Chlamydiales. Bacteria belonging to the Chlamydiales group are obligate intracellular parasites of eukaryotic cells. They are found within vertebrates, invertebrate cells, and amoebae hosts. In 1999 this group was taxonomically reclassified and it now includes a broader range of Gram-negative bacteria based on their similarities with regards to their nucleotide sequences (predominantly ribosomal RNA) and their two-stage developmental cycle of replication (Everett et al., 1999). The Chlamydiales group is now divided into four families, Chlamydiaceae, Simkaniaceae, Parachlamydiaceae and Waddliaceae. The Chlamydiaceae family itself is divided into the genera Chlamydia and Chlamydophila (meaning Chlamydia-like). They have a common evolutionary lineage that started around 2 billion years ago, which makes them unique compared to other types of bacteria. Chlamydiae are one of the commonest causes of sexually transmitted diseases (STDs) and if left untreated may cause infertility in women. They are transmitted by direct contact or aerosols, and can cause various diseases, while also being able to coexist with the host in an apparently asymptomatic state.

Chlamydophila abortus. Some strains of Chlamydophila abortus were previously classed under Chlamydia psittaci. Endemic among ruminants, the bacteria mainly resides in the placenta, which leads to weak newborns or abortions. Chlamydophila abortus is the reference strain for determining whether a new strain belongs to the Chlamydiaceae family.