Comparison of the morphology, antigenicity, and pathology of Nichols and Birmingham isolates of Treponema pallidum.

A strain of Treponema pallidum isolated from a homosexually acquired penile chancre in Birmingham in 1979 was serially passaged in rabbits and compared with the Nichols strain. The two strains appeared identical morphologically, and no gross differences were detected on antigenic analysis by two-dimensional immunoelectrophoresis. The Birmingham strain was, however, considerably less virulent for rabbits; the number of organisms required for equivalent time of development and severity of skin lesions was 10-fold to 100-fold greater for the Birmingham than for the Nichols strain. The pathology of testicular infection was broadly similar for both strains.