Lyme disease spirochetes, Borrelia burgdorferi sensu lato, are maintained in zoonotic cycles involving ticks and small mammals. In unfed ticks, the spirochetes produce one outer surface protein, OspA, but not OspC. During infection in mammals, immunological data suggest that the spirochetes have changed their surface, now expressing OspC but little or no OspA. We find by in vitro growth experiments that this change is regulated in part by temperature; OspC is produced by spirochetes at 32-370C but not at 24°C. Furthermore, spirochetes in the midgut of ticks that have fully engorged on mice now have OspC on their surface. Thus two environmental cues, an increase in temperature and tick feeding, trigger a major alteration of the spirochetal outer membrane. This rapid synthesis of OspC by spirochetes during tick feeding may play an essential role in the capacity of these bacteria to successfully infect mammalian hosts, including humans, when transmitted by ticks.Many infectious agents pathogenic in humans are maintained in natural zoonotic cycles involving wild vertebrates and obligate blood-feeding arthropods (1, 2). Although much is known about the clinical description and diagnosis of these human diseases, the physiological and morphological adaptations of these agents, especially bacteria, while in their arthropod vectors are not well understood. Several non-vector-borne bacterial pathogens, including species of Salmonella, Shigella, and Bordetella, display an impressive repertoire of adaptive molecular responses to environmental signals on entry into mammalian hosts (3-5). Specific changes by bacterial pathogens in arthropod vectors during feeding on blood have not been described. Identifying such events would broaden our knowledge of how these agents are perpetuated and transmitted in nature and assist in the development of effective vaccines and diagnostic tests.Borrelia burgdorferi is one of at least three closely related species of spirochetes that cause a spectrum of clinical syndromes in humans, collectively called Lyme disease or Lyme borreliosis (6-8). These spirochetes are maintained in zoonotic cycles involving a diversity of wild mammals and ticks primarily in the genus Ixodes (9). These microbes' adaptation to tick and mammalian environments likely involves very different surface components so as to ensure their transmission and survival in two very different hosts. Several lipoproteins have been described on the surface of B. burgdorferi (10)(11)(12)(13)(14)(15)(16), some of which are variably expressed during serial passage in culture (17)(18)(19). The apparent flexibility in the spirochete's synthesis of some outer surface proteins (Osps) in vitro may have relevance to its alternation of hosts. Previous studies demonstrate that B. burgdorferi produces one surface protein, OspA, and likely OspB, in the midgut of Ixodes ticks that have not yet engorged on blood (10). However, it is unclear how long spirochetes continue to produce this protein after entering mammalian hosts because few anim...

Ticks are parasitiform mites that are obligate hematophagous ectoparasites of amphibians, reptiles, birds, and mammals. A phylogeny for tick familie, subfamie, and genera has been described based on morphological characters, life histories, and host actions. To test the exring phylogeny, we sequenced -460 bp from the 3' end of the mitochondrial 16S rRNA gene (rDNA)

Lyme borreliosis, caused by the tick-borne bacterium Borrelia burgdorferi, has become the most common vector-borne disease in North America over the last three decades. To understand the dynamics of the epizootic spread and to predict the evolutionary trajectories of B. burgdorferi, accurate information on the population structure and the evolutionary relationships of the pathogen is crucial. We, therefore, developed a multilocus sequence typing (MLST) scheme for B. burgdorferi based on eight chromosomal housekeeping genes. We validated the MLST scheme on B. burgdorferi specimens from North America and Europe, comprising both cultured isolates and infected ticks. These data were compared with sequences for the commonly used genetic markers rrs-rrlA intergenic spacer (IGS) and the gene encoding the outer surface protein C (ospC). The study demonstrates that the concatenated sequences of the housekeeping genes of B. burgdorferi provide highly resolved phylogenetic signals and that the housekeeping genes evolve differently compared with the IGS locus and ospC. Using sequence data, the study reveals that North American and European populations of B. burgdorferi correspond to genetically distinct populations. Importantly, the MLST data suggest that B. burgdorferi originated in Europe rather than in North America as proposed previously.evolution ͉ Lyme borreliosis ͉ ticks

The genome of Borrelia burgdorferi encodes a large number of lipoproteins, many of which are expressed only at certain stages of the spirochete's life cycle. In the current study we describe the B. burgdorferi population structure with respect to the production of two lipoproteins [outer surface protein A (OspA) and outer surface protein C (OspC)] during transmission from the tick vector to the mammalian host. Before the blood meal, the bacteria in the tick were a homogeneous population that mainly produced OspA only. During the blood meal, the population became more heterogeneous; many bacteria produced both OspA and OspC, whereas others produced only a single Osp and a few produced neither Osp. From the heterogeneous spirochetal population in the gut, a subset depleted of OspA entered the salivary glands and stably infected the host at time points >53 hr into the blood meal. We also examined genetic heterogeneity at the B. burgdorferi vlsE locus before and during the blood meal. In unfed ticks, the vlsE locus was stable and one predominant and two minor alleles were detected. During the blood meal, multiple vlsE alleles were observed in the tick. Tick feeding may increase recombination at the vlsE locus or selectively amplify rare vlsE alleles present in unfed ticks. On the basis of our data we propose a model, which is different from the established model for B. burgdorferi transmission. Implicit in our model is the concept that tick transmission converts a homogeneous spirochete population into a heterogeneous population that is poised to infect the mammalian host.

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