Phyla Related to Planctomycetes: Members of Phylum Chlamydiae and Their Implications for Planctomycetes Cell Biology

Planctomycetes and Chlamydiae are the best studied phylum members of a recently recognized superphylum, which also includes phyla Verrucomicrobia, Lentisphaerae , and Poribacteria . The phylum Chlamydiae includes established human pathogens such as Chlamydia trachomatis , species of major veterinary and economic importance such as Chlamydia abortus , as well as a large biodiversity of recently discovered environmental bacteria commonly termed Chlamydia- related bacteria, which include emerging pathogens. In this chapter, we summarize the recent knowledge gathered on the biology and the evolution of phylum Chlamydiae , comprising the single order Chlamydiales, especially focusing on specific features shared with Planctomycetes . All members of the Chlamydiales order, which diverged more than one billion years ago, have a biphasic life cycle with metabolically active reticulate bodies (RBs) and infectious elementary bodies (EBs). The ultrastructure of both developmental stages shows morphological differences in cell size and structure as well as a DNA condensation that occurs during the differentiation from RBs to EBs. Like Planctomycetes, Chlamydiae genomes encode two DnaA replication initiator proteins but no FtsZ, the protein forming the division ring in most other bacteria and even in chloroplasts. Finally, biological and genomic data on Chlamydia and Chlamydia -related bacteria revealed their very long history as intracellular parasites of eukaryotes and their possible role in a chimerical event that may have led to the formation of current chloroplasts. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.