Bacterial endosymbionts of free-living amoebae
Trophozoite of Acanthamoeba sp. | 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 diversity of these bacterial endosymbionts and their interaction with their eukaryotic hosts was the main focus of our recent studies.
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The obligate endosymbionts of several isolates of free-living amoebae could be 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.
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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 (Fritsche at al., 1998). 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 (Horn et al., 2002). Another endosymbiont, tentatively classified as "Candidatus Amoebophilus asiaticus", was identified as a novel member of the phylum Cytophaga-Flavobacterium-Bacteroides (Horn et al., 2001). | 
Phylogenetic relationship of Acanthamoeba symbionts |
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 (Horn et al., 1999).
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Several coccoid endosymbionts of acanthamoebae 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 (Amann et al., 1997; Horn et al., 2000; Fritsche et al., 2000). Development and application of a PCR assay specific for these novel members of the Chlamydiales demonstrated that the diversity of chlamydiae in the environment is not yet fully discovered (Horn and Wagner, 2001).
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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.
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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.
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Links
 Environmental chlamydiae
Nucleotide transport proteins in environmental chlamydiae
Environmental chlamydia genomics and post-genomics
Investigated by: Matthias Horn
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