Unique ultrastructure in the elementary body of Chlamydia sp. strain TWAR

The ultrastructure of two prototype strains (TW-183 and AR-39) of Chlamydia sp. strain TWAR was described. The TWAR elementary body (EB) demonstrated a unique morphology and structure distinct from those of other chlamydial organisms. It was pleomorphic but typically pear shaped. The average size was 0.38 micron, with a long axis of 0.44 micron, a short axis of 0.31 micron, and a ratio of the long to the short axes of 1.42. The cytoplasmic mass was round, with an average diameter of 0.24 micron. There was a large periplasmic space. Small, round electron-dense bodies (0.05 micron in diameter), which were attached to the cytoplasm by a stringlike structure, were seen in the periplasmic space. These features are in contrast to those of other chlamydiae, which are typically round with a narrow or barely discernible periplasmic space. The TWAR reticulate body (RB) was morphologically and structurally similar to those of other Chlamydia species, having an average diameter of 0.51 micron and being circular in shape. The ultrastructural observations of the intracellular growth of TWAR in HeLa cells revealed that TWAR underwent the same developmental cycle as do other chlamydiae, i.e., transformation of EB to RB, multiplication by binary fission, and maturation by transformation of RB to EB via the intermediate-form stage.