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

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...

Environmentally responsive synthesis of surface proteins represents a hallmark of the infectious cycle of the Lyme disease agent, Borrelia burgdorferi. Here we created and analyzed a B. burgdorferi mutant lacking outer-surface protein C (OspC), an abundant Osp that spirochetes normally synthesize in the tick vector during the blood meal and down-regulate after transmission to the mammal. We demonstrate that B. burgdorferi strictly requires OspC to infect mice but not to localize or migrate appropriately in the tick. The induction of a spirochetal virulence factor preceding the time and host in which it is required demonstrates a developmental sequence for transmission of this arthropod-borne pathogen.