Spirochætes occur in the crystalline style and digestive tract of many bivalve molluscs. The first molluscan spirochæte to be studied was that of the oyster, named by Certes (1882) “Trypanosoma” balbianii (fig. 53). Similar spirochætes, probably belonging to the same species, occur in various species of Tapes and in Pecten (the scallop). S. balbianii has rounded ends (fig. 53). Other spirochætes occur in freshwater mussels (Anodonta spp). S. anodontæ, studied by Keysselitz (1906) and by Fantham (1907), has pointed ends. Gross (1911) suggested the generic name Cristispira for molluscan spirochætes, because they possess a well-marked membrane or “crista,” which appears to be absent from S. plicatilis, according to Zuelzer’s researches.
Schaudinn in 1905 founded the genus Treponema for the parasite of syphilis (T. pallidum), discovered by him and by Hoffmann. According to Schaudinn the Treponemata have no membrane or crista. The pathogenic agent of yaws or frambœsia, discovered by Castellani, is also placed in the genus Treponema, as T. pertenue.
There remain the blood spirochætes. It is somewhat disputed as to whether these organisms possess a membrane. The present writer considers that they have a slight membrane or crista. The name of the genus in which to place the blood-inhabiting forms is somewhat uncertain and disputed. Various generic names given to them are Spirochæta, Treponema, Spiroschaudinnia (Sambon) and Borrelia (Swellengrebel). Included in this division are the causal agents of relapsing or recurrent fever. These Protists will be named, for description, Spirochætes without prejudice as to the ultimate correct generic name.
It is sometimes made a matter of argument as to whether the spirochætes are Protozoa or Bacteria. Such arguments are somewhat unprofitable. Morphologically the spirochætes are like the Bacteria in possessing a diffuse nucleus. They differ from Spirillum, an undoubted bacterial genus, in being flexible and not possessing flagella. Molluscan spirochætes, however, may appear to have flagella if their membrane becomes frayed or ruptured, when the myonemes therein (fig. 53), becoming separated, form apparent threads or flagella (Fantham, 1907–08).[142]
Again, the mode of division of spirochætes has been used as a criterion of their bacterial or protozoal affinity. They have been stated to divide transversely, longitudinally, and by “incurvation,” or bending on themselves in the form of a U, “a form of transverse fission.” The present writer believes that they divide both transversely and longitudinally, and that there is a periodicity in their mode of division at first longitudinal (when there are few spirochætes in, say, the blood) and then transversely (when spirochætes are numerous in the blood).[143] Some authors consider that longitudinal division is explained by “incurvation.”
The spirochætes of relapsing fever show a remarkable periodic increase and decrease in numbers in the blood. They are transmitted by ticks or by lice. They react to drugs (e.g., salvarsan or “606”) rather like trypanosomes, and—like Protozoa, but unlike Bacteria—they are cultivated with difficulty. These and other criteria have been used to endeavour to determine whether they are Protozoa or Bacteria. The present writer believes that they are intermediate in character, showing morphological affinities with the Bacteria and physiological and therapeutical affinities with the Protozoa. The group Spirochætacea, as an appendix to the Protozoa, has been created for them by the present writer (Jan., 1908). Others have placed them in the Spirochætoidea of the Bacteria or with the Spirillacea. Doflein (1909) called them Proflagellata. Further discussion is unnecessary, as they are undoubtedly Protista (see p. [29]).
There is no true conjugation, sex or encystment in spirochætes, but morphological variation may occur.[144] They may agglomerate.
The Spirochætes form an interesting chapter in the evolution of parasites. There are free living forms, parasitic forms in the guts of both vertebrates and invertebrates, and blood-inhabiting forms. These probably represent the order of evolution of parasitism. The blood-inhabiting forms are pathogenic to warm-blooded hosts.
We must now consider the blood Spirochætes and the Treponemata (organisms of syphilis and of yaws).