Environmental chlamydiae and amoebae

Chlamydiae are a unique group of bacteria that infect and live exclusively within eukaryotic host cells. While chlamydial pathogens of humans and animals, such as Chlamydia trachomatis and Chlamydia pneumoniae, are known for more than 100 years, chlamydiae associated with protists have been identified only recently. This project aimed to improve our understanding of these environmental chlamydiae and their similarities with and differences to their pathogenic counterparts by a multiphasic molecular biology approach. We revealed that the diversity of chlamydiae in the environment exceeds the known diversity by orders of magnitude; a vast number of unknown chlamydiae is hidden primarily in marine environments. We developed and successfully applied novel methods to harvest this diversity by isolation of novel chlamydiae from complex environmental samples, resulting in the identification and characterization of several new environmental chlamydiae. Using state-of-the-art genomics analyses, we gained insights into the genetic repertoire of environmental chlamydiae, and we were able to highlight similarities and major differences with their pathogenic counterparts. Using this data we reconstructed the evolutionary history of chlamydiae and revealed that chlamydiae represent probably the oldest group of bacteria able to infect eukaryotic cells; an essential contribution of chlamydiae to the evolution of algae and plants, as suggested recently, seems plausible. Challenging long-standing text-book knowledge, we provide evidence that chlamydiae remain physiologically active even outside of their host cells, and we for the first time were able to demonstrate the presence of peptidoglycan in the cell wall of chlamydiae, a key component of the cell wall of most bacteria whose existence in chlamydiae has been debated controversially for decades. These findings do not only provide a fresh perspective on the cell biology of these bacteria but also have potential implications for anti-chlamydia therapy. Finally, the in-depths analyses of cell envelope associated proteins of environmental chlamydiae such as outer membrane proteins and nucleotide transport proteins provided new insights into the adaptation of chlamydiae to and the interaction with their different host cells. In summary, the knowledge gained in this project contributes to a better understanding of diversity, biology, host interaction and evolution of chlamydiae and demonstrated the power of comparative analysis of pathogens and symbionts.