Population dynamics of a naturally occurring heterogeneous mixture of Borrelia burgdorferi clones

Two unique isolates of Borrelia burgdorferi, differing in plasmid content a nd outer surface protein C expression, were cultured on sequential captures of a single free-living Peromyscus leucopus mouse and were examined for di fferences in transmissibility. Both isolates were transmissible from inocul ated C.B-17 mice to larval Ixodes scapularis ticks and, subsequently, from infected nymphal ticks to C3H/HeJ mice. Plasmid and protein analyses sugges ted that the original isolates were a mixed population of B. burgdorferi, a nd cloning by limiting dilution resulted in the identification of two clona l groups. In addition to being heterogeneous in plasmid and genomic macrore striction analyses, the clones varied with respect to the electrophoretic m obilities and antigenicity of their OspC proteins, as shown by their reacti vity to a panel of monoclonal antibodies. Plasmid analysis of sequential is olates from C3H mice experimentally infected with the primary isolate or va rious mixtures of its subclones showed an apparently random fluctuation in clonal dominance in the majority of mice. Surprisingly, mice infected with each subclone were permissive to superinfection with the heterologous subcl one, despite the presence of anti-B. burgdorferi antibodies at the time of the secondary challenge. These results show conclusively that mice captured at Lyme disease enzootic sites may be infected by mixed populations of gen etically and antigenically distinct B. burgdorferi clones and that these in fections can be acquired by coinfection or by sequential infection. The lac k of cross-immunization between clones existing within a naturally occurrin g population may play a role in the maintenance of the genetic heterogeneit y of B. burgdorferi in nature.