Acanthamoebae are ubiquitous free-living amoebae that play an important role as predators controlling microbial communities. In addition, acanthamoebae have been recognized as opportunistic human pathogens. Despite being a food source several bacteria are able to survive the uptake and temporarily multiply in free-living amoebae. Furthermore, one fourth of Acanthamoeba isolates contain unknown obligate intracellular bacterial symbionts. The symbiosis between these bacteria and their amoebal host cells was subject of this thesis.   The obligate endosymbionts of nineteen isolates of free-living amoebae were assigned to five novel evolutionary lineages by means of the full cycle rRNA approach, including comparative rRNA sequence analysis and fluorescence in situ hybridization. Four of these lineages appeared to be cosmopolitan in distribution since phylogenetically almost identical strains were found in Acanthamoeba isolates from geographically distant sources. Interestingly, these novel lineages are most closely related to other obligate intracellular bacteria including several important human pathogens.   Two rod-shaped Gram-negative endosymbionts formed a novel lineage within the Rickettsiales, a diverse group within the alpha-subclass of Proteobacteria comprising recognized pathogens of animal and humans as well as a number of insect symbionts. Several other, rod-shaped Gram-negative endosymbionts, for which the provisional genus "Candidatus Procabacter" was proposed, were members of a previously not recognized evolutionary lineage within the beta-subclass of Proteobacteria. Another endosymbiont, tentatively classified as "Candidatus Amoebophilus asiaticus", was identified as a novel member of the phylum Cytophaga-Flavobacterium-Bacteroides.   Several Acanthamoeba endosymobionts were identified as being related to the paramecium symbiont Caedibacter caryophilus and were therefore grouped into the new genus "Candidatus Paracaedibacter". Comparison of the phylogenetic affiliation of the Caedibacter-affiliated endosymbionts and their amoebal host cells suggested co-evolution between these symbionts and their Acanthamoeba hosts.   Several coccoid endosymbionts of Acanthamoeba were demonstrated to represent at least two new genera (designated Parachlamydia and Neochlamydia) within the Chlamydiales, indicating a previously underestimated genus-level diversity within this medically important order. Development and application of a PCR assay specific for these novel members of the Chlamydiales demonstrated that the genus-level diversity of chlamydiae in the environment is yet not fully discovered.   First indications for a molecular basis of the intimate association between free-living amoebae and their bacterial endosymbionts are provided by the detection of genes coding for putative non-mitochondrial ATP/ADP transporter proteins in members of three of the five endosymbiont lineages, suggesting that these bacteria can thrive as energy-parasites within their Acanthamoeba host cells.   The analysis of the symbiosis between free-living amoebae and their intracellular bacteria might eventually help to understand the process of adaptation of bacteria to an intracellular way of life. Long before humans evolved, these adaptations might have represented the first steps in the development of microorganisms pathogenic for humans.       «

Acanthamoebae are ubiquitous free-living amoebae that play an important role as predators controlling microbial communities. In addition, acanthamoebae have been recognized as opportunistic human pathogens. Despite being a food source several bacteria are able to survive the uptake and temporarily multiply in free-living amoebae. Furthermore, one fourth of Acanthamoeba isolates contain unknown obligate intracellular bacterial symbionts. The symbiosis between these bacteria and their amoebal host...     »