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Chlamydial structure

The elementary body.

Chlamydial elementary bodies (EBs) are small, round or occasionally pear shaped, electron-dense structures approximately 0.3 microns in diameter [a micron is one thousandth of a millimetre, just above the ~ 0.25 micron limit of resolution of a light microscope]. The EB is the only infectious stage of the chlamydial developmental cycle. It functions as a tough "spore-like" body whose purpose is to permit chlamydial survival in the non-supportive (to chlamydiae) environment outside the host cell. The EB is thought to be metabolically inert until it attaches to, and is endocytosed by, a susceptible host cell. It contains only small amounts of the usual bacterial cell wall strengthening substance, peptidoglycan. Instead it derives its strength, among other things, from cross links [-S-S- bridges] formed between the sulphur atoms of its sulphur amino acid [cysteine and methionine] rich proteins in the outer envelope, see: outer envelope proteins. The ultrastructure of chlamydial EBs has been extensively studied, most notably in laboratories in Amiens, France and in Japan [Eb et al., 1976; Louis et al., 1980; Matsumoto, 1982a,1988; Rockey & Matsumoto, 2000; Solof et al., 1982]. Although pear-shaped EBs were originally regarded as a species characteristic of C. pneumoniae, it is now clear that this is erroneous [Miyashita et al., 1993].

Figure EB1. A transmission electron microscope picture of a thin section through an elementary body of C. psittaci Cal 10. The sample has been treated with ruthenium red to enhance the electron opacity of the surface projections (arrows). The most obvious feature is the eccentric, electron dense (black) DNA core (n) which is  tightly compacted onto chlamydial histone protein. Histones are basic, DNA-binding proteins commonly found in higher plant or animal cells, but unusual in bacteria. Note that the EB cytoplasm (c) has a granular appearance due to the presence of 70S ribosomes [responsible for protein synthesis]. Surrounding the cytoplasm is a lipid cytoplasmic membrane and a rigid outer envelope (both ~8 nm) containing a regularly packed hexagonal structure of 16.7 nm. This structure, together with extensive -S-S- bridging of the sulphur amino-acid rich outer envelope proteins, probably accounts for much of the outer envelope strength.  The bar represents 0.15 microns. [Electron micrograph kindly provided by A. Matsumoto Okayama University Medical School, Japan. From:  Matsumoto A., (1988) Structural characteristics of chlamydial bodies. Pages 21-45. In:  Microbiology of Chlamydia. (Baron, A. L. ed.).  CRC Press., Boca Raton, Fl., USA ISBN 0-8493-6877-4] 

Figure EB2.  A replica of freeze deep-etching of chlamydial elementary bodies of C. trachomatis D-12N within a McCoy cell inclusion 44 hours post infection. The bar is 0.1 microns. Note the projections radiating from the surface of each EB. These projections were first observed by Matsumoto et al., 1976 [Unpublished electron micrograph kindly provided by A. Matsumoto.]

Figure EB3. Electron micrograph of a carbon replica of the freeze fractured surface of an EB of plasmid and glycogen-free C. trachomatis D9-3. Pits in the carbon layer corresponding to the projections on the surface of the EB can clearly be seen. The bar represents 0.1 microns. [Electron micrograph kindly provided by A. Matsumoto from: Matsumoto, A., Ikegami, M., Uehira, K., Ohmori, S. & Tanaka, Y (1999). The morphology of Chlamydia trachomatis plasmid-free strains. Symposium on Chlamydial Infections. Japan Society for Chlamydia research. Pages 15-18. Life Science Medica Co., Tokyo, Japan. ISBN4-947628-43-X].

Figure EB4. Similar carbon replica of the surface of a purified suspension of EBs of C. psittaci Cal 10, with the exception that this shows a positive image of the radiating projections and the EBs are not in an inclusion. Projections are arranged hexagonally, with an approximate centre to centre spacing of 50 nm. The bar is 0.1 microns. [Electron micrograph kindly provided by A. Matsumoto from: Matsumoto, A., (1979). Recent progress of electron microscopy in microbiology and its development in future: from a study of the obligate intracellular parasites, Chlamydia organisms. J. Electron Microsc. 28 (Suppl.) 57-64.].

Figure EB5. Negatively stained envelopes of C. psittaci Cal 10 EBs. A) An envelope showing projections (arrows) and the 30 nm rosettes (arrowheads) from which projections split off. B) Vertical view of projections and rosettes (arrows). The bars are 0.1 microns. im = inner membrane; om = outer membrane. [Electron micrograph kindly provided by A. Matsumoto from: A) Matsumoto A., (1988) in Baron, A. L. (ed). Microbiology of Chlamydia. CRC Press. B) Matsumoto, A. (1982). Morphology of Chlamydia psittaci elementary bodies as revealed by electron microscopy. Kawasaki Med. J. 8, 149-157].

Figure EB6. Negatively stained envelope fragment of a C. psittaci Cal 10 EB. Note the dark centred rosettes, (arrows), the rotational symmetry of which (inset) shows that they are made up of 9 sub units. [Electron micrograph kindly provided by A. Matsumoto from: Matsumoto, A., (1979). Recent progress of electron microscopy in microbiology and its development in future: from a study of the obligate intracellular parasites, Chlamydia organisms. J. Electron Microsc. 28 (Suppl.) 57-64.].

Figure EB7. Rosettes on the surface of C. trachomatis D9-3 exposed by deep freeze etching. The rosette is composed of 8 subunits, unlike the 9 subunits for C. psittaci shown in Fig EB6. [Electron micrograph kindly provided by  A. Matsumoto from: Matsumoto, A., Ikegami, M., Uehira, K., Ohmori, S. & Tanaka, Y (1999). The morphology of Chlamydia trachomatis plasmid-free strains. Symposium on Chlamydial Infections. Japan Society for Chlamydia research. Pages 15-18. Life Science Medica Co., Tokyo, Japan. ISBN4-947628-43-X].

Figure EB8. Envelopes (outer membrane-inner membrane complexes) of C. psittaci Cal 10 EBs treated with tannic acid to enhance the electron opacity. Arrows indicate the sites of the projections and arrowheads indicate DNA protruding from the nucleus and binding at the inner membrane at sites coincident and adjacent to the projections. The bar represents 0.1 microns. [Electron micrograph kindly provided by A. Matsumoto  from: Matsumoto, A. (1981). Electron microscopic observations of surface projections and related intracellular structures of Chlamydia organisms. J. Electron Microsc. 30, 315-320].

Figure EB9. Purified isolated projections from C. psittaci Cal 10 at high magnification. Each projection appears to be composed of fine subunits. The bar represents 0.05 microns. [Electron micrograph kindly provided by A. Matsumoto  from: Matsumoto, A. (1988). In: Baron, A. L. (ed). Microbiology of Chlamydia. CRC Press].

Figure EB10. Dr Matsumoto's model of the arrangement of the EB projections in the centre of the 8 or 9 subunit "flower" rosette. The rosette in turn is embedded in the envelope and has DNA strands extending to it  from the interior. There is speculation, but as yet no proof, that the Projection / Rosette complex may represent a type three secretion in chlamydiae, serving to introduce active chlamydial proteins across the inclusion membrane into the host cell cytoplasm. Modified from: Matsumoto, A. (1988). In: Baron, A. L. (ed). Microbiology of Chlamydia. CRC Press

[MEW], May 2002

NEXT: Reticulate Body structure.

References.

Carter, M. W., al-Mahdawi, S. A., Giles, I. G., Treharne, J. D., Ward, M. E. & Clarke I. N. (1991). Nucleotide sequence and taxonomic value of the major outer membrane protein gene of Chlamydia pneumoniae IOL-207. Journal of General Microbiology 137, 465 - 475. [States pear shaped EBs not a reliable taxonomic indicator for C. pneumoniae].

Eb, F., Orfila, J. & Lefebvre, J. F. (1976) Ultrastructural study of the development of the agent of ewe's abortion. Journal of Ultrastructure Research 56, 177 - 185.

Louis, C., Nicolas, G., Eb, F., Lefebvre, J. F. & Orfila, J. (1980). Modifications of the envelope of Chlamydia psittaci during its developmental cycle: freeze-fracture study of complementary replicas. Journal of Bacteriology 141, 868 - 875.

Matsumoto, A. (1973). Fine structures of cell envelopes of Chlamydia organisms as revealed by freeze-etching and negative staining techniques. Journal of Bacteriology 116, 1355 - 1363.

Matsumoto A, Fujiwara E, Higashi N. (1976). Observations of the surface projections of infectious small cell of Chlamydia psittaci in thin sections. Journal of Electron Microscopy (Tokyo). 25, 169 - 170.

Matsumoto, A. (1982a). Surface projections of Chlamydia psittaci elementary bodies as revealed by freeze-deep-etching. Journal of Bacteriology 151, 1040 - 1042.

Matsumoto, A. (1982b). Electron microscopic observations of surface projections on Chlamydia psittaci reticulate bodies. Journal of Bacteriology 150, 358 - 364.

Matsumoto, A. (1988) Structural characteristics of chlamydial bodies. Pages 21-45. In:  Microbiology of Chlamydia. (Baron, A. L. ed.).  CRC Press., Boca Raton, Fl., USA ISBN 0-8493-6877-4

Miyashita, N., Kanamoto, Y. & Matsumoto, A. (1993). The morphology of Chlamydia pneumoniae. Journal of  Medical  Microbiology 38, 418 - 425. [C. pneumoniae EBs often not pear shaped].

Soloff, B., Rank, R. G., & Barron, A. L. (1982). Ultrastructural studies of chlamydial infection in guinea-pig urogenital tract. Journal of Comparative Pathology 92, 547.