Establishment of the Nichols strain as the type strain of Treponema pallidum.

Abstract

In this article, it is proposed that the Nichols strain of Treponema pallidum be established as the type strain. T. pallidum was first identified as the causative agent of syphilis in 1905, and the Nichols strain was isolated in 1912 by inoculation of a rabbit with cerebrospinal fluid from a patient with neurosyphilis. The Nichols strain has been maintained by serial passage in rabbits for over a century, and historically most studies of T. pallidum have been conducted using this strain. In recent years, a procedure for continuous in vitro culture of T. pallidum in a tissue culture system has been developed, making propagation of this spirochaete easier and hence facilitating research. The Nichols strain has >99% DNA homology with a group of organisms that cause syphilis, bejel/endemic syphilis and yaws in humans, a yaws-like disease in primates and spirochaetosis in rabbits and hares. This group is highly similar in terms of their gene and G+C content, genome synteny, cell morphology, natural dependence on mammalian hosts and ability to cause long-term infections; variation occurs in host range, modes of transmission, aptitude for dissemination, manifestations, congenital infection and geographical distribution. Availability of a type strain will aid in the formal acceptance of T. pallidum subspecies first described in 1984 and supported by recent whole-genome analyses of numerous strains from the T. pallidum-related group.