Evolution of Symbiosis in the Bacteroidetes

Free-living amoebae are abundant in terrestrial habitats and are among the most important predators of microorganisms, thereby having a major impact on microbial community composition. During evolution some bacteria have learned to evade phagocytosis by amoebae, to use them as hosts, and to establish a stable and symbiotic association with these protozoa. In total, five groups of symbionts of Acanthamoeba sp., belonging to three different phyla, are known to date. All Acanthamoeba symbionts are obligate intracellular, i.e. they can not be cultivated in cell-free media. Due to their only recent discovery and their intracellular life style our knowledge about these elusive bacteria is still scarce. Whole genome sequence analysis of a chlamydia-related symbiont recently revealed that the interaction between these symbionts and amoebae is ancient and has led to the development of interaction mechanisms, which are still today employed by modern pathogenic chlamydiae (Horn et al., 2004; Science 304: 728). This study demonstrated that genomics is the most efficient approach to investigate obligate intracellular bacterial symbionts of protozoa. In this study we plan to analyze the Acanthamoeba symbiont `Candidatus Amoebophilus asiaticus`, which belongs to the phylum Bacteroidetes (Horn et al., 2001; Environ Microbiol 3: 440), with respect to its biology, its interaction with its host, and its evolutionary history. Members of the highly diverse phylum Bacteroidetes are ubiquitous and abundant in many habitats; they are considered to play an important role in biogeochemical cycles. The Bacteroidetes contain only four groups of endosymbiotic bacteria, the cockroach symbionts Blattabacterium spp. and the related cicada symbiont `Candidatus Sulcia muelleri`, the parasitic wasp symbiont `Candidatus Cardinium hertigii`, and the amoeba symbiont `Candidatus A. asiaticus`. The A. asiaticus genome sequence will allow us to investigate the evolution of the intracellular life style within the phylum Bacteroidetes. We will be able to determine molecular interaction mechanisms between symbionts and host cell, and to compare these to mechanisms found in other intracellular bacteria. The A. asiaticus genome sequence (which is with 1.5-1.8 Mb small compared to known Bacteroidetes genomes) will furthermore enable us to analyze the process of genome reduction in the Bacteroidetes - a key process during evolution of intracellular bacterial symbionts and pathogens, currently almost exclusively studied using Proteobacteria as examples. As genome sequencing is a hypothesis-generating research, we will further use the `Cand. Amoebophilus asiaticus` genome sequence to test and verify in-silico predictions experimentally, thereby mainly focusing on the interaction between `Cand. Amoebophilus asiaticus` and its amoeba host.

Free-living amoebae are abundant in terrestrial habitats and are among the most important predators of microorganisms, thereby having a major impact on microbial community composition. During evolution some bacteria have learned to evade phagocytosis by amoebae, to use them as hosts, and to establish a stable and symbiotic association with these protozoa. In total, five groups of symbionts of Acanthamoeba sp., belonging to three different phyla, are known to date. All Acanthamoeba symbionts are obligate intracellular, i.e. they can not be cultivated in cell-free media. Due to their only recent discovery and their intracellular life style our knowledge about these elusive bacteria is still scarce. Whole genome sequence analysis of a chlamydia-related symbiont recently revealed that the interaction between these symbionts and amoebae is ancient and has led to the development of interaction mechanisms, which are still today employed by modern pathogenic chlamydiae (Horn et al., 2004; Science 304: 728). This study demonstrated that genomics is the most efficient approach to investigate obligate intracellular bacterial symbionts of protozoa. In this study we plan to analyze the Acanthamoeba symbiont "Candidatus Amoebophilus asiaticus", which belongs to the phylum Bacteroidetes (Horn et al., 2001; Environ Microbiol 3: 440), with respect to its biology, its interaction with its host, and its evolutionary history. Members of the highly diverse phylum Bacteroidetes are ubiquitous and abundant in many habitats; they are considered to play an important role in biogeochemical cycles. The Bacteroidetes contain only four groups of endosymbiotic bacteria, the cockroach symbionts Blattabacterium spp. and the related cicada symbiont "Candidatus Sulcia muelleri", the parasitic wasp symbiont "Candidatus Cardinium hertigii", and the amoeba symbiont "Candidatus A. asiaticus". The A. asiaticus genome sequence will allow us to investigate the evolution of the intracellular life style within the phylum Bacteroidetes. We will be able to determine molecular interaction mechanisms between symbionts and host cell, and to compare these to mechanisms found in other intracellular bacteria. The A. asiaticus genome sequence (which is with 1.5-1.8 Mb small compared to known Bacteroidetes genomes) will furthermore enable us to analyze the process of genome reduction in the Bacteroidetes - a key process during evolution of intracellular bacterial symbionts and pathogens, currently almost exclusively studied using Proteobacteria as examples. As genome sequencing is a hypothesis-generating research, we will further use the "Cand. Amoebophilus asiaticus" genome sequence to test and verify in-silico predictions experimentally, thereby mainly focusing on the interaction between "Cand. Amoebophilus asiaticus" and its amoeba host.