|
The chlamydial developmental cycle in pictures.
Introduction.
Fig 1. Diagram
of an idealised chlamydial developmental cycle, courtesy of Dr Karin D Everett. The small, infectious elementary bodies
are in red; the larger, replicating reticulate bodies
are in green.
Chlamydial infection is initiated
by attachment of a chlamydial elementary body to the host
cell, followed by its entry into the cell.
The chlamydial elementary bodies
are internalised in tight, endocyctic vesicles, within which they differentiate
into reticulate bodies.
|
|
|
Fig 2.
Multiple invasion of a laboratory-infected HeLa 229 cell. The matured infectious elementary body (i), 0.3 microns in size, has
an electron dense "black" core of nucleic acid condensed onto chlamydial histone
protein. By 3 hours after infection, some of the elementary bodies are already
beginning to differentiate. Note at (ii) that the nucleic acid core is less marked;
the spotty cytoplasm is due to the appearance of protein-synthesizing ribosomes. At (iii)
the elementary body has enlarged further, the cytoplasm is less dense and the chlamydial
DNA core even less prominent. Note the host cell membrane surrounding the chlamydial
particle at (m). The bar represents 0.1 microns. Electron micrograph by M.
E. Ward modified from Ward, M. E. (1983). Chlamydial classification, development and
structure. British Medical Bulletin 39, 109-115.
|
Fig 3. By 9
hours post infection, chlamydial endosomes containing single, 1 micron, reticulate
bodies (R) can be seen. These originated from the differentiation of single
elementary bodies. Note that some of the chlamydiae have a division septum (ds) and
are already dividing by binary fission in typical bacterial manner. Note also the
endosomal membrane (e) and the presence of many vesicles of host cell membrane (m).
At this stage the chlamydial endosome enlarges by intercepting exocytic membrane traffic
from the Golgi apparatus (see: reticulate bodies; membrane
trafficking). The bar represents 1 micron. Electron micrograph by M. E. Ward
as above.
|
Table:
Comparison of chlamydial elementary bodies and reticulate bodies:
| Characteristic |
Elementary Body |
Reticulate Body |
| Size |
0.2 - 0.3 microns |
1 micron |
| Morphology |
Electron dense core; rigid |
Fragile, pleomorphic |
| Infectivity to host |
Infectious |
Non-infectious |
| RNA : DNA ratio |
1 : 1 (condensed DNA core) |
3 : 1 (increased ribosomes) |
| Metabolic activity |
Relatively inactive |
Active, replicating stage |
| Trypsin digestion |
Resists |
Sensitive |
| Projections
and rosettes |
Few |
More |
Table Development 1. A
simplified table contrasting the basic properties of chlamydial elementary and reticulate
bodies. Modified from: Ward, M. E. (1983). Chlamydial classification, development and
structure. Brit Med Bull 39, 109-115.
Figures 4 and 5:
|
|
|
| Fig 4. A
chlamydial inclusion 15 hours post infection containing many reticulate bodies (RB)
of C. trachomatis LGV 404. Note the endosome membrane (em) and the blebs of
membranous material (mb) in the inclusion, probably derived from the reticulate
body outer envelope. Chlamydial lipopolysaccharide, exported from reticulate body
inclusions, is the basis of some of the enzyme immunoassay tests for the diagnosis of
chlamydial infection by the detection of chlamydial antigen. The bar represents 1 micron.
Electron micrograph by M. E. Ward, as above. |
Fig 5. Thin
section of part of the contents of a mature C. trachomatis UW4
inclusion, 40 hours after infection of a HeLa 229 cell. The picture shows
the large, fragile, reticulate bodies (R), the smaller intermediate
bodies (I) which develop from them with their characteristic
condensed nucleoids of nucleic acid, and the slightly smaller elementary
bodies (E) with their dense gene core. The bar represents 1 micron.
Previously unpublished electron micrograph by M. E. Ward.
|
|
|
|
| Fig
6.
Electron micrograph showing the connection of three different reticulate
bodies of C. psittaci Cal 10 to the inclusion membrane. Tannic acid
staining was used to enhance the opacity of the projections which connect
to, and penetrate, the inclusion membrane. Electron micrographs courtesy
of A. Matsumoto, Okayama University Medical School, Japan.
From: Matsumoto, A. (1981). Journal of Bacteriology 145, 605
- 612. |
Fig 7. Carbon
replica of a freeze-fractured face of a C. psittaci Cal 10
inclusion at 18 hours post infection. The arrows show the projections
studding the inclusion membrane. Electron micrograph courtesy of A.
Matsumoto. Modified from: Rockey, D. D. & Matsumoto, A. (1999).
The chlamydial developmental cycle. In: Prokaryotic development (Brun
& Shimkets, eds.) ASM Press.
|
 |
|
|
Fig 8. Some 18-22
hours post infection, reticulate bodies begin to differentiate again into elementary
bodies (E), inside the chlamydial inclusion. The initial sign of this is
the re-condensation of chlamydial nucleic acid on to histone protein. This stage is called
the intermediate body, (I). Although it is usually thought that 1
reticulate body gives rise to 1 elementary body, often more than 1 elementary body may be
formed as shown in this photograph, where two intermediate bodies can be seen in the
act of division. The intermediate bodies are approximately 0.5 microns in size. C.
trachomatis LGV 404. Electron micrograph by M. E. Ward. |
Fig 9.
Four mature elementary bodies surrounded by the envelope of the reticulate
body that produced them. The bar represents .15
microns. Electron micrograph by M. E. Ward from: Ward,
M. E. The chlamydial
developmental cycle. In: Microbiology of Chlamydia, (Barron, A. L. ed). CRC Press, (1988).
|
|
|
|
Fig 10. Mature
inclusion of C. trachomatis LGV 404 in a BGMK cell, 48 hrs after infection. The
cell cytoplasm is packed with small, dark-staining, chlamydial elementary bodies and the
larger, grey, decaying remains of the chlamydial reticulate bodies that produced them. The
host cell nucleus has been pushed into the left-hand corner of the cell. A cell like this
will eventually lyse to release infectious chlamydial elementary bodies to restart the
cycle. Electron micrograph by M. E. Ward.
|
Fig 11.
Beautiful
"comets" of 48 hr mature inclusions of C. trachomatis LGV 404 in HeLa
229 cells. The preparation has been stained with Hoechst 33258, which forms a fluorescent
complex in DNA. Round or kidney bean-shaped host cell nuclei can be seen together with
tiny dots of fluorescing chlamydial elementary bodies. The comet "body" is made
of DNA-containing, fluorescent chlamydial particles packed into mature cytoplasmic
inclusions. The preparation was photographed under short wave ultra violet light.
Fluorescence micrograph by M. E. Ward. Optical magnification 450x. |
|
|
| Fig 12. A
HeLa 229 cell infected for 40 hrs with C. trachomatis LGV 404, critical point
dried, then freeze-fractured open. Ice has removed most of the chlamydial particles,
revealing the inclusion membrane. Chlamydiae extensively modify this
membrane with Inc and other proteins. The inclusion membrane
enlarges by intercepting exocytic membrane vesicles
from the host cell's Golgi apparatus. Field emission scanning electron micrograph by M.
E. Ward and C. Inman, Southampton Biomedical Imaging Unit. |
Fig 13. Specimen
and preparation as for Fig 10. Detail of the freeze-fractured interior of a mature
chlamydial inclusion showing, slightly left of centre, the decaying remains of a
reticulate body surrounded by membrane blebs of chlamydial antigen similar to those in Fig
4. The small round structures are chlamydial elementary bodies and the fine granular
matrix is due to the glycogen-like reserve carbohydrate that C. trachomatis deposits in
inclusions. Field emission scanning electron micrograph by M. E. Ward
and C. Inman, Southampton Biomedical Imaging Unit. |
|
|
|
Figure 14. Field emission
scanning electron micrograph of a mature inclusion of C. trachomatis
serovar L1 in a BGMK cell. Specimen and preparation as for Fig 10. Electron micrograph by M.
E. Ward and C. Inman. |
For illustrated descriptions of the chlamydial
developmental cycle see Rockey & Matsumoto, 2000 and Ward
1983; 1988. Although the chlamydial life cycle is widely
regarded as being unique, a superficially similar cycle has evolved in an
unrelated lineage of gamma Proteobacteria formed by the intracellular pathogens
Coxiella and Legionella [Corsaro et al., 2003; see
Rhabdochlamydia].
Thus the life cycle alone, unless accompanied by 16S rDNA sequence data, is
insufficient to prove that an organism belongs to the Chlamydiales.
For a description of the developmental cycle of
environmental chlamydiae, see: Parachlamydiaceae.
See also the description of the developmental
cycle of Rhabdochlamydia.
[MEW] May 2004
NEXT: Temporal
regulation of the developmental cycle
References
Corsaro, D., Valassina, M. & Venditti, D. (2003).
Increasing diversity within Chlamydiae. Critical Reviews
of Microbiology 29, 37 - 78.
[Excellent scholarly review and taxonomic study]
Rockey, D. D. & Matsumoto, A. (2000). The chlamydial
developmental cycle. Pages 403-425. In: Prokaryotic Development (Brun, Y. V. &
Shimkets, L. J. eds.). ASM Press, Washington D. C.
Ward, M. E. (1983). Chlamydial
classification, development and structure. British Medical Bulletin 39,
109 - 115. [An ancient issue devoted to chlamydiae, but
still good stuff]
Ward, M. E. (1988). The chlamydial
developmental cycle. Chapter 4 In: Microbiology of Chlamydia (Barron, A.
L. ed)., pp 71 - 95, Boca Raton Florida, CRC Press ISBN 0-8493-6877-4 [Shows
multiple EBs arising from single RBs].
NEXT: Temporal
regulation of the developmental cycle
|
