Effect of serum concentration and metabolic inhibitors on the attachment of Treponema pallidum to rabbit cells.

The effect of host-cell metabolism on the attachment of Treponema pallidum to mammalian cells in vitro was studied. The growth of baby rabbit genital organ (BRGO) cells was enhanced by increasing the concentration of serum ("serum shift-up") in the growth medium. Cells starved for 24 h in serum-free medium showed a burst of DNA synthesis when shifted to fresh medium containing 20% serum. In aerobic conditions, they were much more heavily coated with attached T. pallidum than cells shifted to 20% serum after maintenance at serum concentrations of 2.5%, 5% or 10%. This effect was very pronounced during the first few hours of co-incubation. In microaerophilic conditions, the extent of T. pallidum adherence also paralleled the increase in DNA synthesis by BRGO cells. Cycloheximide and methotrexate greatly inhibited DNA and protein synthesis in BRGO cells, but did not affect the motility of T. pallidum. When BRGO cell metabolism was inhibited by these two drugs, attachment of T. pallidum was significantly decreased. These results indicate that T. pallidum attaches best to actively growing BRGO cells in tissue culture. This may explain the apparently preferential parasitism of actively growing tissues by T. pallidum in syphilis in man.