When many cultures of Treponema pallidum, whether obtained from the testicular lesions produced in rabbits or directly from human cases of syphilis, are compared, certain definite differences in morphological character become apparent. The different specimens can be divided into thicker and thinner forms or types, and an average or normal form or type. The last is the common or most frequent variety, but the other two occur with sufficient frequency and retain their characters with such constancy as to constitute distinct varieties. Indeed, two of the varieties—the average and the thinner—occurred in association in a chancre and were separated afterwards in cultures. The gross cultural properties of the three varieties present no points of distinction.
The lesions caused in the testicle of the rabbit differ according to the variety inoculated, and consist either of a diffuse or of a nodular orchitis. This is a highly important distinction, and if, in the study of a still larger number of specimens of pallida, it is maintained, it is capable of throwing light on certain important clinical features of the human syphilitic disease.
The thinner variety of Treponema pallidum resembles in morphology Treponema microdentium, from which it is unmistakably distinguished by cultural characters.
The morphological and pathogenic variations in cultures of the pallidum may constitute racial differences within the species.