Genetic stability of Borrelia burgdorferi recovered from chronically infected immunocompetent mice.

Persistent infection with Borrelia burgdorferi in the presence of a vigorous host immune response has been demonstrated in humans and in animal models of Lyme disease. Long-term persistence of B. burgdorferi was documented recently in our studies of BALB/c and C3H mice infected with cloned and uncloned strains of B. burgdorferi. From mice inoculated with the cloned strain, 11 isolates were recovered from the skin, bladder, and blood after 1 year of infection. Analysis of the genes encoding the major outer surface proteins (OspA and OspB) by restriction digestion and DNA sequencing showed no evidence of point mutations or other small genetic alterations after 1 year. Genomic macrorestriction analysis of whole B. burgdorferi showed no loss or gross alterations of the plasmids encoding OspA, OspB, or OspC. However, in two isolates, loss of a 38-kb plasmid encoding outer surface protein D was noted. Our studies suggest that loss or alteration of the genes encoding OspA and OspB is not a common occurrence during persistent spirochetal infection and that other possible mechanisms, including invasion of immunologically privileged sites, should be actively explored.