Maintenance of infective Borrelia burgdorferi Sh-2-82 in 4% oxygen - 5% carbon dioxide in vitro.

The infectivity of Borrelia burgdorferi depends on the number of passages in Barbour-Stoenner-Kelly (BSK-II) medium in ambient air. In this study the parameters of medium formulation or O2-CO2 atmosphere were altered to define conditions that would retain borrelial infectivity upon serial passage. The infective strain Sh-2-82 was passaged 20 times in BSK-II and BSK-A (a modified BSK-II medium) in ambient O2-CO2 and BSK-II in 4% O2 - 5% CO2 - 91% N2. Spirochetes of every fifth passage were inoculated intraperitoneally into neonatal Lewis rats. Infectivity was lost after 15 passages in media in ambient O2-CO2. However, infectivity was maintained through all 20 passages in BSK-II in 4% O2 - 5% CO2 - 91% N2. There were no pH differences, but the dissolved O2 concentration in ambient O2-CO2 was approximately twice that in 4% O2 - 5% CO2 - 91% N2. Therefore, differences in infectivity were due to culturing under a constant environment of decreased O2 and increased CO2. This environment may place a selective pressure on the borreliae for retention of the infective phenotype. The results suggest that the levels of O2 and CO2 in the environment influence infectivity by preventing the loss of genetic information or inducing the expression of virulence determinants in B. burgdorferi.