Eukaryotic genes in vacuolar pathogens and symbionts (EUGENPATH)

The phylogenetic diverse group of intracellular environmental bacteria comprises many human pathogens. Some of these resist degradation by free-living protozoa and likely due to training in amoebae also by macrophages. Macrophage resistance is a prerequisite for several life-threatening human respiratory diseases, including a severe pneumonia termed Legionnaires disease caused by Legionella spp., Q fever provoked by Coxiella burnetii, and respiratory diseases possibly caused by the amoeba-associated symbionts related to the Parachlamydiaceae. Intracellular bacterial pathogens and symbionts maintain intimate interactions with their eukaryotic hosts, which have been shaped by mutual trans-kingdom co-evolution over extended periods of time. To adapt to their intracellular niches, these bacteria acquired and functionally integrated genes from their unicellular hosts. Eukaryotic genes present in bacterial genomes, here termed EUGENs, represent a hallmark of intracellular bacteria, and are rarely if at all present in free-living bacteria. Thus, EUGENs are expected to define intracellular parasitic or symbiotic bacterial life styles, yet it is currently largely unknown to what extent and how. The genomes of members of the families Legionellaceae, Coxiellaceae and Parachlamydiaceae harbor dozens if not hundreds of EUGENs. Many EUGEN products are effector proteins that are translocated into host cells through dedicated type III or type IV secretion systems, and some have indeed been shown to interfere with host vesicle trafficking or signal transduction pathways. Yet, most EUGENs and their products have not been functionally studied on a molecular and cellular level. Here, we propose to analyze the role of distinct EUGENs in Legionella and Coxiella virulence, Protochlamydia symbiosis, and pathogen or symbiont metabolism. Furthermore, we will study for the first time the role of EUGENs in co-infection and bacterial ecology. By analyzing distinct groups of EUGENs and their products from different families of intracellular environmental bacteria, our studies will (i) provide mechanistic insights into parasitic and symbiotic processes, (ii) contribute to elucidating the relationship between parasitism and symbiosis, and (iii) establish a link between bacterial virulence or symbiosis and intracellular bacterial metabolism. These studies will reveal novel targets for anti- bacterial compounds and identify effectors useful as molecular probes and for vaccine development.

Bacteria interact with animals and humans in different ways, causing disease (pathogens) or providing benefit (symbionts). The mechanisms mediating these interactions are often poorly understood but frequently employ gene products originally acquired from their animal partners. This collaborative research project, involving seven partners from four European countries aimed at better understanding the role of those genes in a selection of bacterial pathogens and symbionts. The Austrian subproject, led by Matthias Horn at the Department of Microbiology and Ecosystem Science (University of Vienna), investigated bacteria able to infect both humans as well as unicellular protists such as amoeba. The scientists have discovered a novel molecular harpoon mediating bacterial cell-cell interactions. Their work revealed novel insights into the regulations and evolution of bacterial proteins used for the invasion of animal and protist cells, and they learned that bacterial symbionts and pathogens infecting the same amoeba cell interfere with each other, eventually leading to eradication of the pathogen. In the focus of the subproject were Legionella pneumophila, the causative agent of Legionnaires disease, and chlamydiae, which are well known as human pathogens but also include symbionts living within amoeba widespread in the environment. The research team found that chlamydia symbionts efficiently inhibit the replication of L. pneumophila in Acanthamoeba castellanii. Importantly, L. pneumophila released from amoeba containing the symbiont showed decreased virulence. Their experiments revealed unexpected interactions of human pathogens with environmental microbes and suggest a possible route to control spread of L. pneumophila. Using cutting-edge approaches in genomics, systems biology, and evolutionary biology, the scientists further analyzed host interaction mechanisms of symbiotic chlamydiae. They discovered hitherto unmatched large groups of genes (gene families) in these amoeba- associated bacteria, suggesting that the symbionts manipulate their host cells using an armada of originally protist-derived effectors simultaneously. Together, results from this project helped to better understand molecular mechanisms of microbes for invading and exploiting animal and protist host cells, contributing important puzzle pieces in our quest for new strategies and molecular targets for anti-bacterial therapy and vaccine development.