Loss of Virulence of Motile Treponema pallidum in vitro.

Abstract

Although Treponema pallidum, the causative agent of syphilis, has not been cultivated in vitro, the organism may remain motile under optimum conditions for periods up to 3 weeks. It has been demonstrated repeatedly that organisms maintained in vitro may cause syphilis when injected into rabbits. There are, however, conflicting statements in the literature on whether motile T. pallida are invariably virulent, i.e. whether all motile organisms can cause syphilis when injected into a suitable host. Since this question could have important implications in the problem of culturing the organisms, we are reporting experiments designed to give a definite answer. Nelson (1948) stated that, when T. pallidum was maintained in vitro for periods of 54, 72, 144, or 192 hours at 30°C. and subsequently injected into rabbits, the animals developed darkfield-positive lesions. Further, when the number of viable organisms was calculated from the total count and the percentage of motile organisms, the incubation periods found were consistent with the ranges obtained by Magnuson, Eagle, and Fleischman (1948) with freshly isolated T. pallidum. This result suggested to Nelson that there was no loss of virulence when the organisms were maintained in vitro. It should be pointed out, however, that the standard deviation in the determination of the incubation period (as a function of number of organisms injected) is so large that only large differences between motility and virulence would be significant. By contrast, Thompson, Greenberg, and Magnuson (1950), in studying the effect of the immobilizing antibody on the loss of infectiousness of T. pallidum, concluded that virulence might in fact be lost slightly before motility ceased. Weber (1960) (in a report published after the completion of the work described in the present paper) stated that there was a signifi-