The growth cycle of Simkania negevensis

Simkania negevensis, a bacterium formerly referred to as 'the micro-organism Z' or 'Simkania Z', belongs to the order Chlamydiales, assigned to the family Simkaniaceae: The purpose of this study was to investigate the production of Simkania negevensis progeny in infected cells in comparison with the well-documented Chlamydiaceae developmental cycle. It was found that replicating Simkania negevensis in Vero cells resembled the reticulate bodies of all known chlamydial species: in electron micrographs they were reticulated, homogeneously staining, and often caught in the process of binary division. These replicative forms were found in low abundance shortly after infection, but by 3 days post-infection they were the most prevalent particles in host cells. Electron-dense forms of Simkania negevensis began to appear on the third day post-infection, but quantitatively did not account for the high titre of infectivity in extracts from these host cells. These had both electron-dense and electron-lucent areas, a characteristic seen only in a few chlamydial species. Simkania negevensis infectivity did not appreciably change during the ensuing 12 days required for host cell lysis, despite an eightfold increase in the proportion of electron-dense bacteria over this time. The emergence of electron-dense bodies, increase in infectivity and host-cell lysis were not synchronized developmental events. This is a novel finding in Chlamydiales spp. and suggests that Simkania negevensis will provide new perspectives in the mechanisms of chlamydial intracellular growth.