The Chlamydial Cell Envelope

Abstract

Chlamydial development is punctuated by changes in protein-protein interactions on elementary body (EB) and reticulate body (RB) surfaces. Reduction of disulfide cross-links in the chlamydial outer membrane complex (COMC) concomitant with attachment and entry of the EB is rapidly followed by transition to the fragile RB, which is specialized for acquisition of nutrients during chlamydial growth and differentiation. This chapter reviews knowledge about the progression starting with the structure of the EB envelope in the extracellular environment and the way in which this surface interacts with, and is altered during, the process of chlamydial attachment, entry, development, and exit from host cells. The presence of gram-negative double membranes was confirmed by early transmission electron microscopy (TEM) studies of RBs and EBs, but challenges in purification and fractionation of RB membranes shifted emphasis toward EB membranes in subsequent studies. Regularly spaced hexagonal lattices were observed in negatively stained EB envelope preparations. Hexagonal lattices, located between the outer and cytoplasmic EB membranes, and similar structures were present in multiple Chlamydia species. Varying degrees of evidence suggest that additional EB OMPs may exist. The envelopes of the extracellular EB and intracellular RB differ markedly and reflect requirements for survival of chlamydiae in two unique environments. Outside the cell, the oxidized COMC provides osmotic protection, restricts loss of metabolites across the OM, and may mask immunogenic determinants that could alert host cells to the presence of these pathogens.