AN EXPERIMENTAL CHAIN OF INFECTION REVEALS THAT DISTINCT BORRELIA-BURGDORFERI POPULATIONS ARE SELECTED IN ARTHROPOD AND MAMMALIAN HOSTS

The prokaryotic, spirochaetal microorganism Borrelia burgdorferi is th e causative agent of Lyme disease, an arthropod-borne disease of a var iety of vertebrates and the most prevalent arthropod-borne disease of humans in the United States. In order to understand better the normal life cycle of B. burgdorferi, an experimental chain of infection was d evised that involved multiple sequential arthropod and mammalian passa ges. By examining populations of B. burgdorferi emerging from differen t points in this infectious chain, we demonstrate that selection of a, burgdorferi populations peculiar to arthropod or vertebrate hosts is a property of at least one of the two ecologically distinct strains we examined. Distinct B. burgdorferi populations were identified using a n antigenic profile, defined by a set of monoclonal antibodies to eigh t B. burgdorferi antigens, and a plasmid profile, defined by the natur ally occurring plasmids in the starting clonal populations. These two profiles constituted the phenotypical signature of the population. In the strain exhibiting selection in the different hosts, transition fro m one host to another produced a striking series of alternating phenot ypical signatures down the chain of infection. At the molecular level, the alternating signatures were manifested as a reciprocal relationsh ip between the expression of certain antigenic forms of outer surface protein (Osp) B and OspC, In the case of OspC, the antigenic changes c ould be correlated to the presence of one of two distinctly different alleles of the ospC gene in a full-length and presumably transcription ally active state. In the case of OspB, two alleles were again identif ied. However, their differences were minor and their relationship to O spB antigenic variation more complicated. In addition to the reciproca ting changes in the antigenic profile, a reciprocating change in the s ize (probably the multimeric state) of a 9.0 kbp supercoiled plasmid w as also noted. Selection of distinct populations in the tick may be re sponsible for the microorganism's ability to infect a wide range of ve rtebrate hosts efficiently, in that the tick might provide selective p ressure for the elimination of the population selected in the previous host.