Overview and General Characteristics of Chlamydiae

Definition and classification. The bacteria in the taxonomic family Chlamy-diaceae are small (0.3-1 im) obligate cell parasites with a Gram-negative cell wall. The reproductive cycle of the chlamydiae comprises two developmental stages: The elementary bodies are optimally adapted to survival outside of host cells. The initial bodies, also known as reticulate bodies, are the form in which the chlamydiae reproduce inside the host cells by means of transverse fission. Three human pathogen species of chlamydiae are known: C. psittaci, C. trachomatis (with the biovars trachoma and lymphogranuloma venereum), and C. pneumoniae.

Morphology and developmental cycle. Two morphologically and functionally distinct forms are known:

■ Elementary bodies. The round to oval, optically dense elementary bodies have a diameter of approximately 300 nm. They represent the infectious form of the pathogen and are specialized for the demands of existence outside the host cells. Once the elementary bodies have attached themselves to specific host cell receptors, they invade the cells by means of endocytosis (Fig. 4.27). Inside the cell, they are enclosed in an endocytotic membrane vesicle or inclusion, in which they transform themselves into the other form—initial bodies—within a matter of hours.

■ Initial bodies. Chlamydiae in this spherical to oval form are also known as reticular bodies. They have a diameter of approximately 1000 nm. The initial bodies reproduce by means of transverse fission and are not infectious while in this stage. At the end of the cycle, the initial bodies are transformed back into elementary bodies. The cell breaks open and releases the elementary bodies to continue the cycle by attaching themselves to new host cells.

— Reproduction Cycle of Chlamydiae -

i—i—i—i—i—i—i—i—i— 0 6 12 18 24 30 36 42 48

Hours after infection

Fig. 4.27 Two chlamydial stages: elementary body and initial body. a Attachment of elementary body to cell membrane. b Endocytosis.

c Transformation of elementary body into initial body inside the endosome. d Reproduction of initial bodies by transverse fission. e Transformation of some initial bodies back into elementary bodies. f Lysis of inclusion vesicle and cell, release of initial and elementary bodies.

Culture. Chlamydiae exploit energy metabolism processes in their host cells that they themselves are lacking (ATP synthesis). For this reason, they can only be grown in special cell cultures, in the yolk sacs of embryonated hen eggs, or in experimental animals.

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