New view of the surface projections of Chlamydia trachomatis.

Two kinds of surface specializations of chlamydiae have been described: hemispheric projections and spikelike rods. We undertook the present studies to demonstrate chlamydial ultrastructure in greater detail in conventional thin-sectioned specimens. Chlamydia trachomatis (LGV strain L2/434/Bu), cultured for 40 h in L929 mouse fibroblasts, was fixed in glutaraldehyde-acrolein, p-formaldehyde-glutaraldehyde, or glutaraldehyde-osmium tetroxide mixtures, postfixed in osmium tetroxide, stained in uranyl acetate, dehydrated in ethanols, and embedded in Epon. By the use of fixatives that penetrate and fix rapidly, chlamydial outer and plasma membranes were clearly revealed. Our results indicate that the hemispheric projections are specializations of the plasma membrane of elementary bodies. The spikelike projections are found in intermediate forms, originate beneath depressions of the plasma membrane, and extend through the periplasmic space and outer membrane to end with pointed tips. Improved preservation of chlamydiae provides a new, informative view of their complex structure. Significant interactions between chlamydiae and host cells might be influenced by the surface structures shown in this study.