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Current Concepts of the Morphology and Biology of Treponema Pallidum Based on Electron Microscopy

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Current Concepts of the Morphology and Biology of Treponema Pallidum Based on Electron Microscopy Brit. J. vener. Dis. (1971) 47, 315 Br J Vener Dis: first published as 10.1136/sti.47.5.315 on 1 October 1971. Downloaded from Current concepts of the morphology and biology of Treponema pallidum based on electron microscopy N. M. OVCINNIKOV AND V. V. DELEKTORSKIJ Department of Microbiology, Central Research Institute of Skin and Venereal Diseases (Director: Prof. N. M. Turanov), Ministry of Health, Moscow, USSR The development of electron microscopy has ad- describe first the cultivated strains and then the vanced our knowledge of the morphology of Trepo- pathogenic organism. A comparative study of several nema pallidum, but the exact significance of some of cultivated strains (Reiter, Kazan, and Stavropol) the formations observed is not yet clear and requires has already revealed certain variations, but they are further detailed study by chemists as well as morpho- not very significant. One end of the treponeme logists. The literature has been reviewed and our carries an oval formation which we have named methods described in previous reports (Ovcinnikov 'head structure' (Ovcinnikov and Delektorskij, 1968, and Delektorskij, 1968, 1969, 1970a). In this paper 1969). Its shape is somewhat variable and it contains we shall limit ourselves to a description of our most basal granules, which are the sites of attachment of recent results. the fibrils and are possibly the centres controlling The classic spiral shape of T. pallidum, with coils of movement. When living treponemes are examined regular width and depth slightly narrower towards by darkfield microscopy this structure may exhibit ancopyright. the acuminate ends, is to be considered as typical abrupt rotatory motion. The opposite end is usually for this organism. devoid of such a formation. We shall not reproduce here the illustrations During certain periods of growth, probably before already published in various periodicals; the photo- the division of the treponeme, the head structure is graphs illustrating the present study have not pre- shed together with the fibrils. This head structure is viously been published. plainly visible in cultivated strains, but we could not Examination of preparations processed by the clearly discem it in the pathogenic T. pallidum. http://sti.bmj.com/ negative contrast method with a magnification X6-8000 has confirmed the fact that the thickness The basal granules have an annular shape (Fig. 1); of the treponeme is uneven along its length, as the fibrils that are attached to them stretch towards in microscopic studies the opposite end of the treponeme encircling its body. established previously light the view that are on living specimens. It has been proved that the We do not agree with these fibrils thickness changes as the treponeme moves; this inserted in the ends of the treponeme or in one end only, remaining free at the opposite end. Actually, clearly shows that the body of T. pallidum is able on September 27, 2021 by guest. Protected to contract and, consequently, has a special apparatus each fibril has two insertions: one into a basal for this purpose. granule at one end of the treponeme and the other seen that in into some other part of its body. The number of With greater magnification it can be between young cultures the body of the treponeme is covered fibrils differs not only individual specimens the of a in the by a transparent mass forming a wide mucoid but throughout body single treponeme; In middle of the body one may find ten to twelve fibrils sheath (Ovcinnikov and Delektorskij, 1969). older and this sheath cannot be demonstrated, and the but in another part only three, so on. cultures are studied the question of its origin (i.e. whether it is a product of When the organisms by negative or derives from the host contrast method the fibrils appear to lie on the surface the treponeme tissues) seem to be should be studied in relation to the specific locali- of the treponeme and sometimes entirely zation of the organism and the conditions of its free. This impression prompted some authors to existence. suggest that T. pallidum lacks flagella. Bearing in mind certain differences in the structure Mesosomes, rounded formations of various sizes, of cultivated and pathogenic treponemes, we shall are distributed throughout the body of the tre- poneme. They probably differ in their structure and Received for publication March 1971 functions, and are called mesosomes only because Paper presented at the International Colloquium on the Late Trepo- nematoses, Miami, Florida, U.S.A., on January 4-8, 1971 they cannot yet be further indentified. 316 British Journal of Venereal Diseases Br J Vener Dis: first published as 10.1136/sti.47.5.315 on 1 October 1971. Downloaded from The treponeme has a granular surface, and delicate ('spheroids', according to their terminology) and the parallel bands course along its body. encystment of treponemes under unfavourable In pathogenic treponemes (Nichols, I, Budapest conditions, reached the unexpected conclusion that strains) studied by the negative contrast method, the the cysts were a product of degeneration and not a terminal formations at the two opposite ends differ means of safeguarding the existence of the organism. radically in structure. One end of the organism is As a proof, they mention the fact that 'spheroids' are acuminate and consists of tubular formations con- formed under the influence of diverse stressful tained in a round cavity, but the other end is devoid of factors. In our opinion, this fact affirms rather than them. denies the assumption that the encystment of tre- Each end is provided with a spongy mass adjacent ponemes is a protective mechanism ensuring survival to the outer wall and with basal granules carrying in adverse circumstances. fibrils. If the stress causing encystment surpasses the ad- In past years opinions have differed as to the mode aptive capabilities of the treponeme, the organism of reproduction of T. pallidum. It has now been perishes and then undergoes degeneration. If the proved beyond any doubt that transverse fission is stress is not lethal, accessory envelopes are formed the main mode of reproduction. The treponeme and the treponemes become well encapsulated and divides into halves or undergoes multiple division. may survive new stresses many times stronger than The new organisms may be so numerous that some the initial one. Encystment as a mechanism of of them measure hardly more than one coil of the survival and mode of reproduction is widespread in original. This fact is very important, both from the nature, especially among protozoa. purely morphological point of view, and because it Under stressful conditions, the treponeme 'packs' testifies to the possible existence of non-spiraJ forms itself into a compact roll (Fig. 8) and becomes of T. pallidum. During the process of division into covered with a transparent mucoid capsule, which copyright. halves, an isthmus is formed; old fibrils are shed and resists the penetration of drugs and antibodies. The new ones appear on either side of this isthmus, which organisms may persist in this form for a prolonged persists for some time after the fission. Subsequently period without any reaction from the host. The en- it ruptures and the residual bodies are gradually cysted treponemes and the host coexist more or less resorbed. peacefully, but under propitious circumstances the The possibility of longitudinal fission may be cysts may be transformed again into the usual spiral, categorically denied at present. Sexual activity on the which damages the cells of the host and elicits a part of the treponemes cannot be ruled out, and there response. We shall return to this problem in our http://sti.bmj.com/ is some evidence to suggest this mode ofreproduction. description of ultrathin sections of cysts. Sometimes we may observe thick and thin speci- Studies of ultrathin sections of T. pallidum have mens which are interlaced or lie in apposition. In considerably enriched our knowledge of its structure. other pairs one treponeme is dark or smooth and its The methods of processing the material for ultrathin partner is light or granular. It may be asked whether sections have been described in our previous com- these are not males and females. Their bodies inter- munications. twine and come into contact over certain areas cor- on September 27, 2021 by guest. Protected responding to the mesosomes which are located near Outer membranes the head structure. This seems to admit the possi- In ultrathin sections (approximately 200-250 A bility of sexual reproduction. thick) the outer wall (me) of T. pallidum consists of Another mode of reproduction resorted to in ad- two electron dense layers and one electron transparent verse circumstances consists in the formation of layer (Fig. 2). The cytoplasmic membrane (cm) also spores which subsequently develop into new has three layers. It is separated from the outer wall treponemes. The breakdown into granules is especi- (co) by a space of varying width that contains ally pronounced under the action of penicillin and fibrils (f). In longitudinal sections they appear immune sera. punctate or elongated depending on the plane of Of extreme importance is the ability of treponemes cutting. The cytoplasmic membrane lies over the to form cysts under unfavourable conditions. In- cytoplasm (c) (Fig. 4), where ribosomes (r) are sus- vestigators using light microscopy have expressed pended. Here and there light steUate or polygonal divergent opinions about the existence of cysts. structures are visible; these are the nuclear vacuoles Positive electron microscopic evidence has been (N) (Figs 12 and 13). Mesosomes (M) are met with offered only by Ryter and Pillot (1963) as well as occasionally (Fig. 2), looking like laminar formations. ourselves. However, Ryter and Pillot (1963) and The same structures can be found in a transverse Pillot (1965), having correctly described the cysts section.
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