Genetic Diversity of the Outer Surface Protein C Gene of SouthernBorreliaIsolates and Its Possible Epidemiological, Clinical, and Pathogenetic Implications

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Abstract
TheospCgenes of 20 southernBorreliastrains were sequenced. The strains consisted ofB. burgdorferisensu stricto,B. andersonii,B. bissettii, one undescribed genospecies, MI-8, and one probably newBorreliaspecies, TXW-1. A high degree of similarity exists betweenB. burgdorferisensu stricto andB. bissettiiand betweenB. bissettiiandB. andersonii. Lateral transfers of theospCgene probably occurred betweenB. burgdorferisensu stricto andB. bissettiiand betweenB. bissettiiandB. andersonii. Internal gene recombination appears to occur among them. The highest degree of genetic diversity among them was observed in the two variable domains (V1 and V2), semivariable domain (SV), and the species-specific epitopes (between amino acids 28 and 31). Differences inospCsequences among southern strains reflect diversity at the strain and genospecies levels. MI-8, which was recognized as an undescribed genospecies in our previous reports, remains distinguishable in our current analysis ofospCgenes and is distinct fromB. burgdorferisensu stricto. Interestingly, another undescribed southern isolate, TXW-1, was not amplified under various PCR conditions. Compared to EuropeanB. burgdorferisensu stricto strains, AmericanB. burgdorferisensu stricto strains show greater genetic heterogeneity. SouthernB. burgdorferisensu stricto,B. andersonii, andB. bissettiiisolates were intermixed with each other in the phylogenetic trees. In the derived trees in our work, at least one southeastern strain ofB. burgdorferi, MI-2, most closely aligns with a so-called invasive cluster that possesses many proven human-invasive strains. Transmission experiments show that MI-2 and the strains in this group of southern spirochetes are able to infect mice and hamsters and that the typical vector of Lyme disease,Ixodes scapularis, can acquire the spirochetes from infected mammals. Currently, strain MI-2 appears to be the only southern isolate among the 20 we analyzed that clusters with an OspC invasive group and thus might be invasive for humans.

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