Prolonged survival of virulent Treponema pallidum (Nichols strain) in cell-free and tissue culture systems

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Abstract
Survival of T. pallidum was prolonged in the presence of tissue culture. Of the 12 cultures studied, cottontail rabbit epithelium (Sf1Ep) supported T. pallidum for the longest time. In horizontal Leighton tubes with reduced medium and an atmosphere of 5% CO2 in N2, the 50% survival time (ST50) was 5-6 days for treponemes associated with monolayers of Sf1Ep cells. Comparable cell-free tubes had ST50 values of < 4 days. In vertical Leighton tubes containing 6 ml of prereduced medium incubated aerobically, gradients of O2 tension and redox potential were established. Attachment and survival of T. pallidum were greatest at a depth of about 10-20 mm. Motility was between 70-95% in this area throughout the first 14 days of incubation. Occasionally, > 50% motility was observed for as long as 21 days. The redox potential and O2 tension in the optimal area of gradient cultures were reproduced by adjusting the medium depth in a shell vial culture system containing cells on a horizontal cover slip. Treponemes associated with the cell monolayer in gradient and shell vial cultures were still virulent after 21 days in vitro. Dilution of [rabbit] testis extract and concentration of T. pallidum were important factors in survival of T. pallidum.