Borrelia burgdorferi, the
Lyme disease spirochete, has a genome comprised of a linear chromosome and up to 21 plasmids. Loss of plasmids is associated with decreased infectivity and pathogenicity. Sixteen transformants were generated by transforming the noninfectious clone 5A13 with the recombinant plasmid pBBE22. The transformants were classified into nine groups based on plasmid content analysis. An infectivity study revealed that all nine transformants examined, each of which represented one of the plasmid patterns, were infectious in mice with severe combined immunodeficiency (SCID) regardless of their genomic compositions. Tissue bacterial quantification revealed that the loss of plasmids significantly reduced the spirochete burden in the heart and joint tissues, not in the skin, suggesting virulence factors may be tissue specific. Four transformants containing lp28-1 induced severe arthritis in SCID mice, in contrast to the five transformants lacking lp28-1. These pathogenicity studies associated lp28-1 with an arthritic phenotype and further studies may identify factors that contribute to arthritic pathology.