Four clones of Borrelia burgdorferi sensu stricto cause invasive infection in humans

Genetic diversity of the outer surface protein C gene of southern Borrelia isolates and its possible epidemiological, clinical, and pathogenetic implications

The ospC genes of 20 southern Borrelia strains were sequenced. The strains consisted of B. burgdorferi sensu stricto, B. andersonii, B. bissettii, one undescribed genospecies, MI-8, and one probably new Borrelia species, TXW-1. A high degree of similarity exists between B. burgdorferi sensu stricto and B. bissettii and between B. bissettii and B. andersonii. Lateral transfers of the ospC gene probably occurred between B. burgdorferi sensu stricto and B. bissettii and between B. bissettii and B. andersonii. Internal gene recombination appears to occur among them. The highest degree of genetic diversity among them was observed in the two variable domains (V1 and V2), semivariable domain (SV), and the species-specific epitopes (between amino acids 28 and 31). Differences in ospC sequences among southern strains reflect diversity at the strain and genospecies levels. MI-8, which was recognized as an undescribed genospecies in our previous reports, remains distinguishable in our current analysis of ospC genes and is distinct from B. burgdorferi sensu stricto. Interestingly, another undescribed southern isolate, TXW-1, was not amplified under various PCR conditions. Compared to European B. burgdorferi sensu stricto strains, American B. burgdorferi sensu stricto strains show greater genetic heterogeneity. Southern B. burgdorferi sensu stricto, B. andersonii, and B. bissettii isolates were intermixed with each other in the phylogenetic trees. In the derived trees in our work, at least one southeastern strain of B. burgdorferi, MI-2, most closely aligns with a so-called invasive cluster that possesses many proven human-invasive strains. Transmission experiments show that MI-2 and the strains in this group of southern spirochetes are able to infect mice and hamsters and that the typical vector of Lyme disease, Ixodes scapularis, can acquire the spirochetes from infected mammals. Currently, strain MI-2 appears to be the only southern isolate among the 20 we analyzed that clusters with an OspC invasive group and thus might be invasive for humans.

Neurologic aspects of Lyme disease

Lyme disease has emerged as a major infection with frequent neurologic manifestations. These manifestations probably reflect several predominantly indirect pathogenetic mechanisms and involve host, vector, and organism factors. With early diagnosis and appropriate antibiotic treatment, patients do well. Because culture is not reliable, diagnosis has relied on positive serology to document exposure. Serology should improve as second-generation assays become available. Although there is a preventive vaccine based on the lipoprotein OspA, newer vaccines in development may prove more desirable. Lyme disease provides a valuable model to study how infectious pathogens cause neurologic disease.

Geographic uniformity of the Lyme disease spirochete (Borrelia burgdorferi) and its shared history with tick vector (Ixodes scapularis) in the Northeastern United States

Over 80% of reported cases of Lyme disease in the United States occur in coastal regions of northeastern and mid-Atlantic states. The genetic structure of the Lyme disease spirochete (Borrelia burgdorferi) and its main tick vector (Ixodes scapularis) was studied concurrently and comparatively by sampling natural populations of I. scapularis ticks along the East Coast from 1996 to 1998. Borrelia is genetically highly diverse at the outer surface protein ospC. Since Borrelia is highly clonal, the ospC alleles can be used to define clones. A newly designed reverse line blotting (RLB) assay shows that up to 10 Borrelia clones can infect a single tick. The clone frequencies in Borrelia populations are the same across the Northeast. On the other hand, I. scapularis populations show strong regional divergence (among northeastern, mid-Atlantic, and southern states) as well as local differentiation. The high genetic diversity within Borrelia populations and the disparity in the genetic structure between Borrelia and its tick vector are likely consequences of strong balancing selection on local Borrelia clones. Demographically, both Borrelia and I. scapularis populations in the Northeast show the characteristics of a species that has recently expanded from a population bottleneck. Major geological and ecological events, such as the last glacial maximum (18,000 years ago) and the modern-day expansion of tick habitats, are likely causes of the observed "founder effects" for the two organisms in the Northeast. We therefore conclude that the genetic structure of B. burgdorferi has been intimately shaped by the natural history of its main vector, the northern lineage of I. scapularis ticks.

Recombinant assay for serodiagnosis of Lyme disease regardless of OspA vaccination status

All current seroassays using cultured Borrelia burgdorferi as their antigen source have been rendered obsolete by the recombinant OspA Lyme disease vaccine. OspA is the major outer surface protein expressed in cultured B. burgdorferi, and any seroassay that uses whole organisms as its antigen source cannot differentiate between subjects who received the vaccine and those who were naturally infected. We developed a new sensitive and specific enzyme-linked immunosorbent assay (ELISA) utilizing recombinant chimeric borrelia proteins devoid of OspA (rNon-OspA) that can be used to detect antibodies to diagnostically important B. burgdorferi antigens in both OspA-vaccinated and nonvaccinated individuals. We tested sera from patients with Lyme disease and with conditions associated with false-positive serologies, OspA-vaccinated individuals, and healthy high-risk workers from an area of endemicity and normal sera from individuals from areas of nonendemicity. The rNon-OspA test was compared with two commercially available whole-cell immunoassays. The rNon-OspA assay is as sensitive and specific as the whole-cell assay (P > 0.05) for detection of anti-B. burgdorferi antibodies. However, the rNon-OspA assay can differentiate between populations comprised of naturally infected and OspA-vaccinated individuals (P < 0.05). Our data demonstrate that this new sensitive rNon-OspA ELISA can be used for the laboratory detection of B. burgdorferi antibodies regardless of vaccination status and could replace existing serologic assays for Lyme disease.

Lyme disease: an update

Lyme disease is a multisystem illness caused by the spirochete Borrelia burgdorferi, and it is the most common vector-borne illness in the United States. Lyme disease is also endemic in Europe and Asia. There have been major advances in the field since the disease was first described, including the sequencing of the B. burgdorferi genome; an increase in understanding of the interactions among the spirochete, the tick, and the mammalian host; new and improved laboratory tests; and a vaccine for prevention of the disease. Still, the diagnosis of Lyme disease remains based on history and clinical findings, supplemented by careful use of laboratory tests, and requires that the physician be familiar with the disease's clinical manifestations and the shortcomings of the available diagnostic tests.

Asymptomatic Borrelia burgdorferi infection

Little is known about the natural history of asymptomatic Borrelia burgdorferi infection. Our analysis of the asymptomatic infections diagnosed serologically in a recent OspA vaccine trial conducted in the United States (N Engl J Med 1998;339: 209-215), suggests that the natural history of this event is more benign than that reported for untreated patients with erythema migrans (Ann Intern Med 1987;107: 725-731). We hypothesize that this is due either to incorrect diagnosis since the specificity of the serologic criteria used to diagnose asymptomatic infection in the vaccine study is unknown, or to infection with non-pathogenic strains of B. burgdorferi. Increasing evidence indicates that the invasive potential of strains of B. burgdorferi varies according to the specific subtype. Theoretically, a serologic testing method could be devised which would distinguish infection with invasive versus non-invasive strains of B. burgdorferi, and allow testing of the second hypothesis.

Coinfecting deer-associated zoonoses: Lyme disease, babesiosis, and ehrlichiosis

The heightened worldwide recognition of the health burden of tickborne infection derives largely from the increasing incidence of Lyme disease, human babesiosis, and human granulocytic ehrlichiosis, both individually and in concert. Because these infections share the same rodent reservoir and tick vector hosts, they can be cotransmitted to human hosts. Indeed, human coinfections involving various combinations of these pathogens are common, and some tend to be particularly severe. Diagnostic procedures and clinical management of the resulting disease syndrome is rendered complex by the diversity of pathogens involved and by the unusual diversity and duration of symptoms.

The implications of a low rate of horizontal transfer in Borrelia

The nature and rate of recombination can be studied by comparing the sequences of multiple genes across a set of strains. When this approach is applied to Borrelia burgdorferi, four results emerge: (1) chromosomal genes are clonal; (2) there is little or no plasmid exchange; (3) the major mode of horizontal transfer of genetic material inserts a small fragment of DNA, typically <1 kb, during recombination; and (4) the level of horizontal transfer in Borrelia is so low that there is evidence for horizontal transfer only in genes where there is positive selection for diversity, that is, positive selection for the recombinant. Thus, Borrelia can serve as a model of a low recombination taxon. The implications of these results lead us to postulate that an unknown agent that is part of the Borrelia genome mediates the horizontal transfer of small fragments of DNA; the rare transfer of small fragments of DNA excludes both DNA parasites and virulence factors from the genome.

Impact of genotypic variation of Borrelia burgdorferi sensu stricto on kinetics of dissemination and severity of disease in C3H/HeJ mice

Various genotypes of Borrelia burgdorferi sensu stricto have been previously identified among a large collection of isolates cultured from patients with Lyme disease in the United States. Furthermore, association of specific genotypes with hematogenous dissemination early in the disease course has been observed. The present study assessed kinetics of spirochete dissemination and disease severity in C3H/HeJ mice infected with two different genotypes of B. burgdorferi. Spirochete load in plasma and ear and other tissue samples of infected mice was measured by quantitative PCR, and these data were compared to those obtained by culture and histopathologic analysis. In mice infected with isolate BL206 (a type 1 strain), the peak number of spirochetes was observed in plasma between day 4 and 7, in heart and ear tissue on day 14, and in joints on day 28 postinoculation. There was a correlation between the peak number of spirochetes in plasma on day 4 or 7 and that in ear biopsy and joint specimens on day 14. By contrast, spirochete burdens in plasma of mice infected with isolate B356 (a type 3 strain) were 16- and 5-fold lower than those of BL206-infected mice on days 7 and 14 of infection, respectively. Similarly, approximately 6- and 13-fold fewer spirochetes were detected in the heart tissues of B356-infected mice compared to BL206-infected mice. Histopathologically, severe arthritis and aortitis were noted only in mice infected with isolate BL206. Spirochete dissemination and disease severity vary significantly in mice infected with distinct genotypes of B. burgdorferi, suggesting that genotypic differences in the infecting spirochetes play a key role in the pathogenesis and development of clinical disease.

Distinct levels of genetic diversity of Borrelia burgdorferi are associated with different aspects of pathogenicity

Different species of pathogenic Borrelia show different symptoms and tick vector specificity. Even within regions where only one species is found, Lyme disease progresses very differently from one patient to another. Since Borrelia shows very little recombination either within or between species, alleles of a gene can be used to mark clones. The ospC gene is highly variable within each species and can be used to define groups of related clones. It has been previously shown that only four out of seventeen ospC groups of Borrelia burgdorferi sensu stricto cause invasive forms of the disease. Other groups cause erythema migrans, a skin rash at the site of the tick bite, but not invasive disease, while still other groups seem to be nonpathogenic to humans. In this study we extend the analysis of the ospC gene to the other pathogenic species, Borrelia garinii and Borrelia afzelii. Only two groups in B. afzelii and four groups in B. garinii cause invasive disease. Thus, only ten out of the 58 defined ospC groups cause invasive and presumably chronic Lyme disease.

Transmission of Borrelia garinii OspA serotype 4 to BALB/c mice by Ixodes ricinus ticks collected in the field

In Europe, Borrelia garinii OspA serotype 4 has been isolated from the cerebrospinal fluid of patients but, up to now, has never been identified among culture isolates from Ixodes ricinus ticks. This information raises the question of whether OspA serotype 4 is transmitted by I. ricinus in nature. In the present study, I. ricinus nymphs collected in an area of endemicity in southern Germany were allowed to feed on mice. Cultivation of ear biopsy specimens showed that six of seven B. garinii-infected mice were infected by OspA serotype 4. In contrast, very few B. garinii OspA serotype 4 organisms were isolated directly from the ticks which infected the mice; most isolates were B. afzelii. The infected mice transmitted mainly OspA serotype 4 to xenodiagnostic ticks, preferentially in combination with B. afzelii.

Crystal structure of outer surface protein C (OspC) from the Lyme disease spirochete, Borrelia burgdorferi

Outer surface protein C (OspC) is a major antigen on the surface of the Lyme disease spirochete, Borrelia burgdorferi, when it is being transmitted to humans. Crystal structures of OspC have been determined for strains HB19 and B31 to 1.8 and 2.5 A resolution, respectively. The three-dimensional structure is predominantly helical. This is in contrast to the structure of OspA, a major surface protein mainly present when spirochetes are residing in the midgut of unfed ticks, which is mostly beta-sheet. The surface of OspC that would project away from the spirochete's membrane has a region of strong negative electrostatic potential which may be involved in binding to positively charged host ligands. This feature is present only on OspCs from strains known to cause invasive human disease.

Correlation between plasmid content and infectivity in Borrelia burgdorferi

Infectivity-associated plasmids were identified in Borrelia burgdorferi B31 by using PCR to detect each of the plasmids in a panel of 19 clonal isolates. The clones exhibited high-, low-, and intermediate-infectivity phenotypes based on their frequency of isolation from needle-inoculated C3H/HeN mice. Presence or absence of 21 of the 22 plasmids was determined in each of the clones by using PCR primers specific for regions unique to each plasmid, as identified in the recently available genome sequence. Southern blot hybridization results were used to confirm the PCR results in some cases. Plasmid lp25 exhibited a direct correlation with infectivity in that it was consistently present in all clones of high or intermediate infectivity and was absent in all low-infectivity clones. lp28-1, containing the vmp-like sequence locus, also correlated with infectivity; all clones that lacked lp28-1 but contained lp25 had an intermediate infectivity phenotype, in which infection was primarily restricted to the joints. Plasmids cp9, cp32-3, lp21, lp28-2, lp28-4, and lp56 apparently are not required for infection in this model, because clones lacking these plasmids exhibited a high-infectivity phenotype. Plasmids cp26, cp32-1, cp32-2 and/or cp32-7, cp32-4, cp32-6, cp32-8, cp32-9, lp17, lp28-3, lp36, lp38, and lp54 were consistently present in all clones examined. On the basis of these results, lp25 and lp28-1 appear to encode virulence factors important in the pathogenesis of B. burgdorferi B31.

Identification of 11 pH-regulated genes in Borrelia burgdorferi localizing to linear plasmids

When Borrelia burgdorferi is transmitted from the tick vector to the mammalian host, the bacterium experiences alterations in its environment, such as changes in temperature and pH. Previously, we observed numerous alterations in the membrane protein profile when B. burgdorferi B31 was grown at pH 7.0 compared to pH 8.0. Here we identify 11 genes localizing to linear plasmids that are up-regulated at pH 7.0 relative to pH 8.0 in vitro. Seven genes (bba03, bba24, bba64, bba66, bbe31, bbj41/bbi39 [encoding products that are 99% identical], and bbk01) were indirectly identified by proteomic analysis of membrane proteins. Another gene, bba36, was identified by screening a B. burgdorferi B31 genomic library with cross-adsorbed hyperimmune rabbit serum. Two additional genes, bba65 and bba73, were identified by Northern blot analysis. Genes bba64, bba65, bba66, bbj41/bbi39, and bba73 are members of paralogous gene family 54, and bbe31 is a member of the closely related paralogous gene family 60. Gene bba24 is part of a bicistronic operon with bba25 that encodes the well-characterized decorin binding proteins A and B. All 11 genes were transcriptionally regulated, yet the degree of pH regulation varied, with some genes more tightly regulated than others. The regions upstream of these pH-regulated genes appeared to be unrelated, yet many contained dyad repeats ranging from 12 to 25 nucleotides in length that may be involved in the regulation of these genes.

Resolution of Lyme arthritis, acute or prolonged: a new look

I have presented a paradigm for the pathogenesis and resolution of Lyme arthritis founded upon the clinical facts that describe its natural history, and centered about a process of intermittent immune recognition of what may sometimes be an array of antigenic variants. The model applies both to acute and to prolonged (chronic) disease. If it proves to be correct, it represents a variation of the struggle between parasite and host that is probably not limited to Lyme disease.

Recombinant chimeric Borrelia proteins for diagnosis of Lyme disease

Current serologic Lyme disease tests use whole borrelia cells as the source of antigen. These assays are difficult to standardize and to optimize for sensitivity and specificity. To help solve these problems, we constructed a library of recombinant chimeric proteins composed of portions of key antigens of Borrelia burgdorferi. These proteins were then used to develop an enzyme-linked immunosorbent assay. We compared our assay with the most sensitive of three whole-cell borrelia assays. We found that the recombinant assay could detect antibodies significantly better from early Lyme disease sera (P<0.05), and had the same sensitivity for late Lyme disease sera, as the most sensitive whole-cell borrelia assay. On potentially cross-reactive sera, the recombinant assay was more specific, but not significantly so, than the best whole-cell borrelia assay. Optimization of the recombinant assay offers the potential for a significant improvement in both sensitivity and specificity.

Molecular and evolutionary characterization of the cp32/18 family of supercoiled plasmids in Borrelia burgdorferi 297

In this study, we characterized seven members of the cp32/18 family of supercoiled plasmids in Borrelia burgdorferi 297. Complete sequence analysis of a 21-kb plasmid (cp18-2) confirmed that the strain 297 plasmids are similar in overall content and organization to their B31 counterparts. Of the 31 open reading frames (ORFs) in cp18-2, only three showed sequence relatedness to proteins with known functions, and only one, a ParA/SopA ortholog, was related to nonborrelial polypeptides. Besides the lipoproteins, none of the ORFs appeared likely to encode a surface-exposed protein. Comparison with the B31 genomic sequence indicated that paralogs for most of the ORFs in cp18-2 can be identified on other genetic elements. cp18-2 was found to lack a 9- to 10-kb fragment present in the 32-kb homologs which, by extrapolation from the B31 cp32 sequences, contains at least 15 genes presumed to be unnecessary for plasmid maintenance. Sequence analysis of the lipoprotein-encoding variable loci provided evidence that recombinatorial processes within these regions may result in the acquisition of exogenous DNA. Pairwise analysis with random shuffling revealed that the multiple lipoproteins (Mlp; formerly designated 2.9 LPs) fall into two distinct homology groups which appear to have arisen by gene fusion events similar to those recently proposed to have generated the three OspE, OspF, and Elp lipoprotein families (D. R. Akins, M. J. Caimano, X. Yang, F. Cerna, M. V. Norgard, and J. D. Radolf, Infect. Immun. 67:1526-1532, 1999). Comparative analysis of the variable regions also indicated that recombination within the loci of each plasmid may occur independently. Last, comparison of variable loci revealed that the cp32/18 plasmid complements of the B31 and 297 isolates differ substantially, indicating that the two strains have been subject to divergent adaptive pressures. In addition to providing evidence for two different types of recombinatorial events involving cp32/18 plasmids, these findings underscore the need for genetic analysis of diverse borrelial isolates in order to elucidate the Lyme disease spirochete's complex parasitic strategies.