Decreased infectivity in Borrelia burgdorferi strain B31 is associated with loss of linear plasmid 25 or 28-1

Characterization and in vitro reaction properties of 19 unique hairpin telomeres from the linear plasmids of the lyme disease spirochete

The genome of the Lyme disease pathogen Borrelia burgdorferi contains about a dozen linear DNA molecules that carry covalently closed hairpin telomeres as a specialized mechanism for dealing with the end-replication problem. The hairpin telomeres are generated from replicative intermediates through a two-step transesterification promoted by the telomere resolvase ResT. Although the genome of B. burgdorferi has been sequenced, the sequence of most telomeres has remained unknown because of difficulties in recovering and completely sequencing the covalently closed hairpin ends. In this study we report a new approach for the direct sequencing Borrelia telomeres and report the sequence, characterization, and in vitro reaction properties of 19 unique telomeres. Surprisingly, a variation of greater than 160-fold in the initial reaction rates of in vitro ResT-mediated telomere resolution was observed between the most active and least active telomeres. Moreover, three of the hairpin telomeres were completely inactive in vitro, but their in vivo functionality was demonstrated. Our results provide important new information on the structure and function of the B. burgdorferi telomeres and suggest the possibility that factors besides the telomere resolvase ResT may influence the reaction in vivo and rescue those telomeres that are not functional in vitro with ResT alone.

Detailed analysis of sequence changes occurring during vlsE antigenic variation in the mouse model of Borrelia burgdorferi infection

Lyme disease Borrelia can infect humans and animals for months to years, despite the presence of an active host immune response. The vls antigenic variation system, which expresses the surface-exposed lipoprotein VlsE, plays a major role in B. burgdorferi immune evasion. Gene conversion between vls silent cassettes and the vlsE expression site occurs at high frequency during mammalian infection, resulting in sequence variation in the VlsE product. In this study, we examined vlsE sequence variation in B. burgdorferi B31 during mouse infection by analyzing 1,399 clones isolated from bladder, heart, joint, ear, and skin tissues of mice infected for 4 to 365 days. The median number of codon changes increased progressively in C3H/HeN mice from 4 to 28 days post infection, and no clones retained the parental vlsE sequence at 28 days. In contrast, the decrease in the number of clones with the parental vlsE sequence and the increase in the number of sequence changes occurred more gradually in severe combined immunodeficiency (SCID) mice. Clones containing a stop codon were isolated, indicating that continuous expression of full-length VlsE is not required for survival in vivo; also, these clones continued to undergo vlsE recombination. Analysis of clones with apparent single recombination events indicated that recombinations into vlsE are nonselective with regard to the silent cassette utilized, as well as the length and location of the recombination event. Sequence changes as small as one base pair were common. Fifteen percent of recovered vlsE variants contained “template-independent” sequence changes, which clustered in the variable regions of vlsE. We hypothesize that the increased frequency and complexity of vlsE sequence changes observed in clones recovered from immunocompetent mice (as compared with SCID mice) is due to rapid clearance of relatively invariant clones by variable region-specific anti-VlsE antibody responses.

Lyme borreliosis is the most common vector-transmitted infection in Europe and North America, and is caused by the spirochete Borrelia burgdorferi and other closely related Borrelia species. Lyme disease Borrelia have an elaborate mechanism for varying the sequence of VlsE, a surface-localized, immunogenic lipoprotein. This antigenic variation is thought to be important in immune evasion and thus in the ability of Lyme disease Borrelia to cause long-term infection. In this study, we examined 1,399 B. burgdorferi clones isolated from infected immunocompetent and immunodeficient mice to gain a better understanding of the rate and variety of VlsE sequence changes that occur during infection. We determined that clones with few or no VlsE sequence changes are rapidly cleared in mice with active immune responses, whereas clones with many VlsE changes persist. The vls antigenic variation system can utilize any of the 15 silent cassette sequences as sequence “donors,” and does not exhibit obvious preferences in the location of changes within the vlsE cassette region or the types of VlsE sequence variations found in different tissues, such as in joints or in the heart. Our findings provide further evidence that the vls locus represents a remarkably robust recombination system and immune evasion mechanism.

Biological aspects of Lyme disease spirochetes: unique bacteria of the Borrelia burgdorferi species group

BACKGROUND

Borrelia burgdorferi sensu lato is a group of at least twelve closely related species some of which are responsible for Lyme disease, the most frequent zoonosis in Europe and the USA. Many of the biological features of Borrelia are unique in prokaryotes and very interesting not only from the medical viewpoint but also from the view of molecular biology.

METHODS

Relevant recent articles were searched using PubMed and Google search tools.

RESULTS AND CONCLUSION

This is a review of the biological, genetic and physiological features of the spirochete species group, Borrelia burgdorferi sensu lato. In spite of a lot of recent articles focused on B. burgdorferi sensu lato, many features of Borrelia biology remain obscure. It is one of the main reasons for persisting problems with prevention, diagnosis and therapy of Lyme disease. The aim of the review is to summarize ongoing current knowledge into a lucid and comprehensible form.

Relapsing fever spirochetes retain infectivity after prolonged in vitro cultivation

Borrelia hermsii and Borrelia burgdorferi, two closely related spirochetes, are the etiological agents of tick-borne relapsing fever and Lyme disease, respectively. Previous studies have shown the loss of infectivity of B. burgdorferi is associated with in vitro cultivation. This diminished infectivity of B. burgdorferi has occurred as early as three in vitro passages, and the loss of plasmids have been observed with these less virulent to noninfective cultures. The effects of long-term in vitro cultivation on B. hermsii have not been investigated. However, understanding the degree of genomic degradation during in vitro cultivation is important for investigating pathogenic mechanisms of spirochetes. In this study, we analyzed the effects of continuous in vitro cultivation on the genomic composition and infectivity of B. hermsii and B. turicatae.We report that all seven isolates of B. hermsii and the one isolate of B. turicatae examined retained infectivity in mice after 1 year of continuous in vitro cultivation. Furthermore, there were few apparent differences in the plasmid profiles after long-term cultivation. Two isolates of B. hermsii remained infective after high passage despite losing a portion of the 200-kb linear plasmid containing the fhbA gene encoding the factor H binding protein. Also, sequence analysis of multiple B. hermsii isolates demonstrated two types of fhbA with complete congruence with the two genomic groups of B. hermsii spirochetes. Therefore, these results suggest that relapsing fever spirochetes are genetically stable during in vitro cultivation, and the fhbA-containing segment of DNA that is lost during cultivation is not required for infection.

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

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

In vitro CpG methylation increases the transformation efficiency of Borrelia burgdorferi strains harboring the endogenous linear plasmid lp56

Borrelia burgdorferi is the causative agent of Lyme disease, the most common vector-borne illness in the Northern hemisphere. Low-passage-number infectious strains of B. burgdorferi exhibit extremely low transformation efficiencies-so low, in fact, as to hinder the genetic study of putative virulence factors. Two putative restriction-modification (R-M) systems, BBE02 contained on linear plasmid 25 (lp25) and BBQ67 contained on lp56, have been postulated to contribute to this poor transformability. Restriction barriers posed by other bacteria have been overcome by the in vitro methylation of DNA prior to transformation. To test whether a methylation-sensitive restriction system contributes to poor B. burgdorferi transformability, shuttle plasmids were treated with the CpG methylase M.SssI prior to the electroporation of a variety of strains harboring different putative R-M systems. We found that for B. burgdorferi strains that harbor lp56, in vitro methylation increased transformation by at least 1 order of magnitude. These results suggest that in vitro CpG methylation protects exogenous DNA from degradation by an lp56-contained R-M system, presumably BBQ67. The utility of in vitro methylation for the genetic manipulation of B. burgdorferi was exemplified by the ease of plasmid complementation of a B. burgdorferi B31 A3 BBK32 kanamycin-resistant (B31 A3 BBK32::Kan(r)) mutant, deficient in the expression of the fibronectin- and glycosaminoglycan (GAG)-binding adhesin BBK32. Consistent with the observation that several surface proteins may promote GAG binding, the B. burgdorferi B31 A3 BBK32::Kan(r) mutant demonstrated no defect in the ability to bind purified GAGs or GAGs expressed on the surfaces of cultured cells.

Borrelia burgdorferi lacking DbpBA exhibits an early survival defect during experimental infection

Several Borrelia burgdorferi genes induced under mammalian host conditions have been purported to be important in Lyme disease pathogenesis based on their binding to host structures. These genes include the dbpBA locus, whose products bind host decorin and glycosoaminoglycans. Recently, the dbpBA genes were reported to be involved in borrelial infectivity. Here we extended the previous observations by using culture and quantitative PCR to evaluate low- and high-dose murine infection by a Delta dbpBA::Gent(r) derivative of B. burgdorferi strain B31. The results indicate that the Delta dbpBA::Gent(r) mutant is attenuated in the ability to initially colonize and then persist in multiple tissues. The mutant exhibited a colonization defect as early as 3 days postinfection, before the development of an adaptive immune response, and after low-dose infection of SCID mice, which are deficient in adaptive immunity. These findings suggest that the inability to adhere to host decorin may promote clearance of B. burgdorferi, presumably via innate immune mechanisms. In a high-dose infection, the mutant disseminated to several tissues, particularly joint tissue, but it was generally cleared from these tissues by 3 weeks postinfection. Finally, following high-dose infection of SCID mice, the dbpBA mutant exhibited only a mild colonization defect, suggesting that the adaptive response is involved in the clearance of the mutant in immunocompetent mice. Taken together, these results suggest that the DbpBA proteins facilitate the colonization of multiple tissues by B. burgdorferi and are required for optimal resistance to both innate and adaptive immune mechanisms following needle inoculation.

Borrelia burgdorferi expression of the bba64, bba65, bba66, and bba73 genes in tissues during persistent infection in mice

Borrelia burgdorferi, the etiological agent of Lyme disease in humans, is vectored between mammalian hosts in nature by Ixodes ticks. The organism adapts to diverse environments encountered throughout the enzootic cycle by differentially expressing essential gene products to survive the specialized conditions, whether in ticks or warm-blooded hosts. However, little is known regarding the identity and/or function of B. burgdorferi genes expressed during colonization of tissues during mammalian infection. Experimental evidence has shown that a group of genes (formerly classified as paralogous gene family 54) contiguously localized on the 54-kilobase linear plasmid of B. burgdorferi, are among the most highly regulated by in vitro conditions resembling mammalian infection. In this study, we employed quantitative reverse transcription-PCR to measure temporal gene expression of a subset of this B. burgdorferi gene family (bba64, bba65, bba66, and bba73) in tissues during chronic murine infection. The goal was to gain insight into the role of these genes in infectivity and pathogenesis by identifying when the genes are induced and whether they are expressed in specific target tissues. B. burgdorferi bba64, bba65, bba66, and bba73 expression was measured from infected mouse tissues relative to expression in in vitro culture conditions at specific times post-infection. bba64 expression was highly upregulated in bladder, heart, and spleen tissues throughout the infection period, contrasting with the sharp downregulation previously observed in ear tissues. bba65, bba66, and bba73 demonstrated upregulated differential expression in various tissues over 1 year post-infection. These results suggest an essential role for these genes in borrelial survival, persistence, and/or pathogenesis.

Deletion of BBA64, BBA65, and BBA66 loci does not alter the infectivity of Borrelia burgdorferi in the murine model of Lyme disease

Borrelia burgdorferi, the causative agent of Lyme disease, alters its gene expression in response to highly disparate environmental signals encountered in its tick vector versus vertebrate hosts. Whole-genome transcriptional profile analysis of B. burgdorferi, propagated in vitro under mammalian-host-specific conditions, revealed significant upregulation of several linear plasmid 54 (lp54)-encoded open reading frames (ORFs). Among these ORFs, BBA64, BBA65, and BBA66 have been shown to be upregulated in response to multiple mammalian-host-specific signals. Recently, we determined that there was no significant difference in the ability of BBA64(-) mutant to infect C3H/HeN mice compared to its isogenic control strains, suggesting that B. burgdorferi might utilize multiple, functionally related determinants to establish infection. We further generated BBA65(-) and BBA66(-) single mutants in a noninfectious, lp25(-) clonal isolate of B. burgdorferi strain B31 (ML23) and complemented them with the minimal region of lp25 (BBE22) required for restoring the infectivity. In addition, we generated a BBA64(-) BBA65(-) BBA66(-) triple mutant using an infectious, clonal isolate of B. burgdorferi strain B31 (5A11) that has all of the infection-associated plasmids. There were no significant differences in the ability to isolate viable spirochetes from different tissues of C3H/HeN mice infected via intradermal needle inoculation with either the individual single mutants or the triple mutant compared to their respective isogenic parental strains at days 21 and 62 postinfection. These observations suggest that B. burgdorferi can establish infection in the absence of expression of BBA64, BBA65, and BBA66 in the murine model of Lyme disease.

Borrelia burgdorferi vlsE antigenic variation is not mediated by RecA

RecA is a key protein linking genetic recombination to DNA replication and repair in bacteria. Previous functional characterization of Borrelia burgdorferi RecA indicated that the protein is mainly involved in genetic recombination rather than DNA repair. Genetic recombination may play a role in B. burgdorferi persistence by generation of antigenic variation. We report here the isolation of a recA null mutant in an infectious B. burgdorferi strain. Comparison of the in vitro growth characteristics of the mutant with those of the wild-type strain under various conditions showed no significant differences. While the RecA mutant was moderately more sensitive to UV irradiation and mitomycin C than the wild-type strain, the lack of RecA abolished allelic exchange in the mutant. Absence of RecA did not affect the ability of the mutant to infect mice. However, the RecA mutant was attenuated for joint infection in competitive-infection assays with the wild-type strain. vlsE sequence variation in mice was observed in both wild-type and RecA mutant spirochetes, indicating that the mechanism of antigenic variation is not homologous genetic recombination.

Presence of multiple variants of Borrelia burgdorferi in the natural reservoir Peromyscus leucopus throughout a transmission season

White-footed mice (Peromyscus leucopus) serve as the principal reservoir for Borrelia burgdorferi and have been shown to remain infected for life. Complex infections with multiple genetic variants of B. burgdorferi occur in mice through multiple exposures to infected ticks or through exposure to ticks infected with multiple variants of B. burgdorferi. Using a combination of cloning and single strand conformation polymorphism (SSCP), B. burgdorferi ospC variation was assessed in serial samples collected from individual P. leucopus during a single transmission season. In individuals with ospC variation, at least seven ospC variants were recognized at each time point. One to four of these variants predominated at each time point; however, the predominant variants seldom remained consistent in an individual mouse throughout the entire sampling period. These results confirmed that mice in southern Maryland were persistently infected with multiple variants of B. burgdorferi throughout the transmission season. However, the presence of multiple ospC variants and the fluctuations in the frequency of these variants indicates that either new ospC variants are regularly introduced to this mouse population and predominate while the existing infections are cleared, or that the variation detected in the genetic profile at different time points reflects a complex mixture of B. burgdorferi populations whose relative frequencies may continually change.

Borrelia burgdorferi complement regulator-acquiring surface protein 2 does not contribute to complement resistance or host infectivity

Borrelia burgdorferi, the pathogen of Lyme disease, cycles in nature through Ixodes ticks and mammalian hosts. At least five Complement Regulator-Acquiring Surface Proteins (BbCRASPs) are produced by B. burgdorferi, which are thought to assist spirochetes in host immune evasion. Recent studies established that BbCRASP-2 is preferentially expressed in mammals, and elicits robust antibody response in infected hosts, including humans. We show that BbCRASP-2 is ubiquitously expressed in diverse murine tissues, but not in ticks, reinforcing a role of BbCRASP-2 in conferring B. burgdorferi defense against persistent host immune threats, such as complement. BbCRASP-2 immunization, however, fails to protect mice from B. burgdorferi infection and does not modify disease, as reflected by the development of arthritis. An infectious BbCRASP-2 mutant was generated, therefore, to examine the precise role of the gene product in spirochete infectivity. Similar to wild type B. burgdorferi, BbCRASP-2 mutants remain insensitive to complement-mediated killing in vitro, retain full murine infectivity and induce arthritis. Quantitative RT-PCR assessment indicates that survivability of BbCRASP-2-deficient B. burgdorferi is not due to altered expression of other BbCRASPs. Together, these results suggest that the function of a selectively expressed B. burgdorferi gene, BbCRASP-2, is not essential for complement resistance or infectivity in the murine host.

The failure of immune response evasion by linear plasmid 28-1-deficient Borrelia burgdorferi is attributable to persistent expression of an outer surface protein

Infectivity and persistence by Borrelia burgdorferi, the etiologic agent of Lyme disease, rely stringently on regulatory events. Among these is the downregulation of lipoprotein antigen expression, exemplified by outer surface protein C (OspC), at the advent of specific immunity in the mammalian host. B. burgdorferi spirochetes that lack the linear plasmid 28-1 (lp28-1) succumb to the host's immune response. We thus explored the notion that these two phenomena were related--that lp28-1(-) organisms fail to downregulate ospC and thus are cleared following the appearance of anti-OspC antibody in the murine host. The lp-28-1(-) isolate and a wild-type (wt) isolate bearing the complete set of plasmids were grown in dialysis membrane chambers that were implanted into rat peritoneal cavities. Analysis of mRNA and protein from these cultures showed that OspC expression levels by lp28-1(-) organisms are abnormally high in vivo. A time course analysis of ospC expression in tissues following infection indicates also that temporal diminution of the dominant antigen OspC is impaired in lp28-1(-) spirochetes. Finally, passive transfer of monoclonal OspC-specific antibody into SCID mice 8 days postinfection cleared lp28-1(-) spirochetes, yet the wt organisms persisted in a majority of animals. These findings indicate that incomplete repression of OspC by lp28-1(-) organisms renders them susceptible to immune-mediated clearance. The lp28-1 plasmid must harbor one or more genes involved in OspC downregulation.

Essential role of the response regulator Rrp2 in the infectious cycle of Borrelia burgdorferi

Alteration of surface lipoprotein profiles is a key strategy that the Lyme disease pathogen, Borrelia burgdorferi, has evolved to be maintained within its enzootic cycle between arthropods and mammals. Accumulated evidence indicates that the central regulatory pathway controlling differential gene expression by B. burgdorferi is the RpoN-RpoS pathway (the sigma(54)-sigma(S) sigma factor cascade). It was previously shown that activation of the RpoN-RpoS pathway is controlled by Rrp2, a two-component response regulator and sigma(54)-dependent transcriptional activator. The role of Rrp2 in the infectious cycle of B. burgdorferi has not been determined heretofore. In this report, we demonstrate that an rrp2 mutant defective in activating sigma(54)-dependent transcription was unable to establish infection in mice, but the rrp2 mutant was capable of surviving within ticks during and after tick feeding. Because the rrp2 mutant was defective in the production of OspC, an outer surface lipoprotein essential for mammalian host infection, we further examined whether the loss of infectivity of the rrp2 mutant was solely due to the inability to produce OspC. While transformation with a shuttle vector carrying ospC under the control of a constitutive flaB promoter restored infection to an ospC mutant in immunodeficient SCID mice, it could not rescue the avirulent phenotype of the rrp2 mutant. These data indicate that, in addition to controlling OspC, Rrp2 controls another factor(s) essential for B. burgdorferi to establish infection in mammals. Furthermore, microarray analyses revealed that 125 and 19 genes were positively and negatively regulated, respectively, by Rrp2, which provides a foundation for future identification of additional Rrp2-dependent virulence determinants in B. burgdorferi.

Biology of infection with Borrelia burgdorferi

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

Purification and properties of the plasmid maintenance proteins from the Borrelia burgdorferi linear plasmid lp17

The Lyme disease spirochete Borrelia burgdorferi carries more plasmids than any other bacterium, many of which are linear with covalently closed hairpin ends. These plasmids have also been referred to as mini-chromosomes and essential genetic elements and are integral components of its segmented genome. We have investigated two plasmid maintenance proteins, BBD14 (the replication initiator) and BBD21 (a presumptive ParA orthologue), encoded by the linear plasmid lp17; these proteins are representatives of paralogous families 62 and 32, respectively. We have purified recombinant 6-his-BBD21 and shown it possesses an ATPase activity. 6-his-BBD14 initially could not be overexpressed in Escherichia coli by itself. It was only effectively overproduced in recombinant form through coexpression with other B. burgdorferi proteins and codon optimization. Although the mechanism for increased production through coexpression is not clear, this method holds promise for expression and purification of other B. burgdorferi proteins, a number of which have remained recalcitrant to purification from E. coli. Finally, we present evidence for the physical interaction of BBD14 and BBD21, a feature suggesting that BBD21 and the paralogous family 32 proteins are more likely involved in DNA replication than functioning as simple ParA orthologues as previously surmised based upon sequence homology. Such a role would not preclude a function in plasmid partitioning through interaction with the replication initiator.

Analysis of the determinants of bba64 (P35) gene expression in Borrelia burgdorferi using a gfp reporter

The bba64 (P35) gene of Borrelia burgdorferi, the agent of Lyme disease, encodes a surface-exposed lipoprotein. The expression of bba64 in vitro is tightly regulated and dependent on several environmental factors. In nature, its expression is induced in the tick vector during feeding and maintained during infection of the vertebrate host. The pattern of expression of bba64 suggests that it imparts a critical function to the pathogen. A previous study has shown that the expression of bba64 is down-regulated in the absence of RpoS, suggesting that the alternative sigma factor may be involved in its expression. A DNA-binding protein has also been shown to specifically recognize a sequence in the 5' regulatory region of the gene. Therefore, the contribution of these putative determinants to the differential expression of bba64 was investigated. The role of RpoS was critically evaluated by genetic complementation of the rpoS mutant using a chromosomally targeted copy of the wild-type gene. The results confirm that RpoS is indeed required for the expression of bba64. The role of the upstream DNA-binding site was examined using bba64 promoter-gfp transcriptional fusions in a shuttle vector. The DNA-binding site was studied by targeting mutations to an inverted repeat sequence (IRS), the most prominent feature within the binding site, as well as by deletion of the entire sequence upstream of the basal promoter. Quantitative assessment of gene expression demonstrated that neither the IRS nor the sequence upstream of the promoter was essential for expression. Moreover, the expression of the reporter (GFP) appeared to remain RpoS-dependent in all cases, based on the co-expression of GFP and OspC in a subpopulation of spirochaetes and the selective expression of GFP in the stationary phase. Collectively, the data indicate that RpoS is the sole determinant of differential bba64 expression in cultured spirochaetes.

Persistence of Borrelia burgdorferi following antibiotic treatment in mice

The effectiveness of antibiotic treatment was examined in a mouse model of Lyme borreliosis. Mice were treated with ceftriaxone or saline solution for 1 month, commencing during the early (3 weeks) or chronic (4 months) stages of infection with Borrelia burgdorferi. Tissues from mice were tested for infection by culture, PCR, xenodiagnosis, and transplantation of allografts at 1 and 3 months after completion of treatment. In addition, tissues were examined for the presence of spirochetes by immunohistochemistry. In contrast to saline solution-treated mice, mice treated with antibiotic were consistently culture negative, but tissues from some of the mice remained PCR positive, and spirochetes could be visualized in collagen-rich tissues. Furthermore, when some of the antibiotic-treated mice were fed on by Ixodes scapularis ticks (xenodiagnosis), spirochetes were acquired by the ticks, as determined based upon PCR results, and ticks from those cohorts transmitted spirochetes to naïve SCID mice, which became PCR positive but culture negative. Results indicated that following antibiotic treatment, mice remained infected with nondividing but infectious spirochetes, particularly when antibiotic treatment was commenced during the chronic stage of infection.

Both decorin-binding proteins A and B are critical for the overall virulence of Borrelia burgdorferi

Both decorin-binding proteins (DbpA and DbpB) of the Lyme disease spirochete Borrelia burgdorferi bind decorin and glycosaminoglycans, two important building blocks of proteoglycans that are abundantly found in the extracellular matrix (ECM) and connective tissues as well as on cell surfaces of mammals. As an extracellular pathogen, B. burgdorferi resides primarily in the ECM and connective tissues and between host cells during mammalian infection. The interactions of B. burgdorferi with these host ligands mediated by DbpA and DbpB potentially influence various aspects of infection. Here, we show that both DbpA and DbpB are critical for the overall virulence of B. burgdorferi in the murine host. Disruption of the dbpBA locus led to nearly a 10(4)-fold increase in the 50% infectious dose (ID50). Complementation of the mutant with either dbpA or dbpB reduced the ID50 from over 10(4) to roughly 10(3) organisms. Deletion of the dbpBA locus affected colonization in all tissues of infected mice. The lack of dbpA alone precluded the pathogen from colonizing the heart tissue, and B. burgdorferi deficient for DbpB was recovered only from 42% of the heart specimens of infected mice. Although B. burgdorferi lacking either dbpA or dbpB was consistently grown from joint specimens of almost all infected mice, it generated bacterial loads significantly lower than the control. The deficiency in either DbpA or DbpB did not reduce the bacterial load in skin, but lack of both significantly did. Taken together, the study results indicate that neither DbpA nor DbpB is essential for mammalian infection but that both are critical for the overall virulence of B. burgdorferi.

Role of the BBA64 locus of Borrelia burgdorferi in early stages of infectivity in a murine model of Lyme disease

Borrelia burgdorferi, the causative agent of Lyme disease, undergoes rapid adaptive gene expression in response to environmental signals encountered during different stages of its life cycle in the arthropod vector or the mammalian host. Among all the plasmid-encoded genes of B. burgdorferi, several linear plasmid 54 (lp54)-encoded open reading frames (ORFs) exhibit the greatest differential expression in response to mammalian host-specific temperature, pH, and other uncharacterized signals. These ORFs include members of the paralogous gene family 54 (pgf 54), such as BBA64, BBA65, and BBA66, present on lp54. In an attempt to correlate transcriptional up-regulation of these pgf 54 members to their role in infectivity, we inactivated BBA64 and characterized the phenotype of this mutant both in vitro and in vivo. There were no major differences in the protein profiles between the BBA64 mutant and the control strains, while immunoblot analysis indicated that inactivation of BBA64 resulted in increased levels of BBA65. Moreover, there was no significant difference in the ability of the BBA64 mutant to infect C3H/HeN mice compared to that of its parental or complemented control strains as determined by culturing of viable spirochetes from infected tissues. However, enumeration of spirochetes using quantitative real-time PCR revealed tissue-specific differences, suggesting a minimal role for BBA64 in the survival of B. burgdorferi in select tissues. Infectivity analysis of the BBA64 mutant suggests that B. burgdorferi may utilize multiple determinants to establish infection in mammalian hosts.

Genetic basis for retention of a critical virulence plasmid of Borrelia burgdorferi

The genome of Borrelia burgdorferi is composed of one linear chromosome and approximately 20 linear and circular plasmids. Although some plasmids are required by B. burgdorferi in vivo, most plasmids are dispensable for growth in vitro. However, circular plasmid (cp) 26 is present in all natural isolates and has never been lost during in vitro growth. This plasmid carries ospC, which is critical for mammalian infection. We previously showed that cp26 encodes essential functions, including the telomere resolvase, ResT, and hence cannot be displaced. Here we identify two additional essential genes on cp26, bbb26 and bbb27, through a systematic attempt to inactivate each open reading frame (ORF). Furthermore, an incompatible plasmid carrying resT, bbb26 and bbb27 could displace cp26. Computational and experimental analyses suggested that both BBB26 and BBB27 are membrane-associated, periplasmic proteins. These data indicate that bbb26 and bbb27 encode essential but possibly redundant functions and that one or the other of these cp26 genes, in addition to resT, is required for bacterial viability. We conclude that the genetic linkage of critical physiological and virulence functions on cp26 is pertinent to its stable maintenance throughout the evolution of B. burgdorferi.

Delineation of species-specific binding properties of the CspZ protein (BBH06) of Lyme disease spirochetes: evidence for new contributions to the pathogenesis of Borrelia spp

Borrelia burgdorferi CspZ (TIGR open reading frame designation, BBH06) is part of a functionally related group of proteins that bind one or more members of the factor H (FH) protein family. In this report we assess the conservation, distribution, properties, and ligand binding abilities of CspZ from the three main Borrelia species associated with Lyme disease infections in humans. CspZ (also referred to as BbCRASP-2 in the literature) was found to be highly conserved at the intraspecies level but divergent at the interspecies level. All CspZ orthologs that originated from B. burgdorferi isolates bound FH from a diverse group of mammals. In contrast, CspZ derived from B. garinii and B. afzelii did not. Regardless of the Borrelia species of origin, all CspZ proteins tested bound to unknown approximately 60-kDa serum proteins produced by different mammals. To further define the molecular basis for the differential binding of CspZ orthologs to host proteins, DNA sequence, truncation, and site-directed mutagenesis analyses were performed. DNA sequence analyses revealed that B. garinii and B. afzelii CspZ orthologs possess a 64-amino-acid N-terminal domain that is absent from B. burgdorferi CspZ. However, binding analyses of recombinant proteins revealed that this domain does not in and of itself influence ligand binding properties. Truncation and mutagenesis analyses further revealed that the key determinants required for ligand binding are discontinuous and that the presentation of the ligand binding pocket is dependent on alpha helices with high coiled-coil formation probability. The data presented here provide insight into the molecular basis of CspZ-ligand interactions and suggest that CspZ orthologs from diverse Borrelia species can contribute to the host-pathogen interaction through their interaction with serum proteins.

The role of VlsE antigenic variation in the Lyme disease spirochete: persistence through a mechanism that differs from other pathogens

The linear plasmid, lp28-1, is required for persistent infection by the Lyme disease spirochete, Borrelia burgdorferi. This plasmid contains the vls antigenic variation locus, which has long been thought to be important for immune evasion. However, the role of the vls locus as a virulence factor during mammalian infection has not been clearly defined. We report the successful removal of the vls locus through telomere resolvase-mediated targeted deletion, and demonstrate the absolute requirement of this lp28-1 component for persistence in the mouse host. Moreover, successful infection of C3H/HeN mice with an lp28-1 plasmid in which the left portion was deleted excludes participation of other lp28-1 non-vls genes in spirochete virulence, persistence and the process of recombinational switching at vlsE. Data are also presented that cast doubt on an immune evasion mechanism whereby VlsE directly masks other surface antigens similar to what has been observed for several other pathogens that undergo recombinational antigenic variation.

The critical role of the linear plasmid lp36 in the infectious cycle of Borrelia burgdorferi

Borrelia burgdorferi, the aetiological agent of Lyme disease, follows a life cycle that involves passage between the tick vector and the mammalian host. To investigate the role of the 36 kb linear plasmid, lp36 (also designated the B. burgdorferi K plasmid), in the infectious cycle of B. burgdorferi, we examined a clone lacking this plasmid, but containing all other plasmids known to be required for infectivity. Our results indicated that lp36 was not required for spirochete survival in the tick, but the clone lacking lp36 demonstrated low infectivity in the mammal. Restoration of lp36 to the mutant strain confirmed that the infectivity defect was due to loss of lp36. Moreover, spirochetes lacking lp36 exhibited a nearly 4-log increase in ID₅₀ relative to the isogenic lp36⁺ clone. The infectivity defect of lp36-minus spirochetes was localized, in part, to loss of the bbk17 (adeC) gene, which encodes an adenine deaminase. This work establishes a vital role for lp36 in the infectious cycle of B. burgdorferi and identifies the bbk17 gene as a component of this plasmid that contributes to mammalian infectivity.

Role of outer surface protein D in the Borrelia burgdorferi life cycle

Borrelia burgdorferi preferentially induces selected genes in mice or ticks, and studies suggest that ospD is down-regulated in response to host-specific signals. We now directly show that ospD expression is generally elevated within Ixodes scapularis compared with mice. We then assessed the importance of OspD throughout the spirochete life cycle by generating OspD-deficient B. burgdorferi and examining the mutant in the murine model of tick-transmitted Lyme borreliosis. The lack of OspD did not influence B. burgdorferi infectivity in mice or the acquisition of spirochetes by I. scapularis. OspD adhered to tick gut extracts in vitro, and the OspD-deficient B. burgdorferi strain had a threefold decrease in colonization of the tick gut in vivo. This decrease, however, did not alter subsequent spirochete transmission during a second blood meal. These data suggest that B. burgdorferi can compensate for the lack of OspD in both ticks and mice and that OspD may have a nonessential, secondary, role in B. burgdorferi persistence within I. scapularis.

Antigenicity and recombination of VlsE, the antigenic variation protein of Borrelia burgdorferi, in rabbits, a host putatively resistant to long-term infection with this spirochete

Borrelia burgdorferi, the Lyme disease pathogen, employs several immune-evasive strategies to survive in mammals. Unlike mice, major reservoir hosts for B. burgdorferi, rabbits are considered to be nonpermissive hosts for persistent infection. Antigenic variation of the VlsE molecule is a probable evasion strategy known to function in mice. The invariable region 6 (IR6) and carboxyl-terminal domain (Ct) of VlsE elicit dominant antibody responses that are not protective, perhaps to function as decoy epitopes that protect the spirochete. We sought to determine if either of these characteristics of VlsE differed in rabbit infection, contributing to its reputed nonpermissiveness. VlsE recombination was observed in rabbits that were given inoculations with either cultured or host-adapted spirochetes. Early observations showed a lack of anti-C6 (a peptide encompassing the IR6 region) response in most rabbits, so the anti-Ct and anti-C6 responses were monitored for 98 weeks. Anti-C6 antibody appeared as late as 20 weeks postinoculation, and the anti-Ct response, evident within the first 2 weeks, oscillated for prolonged periods of time. These observations, together with the recovery of cultivable spirochetes from tissue of one animal at 98 weeks postinoculation, challenge the notion that the rabbit cannot harbour a long-term B. burgdorferi infection.

Identification of an ospC operator critical for immune evasion of Borrelia burgdorferi

Timely expression of the outer surface protein C (OspC) is crucial for the pathogenic strategy of the Lyme disease spirochete Borrelia burgdorferi. The pathogen abundantly expresses OspC during initial infection when the antigen is required, but downregulates when its presence poses a threat to the spirochetes once the anti-OspC humoral response has developed. Here, we show that a large palindromic sequence immediately upstream of the ospC promoter is essential for the repression of ospC expression during murine infection and for the ability of B. burgdorferi to evade specific OspC humoral immunity. Deletion of the sequence completely diminished the ability of B. burgdorferi to avoid clearance by transferred OspC antibody in SCID mice. B. burgdorferi lacking the regulatory element was able to initiate infection but unable to persist in immunocompetent mice. Taken together, the regulatory element immediately upstream of the ospC promoter serves as an operator that may interact with an unidentified repressor(s) to negatively regulate ospC expression and is essential for the immune evasion of B. burgdorferi.

Temperature-induced regulation of RpoS by a small RNA in Borrelia burgdorferi

The alternative sigma factor RpoS (sigma38 or sigmaS) plays a central role in the reciprocal regulation of the virulence-associated major outer surface proteins OspC and OspA in Borrelia burgdorferi, the Lyme disease spirochete. Temperature is one of the key environmental signals controlling RpoS, but the molecular mechanism by which the signal is transduced remains unknown. Herein, we identify and describe a small non-coding RNA, DsrABb, that regulates the temperature-induced increase in RpoS. A novel 5' end of the rpoS mRNA was identified and DsrABb has the potential to extensively base-pair with the upstream region of this rpoS transcript. We demonstrate that B. burgdorferi strains lacking DsrABb do not upregulate RpoS and OspC in response to an increase in temperature, but do regulate RpoS and OspC in response to changes in pH and cell density. Analyses of the rpoS and ospC steady-state mRNA levels in the dsrABb mutant indicate that DsrABb regulates RpoS post-transcriptionally. The 5' and 3' ends of DsrABb were mapped, demonstrating that at least four species exist with sizes ranging from 213 to 352 nucleotides. We hypothesize that DsrABb binds to the upstream region of the rpoS mRNA and stimulates translation by releasing the Shine-Dalgarno sequence and start site from a stable secondary structure. Therefore, we postulate that DsrABb is a molecular thermometer regulating RpoS in Borrelia burgdorferi.

Artificial regulation of ospC expression in Borrelia burgdorferi

Outer surface lipoprotein (Osp) C is a virulence factor required for transmission of the Lyme disease agent, Borrelia burgdorferi. We have constructed an inducible promoter system to study the function and regulation of OspC by integrating regulatory elements from the Escherichia coli lac operon into the B. burgdorferi genome. An inducible promoter (flacp) was constructed by inserting a synthetic lac operator sequence between the transcriptional start site and the ribosomal binding site of the B. burgdorferi flgB promoter; flacp was then used to replace the native ospC and rpoS promoters in B. burgdorferi derivatives that constitutively express the E. coli Lac repressor protein (LacI). In vitro, the expression of ospC and rpoS from flacp was dependent on the inducer isopropyl beta-D-thiogalactopyranoside and was unaffected by temperature or pH, conditions commonly used to mimic different aspects of the B. burgdorferi life cycle. Our results suggest that OspC is essential immediately upon injection into a mouse and OspC expression must be maintained during the early stages of infection. In addition, the mouse infectivity experiment indicates that this system can be used to regulate B. burgdorferi genes in vivo, within the context of an experimental tick-mouse infectious cycle. RpoS is an alternative sigma factor that is required for ospC transcription. However, the role of other temperature-dependent factors has not previously been addressed. Our results with the inducible rpoS strain demonstrate that RpoS alone is sufficient to activate OspC expression, even at 23 degrees C. This is the first functional inducible promoter system developed for use in B. burgdorferi and, for the first time, will provide researchers with the ability to artificially regulate the expression of genes in this pathogenic spirochaete.

Borrelia burgdorferi alters its gene expression and antigenic profile in response to CO2 levels

The etiologic agent of Lyme disease, Borrelia burgdorferi, must adapt to the distinct environments of its arthropod vector and mammalian host during its complex life cycle. B. burgdorferi alters gene expression and protein synthesis in response to temperature, pH, and other uncharacterized environmental factors. The hypothesis tested in this study is that dissolved gases, including CO(2), serve as a signal for B. burgdorferi to alter protein production and gene expression. In this study we focused on characterization of in vitro anaerobic (5% CO(2), 3% H(2), 0.087 ppm O(2)) and microaerophilic (1% CO(2), 3.48 ppm O(2)) growth conditions and how they modulate protein synthesis and gene expression in B. burgdorferi. Higher levels of several immunoreactive proteins, including BosR, NapA, DbpA, OspC, BBK32, and RpoS, were synthesized under anaerobic conditions. Previous studies demonstrated that lower levels of NapA were produced when microaerophilic cultures were purged with nitrogen gas to displace oxygen and CO(2). In this study we identified CO(2) as a factor contributing to the observed change in NapA synthesis. Specifically, a reduction in the level of dissolved CO(2), independent of O(2) levels, resulted in reduced NapA synthesis. BosR, DbpA, OspC, and RpoS synthesis was also decreased with the displacement of CO(2). Quantitative reverse transcription-PCR indicated that the levels of the dbpA, ospC, and BBK32 transcripts are increased in the presence of CO(2), indicating that these putative borrelial virulence determinants are regulated at the transcriptional level. Thus, dissolved CO(2) may be an additional cue for borrelial host adaptation and gene regulation.

Borrelia burgdorferi genetic markers and disseminated disease in patients with early Lyme disease

Three genetic markers of Borrelia burgdorferi have been associated with disseminated disease: the OspC type, the 16S-23S rRNA intergenic spacer type (RST), and vlsE. Here, we modified previous methods so as to identify the three markers by PCR and restriction fragment length polymorphism in parallel, analyzed B. burgdorferi isolates from erythema migrans (EM) skin lesions in 91 patients, and correlated the results with evidence of dissemination. OspC type A was found approximately twice as frequently in patients with disseminated disease, whereas type K was identified approximately twice as often in those without evidence of dissemination, but these trends were not statistically significant. The remaining seven types identified were found nearly equally in patients with or without evidence of dissemination. RST 1 strains were significantly associated with dissemination (P=0.03), whereas RST 2 and RST 3 strains tended to have an inverse association with this outcome. The vlsE gene was identified in all 91 cases, using primer sets specific for an N-terminal sequence of B. burgdorferi strain B31 (vlsEB31) or strain 297 (vlsE297), but neither marker was associated with dissemination. Specific combinations of the three genetic markers usually occurred together. OspC type A was always found with RST 1 and vlsEB31, type K was always identified with RST 2 and more often with vlsE297, and types E and I were almost always found with RST 3 and equally often with vlsEB31 and vlsE297. We conclude that B. burgdorferi strains vary in their capacity to disseminate, but almost all strains isolated from EM lesions sometimes caused disseminated disease.

Identification of potential virulence determinants by Himar1 transposition of infectious Borrelia burgdorferi B31

Lyme disease Borrelia organisms are highly invasive spirochetes that alternate between vertebrate and arthropod hosts and that establish chronic infections and elicit inflammatory reactions in mammals. Although progress has been made in the targeted mutagenesis of individual genes in infectious Borrelia burgdorferi, the roles of the vast majority of gene products in pathogenesis remain unresolved. In this study, we examined the feasibility of using transposon mutagenesis to identify infectivity-related factors in B. burgdorferi. The transformable, infectious strain 5A18 NP1 was transformed with the spirochete-adapted Himar1 transposon delivery vector pMarGent to create a small library of 33 insertion mutants. Single mouse inoculations followed by culture of four tissue sites and serology were used to screen the mutants for infectivity phenotypes. Mutants that appeared attenuated (culture positive at some sites) or noninfectious (negative at all sites) and contained the virulence-associated plasmids lp25 and lp28-1 were examined in more extensive animal studies. Three of these mutants (including those with insertions in the putative fliG-1-encoded flagellar motor switch protein and the guaB-encoded IMP dehydrogenase) were noninfectious, whereas four clones appeared to exhibit reduced infectivity. Serological reactivity in VlsE enzyme-linked immunosorbent assays correlated with the assignment of mutants to the noninfectious or attenuated-infectivity groups. The results of this study indicate that random transposon mutagenesis of infectious B. burgdorferi is feasible and will be of value in studying the pathogenesis of Lyme disease Borrelia.

Enhanced culture of Borrelia garinii and Borrelia afzelii strains on a solid BSK-based medium in anaerobic conditions

The growth of 29 human strains from the three main pathogenic species of Borrelia burgdorferi sensu lato on a solid BSK-based medium was compared in two culture atmospheres: 3% CO(2) air and anaerobiosis. All strains grew under anaerobic conditions, whereas only 13 strains were able to grow in aerobiosis with 3% CO(2) (P<0.001). In the latter condition, 75% of the B. burgdorferi sensu stricto strains grew versus 33% of the B. garinii and B. afzelii strains. These data suggest that, especially for B. garinii and B. afzelii species, anaerobic conditions enhance growth yield and speed of low-passage Borrelia strains.

Transcriptional profiling of Borrelia burgdorferi containing a unique bosR allele identifies a putative oxidative stress regulon

Borrelia burgdorferi regulates gene expression in response to environmental conditions, including temperature, pH, redox potential and host factors. B. burgdorferi encodes a PerR homologue designated BosR, which presumably serves as a global regulator of genes involved in the oxidative stress response. Infectious B. burgdorferi strain B31 is resistant to oxidative stressors in vitro, whereas the non-infectious isolate was sensitive due, in part, to a point mutation that converts an arginine to a lysine at residue 39 of BosR. Subsequent insertional inactivation of this bosRR39K allele (bosRR39K : : kanR) restored resistance to oxidative stressors. These observations suggest that the B. burgdorferi non-infectious bosRR39K : : kanR strain may transcribe genes that are also expressed in infectious B. burgdorferi cells, but are repressed in the bosRR39K background, thus explaining the different oxidative stress phenotypes observed between these isolates. To test this hypothesis, macroarray technology and quantitative RT-PCR were utilized to compare the transcriptional profiles from the isogenic bosRR39K and bosRR39K : : kanR isolates. Array data indicated that 88 ORFs were significantly expressed in the absence of BosRR39K. Since most affected genes mapped to the chromosome, it is likely that these genes define an important physiologic response for B. burgdorferi. Included within the genes identified was the detoxification gene sodA, as well as other loci not overtly linked to oxidative stress. These results suggest that a putative BosR regulon, as defined by the bosRR39K allele, is required to combat toxic oxidative intermediates, but may also be involved in adaptive strategies that are independent of reactive oxygen species.

Immunological and molecular analyses of the Borrelia hermsii factor H and factor H-like protein 1 binding protein, FhbA: demonstration of its utility as a diagnostic marker and epidemiological tool for tick-borne relapsing fever

It has been demonstrated that Borrelia hermsii, a causative agent of relapsing fever, produces a factor H (FH) and FH-like protein 1 (FHL-1) binding protein. The binding protein has been designated FhbA. To determine if FH/FHL-1 binding is widespread among B. hermsii isolates, a diverse panel of strains was tested for the FH/FHL-1 binding phenotype and FhbA production. Most isolates (23/24) produced FhbA and bound FH/FHL-1. Potential variation in FhbA among isolates was analyzed by DNA sequence analyses. Two genetically distinct FhbA types, designated fhbA1 and fhbA2, were delineated, and type-specific PCR primers were generated to allow for rapid differentiation. Pulsed-field gel electrophoresis and hybridization analyses demonstrated that all isolates that possess the gene carry it on a 200-kb linear plasmid (lp200), whereas isolates that lack the gene lack lp200 and instead carry an lp170. To determine if FhbA is antigenic during infection and to assess the specificity of the response, recombinant FhbA1 (rFhbA1) and rFhbA2 were screened with serum from infected mice and humans. FhbA was found to be expressed and antigenic and to elicit a potentially type-specific FhbA response. To localize the epitopes of FhbA1 and FhbA2, truncations were generated and screened with infection serum. The epitopes were determined to be conformationally defined. Collectively, these analyses indicate that FH/FHL-1 binding is a widespread virulence mechanism for B. hermsii and provide insight into the genetic and antigenic structure of FhbA. The data also have potential implications for understanding the epidemiology of relapsing fever in North America and can be applied to the future development of species-specific diagnostic tools.

Differential telomere processing by Borrelia telomere resolvases in vitro but not in vivo

Causative agents of Lyme disease and relapsing fever, including Borrelia burgdorferi and Borrelia hermsii, respectively, are unusual among bacteria in that they possess a segmented genome with linear DNA molecules terminated by hairpin ends, known as telomeres. During replication, these telomeres are processed by the essential telomere resolvase, ResT, in a unique biochemical reaction known as telomere resolution. In this study, we report the identification of the B. hermsii resT gene through cross-species hybridization. Sequence comparison of the B. hermsii protein with the B. burgdorferi orthologue revealed 67% identity, including all the regions currently known to be crucial for telomere resolution. In vitro studies, however, indicated that B. hermsii ResT was unable to process a replicated B. burgdorferi type 2 telomere substrate. In contrast, in vivo cross-species complementation in which the native resT gene of B. burgdorferi was replaced with B. hermsii resT had no discernible effect, even though B. burgdorferi strain B31 carries at least two type 2 telomere ends. The B. burgdorferi ResT protein was also able to process two telomere spacing mutants in vivo that were unresolvable in vitro. The unexpected differential telomere processing in vivo versus in vitro by the two telomere resolvases suggests the presence of one or more accessory factors in vivo that are normally involved in the reaction. Our current results are also expected to facilitate further studies into ResT structure and function, including possible interaction with other Borrelia proteins.

Transcriptional regulation of the Borrelia burgdorferi antigenically variable VlsE surface protein

The Lyme disease agent Borrelia burgdorferi can persistently infect humans and other animals despite host active immune responses. This is facilitated, in part, by the vls locus, a complex system consisting of the vlsE expression site and an adjacent set of 11 to 15 silent vls cassettes. Segments of nonexpressed cassettes recombine with the vlsE region during infection of mammalian hosts, resulting in combinatorial antigenic variation of the VlsE outer surface protein. We now demonstrate that synthesis of VlsE is regulated during the natural mammal-tick infectious cycle, being activated in mammals but repressed during tick colonization. Examination of cultured B. burgdorferi cells indicated that the spirochete controls vlsE transcription levels in response to environmental cues. Analysis of PvlsE::gfp fusions in B. burgdorferi indicated that VlsE production is controlled at the level of transcriptional initiation, and regions of 5' DNA involved in the regulation were identified. Electrophoretic mobility shift assays detected qualitative and quantitative changes in patterns of protein-DNA complexes formed between the vlsE promoter and cytoplasmic proteins, suggesting the involvement of DNA-binding proteins in the regulation of vlsE, with at least one protein acting as a transcriptional activator.

The BBA01 protein, a member of paralog family 48 from Borrelia burgdorferi, is potentially interchangeable with the channel-forming protein P13

The Borrelia burgdorferi genome exhibits redundancy, with many plasmid-carried genes belonging to paralogous gene families. It has been suggested that certain paralogs may be necessary in various environments and that they are differentially expressed in response to different conditions. The chromosomally located p13 gene which codes for a channel-forming protein belongs to paralog family 48, which consists of eight additional genes. Of the paralogous genes from family 48, the BBA01 gene has the highest homology to p13. Herein, we have inactivated the BBA01 gene in B. burgdorferi strain B31-A. This mutant shows no apparent phenotypic difference compared to the wild type. However, analysis of BBA01 in a C-terminal protease A (CtpA)-deficient background revealed that like P13, BBA01 is posttranslationally processed at its C terminus. Elevated BBA01 expression was obtained in strains with the BBA01 gene introduced on the shuttle vector compared to the wild-type strain. We could further demonstrate that BBA01 is a channel-forming protein with properties surprisingly similar to those of P13. The single-channel conductance, of about 3.5 nS, formed by BBA01 is comparable to that of P13, which together with the high degree of sequence similarity suggests that the two proteins may have similar and interchangeable functions. This is further strengthened by the up-regulation of the BBA01 protein and its possible localization in the outer membrane in a p13 knockout strain, thus suggesting that P13 can be replaced by BBA01.

Detection of Borrelia burgdorferi gene expression during mammalian infection using transcriptional fusions that produce green fluorescent protein

A novel, infectious Borrelia burgdorferi that expresses green fluorescent protein (GFP) was developed to examine the utility of this marker protein to label live bacteria during infection processes. Use of a borrelial erpAB promoter to direct gfp transcription supported previous indications that B. burgdorferi expresses erp genes during chronic mammalian infection and during acquisition by feeding ticks. Live bacteria fluoresced and were seen to be located extracellularly in infected mice and within midguts of infected ticks. These results indicate that transcriptional fusions between B. burgdorferi promoters and gfp can be useful tools to examine spirochete gene expression in vivo.

Inactivation of the fibronectin-binding adhesin gene bbk32 significantly attenuates the infectivity potential of Borrelia burgdorferi

Borrelia burgdorferi, the aetiological agent of Lyme disease, utilizes multiple adhesins to interact with both the arthropod vector and mammalian hosts it colonizes. One such adhesive molecule is a surface-exposed fibronectin-binding lipoprotein, designated BBK32. Previous characterization of BBK32-mediated fibronectin binding has been limited to biochemical analyses due to the difficulty in mutagenizing infectious isolates of B. burgdorferi. Here we report an alternative method to inactivate bbk32 via allelic exchange through use of a low-passage variant of B. burgdorferi strain B31 that is more readily transformed. The resulting mutant does not synthesize BBK32, exhibits reduced fibronectin binding in solid phase assays and manifests decreased interactions with mouse fibroblast cells relative to both the infectious parent and genetic complement. Furthermore, the bbk32 knockout was significantly attenuated in the murine model of Lyme disease, whereas a genetically complemented control was not, indicating that BBK32 is necessary for maximal B. burgdorferi infection in the mouse. To our knowledge this is the first mutational analysis of a surface exposed, functional borrelial lipoprotein adhesin whose activity is associated with the mammalian host environment. By analogy with other pathogens that utilize fibronectin binding as an important virulence determinant, the borrelial fibronectin-BBK32 interaction is likely to be important in B. burgdorferi-specific pathogenic mechanisms, particularly in the context of dissemination, secondary colonization and/or persistence.

Borrelia burgdorferi–Traveling incognito?

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

Borrelia burgdorferi lipoproteins are secreted to the outer surface by default

Borrelia spirochaetes are unique among diderm bacteria in their abundance of surface-displayed lipoproteins, some of which play important roles in the pathogenesis of Lyme disease and relapsing fever. To identify the lipoprotein-sorting signals in Borrelia burgdorferi, we generated chimeras between the outer surface lipoprotein OspA, the periplasmic oligopeptide-binding lipoprotein OppAIV and mRFP1, a monomeric red fluorescent reporter protein. Localization of OspA and OppAIV point mutants showed that Borrelia lipoproteins do not follow the '+2' sorting rule which targets lipoproteins to the cytoplasmic or outer membrane of Gram-negative bacteria via the Lol pathway. Fusions of mRFP1 to short N-terminal lipopeptides of OspA, and surprisingly OppAIV, were targeted to the spirochaetal surface. Mutagenesis of the OspA N-terminus defined less than five N-terminal amino acids as the minimal secretion-facilitating signal. With the exception of negative charges, which can act as partial subsurface retention signals in certain peptide contexts, lipoprotein secretion occurs independent of N-terminal sequence. Together, these data indicate that Borrelia lipoproteins are targeted to the bacterial surface by default, but can be retained in the periplasm by sequence-specific signals.

Plasmid requirements for infection of ticks by Borrelia burgdorferi

Borrelia burgdorferi strain B31 MI commonly loses one or more of its complement of 21 extrachromosomal plasmids during normal handling procedures and during genetic manipulations. Certain plasmid losses cause an inability or reduction in the ability of spirochetes to infect mice. In the current study, nine strains of spirochetes with varying plasmid profiles were used to identify plasmids necessary for nymphal tick infection. Nymphal ticks were artificially fed the nine spirochete strains as well as the parental strain containing a full complement of plasmids. The capillary fed nymphs were allowed to feed on mice for at least 63 h and then examined for the presence of spirochetes in their guts and salivary glands. All spirochete strains tested were able to infect ticks guts, but to different degrees. We determined that the plasmids lp5, lp28-1, and cp9 were not required for infecting tick guts, whereas loss of lp25 and lp28-4 was associated with reduced gut infectivity. A reduction in the ability of spirochetes to invade salivary glands was seen in bacteria that did not have lp28-1, whereas cp9 was not required for salivary gland infection. This study has pinpointed specific plasmids whose absence is deleterious to infecting nymphal tick guts and salivary glands.

Fibronectin binding protein BBK32 of the Lyme disease spirochete promotes bacterial attachment to glycosaminoglycans

Borrelia burgdorferi, the agent of Lyme disease, causes a multisystemic illness that can affect the skin, heart, joints, and nervous system and is capable of attachment to diverse cell types. Among the host components recognized by this spirochete are fibronectin and glycosaminoglycans (GAGs). Three surface-localized GAG-binding bacterial ligands, Bgp, DbpA, and DbpB, have been previously identified, but recent studies suggested that at least one additional GAG-binding ligand is expressed on the spirochetal surface when the spirochete is adapted to the mammalian host environment. BBK32 is a surface lipoprotein that is produced during infection and that has been shown to bind to fibronectin. In this study, we show that, when BBK32 was produced from a shuttle vector in an otherwise nonadherent high-passage B. burgdorferi strain, the protein localized on the bacterial surface and conferred attachment to fibronectin and to mammalian cell monolayers. In addition, the high-passage strain producing BBK32 bound to purified preparations of the GAGs dermatan sulfate and heparin, as well as to these GAGs on the surfaces of cultured mammalian cells. Recombinant BBK32 recognized purified heparin, indicating that the bacterial attachment to GAGs was due to direct binding by BBK32. This GAG-binding activity of BBK32 is apparently independent of fibronectin recognition, because exogenous heparin had no effect on BBK32-mediated bacterial binding to fibronectin.

Borrelia burgdorferi sensu stricto invasiveness is correlated with OspC-plasminogen affinity

Borrelia burgdorferi sensu lato, the causative agent of Lyme borreliosis, is transmitted through tick bite. Lyme borreliosis evolves in two stages: a primary red skin lesion called erythema migrans; later on, invasive bacteria disseminate to distant sites inducing secondary manifestations (neuropathies, arthritis, carditis, late skin disorders). It has been previously suggested that the ospC gene could be associated with invasiveness in humans depending on its sequence. Here, we confirm the pattern of invasiveness, according to B. burgdorferi sensu stricto (B. b. ss) ospC group, using the mouse as an experimental host of B. b. ss. As it has been shown that the host plasminogen activation system is used by B. burgdorferi to disseminate throughout the host, we studied the interaction of plasminogen with OspC proteins from invasive and non-invasive groups of B. b. ss. Using two methods, ELISA and surface plasmon resonance, we demonstrate that indeed OspC is a plasminogen-binding protein. Moreover, significant differences in binding affinity for plasminogen are correlated with different invasiveness patterns in mice. These results suggest that the correlation between ospC polymorphism and Borrelia invasiveness in humans is linked, at least in part, to differences in OspC affinity for plasminogen.

Solving a sticky problem: new genetic approaches to host cell adhesion by the Lyme disease spirochete

The Lyme disease spirochetes, comprised of at least three closely related species, Borrelia burgdorferi, Borrelia garinii and Borrelia afzelii, are fascinating and enigmatic bacterial pathogens. They are maintained by tick-mediated transmission between mammalian hosts, usually small rodents. The ability of these bacteria, which have relatively small genomes, to survive and disseminate in both an immunocompetent mammal and in an arthropod vector suggests that they have evolved elegant and indispensable strategies for interacting with their hosts. Recognition of specific mammalian and tick tissues is likely to be essential for successful completion of the enzootic life cycle but, given the historical difficulties in genetic manipulation of these organisms, characterization of factors promoting cell adhesion has until recently largely been confined to either the manipulation of host cells or the analysis of potential bacterial ligands in the form of recombinant proteins. These studies have led to the identification of several mammalian receptors for Lyme disease spirochetes, including glycosaminoglycans, decorin, fibronectin and integrins, as well as a tick receptor for the bacterium, and also candidate cognate bacterial ligands. Recent advances in our ability to genetically manipulate Lyme disease spirochetes, particularly B. burgdorferi, are now providing us with firm evidence that these ligands indeed do promote bacterial adherence to host cells, and with new insights into the roles of these multifacted Borrelia-host cell interactions during mammalian and arthropod infection.

Association of linear plasmid 28-1 with an arthritic phenotype of Borrelia burgdorferi

Borrelia burgdorferi, the Lyme disease spirochete, has a genome comprised of a linear chromosome and up to 21 plasmids. Loss of plasmids is associated with decreased infectivity and pathogenicity. Sixteen transformants were generated by transforming the noninfectious clone 5A13 with the recombinant plasmid pBBE22. The transformants were classified into nine groups based on plasmid content analysis. An infectivity study revealed that all nine transformants examined, each of which represented one of the plasmid patterns, were infectious in mice with severe combined immunodeficiency (SCID) regardless of their genomic compositions. Tissue bacterial quantification revealed that the loss of plasmids significantly reduced the spirochete burden in the heart and joint tissues, not in the skin, suggesting virulence factors may be tissue specific. Four transformants containing lp28-1 induced severe arthritis in SCID mice, in contrast to the five transformants lacking lp28-1. These pathogenicity studies associated lp28-1 with an arthritic phenotype and further studies may identify factors that contribute to arthritic pathology.

Borrelia burgdorferi organisms lacking plasmids 25 and 28-1 are internalized by human blood phagocytes at a rate identical to that of the wild-type strain

Lyme borreliosis caused by Borrelia burgdorferi is a persistent infection capable of withstanding the host's vigorous immune response. Several reports have shown that the spirochete's linear plasmids 25 and 28-1 are essential for its infectivity. In this context, it was proposed that Borrelia burgdorferi organisms control their uptake by macrophages and polymorphonuclear leukocytes (PMNs) through plasmid-encoded proteins and that this mechanism confers resistance to phagocytosis. To investigate this proposal, a precise flow-cytometry-based method with human blood was used to study the impact of the plasmids 25 and 28-1 on B. burgdorferi clearance over 150 min and to investigate whether low-passage organisms are more resistant to phagocytosis than high-passage B. burgdorferi. Exposure of human blood PMNs or blood monocytes to fluorescein isothiocyanate-labeled B. burgdorferi B31 organisms lacking the linear plasmids 25, 28-1, or both revealed that all spirochete populations were internalized at the same rate as the wild-type borrelia parent strain B31. Moreover, no differences in phagocytosis kinetics were detected when low- or high-passage wild-type B. burgdorferi B31 or N40 were cocultured with blood cells. Plasmid loss and probable associated surface protein changes due to serial in vitro propagation of B. burgdorferi do not affect the resistance of these organisms to internalization by phagocytic cells. In particular, we found no evidence for a plasmid-controlled (lp25 and lp28-1) resistance of B. burgdorferi to phagocytosis by leukocytes of the host's innate immune system.

Analysis of the ospC regulatory element controlled by the RpoN-RpoS regulatory pathway in Borrelia burgdorferi

Outer surface lipoprotein C (OspC) is a key virulence factor of Borrelia burgdorferi. ospC is differentially regulated during borrelial transmission from ticks to rodents, and such regulation is essential for maintaining the spirochete in its natural enzootic cycle. Recently, we showed that the expression of ospC in B. burgdorferi is governed by a novel alternative sigma factor regulatory network, the RpoN-RpoS pathway. However, the precise mechanism by which the RpoN-RpoS pathway controls ospC expression has been unclear. In particular, there has been uncertainty regarding whether ospC is controlled directly by RpoS (sigma(s)) or indirectly through a transactivator (induced by RpoS). Using deletion analyses and genetic complementation in an OspC-deficient mutant of B. burgdorferi, we analyzed the cis element(s) required for the expression of ospC in its native borrelial background. Two highly conserved upstream inverted repeat elements, previously implicated in ospC regulation, were not required for ospC expression in B. burgdorferi. Using similar approaches, a minimal promoter that contained a canonical -35/-10 sequence necessary and sufficient for sigma(s)-dependent regulation of ospC was identified. Further, targeted mutagenesis of a C at position -15 within the extended -10 region of ospC, which is postulated to function like the strategic C residue important for Esigma(s) binding in Escherichia coli, abolished ospC expression. The minimal ospC promoter also was responsive to coumermycin A(1), further supporting its sigma(s) character. The combined data constitute a body of evidence that the RpoN-RpoS regulatory network controls ospC expression by direct binding of sigma(s) to a sigma(s)-dependent promoter of ospC. The implication of our findings to understanding how B. burgdorferi differentially regulates ospC and other ospC-like genes via the RpoN-RpoS regulatory pathway is discussed.

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

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