characterization of Borrelia burgdorferi BapA and EppA proteins

A large majority of examined Lyme disease spirochaete isolates were demonstrated to contain one or both of the paralogous genes bapA and eppA. Immunological analyses of serum samples collected from infected patients coupled with comparative sequence analyses indicated that bapA gene sequences are quite stable but the encoded proteins do not provoke a strong immune response in most individuals. Conversely, EppA proteins are much more antigenic but vary widely in sequence between different bacteria. Considerable evidence of insertion, deletion and other mutations within eppA genes was observed. A number of significant recombination events were also found to have occurred in regions flanking bapA genes, while the genes themselves rarely exhibited evidence of mutation, suggesting strong selective pressure to maintain BapA sequences within narrow limits. Data from these and other studies suggest important roles for BapA and EppA during the Borrelia burgdorferi infectious cycle.

Expression of Borrelia burgdorferi OspC and DbpA is controlled by a RpoN-RpoS regulatory pathway

RpoS and RpoN are two alternative sigma factors typically associated with general stress responses in bacteria. To date, there has been no experimental evidence that RpoS and RpoN can directly control the expression of one another. Herein, using a combined strategy of gene disruption and genetic complementation targeting rpoN and rpoS in Borrelia burgdorferi strain 297, we describe a regulatory network for B. burgdorferi. In this network, RpoN controls the expression of RpoS, which, in turn, governs the expression of two important membrane lipoproteins, outer surface protein C and decorin-binding protein A, and likely other proteins of B. burgdorferi. Our findings provide a foundation for elucidating further key regulatory networks that potentially impact many aspects of B. burgdorferi's parasitic strategy, host range, and virulence expression.

Influence of cultivation media on genetic regulatory patterns in Borrelia burgdorferi

Barbour-Stoenner-Kelly II (BSKII) medium and BSKH medium both are routinely used for the cultivation of Borrelia burgdorferi. However, heretofore there have been no studies to compare how these two media affect gene expression patterns in virulent B. burgdorferi. In the present study, we found that some B. burgdorferi strain 297 genes (e.g., ospA, mlp-7A, mlp-8, p22, and lp6.6) that typically are regulated by temperature or pH displayed their predicted pattern of expression when B. burgdorferi was cultivated in BSKH medium; this was not true when spirochetes were cultivated in conventional BSKII medium. The results suggest that BSKH medium is superior to BSKII medium for gene expression studies with B. burgdorferi.

A bacterial genome in flux: the twelve linear and nine circular extrachromosomal DNAs in an infectious isolate of the Lyme disease spirochete Borrelia burgdorferi

We have determined that Borrelia burgdorferi strain B31 MI carries 21 extrachromosomal DNA elements, the largest number known for any bacterium. Among these are 12 linear and nine circular plasmids, whose sequences total 610 694 bp. We report here the nucleotide sequence of three linear and seven circular plasmids (comprising 290 546 bp) in this infectious isolate. This completes the genome sequencing project for this organism; its genome size is 1 521 419 bp (plus about 2000 bp of undetermined telomeric sequences). Analysis of the sequence implies that there has been extensive and sometimes rather recent DNA rearrangement among a number of the linear plasmids. Many of these events appear to have been mediated by recombinational processes that formed duplications. These many regions of similarity are reflected in the fact that most plasmid genes are members of one of the genome's 161 paralogous gene families; 107 of these gene families, which vary in size from two to 41 members, contain at least one plasmid gene. These rearrangements appear to have contributed to a surprisingly large number of apparently non-functional pseudogenes, a very unusual feature for a prokaryotic genome. The presence of these damaged genes suggests that some of the plasmids may be in a period of rapid evolution. The sequence predicts 535 plasmid genes >/=300 bp in length that may be intact and 167 apparently mutationally damaged and/or unexpressed genes (pseudogenes). The large majority, over 90%, of genes on these plasmids have no convincing similarity to genes outside Borrelia, suggesting that they perform specialized functions.

Characterization of cp18, a naturally truncated member of the cp32 family of Borrelia burgdorferi plasmids

We have mapped the genes encoding the antigenic lipoproteins OspE and OspF to an approximately 18-kb circular plasmid in Borrelia burgdorferi N40. Sequencing and restriction mapping have revealed that this plasmid, cp18, is homologous to an 18-kb region of the cp32 circular plasmids found in the Lyme disease spirochetes. Our data show that cp18 may have arisen from an ancestral cp32 plasmid by deletion of a 14-kb region of DNA, indicating that a significant portion of the cp32 plasmid is not essential in cis for plasmid maintenance. These findings suggest that a relatively small recombinant plasmid capable of being stably maintained in B. burgdorferi could be constructed from a cp32 plasmid.

A cheA cheW operon in Borrelia burgdorferi, the agent of Lyme disease

Borrelia burgdorferi sensu stricto homologues of cheA and cheW were cloned and characterized. A combination of strategies such as polymerase chain reaction (PCR) using degenerate primers, random-primed gene walking PCR and construction of a lamda library were used to identify the putative cheA gene. Sequence analysis of the DNA fragments obtained from the CT strain identified a 2,592-bp open reading frame (ORF) encoding an 864-amino-acid protein with significant similarity (53-64.6% identical residues) to the CheA of several genera of eubacteria. In particular, hallmarks of a histidine kinase family were found such as the location of the histidine autophosphorylation domain very close to the NH2 terminus and the nucleotide-binding site. A second ORF located immediately downstream from the putative borrelial cheA gene encoded a 195-amino-acid protein which displayed a high level of similarity to bacterial CheW. Using reverse transcription PCR, we demonstrated that cheA and cheW form an operon with an upstream, unidentified ORF. The cheA and cheW homologues were located at 722-737 kbp, 738-768 kbp and 743-824 kbp on the linear chromosomes of B. burgdorferi sensu stricto, B. garinii and B. afzelii, respectively. Identification of cheA and cheW is the first step toward elucidation of a possible role of chemotaxis in virulence of the Lyme disease borreliae.

PCR-restriction fragment length polymorphism analysis of the ospC gene for detection of mixed culture and for epidemiological typing of Borrelia burgdorferi sensu stricto

Restriction fragment length polymorphism (RFLP) analysis of the outer surface protein C (ospC) gene amplicon was used for rapid screening for genetic variability within Borrelia burgdorferi sensu stricto species and for detection of multiple borreliae in culture. Primers for the ospC gene amplified a fragment of about 600 bp from Borrelia cultures. After cleavage of the amplified products by MboI and DraI, eight different RFLP types were found among 13 B. burgdorferi sensu stricto strains from various sources and geographical areas, and three RFLP types were found among 10 representative isolates from skin biopsy specimens taken from patients residing on the eastern end of Long Island, New York (B. W. Berger, R. C. Johnson, C. Kodner, and L. Coleman, J. Clin. Microbiol. 30:359-361, 1992). These results suggested that the DNA organization of B. burgdorferi sensu stricto is heterogeneous not only globally but also within a localized geographical area and that the ospC-based typing approach could differentiate the B. burgdorferi sensu stricto. From the results obtained using mixed cultures of two different RFLP types of B. burgdorferi sensu stricto, contamination of at least 0.5% of different types of Borrelia cells in culture could be detected. This method could detect a multiple-B. burgdorferi sensu stricto infection in the bladders of mice experimentally infected with two different RFLP type strains. The present study showed that RFLP analysis of ospC-PCR products is a reliable method for epidemiological typing of B. burgdorferi sensu stricto and could be used for rapid detection of mixed Borrelia culture and multiple B. burgdorferi sensu stricto infections in animals, ticks, and patients.

Correlation of plasmids with infectivity of Borrelia burgdorferi sensu stricto type strain B31

The correlation of plasmid profiles with infectivity was investigated by using five clones of Borrelia burgdorferi sensu stricto strain B31 (ATCC 35210). Plasmid profiles were determined by pulsed-field and two-dimensional gel electrophoresis. The 50% infectious dose (ID50) in hamsters was determined. The ID50 of the clone that possessed a full complement of eight linear and three circular plasmids was 10(3) cells. The loss of the 27.5- and 40-kb linear plasmids did not decrease the infectivity of these cells. Rather, the loss of the 27.5-kb linear plasmid was associated with a more disseminated infection. A moderate decrease of the ID50 from 10(3) to 10(5) cells correlated with the loss of the 9.0-kb circular plasmid and the 27.5-kb linear plasmid. A major loss of infectivity (ID50 > 10(3) cells) occurred with cells that lost the 24.7- and 27.5-kb linear plasmids and the 9.0-kb circular plasmid. A 3.0-kb HindIII fragment of the 24.7-kb linear plasmid was used as a probe to determine the presence of the homologous sequences in the three genospecies of Lyme disease spirochetes. An analysis of 21 infectious strains of B. burgdorferi sensu stricto, B. garinii, and B. afzelii revealed a consistent association of infectivity with strains possessing a linear plasmid (size range, 24 to 36 kb) that hybridized with the HindIII fragment. Western immunoblotting with hamster antisera against infectious B31 clone C-3 revealed two proteins with molecular masses of 28 and 43 kDa that were absent in the noninfectious B31 clone C-1. Additionally, a 14-kDa protein was absent in C-1 but present in infectious clone C-9 as shown by two-dimensional polyacrylamide gel electrophoresis.

Analysis of linear plasmid dimers in Borrelia burgdorferi sensu lato isolates: implications concerning the potential mechanism of linear plasmid replication

The Borrelia genome is composed of a linear chromosome and a number of variable circular and linear plasmids. Atypically large linear plasmids of 92 to 105 kb have been identified in several Borrelia burgdorferi sensu lato isolates and characterized. These plasmids carry the p27 and ospAB genes, which in other isolates reside on a 50-kb plasmid. Here we demonstrate that these plasmids are dimers of the 50-kb ospAB plasmid (pAB50). The 94-kb plasmid from isolate VS116, pVS94, was an exception and did not hybridize with any plasmid gene probes. When this plasmid was used as a probe, homologous sequences in other isolates were not detected, suggesting that it is unique to isolate VS116. These analyses provide insight into the mechanism of linear plasmid replication and the mechanisms by which plasmid variability can arise.

Molecular characterization of Borrelia burgdorferi sensu lato from Slovenia revealing significant differences between tick and human isolates

One hundred twenty-nine Slovenian isolates of Borrelia burgdorferi sensu lato derived from patients (69 strains) or Ixodes ricinus ticks (60 strains) were characterized. All of the strains were first- or second-passage isolates obtained in 1992 and 1993 from the same endemic region. The techniques used for the molecular analysis of strains included species-specific polymerase chain reaction (PCR) typing, and pulsed-field gel electrophoretic separation of undigested and MluI-digested genomic DNA. Isolates were identified to the species level by large restriction fragment pattern (LRFP) analysis and the results compared with the species-specific PCR result. Fifty-two patient isolates (75%) were typed as Borrelia afzelii (LRFP MLa1), 6 (9%) as Borrelia garinii (LRFPs MLg1-4), and 11 (16%) as Borrelia burgdorferi sensu stricto. The latter included 9 isolates (13%) with a new LRFP that is not typical of Borrelia burgdorferi sensu stricto and for which the designation MLx is suggested. In contrast, only 32 of 60 (53%) tick isolates were typed as Borrelia afzelii, while 20 strains (33%) were typed as Borrelia garinii and 8 strains (13%) as Borrelia burgdorferi sensu stricto. Three new LRFPs were found among the Borrelia garinii (MLg5 and 6) and Borrelia burgdorferi sensu stricto (MLb15) tick isolates. Large restriction fragment pattern analysis identified new groups of Borrelia burgdorferi sensu lato and revealed an apparent difference in the isolation frequency of different species from patients and ticks in the same endemic region.

Analysis and comparison of plasmid profiles of Borrelia burgdorferi sensu lato strains

The relationship between plasmid profiles and genospecies of the Lyme disease borreliae was investigated by using 40 strains from diverse biological and geographical sources. The genospecies of the strains were determined by examination of rRNA gene restriction patterns with cDNA probes complementary to the 16S and 23S rRNAs of Escherichia coli. Plasmid profiles were obtained by pulsed-field gel electrophoresis. The number of plasmids per strain and the size of these plasmids ranged from 4 to 10 and from 13.3 to 57.7 kb, respectively. The strains all contained a single large plasmid of 50 to 57.7 kb, with the exception of two Borrelia garinii strains that contained two or three of the large plasmids. The large plasmids of Borrelia burgdorferi sensu stricto strains ranged in size from 51.4 to 52.7 kb and were consistently smaller than the 54.0- to 57.7-kb plasmids present in B. garinii and Borrelia afzelii. The exceptions of this observation were the two B. garinii strains with multiple large plasmids; in this case the large plasmids were 50.6 to 53 kb. Although a large degree of heterogeneity in the sizes and frequencies of occurrence of smaller plasmids was observed, there were some differences among the three genospecies. The differences in plasmids were further studied by using two BamHI DNA fragments from a 28.7-kb plasmid of B. burgdorferi sensu stricto 297 as probes. Both probes hybridized with the 27- to 29-kb plasmids of B. burgdorferi sensu stricto strains. In contrast, two patterns of hybridization were observed with B. garinii and B. afzelii. One pattern was the hybridization of both probes, with each probe hybridizing with a different plasmid. The other pattern was the hybridization of a plasmid of a strain with only one of the two probes. some strains of the three genospecies did not hybridize with either probe. Our results suggest that the plasmid profiles of B. burgdorferi sensu lato have genospecies characteristics and that the hybridization patterns of similar-sized plasmids are different for the three genospecies.

Genotypic and phenotypic characterization of Borrelia burgdorferi isolated from ticks and small animals in Illinois

We have characterized 33 isolates of Borrelia burgdorferi from northern Illinois (32 isolates) and Wisconsin (1 isolate) representing the largest series of midwestern isolates investigated to date. The techniques used for molecular analysis of strains included (i) genospecies typing with species-specific PCR primers, (ii) plasmid profiling by pulsed-field gel electrophoresis of total genomic DNA, (iii) large-restriction-fragment pattern (LRFP) analysis by pulsed-field gel electrophoresis of MluI-digested genomic DNA (J. Belfaiza, D. Postic, E. Bellenger, G. Baranton, and I. Saint Girons, J. Clin. Microbiol. 31:2873-2877, 1993), (iv) sodium dodecyl sulfate-polyacrylamide gel electrophoresis of total proteins, (v) microsequencing of high-performance liquid chromatography-purified peptides derived from proteins showing high levels of expression, (vi) amino acid composition analysis of proteins, and (vii) immunological analysis of proteins with a polyclonal antiserum of human origin. Five reference strains as well as two atypical tick isolates from California (DN127) and New York (25015) were included for comparison. All of the Illinois and Wisconsin isolates were typed as B. burgdorferi sensu stricto with genospecies-specific PCR primers. The isolates were found to be heterogeneous with regard to their plasmid and protein profiles. One isolate from Illinois possessed two large-molecular-size plasmids instead of the usual 49-kb plasmid. Fragment patterns resulting from MluI digestion of genomic DNA from the 33 isolates and strains DN127 and 25015 were separable into six distinct LRFPs, five of which have not previously been described. Strain 25015 and an isolate from Illinois (CT39) shared an unusual LRFP that is not typical of other B. burgdorferi sensu stricto strains, suggesting that they may represent a fifth species of B. burgdorferi sensu lato. Five of the 33 isolates and strains DN127 and 25015 showed high-level expression of proteins with molecular masses of approximately 22 kDa. Investigation of these proteins by microsequencing of individual peptides and total amino acid composition analysis indicated that the 22-kDa proteins expressed by the seven strains were polymorphic OspC proteins. By using a polyclonal serum of human origin, expression of OspC could be detected in all 33 Illinois and Wisconsin isolates.

Pulsed field gel electrophoresis analysis of Borrelia burgdorferi sensu lato isolated in Japan and taxonomic implications with Lyme disease spirochetes

Genomic DNAs of Borrelia burgdorferi sensu lato isolates obtained in Japan sharing different rRNA gene ribotypes were digested with rare-cutter restriction endonucleases and the fragments obtained were separated by pulsed field gel electrophoresis (PFGE). The sizes of large restriction cleavage bands with MluI endonuclease were quite similar among isolates in each ribotype group. On the other hand, the PFGE profiles obtained with the other enzymes (NruI, Sal I or SplI) were rather divergent, and Japanese isolates were distinguishable from the United States and European isolates. The Japanese isolates classified as ribotypes group II (Borrelia garinii) and III (B. afzelii) showed different PFGE patterns from that of European isolates. The isolates grouped into ribotype IV revealed distinctively different PFGE profiles. These results indicate that the Japanese isolates may be genetically divergent and distinct from the United States and European isolates.

Isolation of Borrelia burgdorferi from saliva of the tick vector, Ixodes scapularis

A method for cultivating and isolating Lyme disease spirochetes, Borrelia burgdorferi, from the saliva of vector ticks, Ixodes scapularis (formerly known as Ixodes dammini), is described. Saliva was collected from partially engorged ticks after application of pilocarpine to induce salivation. B. burgdorferi was isolated from 8 of 14 (57%) of the saliva samples derived from ticks infected with the bacteria, as determined by direct immunofluorescent-antibody assay of tick hemolymph. A comparison of the protein profiles of the salivary isolates and a highly passaged strain (B31) showed that the salivary isolates all lacked a 22-kDa protein known to increase with continuous passage, but exhibited larger amounts of the OspA and OspB proteins than did the highly passaged B31 strain.

Protective immunity is induced by a Borrelia burgdorferi mutant that lacks OspA and OspB

A mutant of virulent Borrelia burgdorferi 297 was apparently selected for by long-term storage at 5 degrees C. This mutant was found to lack the plasmid which encodes outer surface protein A (OspA) and OspB. In addition to the loss of the OspA and OspB proteins, the mutant lacked two lipoproteins, of 20 and 7.5 kDa, that were observed in the wild type. Since the mutant was not recovered from the tissues or blood of hamsters injected with the mutant, the mutant was determined to be noninfectious. Hamsters vaccinated with noninfectious mutant 297 were protected completely from challenge with virulent wild-type 297 spirochetes. Prechallenge sera from hamsters vaccinated with mutant 297 lacked antibodies to OspA and OspB, while those from hamsters vaccinated with virulent wild-type 297 or avirulent 297 exhibited antibodies to these proteins. Hamsters vaccinated with virulent wild-type 297 or mutant 297 elicited antibodies to OspC and a 39-kDa protein (P39), whereas hamsters vaccinated with avirulent 297 lacked these antibodies. These results suggest that OspC and/or P39 are important for the development of a protective immune response. Study of this mutant may elucidate factors important to the development of a Lyme disease vaccine.

Comparative analysis of genetic variability among Borrelia burgdorferi isolates from Europe and the United States by restriction enzyme analysis, gene restriction fragment length polymorphism, and pulsed-field gel electrophoresis

The genomes of 62 North American and European Borrelia burgdorferi isolates were examined by restriction endonuclease analysis (REA), gene probe restriction fragment length polymorphism, and pulsed-field gel electrophoresis (PFGE). Hybridization of restriction fragments with the immunologically relevant 83-kDa antigen gene revealed polymorphisms and divided the isolates into three major groups. Group I included all but two of the American isolates and some of the European isolates. One of two Californian isolates (DN 127) and one Ixodes dammini isolate from New York (strain 25015), previously described as atypical, were distinct from the isolates in the three groups. Plasmid profile analysis and REA, the method with the highest level of discrimination, revealed extensive heterogeneity among isolates of the same major group. Our study demonstrates the usefulness of the 83-kDa antigen gene probe for dividing the isolates into major genogroups, whereas REA and plasmid profile analysis allow for a distinction of individual strains within these groups.

Genetic diversity among Borrelia burgdorferi isolates from wood rats and kangaroo rats in California

Twenty-nine Borrelia burgdorferi isolates, obtained from dusky-footed wood rats (Neotoma fuscipes) and California kangaroo rats (Dipodomys californicus) in California, were analyzed genetically. Chromosomal DNA was examined by restriction endonuclease analysis (REA) and gene probe restriction fragment length polymorphism. Pulsed-field gel electrophoresis was used to analyze the plasmid profiles of the isolates. REA, the method with the greatest discrimination, disclosed 24 distinct restriction patterns among the 29 isolates. These restriction patterns were sorted into four restriction fragment length polymorphism groups on the basis of their gene hybridization patterns. Results of the REA and plasmid profile analysis supported this grouping. The degree of genetic diversity among Californian isolates demonstrated by our findings is greater than that previously reported among other groups of North American isolates and is similar or greater than the diversity reported among European isolates.