Ravi Kaul and Wanda M. Wenman

Department of Pediatrics, Section of Infectious Diseases, 403 Neurosciences Building, School of Medicine, 1515 Newton Court, University of California, Davis, CA 95616

Received 2/16/ 98 Accepted 2/20/98

3. LIFE CYCLE

Chlamydiae have evolved a complex and unique developmental cycle alternating between two morphologically distinct forms: small (0.2-0.3 mm), extracellular, elementary bodies (EB) and large (1 mm) intracellular, metabolically active reticulate bodies (RB). Infection is initiated by attachment of infectious and metabolically inactive EB to susceptible host cells (8, 9). Attachment is followed by uptake and enlargement within the host cell by circumventing phagosomal fusion to form non-infectious RB which divide within the cytoplasmic inclusions by binary fission (10). The life cycle is completed with the reorganization of RB into EB which subsequently leave the disrupted host cell ready to infect new cells (11). The whole developmental cycle proceeds over 48-72 h depending upon the strain, culminating in completely deranged host cellular and sub-cellular structure. During the morphological transformation of RB to EB, the bacterial chromosome becomes highly condensed and its outer membrane becomes extensively disulfide cross-linked, rendering the EB membrane resistant to osmotic lysis (12, 13). As a result of these changes, the EB nucleoid appears electron dense, localized at the center of the cell, whereas RB chromatin is more relaxed with diffused fine fibrils extending throughout the cell (12). These characteristics are believed to render EB incapable of participating in the transcription and replication processes, as compared to RB whose nucleoid structure is quite relaxed and compatible with replication (14). Wager and Stephens (15) reported the presence of three DNA binding proteins in Chlamydia with molecular masses of 17, 25 and 58 kDa. They suggested that these proteins may be involved in DNA condensation during the transition from RB to EB form. The molecular mass of two of the three DNA binding proteins correspond to previously described HeLa cell binding proteins (16, 17). It is now believed that the unusual composition of DNA binding proteins and their charge are responsible for many of the properties that lead to their affinity for eukaryotic cell surface components.