Borrelia burgdorferi: a (somewhat) clonal bacterial species

Genetic structure of marine Borrelia garinii and population admixture with the terrestrial cycle of Lyme borreliosis

Despite the importance of population structure for the epidemiology of pathogenic bacteria, the spatial and ecological heterogeneity of these populations is often poorly characterized. Here, we investigated the genetic diversity and population structure of the Lyme borreliosis (LB) spirochaete Borrelia garinii in its marine cycle involving colonial seabirds and different host races of the seabird tick Ixodes uriae. Multilocus sequence analyses (MLSA) on eight chromosomal and two plasmid loci (ospA and ospC) indicate that B. garinii circulating in the marine system is highly diverse. Microevolution in marine B. garinii seems to be mainly clonal, but recombination and selection do occur. Sequence types were not evenly distributed among geographic regions, with substantial population subdivision between Atlantic and Pacific Ocean basins. However, no geographic structuring was evident within regions. Results of selection analyses and phylogenetic discordance between chromosomal and plasmid loci indicate adaptive evolution is likely occurring in this system, but no pattern of host or vector-associated divergence was found. Recombination analyses showed evidence for population admixture between terrestrial and marine strains, suggesting that LB spirochaetes are exchanged between these enzootic cycles. Importantly, our results highlight the need to explicitly consider the marine system for a complete understanding of the evolutionary ecology and global epidemiology of Lyme borreliosis.

Functional characterization of Borrelia spielmanii outer surface proteins that interact with distinct members of the human factor H protein family and with plasminogen

Acquisition of complement regulator factor H (CFH) and factor H-like protein 1 (CFHL1) from human serum enables Borrelia spielmanii, one of the etiological agents of Lyme disease, to evade complement-mediated killing by the human host. Up to three distinct complement regulator-acquiring surface proteins (CRASPs) may be expressed by serum-resistant B. spielmanii, each exhibiting an affinity for CFH and/or CFHL1. Here, we describe the functional characterization of the 15-kDa CRASPs of B. spielmanii, members of the polymorphic Erp (OspE/F-related) protein family, that bind two distinct host complement regulators, CFH and factor H-related protein 1 (CFHR1), but not CFHL1. CFH bound to the B. spielmanii CRASPs maintained cofactor activity for factor I-mediated C3b inactivation. Three naturally occurring alleles of this protein bound CFH and CFHR1 while a fourth natural allele could not. Comparative sequence analysis of these protein alleles identified a single amino acid, histidine-79, as playing a significant role in CFH/CFHR1 binding, with substitution by an arginine completely abrogating ligand binding. The mutation of His-79 to Arg did not inhibit binding of plasminogen, another known ligand of this group of borrelial outer-surface proteins.

Diagnosis of lyme borreliosis

A large amount of knowledge has been acquired since the original descriptions of Lyme borreliosis (LB) and of its causative agent, Borrelia burgdorferi sensu stricto. The complexity of the organism and the variations in the clinical manifestations of LB caused by the different B. burgdorferi sensu lato species were not then anticipated. Considerable improvement has been achieved in detection of B. burgdorferi sensu lato by culture, particularly of blood specimens during early stages of disease. Culturing plasma and increasing the volume of material cultured have accomplished this. Further improvements might be obtained if molecular methods are used for detection of growth in culture and if culture methods are automated. Unfortunately, culture is insensitive in extracutaneous manifestations of LB. PCR and culture have high sensitivity on skin samples of patients with EM whose diagnosis is based mostly on clinical recognition of the lesion. PCR on material obtained from extracutaneous sites is in general of low sensitivity, with the exception of synovial fluid. PCR on synovial fluid has shown a sensitivity of up to >90% (when using four different primer sets) in patients with untreated or partially treated Lyme arthritis, making it a helpful confirmatory test in these patients. Currently, the best use of PCR is for confirmation of the clinical diagnosis of suspected Lyme arthritis in patients who are IgG immunoblot positive. PCR should not be used as the sole laboratory modality to support a clinical diagnosis of extracutaneous LB. PCR positivity in seronegative patients suspected of having late manifestations of LB most likely represents a false-positive result. Because of difficulties in direct methods of detection, laboratory tests currently in use are mainly those detecting antibodies to B. burgdorferi sensu lato. Tests used to detect antibodies to B. burgdorferi sensu lato have evolved from the initial formats as more knowledge on the immunodominant antigens has been collected. The recommendation for two-tier testing was an attempt to standardize testing and improve specificity in the United States. First-tier assays using whole-cell sonicates of B. burgdorferi sensu lato need to be standardized in terms of antigen composition and detection threshold of specific immunoglobulin classes. The search for improved serologic tests has stimulated the development of recombinant protein antigens and the synthesis of specific peptides from immunodominant antigens. The use of these materials alone or in combination as the source of antigen in a single-tier immunoassay may someday replace the currently recommended two-tier testing strategy. Evaluation of these assays is currently being done, and there is evidence that certain of these antigens may be broadly cross-reactive with the B. burgdorferi sensu lato species causing LB in Europe.

Characterization of Borrelia lusitaniae isolates collected in Tunisia and Morocco

Borrelia lusitaniae is a species within the complex Borrelia burgdorferi sensu lato and is infrequently isolated in Europe. In contrast, this species is by far the most predominant in North Africa and in Portugal. In this study, we analyzed the genetic diversity, at several loci, of a large population of isolates from free-living Ixodes ricinus ticks collected in Tunisia and Morocco. We found a moderate diversity of the whole genome by using pulsed-field gel electrophoresis as well as in the ospA gene sequences, compared to a high level of strain homogeneity in the small noncoding ribosomal spacer. In contrast, a high diversity of this locus has been previously reported for Portuguese isolates. We hypothesize that B. lusitaniae strains isolated in North Africa constitute a clone of Portuguese origin.